Partial List of Abstracts, 2001

Three pilot studies were performed to assess application of the eddy covariance micrometeorological method in the measurement of carbon dioxide (CO₂) flux of volcanic origin. The selected study area is one of high diffuse CO₂ emission on Mammoth Mountain, CA. Because terrain and source characteristics make this a complex setting for this type of measurement, added consideration was given to source area and upwind fetch. Footprint analysis suggests that the eddy covariance measurements were representative of an upwind elliptical source area (3.8×10³ m²) which can vary with mean wind direction, surface roughness, and atmospheric stability. CO₂ flux averaged 8-16 mg m^-2 s^-1 (0.7-1.4 kg m^-2 day^-1). Eddy covariance measurements of flux were compared with surface chamber measurements made in separate studies and were found to be similar.

This investigation into the occurrence, character, and transformation of dissolved organic matter (DOM) in treatment wetlands in the western United States shows that (i) the nature of DOM in the source water has a major influence on transformations that occur during treatment, (ii) the climate factors have a secondary effect on transformations, (iii) the wetlands receiving treated wastewater can produce a net increase in DOM, and (iv) the hierarchical analytical approach used in this study can measure the subtle DOM transformations that occur. As wastewater treatment plant effluent passes through treatment wetlands, the DOM undergoes transformation to become more aromatic and oxygenated. Autochthonous sources are contributed to the DOM, the nature of which is governed by the developmental stage of the wetland system as well as vegetation patterns. Concentrations of specific wastewater-derived organic contaminants such as linear alkylbenzene sulfonate, caffeine, and ethylenediaminetetraacetic acid were significantly attenuated by wetland treatment and were not contributed by internal loading.

Tracer experiments provide information about aquifer material properties vital for accurate site characterization. Unfortunately, density-induced sinking can distort tracer movement, leading to an inaccurate assessment of material properties. Yet existing criteria for selecting appropriate tracer concentrations are based on analysis of homogeneous media instead of media with heterogeneities typical of field sites. This work introduced a hydraulic-gradient correction for heterogeneous media and applies it to a criterion previously used to indicate density-induced instabilities in homogeneous media. The modified criterion was tested using a series of two-dimensional heterogeneous intermediate-scale tracer experiments and data from several detailed field tracer tests. The intermediate-scale experimental facility (10.0 x 1.2 x 0.06 m) included both homogeneous and heterogeneous (s²_{ln k} = 1.22) zones. The field tracer tests were less heterogeneous (0.24 < s²_{ln k} < 0.37), but measurements were sufficient to detect density-induced sinking. Evaluation of the modified criterion using the experiments and field tests demonstrates that the new criterion appears to account for the change in density-induced sinking due to heterogeneity. The criterion demonstrates the importance of accounting for heterogeneity to predict density-induced sinking and differences in the onset of density-induced sinking in two- and three-dimensional systems.

A multidisciplinary study of a crude-oil contaminated aquifer shows that the distribution of microbial physiologic types is strongly controlled by the aquifer properties and crude oil location. The microbial populations of four physiologic types were analyzed together with permeability, pore-water chemistry, nonaqueous oil content, and extractable sediment iron. Microbial data from three vertical profiles through the anaerobic portion of the contaminated aquifer clearly show areas that have progressed from iron-reduction to methanogenesis. These locations contain lower numbers of iron reducers, and increased numbers of fermenters with detectable methanogens. Methanogenic conditions exist both in the area contaminated by nonaqueous oil and also below the oil where high hydrocarbon concentrations correspond to local increases in aquifer permeability. The results indicate that high contaminant flux either from local dissolution or by advective transport plays a key role in determining which areas first become methanogenic. Other factors besides flux that are important include the sediment Fe(II) content and proximity to the water table. In locations near a seasonally oscillating water table, methanogenic conditions exist only below the lowest typical water table elevation. During 20 years since the oil spill occurred, a laterally continuous methanogenic zone has developed along a narrow horizon extending from the source area to 50-60 m downgradient. A companion paper [J. Contam. Hydrol. 53, 369-386] documents how the growth of the methanogenic zone results in expansion of the aquifer volume contaminated with the highest concentrations of benzene, toluene, ethylbenzene, and xylenes.

Dissolved organic matter (DOM) has been implicated as an important complexing agent for Hg that can affect its mobility and bioavailability in aquatic ecosystems. However, binding constants for natural Hg-DOM complexes are not well known. We employed a competitive ligand approach to estimate conditional stability constants for Hg complexes with DOM isolates collected from Florida Everglades surface waters. The isolates examined were the hydrophobic fraction of DOM from a eutrophic, sulfidic site (F1-HPoA) and the hydrophilic fraction from an oligotrophic, low-sulfide site (2BS-HPiA). Our experimental determinations utilized overall octanol-water partitioning coefficients (D_ow) for ²⁰³Hg at 0.01 M chloride and across pH and DOM concentration gradients. Use of this radioisotope allowed rapid determinations of Hg concentrations in both water and octanol phases without problems of matrix interference.

Conditional stability constants (I = 0.06, 23°C) were log K´ = 11.8 for F1-HPoA and log K´ = 10.6 for 2BS-HPiA. These are similar to previously published stability constants for Hg binding to low-molecular-weight thiols. Further, F1-HPoA showed a pH-dependent decline in D_ow that was consistent with models of Hg complexation with thiol groups as the dominant Hg binding sites in DOM. These experiments demonstrate that the DOM isolates are stronger ligands for Hg than chloride ion or ethylenediamine-tetraacetic acid. Speciation calculations indicate that at the DOM concentrations frequently measured in Everglades, 20 to 40 µM, significant complexation of Hg by DOM would be expected in aerobic (sulfide-free) surface waters.

South-central New Mexico, USA, at the junction of the Rocky Mountains, High Plains and Chihuahuan Desert, is one of the better known regions in the late Quaternary of North America. Plant macrofossils and pollen from a packrat midden series in Rough Canyon, New Mexico allows refinement of plant distributions and paleoclimates in this transitional area since full glacial times. From 17000 to 12000 ¹⁴C yr BP, Pinus edulis-Juniperus scopulorum woodlands dominated limestone substrates between 1800 and 1490 m, with Pseudotsuga menziesii and other mixed-conifer species restricted to shady, north-facing slopes. Juniperus deppeana, the dominant juniper today above 2000 m in southern New Mexico, is conspicuously absent from glacial middens and must have been displaced south of the US-Mexico border. The minimum climatic conditions for P. edulis-J. scopulorum woodlands are ca 20% wetter and 3.5-5 ^oC cooler (July mean maximum temperatures) than the modern climate at Rough Canyon. Holocene warming/drying may have started as early as 12000 ¹⁴C yr BP with the extirpation of J. scopulorum from Rough Canyon, and was completed by at least 10540 ¹⁴C yr BP. The record for arrivals of some desert species is confounded by traces of pollen and macrofossils in some of the glacial middens, which could signify either earliest occurrence or temporal mixing (contamination) of assemblages. AMS (¹⁴C dating can discriminate between early arrival and contamination in midden macrofossils but not in pollen. AMS dates show that Choisya dumosa, presently near its northern (cold) limits at Rough Canyon, endured late glacial winters, possibly as clonal populations. Some Larrea tridentata leaves and pollen occur in middens dominated by conifers and oaks no longer at the site; an AMS date of 3205 ¹⁴C yr BP on Larrea leaves from one midden indicates contamination. Evidence for some macrofossil contamination, however, does not rule out the possibility that pollen of desert elements (e.g. Larrea, Prosopis) in late glacial-early Holocene middens indicates their presence in the Tularosa Basin, well ahead of their local appearance in Rough Canyon. Finally, the increasing dominance of desert elements after 5000 ¹⁴C yr BP in the Rough Canyon series and elsewhere in the northern Chihuahuan Desert could reflect slow, postglacial migration from the south and/or progressive encroachment with gradual stripping of soils formed during the last glacial period.

Climate in the North Pacific and North American sectors has experienced interdecadal shifts during the twentieth century. A network of recently developed tree-ring chronologies for Southern and Baja California extends the instrumental record and reveals decadal-scale variability back to 1661. The Pacific decadal oscillation (PDO) is closely matched by the dominant mode of tree-ring variability that provides a preliminary view of multiannual climate fluctuations spanning the past four centuries. The reconstructed PDO index features a prominent bidecadal oscillation, whose amplitude weakened in the late 1700s to mid-1800s. A comparison with proxy records of ENSO suggests that the greatest decadal-scale oscillations in Pacific climate between 1706 and 1977.

We isolated an obligately anaerobic halophilic bacterium from the Dead Sea that grew by respiration of selenate. The isolate, designated strain DSSe-1, was a gram-negative, non-motile rod. It oxidized glycerol or glucose to acetate + CO₂ with concomitant reduction of selenate to selenite plus elemental selenium. Other electron acceptors that supported anaerobic growth on glycerol were nitrate and trimethylamine-N-oxide; nitrite, arsenate, fumarate, dimethylsulfoxide, thiosulfate, elemental sulfur, sulfite or sulfate could not serve as electron acceptors. Growth on glycerol in the presence of nitrate occurred over a salinity range from 100 to 240 g/l, with an optimum at 210 g/l. Analysis of the 16S rRNA gene sequence suggests that strain DSSe-1 belongs to the order Halanaerobiales, an order of halophilic anaerobes with a fermentative or homoacetogenic metabolism, in which anaerobic respiratory metabolism has never been documented. The highest 16S rRNA sequence similarity (90%) was found with Acetohalobium arabaticum (X89077). On the basis of physiological properties as well as the relatively low homology of 16S rRNA from strain DSSe-1 with known genera, classification in a new genus within the order Halanaerobiales, family Halobacteroidaceae is warranted. We propose the name Selenihalanaerobacter shriftii. Type strain is strain DSSe-1 (ATCC accession number BAA-73).

The physical and chemical characteristics of Lake Bonney, a permanently ice-covered closed basin lake in Taylor Valley, Antarctica are influenced significantly by local climate. The rising lake-levels of the past thirty years indicate a recent change in the local climate. We explored the significance of twentieth century changes in lake-level as a climate-change indicator by using a hydrologic model for the basin and a Monte Carlo simulation based on the variability in the available 30 yrs of hydrologic record. We compared the lake-level in the retrospective simulations with a measurement surveyed by Robert Scott's party in 1903. All the retrospective simulations based on the observed inflows yielded estimates that the lake was dry in 1903. It was necessary to remove 6 yrs from the observed 21-yr record for the retrospective simulation to match the measured 1903 lake-level for 50% of the simulations. From these analyses, we conclude that the period from 1969 to the present has had greater availability of solar radiation for meltwater generation, possibly brought about by changing cloud-cover patterns and coupled with a gradual warming trend. A third simulation indicated that an annual increase in inflow of about 3% between 1903 and 1973 would be required to match the 1903 measurement.

The %N and d¹⁵N values of soils and plants were measured along a chronosequence spanning 3 to 3000 Ky in a California annual grassland. Total soil N decreased with increasing soil age (1.1 to 0.4 kg N m^-2) while the mean d¹⁵N values of the soil N increased by several ^0/00 from the youngest to oldest sites (+3.5 to +6.2 ^0/00). The d¹⁵N values of plants varied along the gradient, reflecting changing soil N pools and differences in the form of N uptake. The decline in total N storage with time is hypothesized to be due to a shift from N to P limitation with increasing soil age. The general increase in d¹⁵N values with time is interpreted using a N mass balance model, and appears to reflect a shift toward an increasing proportional losses of inorganic mineral forms of N (vs. organic forms) with increasing soil age. We develop a quantitative index of this trend (mineral vs. organic forms of N loss) using mass balance considerations and parameters. The %N and d¹⁵N values along the California age gradient were compared to the published data for a comparably aged chronosequence in Hawaii. Most striking in this comparison is the observation that the California soil and plant d¹⁵N values are several ^0/00 greater than those on comparably aged Hawaiian sites. Multiple explanations are plausible, but assuming the sites have a similar range in d¹⁵N values of atmospheric inputs, the isotopic differences suggest that N may be, at least seasonally, in greater excess in the strongly seasonal, semi-arid, California grassland.

The presence of low-chloride fluids in the lowermost sediments drilled at Ocean Drilling Program Site 808, at the Nankai accretionary wedge, has been considered as prime evidence for long-distance, lateral fluid flow from depth. Here, we re-evaluate the potential role of in situ reaction of smectite (S) to illite (I) in the genesis of this low chloride anomaly. This reaction is known to be occurring at Site 808, with both the S content and S to I ratio in the mixed layer clays decreasing substantially with depth. We show that the bulk of the chloride anomaly can generate by in situ clay dehydration, particularly if pre-reaction smectite abundances (Ai) approach ~10-15% of the bulk sediment. The A_i values, however, are not well constrained. At A_i values <10-15%, an additional source of low-Cl fluid centered close to the décollement could be required. Thus, there remains the important possibility that the observed low-Cl anomaly is a compound effect of both lateral flow and in situ smectite dehydration.

The inter-annual variation in the structure of the benthic community of riffles and pools was evaluated in contrasting geomorphic settings. The community structure of riffles and pools was a function of habitat, reach gradient, and discharge and was taxon specific. In years of below average peak discharge, riffles had higher taxon richness than pools (66 versus 47) but richness was similar between habitats during a year of average discharge (56 versus 54). The percentage composition of oligochaetes and elmid beetles was more variable inter-annually in pools and low gradient reaches than in high gradient reaches. Differences in the percentage of collector-gatherers and scrapers in riffles and pools appeared related to inter-annual differences in discharge regimes. Two components of the annual discharge regime appear to differentially affect the composition of the benthic community in the snowmelt dominated stream studied: the magnitude of the annual peak discharge and the duration and timing of the period of extended high flow.

Fluctuations in spring climate in the western United States over the last 4-5 decades are described by examining changes in the blooming of plants and the timing of snowmelt-runoff pulses. The two measures of spring's onset that are employed are the timing of first bloom of lilac and honeysuckle bushes from a long-term cooperative phenological network, and the timing of the first major pulse of snowmelt recorded from high-elevation streams. Both measures contain year-to-year fluctuations, with typical year-to-year fluctuations at a given site of one to three weeks. These fluctuations are spatially coherent, forming regional patterns that cover most of the west. Fluctuations in lilac first bloom dates are highly correlated to those of honeysuckle, and both are significantly correlated with those of the spring snowmelt pulse. Each of these measures, then, probably respond to a common mechanism. Various analyses indicate that anomalous temperature exerts the greatest influence upon both interannual and secular changes in the onset of spring in these networks. Earlier spring onsets since the late 1970s are a remarkable feature of the records, and reflect the unusual spell of warmer-than-normal springs in western North America during this period. The warm episodes are clearly related to larger-scale atmospheric conditions across North America and the North Pacific, but whether this is predominantly an expression of natural variability or also a symptom of global warming is not certain. (on-line pdf file, 1.06 MB, published by the American Meteorological Society withopen-access)

In dealing with the passive transport of organic contaminants from soils to plants (including crops), a partition-limited model is proposed in which (i) the maximum (equilibrium) concentration of a contaminant in any location in the plant is determined by partition equilibrium with its concentration in the soil interstitial water, which in turn is determined essentially by the concentration in the soil organic matter (SOM) and (ii) the extent of approach to partition equilibrium, as measured by the ratio of the contaminant concentrations in plant water and soil interstitial water, a_pt (£1), depends on the transport rate of the contaminant in soil water into the plant and the volume of soil water solution that is required for the plant contaminant level to reach equilibrium with the external soil-water phase. Through reasonable estimates of plant organic-water compositions and of contaminant partition coefficients with various plant components, the model accounts for calculated values of a_pt in several published crop-contamination studies, including near-equilibrium values (i.e.,a_pt @1) for relatively water-soluble contaminants and lower values for much less soluble contaminants; the differences are attributed to the much higher partition coefficients of the less soluble compounds between plant lipids and plant water, which necessitates much larger volumes of the plant water transport for achieving the equilibrium capacities. The model analysis indicates that for plants with high water contents the plant-water phase acts as the major reservoir for highly water-soluble contaminants. By contrast, the lipid in a plant, even at small amounts, is usually the major reservoir for highly water-insoluble contaminants.

Snow-water equivalent (SWE) data measured at several hundred montane sites in the western United States are used to examine the historic effects of El Nino and La Nina events on seasonal snowpack evolution in the major subbasins in the Columbia and Colorado River systems. Results are used to predict annual runoff. In the Columbia River Basin, there is a general tendency for decreased SWE during El Nino years and increased SWE during La Nina years. However, the SWE anomalies for El Nino years are much less pronounced. This occurs in part because midlatitude circulation anomalies in El Nino years are located 35 degrees east of those in La Nina years. This eastward shift is most evident in midwinter, at which time, SWE anomalies associated with El Nino are actually positive in coastal regions of the Columbia River Basin. In the Colorado River Basin, mean anomalies in SWE and annual runoff during El Nino years depict a transition between drier-than-average conditions in the north, and wetter-than-average conditions in the southwest. Associations during La Nina years are generally opposite those in El Nino years. SWE anomalies tend to be more pronounced in spring in the Lower Colorado River Basin. Our predictions of runoff reveal modest skill for scenarios using only historic El Nino and La Nina information. Predictions based on the water stored in the seasonal snowpack are, in almost all cases, much higher than those based on El Nino-Southern Oscillation (ENSO) information alone. However, combining observed midwinter snow conditions with information on seasonal snowpack evolution associated with ENSO improves predictions for basins in which ENSO signals exhibit strong seasonally.

Spatial and temporal patterns in streamflow are rarely monitored for ephemeral streams. Flashy, erosive streamflows common in ephemeral channels create a series of operational and maintenance problems, which makes it impractical to deploy a series of gaging stations along ephemeral channels. Streambed temperature is a robust and inexpensive parameter to monitor remotely, leading to the possibility of analyzing temperature patterns to estimate streamflow frequency and duration along ephemeral channels. A simulation model was utilized to examine various atmospheric and hydrological upper boundary conditions compared with a series of hypothetical temperature-monitoring depths within the streambed. Simulation results indicate that streamflow events were distinguished from changing atmospheric conditions with greater certainty using temperatures at shallow depths (e.g., 10-20 cm) as opposed to the streambed surface. Three ephemeral streams in the American Southwest were instrumented to monitor streambed temperature for determining the accuracy of using this approach to ascertain the long-term temporal and spatial extent of streamflow along each stream channel. Streambed temperature data were collected at the surface or at shallow depth along each stream channel, using thermistors encased in waterproof, single-channel data loggers tethered to anchors in the channel. On the basis of comparisons with site information, such as direct field observations and upstream flow records, diurnal temperature variations successfully detected the presence and duration of streamflow for all sites.

Evaluation of ¹⁴C in tree rings provides a measure of the flux of magmatic CO₂ from Mammoth Mountain both before and after 1994 when copious diffuse emissions were first discovered and linked to tree kill. We analyzed the annual rings of trees with two main purposes: (1) to track changes in the magnitude of magmatic CO₂ emission over time, and (2) to determine the onset of magmatic CO₂ emission at numerous sites on Mammoth Mountain. The onset of CO₂ emission at different areas of tree kill was determined to be in 1990, closely following the seismic events of 1989. At Horseshoe Lake (HSL), CO₂ emission was found to have peaked in 1991 and to have subsequently declined by a factor of two through 1998. The tree-ring data also show that emissions of magmatic carbon from cold springs below the tree-kill areas occurred well before 1989. Trees located on the margins of the kill areas or otherwise away from zones of maximum gas discharge were found to be better integrators of magmatic CO₂ emission than those located in the center of tree kills. Although quantitative extrapolations from our data to a flux history will require that a relationship be established between ¹⁴C depletion in tree rings and average annual magmatic CO₂ flux, the pattern of ¹⁴C depletion in tree rings is likely to be the most reliable indicator of the long-term changes in the magnitude of CO₂ release from Mammoth Mountain.

A 16-year study of a hydrocarbon plume shows that the extent of contaminant migration and compound-specific behavior have changed as redox reactions, most notably iron reduction, have progressed over time. Concentration changes at a small scale, determined from analysis of pore-water samples drained from aquifer cores, are compared with concentration changes at the plume scale, determined from analysis of water samples from an observation well network. The small-scale data show clearly that the hydrocarbon plume is growing slowly as sediment iron oxides are depleted. Contaminants, such as ortho-xylene that appeared not to be moving downgradient from the oil on the basis of observation well data, are migrating in thin layers as the aquifer evolves to methanogenic conditions. However, the plume-scale observation well data show that the downgradient extent of the Fe²⁺ and BTEX plume did not change between 1992 and 1995. Instead, depletion of the unstable Fe (III) oxides near the subsurface crude-oil source has caused the maximum dissolved iron concentration zone within the plume to spread at a rate of approximately 3 m/year. The zone of maximum concentrations of benzene, toluene, ethylbenzene and xylene (BTEX) has also spread within the anoxic plume. In monitoring the remediation of hydrocarbon-contaminated ground water by natural attenuation, subtle concentration changes in observation well data from the anoxic zone may be diagnostic of depletion of the intrinsic electron-accepting capacity of the aquifer. Recognition of these subtle patterns may allow early prediction of growth of the hydrocarbon plume.

A pore network model with cubic chambers and rectangular tubes was used to estimate the nonaqueous phase liquid (NAPL) dissolution rate coefficient, K_dissa₁, and NAPL/water total specific interfacial area, a₁. K_dissa₁ was computed as a function of modified Peclet number (Pe') for various NAPL saturations (S_N) and a₁ during drainage and imbibition and during dissolution without displacement. The largest contributor to a₁ was the interfacial area in the water-filled corners of chambers and tubes containing NAPL. When K_dissa₁ was divided by a₁, the resulting curves of dissolution coefficient, K_diss versus Pe' suggested that an approximate value of K_diss could be obtained as a weak function of hysteresis or S_N. Spatially and temporally variable maps of K_dissa₁ calculated using the network model were used in field-scale simulations of NAPL dissolution. These simulations were compared to simulations using a constant value of K_dissa₁ and the empirical correlation of Powers and others [1994, Water Resour. Res. v. 30, no. 2, p. 321]. Overall, a methodology was developed for incorporating pore-scale processes into field-scale prediction of NAPL dissolution.

Effluent from a large combined sewer overflow (CSO) in Boston and receiving waters near the CSO outfall were sampled during dry and wet weather conditions. Surficial sediments were also collected from the vicinity of the CSO and at nearby sites. The samples were analyzed for a variety of organic constituents including organic carbon and nitrogen, linear alkylbenzenes (LABs), coprostanol and polychlorinated biphenyls (PCBs). As judged by the presence of waste-specific markers (LABs, coprostanol), the CSO effluent contains sewage under both dry and wet weather conditions. When rainfall occurs, the concentration of suspended solids and all organic constituents in the particulate phase increase, ultimately approaching those characteristic of untreated sewage. The concentrations of LABs and PCBs in the effluent are strongly correlated, indicating that PCBs in the CSO are derived from sewage inputs. During heavy rainfall, the vast majority (>90%) of the hydrophobic organic substances are associated with suspended particulate matter, whereas during dry weather, a significant fraction resides in the operationally defined `dissolved' phase. Estimates of the mass emission rates of CSO constituents show that >70% of the suspended particles and >90% of the particulate organic carbon, hydrocarbons and trace organics are discharged during wet weather. Particles in the receiving water appear to be strongly influenced by the CSO effluent during wet weather. Concentrations of PCBs in surficial sediments near the CSO are correlated with those of coprostanol and the LABs, indicating that these compounds are derived from similar sources. Based on the observed correlations, approximately 60-80% of the sedimentary PCBs originate from sewage. Comparison of SLAB/coprostanol ratios of effluent particles, surficial sediments and sewage sludges suggest that the vast majority of the marker compounds and the PCBs in sediments are not from the CSO, but are derived from one of two sewage treatment plants that discharged sludge into the harbor until 1991. The sludge-derived contaminants were probably carried by tidal currents into Dorchester Bay and deposited in shallow, quiescent embayments where sedimentation is favored. These results illustrate the potential importance of long-range transport of waste-derived contaminants in urban harbors and their rapid accumulation in localized depocenters.

Diffuse emissions of CO₂ are known to be large around some volcanoes and hydrothermal areas. Accumulation-chamber measurements of CO₂ flux are increasingly used to estimate the total magmatic or metamorphic CO₂ released from such areas. To assess the performance of accumulation chamber systems at fluxes one to three orders of magnitude higher than normally encountered in soil respiration studies, a test system was constructed in the laboratory where known fluxes could be maintained through dry sand. Steady-state gas concentration profiles and fractionation effects observed in the 30-cm sand column nearly match those predicted by the Stefan-Maxwell equations, indicating that the test system was functioning successfully as a uniform porous medium. Eight groups of investigators tested their accumulation chamber equipment, all configured with continuous infrared gas analyzers (IRGA), in this system. Over a flux range of ~200-12,000 g m^-2 day^-1, 90% of their 203 flux measurements were 0-25% lower than the imposed flux with a mean difference of 12.5%. Although this difference would seem to be within the range of acceptability for many geologic investigations, some potential sources for larger errors were discovered. A steady-state pressure gradient of ^-20 Pa/m was measured in the sand column at a flux of 11,200 g m^-2 day^-1. The derived permeability (50 darcies) was used in the dusty-gas model (DGM) of transport to quantify various diffusive and viscous flux components. These calculations were used to demonstrate that accumulation chambers, in addition to reducing the underlying diffusive gradient, severely disrupt the steady-state pressure gradient. The resultant diversion of the net gas flow is probably responsible for the systematically low flux measurements. It was also shown that the fractionating effects of a viscous CO₂ efflux against a diffusive influx of air will have a major impact on some important geochemical indicators, such as N₂/Ar, d¹⁵N-N₂, and ⁴He/²²Ne.

Mass concentration of suspended sollids is one of the properties needed to understand characteristics of sediment transport in bays and estuaries. Menaingful estimates of total suspended solids concentrations are particularly difficult to obtain because common methods require collection of numerous water samples that tend to under sample the highly variable characteristics of suspended solids. If optical sensors are used they oftne become useless because of biological fouling after short periods of time in highly productive water bodies. Acoustic sensors that are routinely used to measure water velocity hold promise as a means of quantitatively estimating total suspended solids from acoustic backscatter intensity, a by-product used in measurement of velocity. A field experiment was designed and carried out in San Francisco Bay, California in October 1998 to investigate the possibility of estimating total suspended solids concentration from backscatter intensity measured by acoustic Doppler current profilers. Results of estimates from two acoustic Doppler current profilers (1200 kHz and 2400 kHz) deployed at two sites in South San Francisco Bay are described in this paper. Estimated total suspended concentrations are found to agree within about 15 percent (of total range of concentration) of values estimated by optical backscatterance sensors. Success of these estimates provides promise that this technique might be appropriate for determining total suspended solids concentration from commercially available acoustic Doppler current profilers. Some potential pitfalls and limitations of this approach that stem from theoretical limitations of attenuation properties of acoustic waves are discussed.

Advances in technology have resulted in a new instrument that is designed for in-situ determination of particle size spectra. Such an instrument that can measure undisturbed particle size distributions is much needed for sediment transport studies. The LISST-100 (Laser In-Situ Scattering and Transmissometry) uses the principle of laser diffraction to obtain the size distribution and volume concentration of suspended material in 32 size classes logarithmically spaced between 1.25 and 250µm. This paper describes a laboratory evaluation of the ability of LISST-100 to determine particle sizes using suspensions of single size, artificial particles. Findings show the instrument is able to determine particle size to within about 10% with increasing error as particle size increases. The instrument determines volume (or mass) concentration using a volume conversion factor C_v. This volume conversion factor is theoretically a constant. In the laboratory evaluation C_v is found to vary by a factor of about three over the particle size range between 5 and 200µm. Results from field studies in South San Francisco Bay show that values of mass concentration of suspended marine sediments estimated by LISST-100 agree favorably with estimates from optical backscatterance sensors if an appropriate value of C_v, according to mean size, is used and the assumed average particle (aggregate) density is carefully chosen. Analyses of size distribution of suspended materials in South San Francisco Bay over multiple tide cycles suggest the likelihood of different sources of sediment because of different size characteristics during flood and ebb cycles.

Godsy, E.M., Warren, E., and Westjohn, D.B., 2001, Methanogenic biodegradation of charcoal production wastes in groundwater at Kinsford, Michigan, USA: International Association of Hydrologic Sciences Publication No. 269, p. 303-309.

A house exploded in the City of Kingsford, Michigan USA. The explosion was caused by CH₄ that leaked into the basement from the surrounding soil. Evidence suggests that biodegradation of products from the distillation and spillage at or near a former wood carbonization plant site was the major source of CH₄ and CO₂ in the groundwater system. The plant area is directly upgradient from deep groundwater, samples of which are green-yellow in colour, have a very strong odour of burnt wood, contain high concentrations of mononuclear aromatic and phenolic compounds, and extremely high concentrations of volatile fatty acids. The majority of the dissolved compounds in these groundwater samples have been shown, using laboratory microcosms, to be anaerobically biodegradable to CH₄ and CO₂. The biodegradable compounds, and the amounts of CH₄ and CO₂ produced in the microcosms, are consistent with observations from field samples.

Several approaches were used to characterize ground water/surface water interactions near the Willamette River--a large (ninth order) river in Oregon, USA. A series of potentiometric surface maps demonstrated the presence of highly dynamic hydraulic gradients between rivers and the adjacent aquifer. Hyporheic zone gradients extended on the order of hundreds of meters. River gains and losses at the river stretch scale (tens of kilometers) were consistent with fluxes implied by the potentiometric surface maps, and apparently reflect regional ground water/surface water interactions. Gains and losses of up to 5-10% of streamflow were observed at this scale. On the river reach scale (1-2 km), gains and losses on the order of 5% of streamflow were interpreted as representing primarily local hyporheic exchange. Isotopic and chemical data collected from shallow hyporheic zone wells demonstrated interaction between regional ground water and river water. The origin of sampled hyporheic zone water ranged from a mixture dominated by regional ground water to water containing 100% river water. The common assumption that ground and river water mix primarily in the river channel is not applicable in this system. Isotopic and chemical data also indicated that significant (nearly complete) vegetative nitrate uptake and/or nitrate reduction occurred in water from 4 of 12 hyporheic zone sites. In these cases, it was primarily nitrate transported to the hyporheic zone in regional ground water that was removed from solution. Isotopes of water and nitrate indicated that hyporheic zone water sampled at two sites was composed of water originating as river water and demonstrated that significant vegetative nitrate uptake and nitrate reduction occurred along these hyporheic zone flowpaths. Thus, the hyporheic zone may, in some instances, serve to remove nitrate from river water. Additional investigations with chemical tools and microbial enzyme assays were conducted at one hyporheic site. A strong vertical redox gradient was observed, with nitrate-limited denitrification potential in deeper sediment and both nitrification and denitrification potential in shallower sediment. Since nitrogen cycling is strongly affected by redox conditions, nitrogen cycling in the hyporheic zone of this large-river system likely is affected by dynamics of ground water-surface water interactions that control fluxes of nitrogen and other redox species to hyporheic zone sediment.

We compiled time series of hydrothermal discharge consisting of 3593 chloride- or heat-flux measurements from 24 sites in the Yellowstone region, the northern Oregon Cascades, Lassen Volcanic National Park and vicinity, and Long Valley, California. At all of these sites the hydrothermal phenomena are believed to be as yet unaffected by human activity, though much of the data collection was driven by mandates to collect environmental-baseline data in anticipation of geothermal development. The time series average 19years in length and some of the Yellowstone sites have been monitored intermittently for over 30 years. Many sites show strong seasonality but few show clear long-term trends, and at most sites statistically significant decadal-scale trends are absent. Thus, the data provide robust estimates of advective heat flow ranging from ~130MW in the north-central Oregon Cascades to ~6100MW in the Yellowstone region, and also document Yellowstone hydrothermal chloride and arsenic fluxes of 1740 and 15-20g/s, respectively. The discharge time series show little sensitivity to regional tectonic events such as earthquakes or inflation/deflation cycles. Most long-term monitoring to date has focused on high-chloride springs and low-temperature fumaroles. The relative stability of these features suggests that discharge measurements done as part of volcano-monitoring programs should focus instead on high-temperature fumaroles, which may be more immediately linked to the magmatic heat source.

Residence times of groundwater, discharging from springs in the middle Suwannee River Basin, were estimated using chlorofluorocarbons (CFCs), tritium (³H), and tritium/helium-3 (³H/³He) age-dating methods to assess the chronology of nitrate contamination of spring waters in northern Florida. During base-flow conditions for the Suwannee River in 1997-1999, 17 water samples were collected from 12 first, second, and third magnitude springs discharging groundwater from the Upper Floridan aquifer. Extending age-dating techniques, using transient tracers to spring waters in complex karst systems, required an assessment of several models [piston-flow (PFM), exponential mixing (EMM), and binary-mixing (BMM)] to account for different distributions of groundwater age. Multi-tracer analyses of four springs yielded generally concordant PFM ages of around 20±2 years from CFC-12, CFC-113, 3H, and ³He, with evidence of partial CFC-11 degradation. The EMM gave a reasonable fit to CFC-113, CFC-12, and ³H data, but did not reproduce the observed ³He concentrations or ³H/³He ratios, nor did a combination PFM-EMM. The BMM could reproduce most of the multi-tracer data set only if both endmembers had ³H concentrations not much different from modern values. CFC analyses of 14 additional springs yielded apparent PFM ages from about 10 to 20 years from CFC-113, with evidence of partial CFC-11 degradation and variable CFC-12 contamination. While it is not conclusive, with respect to the age distribution within each spring, the data indicate that the average residence times were in the order of 10-20 years and were roughly proportional to spring magnitude. Applying similar models to recharge and discharge of nitrate based on historical nitrogen loading data yielded contrasting trends for Suwanee County and Lafayette County. In Suwanee County, spring nitrate trends and nitrogen isotope data were consistent with a peak in fertilizer input in the 1970s and a relatively high overall ratio of artificial fertilizer/manure; whereas in Lafayette County, spring nitrate trends and nitrogen isotope data were consistent with a more monotonic increase in fertilizer input and relatively low overall ratio of artificial fertilizer/manure. The combined results of this study indicate that the nitrate concentrations of springs in the Suwannee River basin have responded to increased nitrogen loads from various sources in the watersheds over the last few decades; however, the responses have been subdued and delayed because the average residence time of groundwater discharging from springs are in the order of decades.

In January 1999, wastewater influent and effluent from the pretreatment plant at the Stringfellow hazardous waste disposal site were sampled along with groundwater at six locations along the groundwater contaminant plume. The objectives of this sampling and study were to identify at the compound class level the unidentified 40-60% of wastewater organic contaminants, and to determine what organic compound classes were being removed by the wastewater pretreatment plant, and what organic compound classes persisted during subsurface waste migration. The unidentified organic wastes are primarily chlorinated aromatic sulfonic acids derived from wastes from DDT manufacture. Trace amounts of EDTA and NTA organic complexing agents were discovered along with carboxylate metabolites of the common alkylphenolpolyethoxylate plasticizers and nonionic surfactants. The wastewater pretreatment plant removed most of the aromatic chlorinated sulfonic acids that have hydrophobic neutral properties, but the p-chlorobenzenesulfonic acid which is the primary waste constituent passed through the pretreatment plant and was discharged in the treated wastewaters transported to an industrial sewer. During migration in groundwater, p-chlorobenzenesulfonic acid is removed by natural remediation processes. Wastewater organic contaminants have decreased 3- to 45-fold in the groundwater from 1985 to 1999 as a result of site remediation and natural remediation processes. The chlorinated aromatic sulfonic acids with hydrophobic neutral properties persist and have migrated into groundwater that underlies the adjacent residential community.

Molecular weight distributions of fulvic acid from the Suwannee River, Georgia, were investigated by electrospray ionization/quadrupole mass spectrometry (ESI/QMS), and fragmentation pathways of specific fulvic acid masses were investigated by electrospray ionization/ion trap multistage tandem mass spectrometry (ESI/MST/MS). ESI/QMS studies of the free acid form of low molecular weight poly(carboxylic acid) standards in 75% methanol/25% water mobile phase found that negative ion detection gave the optimum generation of parent ions that can be used for molecular weight determinations. However, experiments with poly(acrylic acid) mixtures and specific high molecular weight standards found multiply charged negative ions that gave a low bias to molecular mass distributions. The number of negative charges on a molecule is dependent on the distance between charges. ESI/MST/MS of model compounds found characteristic water loss from alcohol dehydration and anhydride formation, as well as CO₂ loss from decarboxylation, and CO loss from ester structures. Application of these fragmentation pathways to specific masses of fulvic acid isolated and fragmented by ESI/MST/MS is indicative of specific structures that can serve as a basis for future structural confirmation after these hypothesized structures are synthesized.

Rhodamine WT dye-tracer injections in rivers of the Willamette Basin yield concentration-time curves with characteristically long recession times suggestive of active transient storage processes. The scale of drainage areas contributing to the stream reaches studied in the Willamette Basin ranges from 10 to 12,000 km². A transient storage assessment of the tracer studies has been completed using the U.S. Geological Survey's One-dimensional Transport with Inflow and Storage (OTIS) model, which incorporates storage exchange and decay functions along with the traditional dispersion and advection transport equation. The analysis estimates solute transport of the dye. It identifies first-order decay coefficients to be on the order of 10-5/sec for the nonconservative Rhodamine WT. On an individual subreach basis, the first-order decay is slower (typically by an order of magnitude) than the transient storage process, indicating that nonconservative tracers may be used to evaluate transient storage in rivers. In the transient storage analysis, a dimensionless parameter (AS/A) expresses the spatial extent of storage zone area relative to stream cross section. In certain reaches of Willamette Basin pool-and-riffle, gravel-bed rivers, this parameter was as large as 0.5. A measure of the storage exchange flux was calculated for each stream subreach in the simulation analysis. This storage exchange is shown subjectively to be higher at higher stream discharges. Hyporheic linkage between streams and subsurface flows is the probable physical mechanism contributing to a significant part of this inferred active transient storage. Hyporheic linkages are further suggested by detailed measurements of river discharge with an Acoustic Doppler Current Profiler system delineating zones in two large rivers where water alternately enters and leaves the surface channels through gravel-and-cobble riverbeds. Measurements show patterns of hyporheic exchange that are highly variable in time and space.

The nature and chlorine reactivity of organic constituents in reclaimed water (tertiary-treated municipal wastewater) before, during, and after recharge into groundwater at the Montebello Forebay in Los Angeles County, CA, was the focus of this study. Dissolved organic matter (DOM) in reclaimed water from this site is primarily a mixture of aromatic sulfonates from anionic surfactant degradation, N-acetyl amino sugars and proteins from bacterial activity, and natural fulvic acid, whereas DOM from native groundwaters in the aquifer to which reclaimed water was recharged consists of natural fulvic acids. The hydrophilic neutral N-acetyl amino sugars that constitute 40% of the DOM in reclaimed water are removed during the first 3 m of vertical infiltration in the recharge basin. Groundwater age dating with ³H and ³He isotopes, and determinations of organic and inorganic C isotopes, enabled clear differentiation of recent recharged water from older native groundwater. Phenol structures in natural fulvic acids in DOM isolated from groundwater produced significant trihalomethanes (THM) and total organic halogen (TOX) yields upon chlorination, and these structures also were responsible for the enhanced SUVA and specific fluorescence characteristics relative to DOM in reclaimed water. Aromatic sulfonates and fulvic acids in reclaimed water DOM produced minimal THM and TOX yields.

This report documents the Link-AMG (LMG) Package that links MODFLOW-2000, the U.S. Geological Survey modular, transient, three-dimensional, finite-difference ground-water flow model, to an algebraic multigrid (AMG) solver for matrix equations. The LMG Package has some distinct advantages over other solvers available with MODFLOW-2000 for problems with large grids (more than about 40,000 cells) and (or) a highly variable hydraulic-conductivity field. Experience has indicated that, in such problems, execution times using the AMG solver are typically about 2 to 25 times faster than execution times using MODFLOW's PCG2 Package with the modified incomplete Cholesky preconditioner. The drawback to the AMG method used in LMG is the relatively large amount of computer memory required. In problems simulated for this work, the AMG solver used typically required 3 to 8 times more memory than PCG2. For one 465,600-node problem, for example, 151 megabytes (MB) of memory were required when the LMG package was used, whereas PCG2 required 47 MB of memory. The execution time, however, decreased from 942 seconds to 50 seconds, so there is a clear trade-off between execution time and memory requirements. On modern computers, such memory requirements are becoming increasingly attainable.

This report provides a brief description of the AMG method used, an explanation of the convergence criterion, a discussion of its memory requirements, some performance comparisons, and sample data inputs. In addition, detailed instructions on how the LMG Package links the AMG code to MODFLOW-2000 are provided.

Michel, R.L., Silva, S.R., Bemis, B., Godsy, and Westjohn, D.B., 2001, Compound-specific carbon isotope analysis of a contaminant plume in Kinsford, Michigan, USA, in Gehrels, H., Peters, N.E., Hoehn, E., Jensen, K., Leibundgut, C., Griffioen, J., Webb, B., and Zaadnoordijk, W.J., eds., Impact of human activity on groundwater dynamics: International Association of Hydrologic Sciences Publication No. 269, p. 311-316.

Compound-specific isotope analysis was used to study a contaminated site near Kingsford, Michigan, USA. Organic compounds at three of the sites studied had similar d¹³C values indicating that the contaminant source is the same for all sites. At a fourth site, chemical and d¹³C values had evolved due to microbial degradation of organics, with the d¹³C being much heavier than the starting materials. A microcosm experiment was run to observe isotopic changes with time in the methane evolved and in compounds remaining in the water during degradation. The d¹³C values of the methane became heavier during the initial period of the run when volatile fatty acids were being consumed. There was an abrupt decrease in the d¹³C values when fatty acids had been consumed and phenols began to be utilized. The d¹³C value of the propionate remaining in solution also increased, similar to the results found in the field.

The probabilistic response of depth-integrated soil water to given climatic forcing can be described readily using an existing supply-demand-storage model. An apparently complex interaction of numerous soil, climate, and plant controls can be reduced to a relatively simple expression for the equilibrium probability density function of soil water as a function of only two dimensionless parameters. These are the index of dryness (ratio of mean potential evaporation to mean precipitation) and a dimensionless storage capacity (active root zone soil water capacity divided by mean storm depth). The first parameter is mainly controlled by climate, with surface albedo playing a subsidiary role in determining net radiation. The second is a composite of soil (through moisture retention characteristics), vegetation (through rooting characteristics), and climate (mean storm depth). This minimalist analysis captures many essential features of a more general probabilistic analysis, but with a considerable reduction in complexity and consequent elucidation of the critical controls on soil water variability. In particular, it is shown that (1) the dependence of mean soil water on the index of dryness approaches a step function in the limit of large soil water capacity; (2) soil water variance is usually maximized when the index of dryness equals 1, and the width of the peak varies inversely with dimensionless storage capacity; (3) soil water has a uniform probability density function when the index of dryness is 1 and the dimensionless storage capacity is large; and (4) the soil water probability density function is bimodal if and only if the index of dryness is <1, but this bimodality is pronounced only for artificially small values of the dimensionless storage capacity.

A synthesis of available data for the Mississippi River Basin (area 3x10⁶ km²) reveals an upward trend in evaporation during recent decades, driven primarily by increases in precipitation and secondarily by human water use. A cloud-related decrease in surface net radiation appears to have accompanied the precipitation trend. Resultant evaporative and radiative cooling of the land and lower atmosphere quantitatively explains downward trends in observed pan evaporation. These cooling tendencies also reconcile the observed regional atmospheric cooling with the anticipated regional "greenhouse warming." If recent high levels of precipitation (which correlate with the North Atlantic Oscillation) are mainly caused by an internal climatic fluctuation, an eventual return to normal precipitation could reveal heretofore-unrealized warming in the basin. If, instead, they are caused by some unidentified forcing that will continue to grow in the future, then continued intensification of water cycling and suppression of warming in the basin could result.

Spatial determinations of the metal loads in Wightman Fork can be used to identify potential source areas to the stream. In September 1997, a chloride tracer-injection study was done concurrently with synoptic water-quality sampling in Wightman Fork near the Summitville Mine site. Discharge was determined and metal concentrations at 38 sites were used to generate mass-load profiles for dissolved aluminum, copper, iron, manganese, and zinc. The U.S. Environmental Protection Agency had previously identified these metals as contaminants of concern.

Metal loads increased substantially in Wightman Fork near the Summitville Mine. A large increase occurred along a 60-meter reach that is north of the North Waste Dump and generally corresponds to a region of radial faults. Metal loading from this reach was equivalent to 50 percent or more of dissolved aluminum, copper, iron, manganese, and zinc load upstream from the outfall of the Summitville Water Treatment Facility (SWTF). Overall, sources along the entire reach upstream from the SWTF were equivalent to 15 percent of the iron, 33 percent of the copper and manganese, 58 percent of the zinc, and 66 percent of the aluminum load leaving the mine site. The largest increases in metal loading to Wightman Fork occurred as a result of inflow from Cropsy Creek. Aluminum, iron, manganese, and zinc loads from Cropsy Creek were equivalent to about 40 percent of the specific metal load leaving the mine site. Copper, iron, and manganese loads from Cropsy Creek were nearly as large or larger than the load from sources upstream from the SWTF.

For the period 1980-89, we estimate a carbon sink in the coterminous United States between 0.30 and 0.58 petagrams of carbon per year (petagrams of carbon = 10¹⁵ grams of carbon). The net carbon flux from the atmosphere to the land was higher, 0.37 to 0.71 petagrams of carbon per year, because a net flux of 0.07 to 0.13 petagrams of carbon per year was exported by rivers and commerce and returned to the atmosphere elsewhere. These land-based estimates are larger than those from previous studies (0.08 to 0.35 petagrams of carbon per year) because of the inclusion of additional processes and revised estimates of some component fluxes. Although component estimates are uncertain, about one-half of the total is outside the forest sector. We also estimated the sink using atmospheric models and the atmospheric concentration of carbon dioxide (the tracer-transport inversion method). The range of results from the atmosphere-based inversions contains the land-based estimates. Atmosphere- and land-based estimates are thus consistent, within the large ranges of uncertainty for both methods. Atmosphere-based results for 1980-89 are similar to those for 1985-89 and 1990-94, indicating a relatively stable U.S. sink throughout the period.

Scale and sludge deposits formed during oil production can contain elevated levels of Ra, often coprecipitated with barium sulfate (barite). The potential for sulfate-reducing bacteria to release ²²⁶Ra and Ba (a Ra analog) from oil-field barite was evaluated. The concentration of dissolved Ba increased when samples containing pipe scale, tank sludge, or oil-field brine pond sediment were incubated with sulfate-reducing bacteria Desulfovibrio sp., Str LZK1, isolated from an oil-field brine pond. However, Ba release was not stoichiometric with sulfide production in oil-field samples, and < 0.1% of the Ba was released. Potential for the release of ²²⁶Ra was demonstrated, and the ²²⁶Ra release associated with sulfate-reducing activity was predictable from the amount of Ba released. As with Ba, only a fraction of the ²²⁶Ra expected from the amount of sulfide produced was released, and most of the Ra remained associated with the solid material.

Chemical and isotopic properties of water discharging from springs and wells in Shenandoah National Park (SNP), near the crest of the Blue Ridge Mountains, VA, USA were monitored to obtain information on groundwater residence times. Investigated time scales included seasonal (wet season, April, 1996; dry season, August-September, 1997), monthly (March through September, 1999) and hourly (30-min interval recording of specific conductance and temperature, March, 1999 through February, 2000). Multiple environmental tracers, including tritium/helium-3 (³H/³He), chlorofluorocarbons (CFCs), sulfur hexafluoride (SF₆), sulfur-35 (³⁵S), and stable isotopes (d¹⁸O and d²H) of water, were used to estimate the residence times of shallow groundwater discharging from 34 springs and 15 wells. The most reliable ages of water from springs appear to be based on SF₆ and ³H/³He, with most ages in the range of 0-3 years. This range is consistent with apparent ages estimated from concentrations of CFCs; however, CFC-based ages have large uncertainties owing to the post-1995 leveling-off of the CFC atmospheric growth curves. Somewhat higher apparent ages are indicated by ³⁵S (>1.5 years) and seasonal variation of d¹⁸O (mean residence time of 5 years) for spring discharge. The higher ages indicated by the ³⁵S and ¹⁸O data reflect travel times through the unsaturated zone and, in the case of ³⁵S, possible sorption and exchange of S with soils or biomass. In springs sampled in April, 1996, apparent ages derived from the ³H/³He data (median age of 0.2 years) are lower than those obtained from SF₆ (median age of 4.3 years), and in contrast to median ages from ³H/³He (0.3 years) and SF₆ (0.7 years) obtained during the late summer dry season of 1997. Monthly samples from 1999 at four springs in SNP had SF₆ apparent ages of only 1.2 to 2.5±0.8 years, and were consistent with the 1997 SF6 data. Water from springs has low excess air (0-1 cm³ kg^-1) and N₂-Ar temperatures that vary seasonally. Concentrations of He and Ne in excess of solubility equilibrium indicate that the dissolved gases are not fractionated. The seasonal variations in N₂-Ar temperatures suggest shallow, seasonal recharge, and the excess He and Ne data suggest waters mostly confined to gas exchange in the shallow, mountain-slope, water-table spring systems. Water from wells in the fractured rock contains up to 8 cm³ kg^-1 of excess air with ages in the range of 0-25 years. Transient responses in specific conductance and temperature were observed in spring discharge within several hours of large precipitation events in September, 1999; both parameters increased initially, then decreased to values below pre-storm base-flow values. The groundwater residence times indicate that flushing rates of mobile atmospheric constituents through groundwater to streams draining the higher elevations in SNP average less than 3 years in base-flow conditions.

Negatively charged functional groups of fulvic acid compete with inorganic sulfide ion for mercury ion binding. This competition is evaluated here by using a discrete site-electrostatic model to calculate mercury solution speciation in the presence of fulvic acid. Model calculated species distributions are used to estimate a mercury-fulvic acid apparent binding constant to quantify fulvic acid and sulfide ion competition for dissolved inorganic mercury (Hg(II)) ion binding. Speciation calculations done with PHREEQC, modified to use the estimated mercury-fulvic acid apparent binding constant, suggest that mercury-fulvic acid and mercury-sulfide complex concentrations are equivalent for very low sulfide ion concentrations (about 10^-11 M) in Everglades' surface water. Where measurable total sulfide concentration (about 10^-7 M or greater) is present in Everglades' surface water, mercury-sulfide complexes should dominate dissolved inorganic mercury solution speciation. In the absence of sulfide ion (for example, in oxygenated Everglades' surface water), fulvic acid binding should dominate Everglades' dissolved inorganic mercury speciation.

Mammoth Mountain, a dormant volcano in the eastern Sierra Nevada, California, has been passively degassing large quantities of cold magmatic CO₂ since 1990 following a 6-month-long earthquake swarm associated with a shallow magmatic intrusion in 1989. A search for any link between gas discharge and volcanic hazard at this popular recreation area led us to initiate a detailed study of the degassing process in 1997. Our continuous monitoring results elucidate some of the physical controls that influence dynamics in flank CO₂ degassing at this volcano. High coherence between variations in CO₂ efflux and variations in atmospheric pressure and wind speed imply that meteorological parameters account for much, if not all of the variability in CO₂ efflux rates. Our results help explain differences among previously published estimates of CO₂ efflux at Mammoth Mountain and indicate that the long-term (annual) CO₂ degassing rate has in fact remained constant since ~1997. Discounting the possibility of large meteorologically driven temporal variations in gas efflux at other volcanoes may result in spurious interpretations of transients that do not reflect actual geologic processes.

To predict changes in sediment transport, it is essential to know whether transport is regulated mainly by changes in flow or by changes in grain size of sediment on the bed. In flows where changes in suspended sediment transport are regulated purely by changes in flow (grain size of bed sediment is constant), increases in flow strength cause increases in both concentration and grain size of sediment in suspension (because stronger flows are able to suspend more sediment and coarser grains). Under this constraint of constant grain size of bed sediment concentration and median diameter of suspended sediment are positively correlated (because increasing the median diameter of the bed sediment causes the concentration to decrease while causing the median grain size in suspension to increase). Where both flow strength and grain size on the bed are free to vary, the relation between concentration and grain size regulation relative to flow regulation of sediment transport, a measure defined as a. To predict sediment transport in systems that are regulated dominantly by changes in grain size on the bed, it is more useful to measure sediment input events or changes in grain size on the bed than to measure changes in flow. More commonly, grain size of bed sediment may be secondary to flow in regulating transport but may, nevertheless, be important. The relative coarseness of bed sediment (b) can be measured directly or, like a, can be calculated from measurements of concentration and grain size of suspended sediment.

Concentrations of tritium (³H) and dicholorodifluoromethane (CFC-12) in water samples taken from glacial drift and fractured crystalline rock over 4 km² in central New Hampshire are interpreted to identify a conceptual model of matrix diffusion an the magnitude of the diffusion coefficient. Dispersion and mass transfer to and from fractures has affected the 3H concentration to the extent that the peak ³H concentration of the 1960s is no longer distinguishable. Because of heterogeneity in the bedrock the sparsely distributed chemical data do not warrant a three-dimensional transport model. Instead, a one-dimensional model of CFC-12 and ³H migration along flow lines in the glacial drift and bedrock is used to place bounds on the processes affecting kilometer-scale transport, and model parameters are varied to reproduce the measured relation between ³H and CFC-12, rather than their spatial distributions. A model of mass exchange to and from fractures that is dependent on the time-varying concentration gradient at fracture surfaces qualitatively reproduces the measured relation between ³H and CFC-12 with an upper bound for the fracture dispersivity approximately equal to 250 m and a lower bound for the effective matrix diffusion coefficient equal to 1 m^2/yr. The diffusion coefficient at the kilometer scale is at least 3 orders of magnitude greater than laboratory estimates of diffusion in crystalline rock. The large diffusion coefficient indicates that diffusion into an immobile fluid phase (rock matrix) is masked at the kilometer scale by advective mass exchange between fractures with large contrasts in transmissivity. The measured transmissivity of fractures in the study area varies over more than 6 orders of magnitude. Advective mass exchange from high-permeability fractures to low-permeability fractures results in short migration distances of a chemical constituent in low-permeability fractures over an extended period of time before reentering high-permeability fractures; viewed at the kilometer scale, this process is analogous to the chemical constituent diffusing into and out of an immobile fluid phase.

We report a new method for measurement of the isotopic composition of nitrate (NO₃^-) at the natural-abundance level in both seawater and freshwater. The method is based on the isotopic analysis of nitrous oxide (N₂O) generated from nitrate by denitrifying bacteria that lack N₂O^-reductase activity. The isotopic composition of both nitrogen and oxygen from nitrate are accessible in this way. In this first of two companion manuscripts, we describe the basic protocol and results for the nitrogen isotopes. The precision of the method is better than 0.2^0/00 (1 SD) at concentrations of nitrate down to 1 mM, and the nitrogen isotopic differences among various standards and samples are accurately reproduced. For samples with 1 mM nitrate or more, the blank of the method is less than 10% of the signal size, and various approaches may reduce it further.

A groundwater plume enriched in ¹⁵NO₃^- was created upgradient of a mesohaline salt marsh. By measuring thechanges in concentration and isotopic enrichment of NO₃^-, N₂O, N₂, NH₄⁺, and particulate organic nitrogen (PON) during plume transport through the marsh, in situ rates of dissimilatory nitrate reduction to ammonium (DNRA) and denitrification (DNF) were estimated, as well as N storage in the reduced N pools. For groundwater discharge within the top 10 cm of marsh, NO₃^- removal was 90% complete within the 50 cm of marsh nearest the upland 2 3 border. The peak NO₃^- loss rate from the plume ranged from 208 to 645 mM d^-1. Rates of DNRA (180 mM d^-1) and DNF (387–465 mM d^-1) processed 30% and 70% of the NO₃^- load, respectively. Terminal N₂O production was approximately equal to N₂ production rates during DNF. Comparison of ¹⁵N lost from the ¹⁵NO₃^- pool and ¹⁵N gained in each of the reduced products accounted for only 22% of the reduced ¹⁵N, thus indicating N export from the system. Despite high rates of DNRA, the NH₄⁺ produced was not a long-term repository for the groundwater-derived N but was instead rapidly immobilized into marsh PON and retained on longer timescales. The small inventory of ¹⁵N in the N₂O and N₂ pools relative to DNF rates, coincident with an undersaturation of dissolved argon, indicated that denitrified N was exported to the atmosphere on short timescales. The relative magnitudes of DNF and DNRA in conjunction with the immobilization of NH₄⁺ and evasion of N gases dictated the extent of export versus retention of the groundwater NO₃^- load.

In situ stimulation of denitrification has been proposed as a mechanism to remediate groundwater nitrate contamination. In this study, sodium formate was added to a sand and gravel aquifer on Cape Cod, MA, to test whether formate could serve as a potential electron donor for subsurface denitrification. During 16- and 10-day trials, groundwater from an anoxic nitrate-containing zone (0.5-1.5 mM) was continuously withdrawn, amended with formate and bromide, and pumped back into the aquifer. Concentrations of groundwater constituents were monitored in multilevel samplers after up to 15 m of transport by natural gradient flow. Nitrate and formate concentrations were decreased 80-100% and 60-70%, respectively, with time and subsequent travel distance, while nitrite concentrations inversely increased. The field experiment breakthrough curves were simulated with a two-dimensional site-specific model that included transport, denitrification, and microbial growth. Initial values for model parameters were obtained from laboratory incubations with aquifer core material and then refined to fit field breakthrough curves. The model and the lab results indicated that formate-enhanced nitrite reduction was nearly 4-fold slower than nitrate reduction, but in the lab, nitrite was completely consumed with sufficient exposure time. Results of this study suggest that a long-term injection of formate is necessary to test the remediation potential of this approach for nitrate contamination and that adaptation to nitrite accumulation will be a key determinative factor.

At the toe of the northern Barbados accretionary complex, temperature and pore water chemistry data indicate that fluid flow is channeled along the decollement and other shallow thrust faults. We examine mechanisms that may prevent consolidation and maintain high permeability over large sections of the decollement. High-resolution bulk density data from five boreholes show that the decollement is well consolidated at some sites while other sites remain underconsolidated. Underconsolidated decollement behavior is associated with kilometer-scale negative-polarity seismic reflections from the decollement plane that have been interpreted to be fluid conduits. We use a coupled fluid flow/consolidation model to simulate the loading response of a 10-km-long by 680-m-thick slice of sediment as it enters the accretionary complex. The simulations capture 185 ka (5km) of subduction, with a load function representing the estimated effective stress of the overriding accretionary prism (3.8 degree taper angle). Simulation results of bulk density in the decollement 3.2 km arcward of the deformation front are compared with observations. The results show that persistent high pore pressures at the arcward edge of the simulation domain can explain underconsolidated behavior. The scenario is consistent with previous modeling results showing that high pore pressures can propagate intermittently along the decollement from deeper in the complex. Simulated seaward fluxes in the decollement (1-14 cm yr^-1) lie between previous estimates from modeling studies of steady state (<1 cm yr^-1) and transient (>1 m yr^-1) flow. Maximum simulated instantaneous fluid sources (2.5 x 10^-13s^-1) are comparable to previous estimates. The simulations show minor swelling of incoming sediments (fluid sources ~ -3 x 10^-15s^-1) up to 3 km before subduction that may help to explain small-scale shearing and normal faulting proximal to the protodecollement.

Carbon dioxide (CO₂) accumulates under lake ice in winter and degasses to the atmosphere after ice melt. This large springtime CO₂ pulse is not typically considered in surface-atmosphere flux estimates, because most field studies have not sampled through ice during late winter. Measured CO₂ partial pressure (pCO₂) of lake surface water ranged from 8.6 to 4,290 Pa (85-4,230 µatm) in 234 north temperate and boreal lakes prior to ice melt during 1998 and 1999. Only four lakes had surface pCO₂ less than or equal to atmospheric pCO₂, whereas 75% had pCO₂ >5 times atmospheric. The d¹³CDIC (DIC 5 SCO₂) of 142 of the lakes ranged from -26.28^0/00 to +0.95^0/00. Lakes with the greatest pCO₂ also had the lightest d¹³C_DIC, which indicates respiration as their primary CO₂ source. Finnish lakes that received large amounts of dissolved organic carbon from surrounding peatlands had the greatest pCO₂. Lakes set in noncarbonate till and bedrock in Minnesota and Wisconsin had the smallest pCO₂ and the heaviest d¹³C_DIC, which indicates atmospheric and/or mineral sources of C for those lakes. Potential emissions for the period after ice melt were 2.36 ± 1.44 mol CO2 m^-2 for lakes with average pCO₂ values and were as large as 13.7 ± 8.4 mol CO₂ m^-2 for lakes with high pCO₂ values.

Because groundwater discharge along coastal shorelines is often concentrated in zones inhabited by fringing wetlands, accurately estimating discharge is essential for understanding its effect on the function and maintenance of these ecosystems. Most previous estimates of groundwater discharge to coastal wetlands have been temporally limited and have used only a single approach to estimate discharge. Furthermore, groundwater input has not been considered as a major mechanism controlling pore-water flushing. We estimated seasonally varying groundwater discharge into a fringing estuarine wetland using three independent methods (Darcy's Law, salt balance, and Br^- tracer). Seasonal patterns of discharge predicted by both Darcy's Law and the salt balance yielded similar seasonal patterns with discharge maxima and minima in spring and early fall, respectively. They differed, however, in the estimated magnitude of discharge by two- to fourfold in spring and by 10-fold in fall. Darcy estimates of mean discharge ranged between -8.0 and 80 L m^-2d^-1, whereas the salt balance predicted groundwater discharge of 0.6 to 22 L m^-2d^-11. Results from the Br^- tracer experiment estimated discharge at 16 L m^-2d^-1, or nearly equal to the salt balance estimate at that time. Based upon the tracer test, pore-water conductivity profiles, and error estimates for the Darcy and salt balance approaches, we concluded that the salt balance provided a more certain estimate of groundwater discharge at high flow (spring). In contrast, the Darcy method provided a more reliable estimate during low flow (fall). Groundwater flushing of pore water in the spring exported solutes to the estuary at rates similar to tidally driven surface exchange seen in previous studies. Based on pore-water turnover times, the groundwater-driven flux of dissolved organic carbon (DOC), dissolved organic nitrogen (DON), and NH₄⁺ to the estuary was 11.9, 1.6, and 1.3 g C or g N m^-2 wetland for the 90 days encompassing peak spring discharge. Groundwater-induced flushing of the wetland subsurface therefore represents an important mechanism by which narrow fringing marshes may seasonally relieve salt stress and export material to adjacent water masses.

An important problem in hydrologic science is understanding how river flow is influenced by rainfall properties and drainage basin characteristics. In this paper we consider one approach, the use of mass exponents, in examining the relation of river flow to rainfall and the channel network, which provides the primary conduit for transport of water to the outlet in a large basin. Mass exponents, which characterize the power-law behavior of moments as a function of scale, are ideally suited for defining scaling behavior of processes that exhibit a high degree of variability or intermittency. The main result in this paper is an expression relating the mass exponent of flow resulting from an instantaneous burst of rainfall to the mass exponents of spatial rainfall and that of the network width function. Spatial rainfall is modeled as a random multiplicative cascade and the channel network as a recursive replacement tree; these fractal models reproduce certain types of self-similar behavior seen in actual rainfall and networks. It is shown that under these modeling assumptions the scaling behavior of flow mirrors that of rainfall if rainfall is highly variable in space, and on the other hand flow mirrors the structure of the network if rainfall is not so highly variable.

During 1993-97, samples of the full depth of the Rocky Mountain snowpack were collected at 52 sites from northern New Mexico to Montana and analyzed for major-ion concentrations. Concentrations of acidity, sulfate, nitrate, and calcium increased from north to south along the mountain range. In the northern part of the study area, acidity was most correlated (negatively) with calcium. Acidity was strongly correlated (positively) with nitrate and sulfate in the southern part and for the entire network. Acidity in the south exceeded the maximum acidity measured in snowpack of the Sierra Nevada and Cascade Mountains. Principal component analysis indicates three solute associations we characterize as: (1) acid (acidity, sulfate, and nitrate), (2) soil (calcium, magnesium, and potassium), and (3) salt (sodium, chloride, and ammonium). Concentrations of acid solutes in the snowpack are similar to concentrations in nearby wetfall collectors, whereas, concentrations of soil solutes are much higher in the snowpack than in wetfall. Thus, dryfall of acid solutes during the snow season is negligible, as is gypsum from soils. Snowpack sampling offers a cost-effective complement to sampling of wetfall in areas where wetfall is difficult to sample and where the snowpack accumulates throughout the winter.

Five years of global continental water storage variations are used to predict the effects of long-wavelength, long-period variability in water storage on gravity observations. At the sites of existing superconducting gravimeters, the modeled gravity changes have root-mean-square (RMS) values of as much as 7 mu Gals, with ranges of up to 22 mu Gals. Variations much larger than these values can be found over large regions the globe. We find that the gravity effects are predominantly annual in character. We also find that the modeled responses to water loading exhibit long-period variations that could be mistaken for secular tectonic trends when observed over a time span of a few years.

Standard reference water samples (SRWS) were collected from two mine sites, one near Ophir, CO, USA and the other near Redding, CA, USA. The samples were filtered, preserved, and analyzed for rare earth element (REE) concentrations (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) by inductively coupled plasma-mass spectrometry (ICP-MS). These two samples were acid mine waters with elevated concentrations of REEs (0.45-161 µg/l). Seventeen international laboratories participated in a `round-robin' chemical analysis program, which made it possible to evaluate the data by robust statistical procedures that are insensitive to outliers. The resulting most probable values are reported. Ten to 15 of the participants also reported values for Ba, Y, and Sc. Field parameters, major ion, and other trace element concentrations, not subject to statistical evaluation, are provided.

Sources and hydrologic flow paths need to be determined to evaluate remedial options in mining-affected basins. The ⁸⁷Sr/⁸⁶Sr ratios of a suite of water and rock samples from the Middle Fork Mineral Creek basin in the upper Animas River watershed, Colorado, were determined to investigate their possible use as a geochemical tracer for sources and flow paths. Leaching experiments were performed on the dominant lithologies in the study area to determine the more easily weathered constituents including strontium. Variations in whole-rock ⁸⁷Sr/⁸⁶Sr ratios correlate with lithology and hydrothermal alteration intensity. For a given alteration assemblage, the porphyritic quartz monzonite has a lower ⁸⁷Sr/⁸⁶Sr ratio than the surrounding San Juan Volcanics, and for a given lithology the ⁸⁷Sr/⁸⁶Sr ratio is lower for propylitically altered rocks than for quartz-sericite-pyrite altered rocks. The ⁸⁷Sr/⁸⁶Sr ratios of waters draining different lithologies and alteration is relatively young (28-25 million years) compared to the half-life of ⁸⁷Rb, so that the isotopic variation is not great enough to determine mixing ratios for waters derived from multiple sources. In this study area, mine drainage does not have a unique strontium isotopic composition because the mined areas do not have a strontium isotopic composition distinctly different from the unmined, mineralized host rocks.

Numerous regional sediment transport data are used to evaluate three techniques for estimating streamflow sediment yield from ungaged tributaries of the Colorado River in Grand Canyon. These techniques include: (1) a regression equation relating drainage are to sediment yield for all relevant sediment-yield data from northern Arizona, (2) an empirical relation developed by Renard (1972) selected from 8 potentially relevant methods, and (3) a new procedure that combines regional flood-frequency analysis with sediment-rating curves. Results based on techniques (1) and (2) are not significantly different. The third technique requires numerous assumptions, most notably that sediment yield on a decadal average can be described by several floods of recurrence intervals of 2yr., 5yr. and 10 yr. described by regional flood-frequency relations. Using data collected at gaging stations, we develop a relation between peak discharge and total-event sediment yield derived from hydrographs and sediment-rating curves. This third technique produces sediment yield estimates comparable to those of the regional data regression and Renard (1972) relations and may be a more robust technique for estimating sediment yield when streamflow data are available.

Arroyos are channels incised into their former floodplains. In the arid and semiarid parts of the Southwest, most arroyos formed betweend 1862-1909, causing severe agricultural and infrastructure damage. Arroyo cutting and re-filling have occurred episodically in the late Holocene throughout the region. After about 1940, many arroyos in the region partly aggraded and developed floodplains. Many causes have been proposed for arroyo cutting, particularly the introduction of livestock, climatic change (particularly drought), intrinsic geomorphic response, and combinations of these factors. Climatic fluctuations that changed regional flood frequency offer the most parsimonious explanation for arroyo cutting. A series of floods that occurred in discrete periods between 18621-942 are responsible for arroyo incision in the Southwest; a resurgence in flooding between 1964-1993 renewed channel erosion in southern Arizona (with the notable exception of the San Pedro River) and the southern Colorado Plateau as well. The occurrence of these floods was highly influenced over the short term by El Niño-Southern Oscillation (ENSO) conditions, which in turn were influenced by decadal-scale climatic fluctuations. The dynamic channels of arroyos have prompted extensive flood-control and channel-stabilization efforts that continue to the present.

Differential rates of plagioclase and K-feldspar weathering commonly observed in bedrock and soil environments are examined in terms of chemical kinetic and solubility controls and hydrologic permeability. For the Panola regolith, in the Georgia Piedmont Province of southeastern United States, petrographic observations, coupled with elemental balances and ⁸⁷Sr/⁸⁶Sr ratios, indicate that plagioclase is being converted to kaolinite at depths > 6 m in the granitic bedrock. K-feldspar remains pristine in the bedrock but subsequently weathers to kaolinite at the overlying saprolite. In contrast, both plagioclase and K-feldspar remain stable in granitic bedrocks elsewhere in Piedmont Province, such as Davis Run, Virginia, where feldspars weather concurrently in an overlying thick saprolite sequence. Kinetic rate constants, mineral surface areas, and secondary hydraulic conductivities are fitted to feldspar losses with depth in the Panola and Davis Run regoliths using a time-depth computer spreadsheet model. The primary hydraulic conductivities, describing the rates of meteoric water penetration into the pristine granites, are assumed to be equal to the propagation rates of weathering fronts, which, based on cosmogenic isotope dating, are 7 m/10⁶ yr for the Panola regolith and 4 m/10⁶ yr for the Davis Run regolith. Best fits in the calculations indicate that the kinetic rate constants for plagioclase in both regoliths are factors of two to three times faster than K-feldspar, which is in agreement with experimental findings. However, the range for plagioclase and K-feldspar rates (k_r =1.5 × 10^-17 to 2.8 × 10^-16 mol m^-2 s^-1) is three to four orders of magnitude lower than for that for experimental feldspar dissolution rates and are among the slowest yet recorded for natural feldspar weathering. Such slow rates are attributed to the relatively old geomorphic ages of the Panola and Davis Run regoliths, implying that mineral surface reactivity decreases significantly with time.

Differential feldspar weathering in the low-permeability Panola bedrock environment is more dependent on relative feldspar solubilities than on differences in kinetic reaction rates. Such weathering is very sensitive to primary and secondary hydraulic conductivities (q_p and q_s), which control both the fluid volumes passing through the regolith and the thermodynamic saturation of the feldspars. Bedrock permeability is primarily intragranular and is created by internal weathering of networks of interconnected plagioclase phenocrysts. Saprolite permeability is principally intergranular and is the result of dissolution of silicate phases during isovolumetric weathering. A secondary to primary hydraulic conductivity ratio of q_s/q_p = 150 in the Panola bedrock results in kinetically controlled plagioclase dissolution but thermodynamically inhibited K-feldspar reaction. This result is in accord with calculated chemical saturation states for groundwater sampled in the Panola Granite. In contrast, greater secondary conductivities in the Davis Run saprolite, q_s/q_p = 800, produces both kinetically controlled plagioclase and K-feldspar dissolution. Faster plagioclase reaction, leading to bedrock weathering in the Panola Granite but not at Davis Run, is attributed to a higher anorthite component of the plagioclase and a wetter and warmer climate. In addition, the Panola Granite has an abnormally high content of disseminated calcite, the dissolution of which precedes the plagioclase weathering front, thus creating additional secondary permeability.

Carbon dioxide (CO₂) and methane (CH₄) exchange between the atmosphere and a subalpine wetland located in Rocky Mountain National Park, Colorado, at 3200 m elevation were measured during 1996-1998. Respiration, net CO₂ flux, and CH₄ flux were measured using the closed chamber method during snow-free periods and using gas diffusion calculations during snow-covered periods. The ranges of measured flux were 0.2-526 mmol CO₂ m^-2d^-1 (respiration), -1056-100 mmol CO₂ m^-2d^-1 (net CO₂ exchange), and 0.1-36.8 mmol CH₄ m^-2d^-1 (a positive value represents efflux to the atmosphere). Respiration and CH₄ emission were significantly correlated with 5 cm soil temperature. Annual respiration and CH₄ emission were modeled by applying the flux-temperature relationships to a continuous soil temperature record during 1996-1998. Gross photosynthesis was modeled using a hyperbolic equation relating gross photosynthesis, photon flux density, and soil temperature. Modeled annual flux estimates indicate that the wetland was a net source of carbon gas to the atmosphere each of the three years: 8.9 mol C m^-2yr^-1 in 1996, 9.5 mol C m^-2yr^-1 in1997, and 9.6 mol C m^-2yr^-1 in 1998. This contrasts with the long-term carbon accumulation of ~0.7 mol m^-2yr^-1 determined from ¹⁴C analyses of a peat core collected from the wetland.

Geothermal convection of seawater deep in carbonate platforms could provide the necessary supply of magnesium for dolomitization at temperatures high enough to overcome kinetic limitations. We used reactive-transport simulations to predict the rates and spatial patterns of dolomitization during geothermal convection in a platform that was 40 km across and 2 km thick. In the simulations, porosity and permeability decrease with depth to account for sediment compaction.

Dolomitization of a platform consisting of medium grained (~0.05 mm) sediments occurred in a broad band ranging from ~2.5 km depth near the margin to ~1.5 km depth near the platform center. The area of dolomitization is deep enough that temperatures exceed ~50°C but not so deep that low permeabilities restrict mass transport. Complete dolomitization in the center of this zone is estimated to require at least 60 my. Incorporation of permeability contrasts, permeable beds, and reactive beds focused dolomitization strongly and reduced the estimated time required for dolomitization by as much as 50 percent. Dolomitization created magnesium-depleted, calcium-rich fluids in less than 10 ky, and results support a link between dolomitization and anhydrite precipitation where adequate sulfate is available.

Hydrologic landscapes are multiples or variations of fundamental hydrologic landscape units. A fundamental hydrologic landscape unit is defined on the basis of land-surface form, geology, and climate. The basic land-surface form of a fundamental hydrologic landscape unit is an upland separated from a lowland by an intervening steeper slope. Fundamental hydrologic landscape units have a complete hydrologic system consisting of surface runoff, ground-water flow, and interaction with atmospheric water. By describing actual landscapes in terms of land-surface slope, hydraulic properties of soils and geologic framework, and the difference between precipitation and evapotranspiration, the hydrologic system of actual landscapes can be conceptualized in a uniform way. This conceptual framework can then be the foundation for design studies and data networks, syntheses of information on local to national scales, and comparison of process research across small study units in a variety of settings. The Crow Wing River watershed in central Minnesota is used as an example of evaluating stream discharge in the context of hydrologic landscapes. Lake-research watersheds in Wisconsin, Minnesota, North Dakota, and Nebraska are used as an example of using the hydrologic-landscapes concept to evaluate the effect of ground water on the degree of mineralization and major-ion chemistry of lakes that lie within ground-water flow systems.

Oaks (Quercus velutina Lam.) growing over a shallow aquifer contaminated by chlorinated hydrocarbons were studied to determine if it was possible to estimate the approximate year that contamination began. The annual rings of some trees downgradient from the contaminant release site contained elevated concentrations of chloride possibly derived from dechlorination of contaminants. Additionally, a radial-growth decline began in these trees at approximately the same time that chloride became elevated. Growth did not decline in trees that contained smaller concentrations of chloride. The source of elevated chloride and the corresponding reductions in tree growth could not be explained by factors other than contamination. On the basis of tree-ring evidence alone, the release occurred in the late 1960s or early 1970s. Contaminant release at a second location apparently occurred in the mid- to late 1970s, suggesting that the area was used for disposal for at least 5 years and possibly longer.

MODFLOW-2000, the newest version of MODFLOW, is a computer program that numerically solves the three-dimensional ground-water flow equation for a porous medium using a finite-difference method. MT3DMS, the successor to MT3D, is a computer program for modeling multi-species solute transport in three-dimensional ground-water systems using multiple solution techniques, including the finite-difference method, the method of characteristics (MOC), and the total-variation-diminishing (TVD) method. This report documents a new version of the Link-MT3DMS Package, which enables MODFLOW-2000 to produce the information needed by MT3DMS, and also discusses new visualization software for MT3DMS. Unlike the Link-MT3D Packages that coordinated previous versions of MODFLOW and MT3D, the new Link-MT3DMS Package requires an input file that, among other things, provides enhanced support for additional MODFLOW sink/source packages and allows list-directed (free) format for the flow model produced flow-transport link file. The report contains four parts: (a) documentation of the Link-MT3DMS Package Version 6 for MODFLOW-2000; (b) discussion of several issues related to simulation setup and input data preparation for running MT3DMS with MODFLOW-2000; (c) description of two test example problems, with comparison to results obtained using another MODFLOW-based transport program; and (d) overview of post-simulation visualization and animation using the U.S. Geological Survey's Model Viewer.