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Final Report: The Particle Size Distribution of Toxicity in Metal-Contaminated Sediments
EPA Grant Number: R826651Title: The Particle Size Distribution of Toxicity in Metal-Contaminated Sediments
Investigators: Ranville, James , Clements, William , Macalady, Donald L. , Ross, Phillipe
Institution: Colorado School of Mines , Colorado State University
EPA Project Officer: Krishnan, Bala S.
Project Period: October 1, 1998 through September 30, 2001 (Extended to September 30, 2002)
Project Amount: $372,795
RFA: Exploratory Research - Environmental Chemistry (1998)
Research Category: Engineering and Environmental Chemistry
Description:
Objective:The prime objective of the research project was to gain further understanding of the toxicity of oxic, metal-contaminated sediments to aquatic organisms. Of particular interest was the potential role of sediment particle size on metal exposure and bioavailability. A major hypothesis of the work was that the size distribution of acid-extractable metals would significantly affect both organism exposure to, and toxicity of, oxic metal-contaminated sediments. This hypothesis depends directly on three assumptions: first, dietary routes of exposure can dominate over dissolved metal exposure; second, sediment composition varies with particle size; and third, that organisms are size-selective in their ingestion of sediments. Sediment size could also play an indirect role when dissolved metal exposure is dominant, through the influence of specific surface area (m2/g) on metal partitioning between dissolved and sorbed phases.
The study focused on bed sediments collected from streams contaminated by acid mine drainage (AMD). These steams are common throughout the Rocky Mountain region and represent a significant problem with respect to contaminated sediments. The field sites were located on the North Fork of Clear Creek (NFCC), near Denver, Colorado, and the Arkansas River and California Gulch, its AMD impacted tributary, both located near Leadville, Colorado. Sediments in these streams are dominated by metal (Fe, Al, Mn) oxyhydroxide precipitates, which form during the neutralization of low-pH, metal-rich tailings and mine effluents. Major contaminants of concern are Zn, Cu, Cd, and Pb.
In order to further our understanding of the impact of metal-contaminated sediment, three areas of work were performed. The first research direction was to more fully understand the relationship between particle size and metal content for sediments collected from streams contaminated by AMD. This was investigated in order to determine which size fractions might provide the greatest potential for metal exposure to aquatic organisms. The second area of research was to directly measure the uptake and toxicity of metals to aquatic organisms resulting from exposure to sediments collected from AMD-impacted streams. Finally, because of the potential importance of particle size on metal transport and bioavailability and the inadequacy of many current sediment characterization techniques, we focused efforts on the further development of a new technique, namely field flow fractionation-inductively coupled plasma-mass spectrometry (FFF-ICP-MS). This technique promises to provide a means of examining the role of particle size on sediment bioavailability processes at a much more detailed level.
Summary/Accomplishments (Outputs/Outcomes):Sediment Characteristics as Related to Particle Size
Historically the sulfidic ores of Central City and Leadville (Colorado) were principally mined for gold, silver, and lead but substantial amounts of copper, zinc, and uranium were also recovered (Wildeman, et al., 1974; Cunningham, et al., 1994). In waste rock piles and tailings, metal solubilization occurs when metal (Cu, Zn, and Pb) sulfides are oxidized. Pyrite (FeS2) oxidation generates additional acidity, leading to further release of dissolved metals (U. S. Environmental Protection Agency [EPA], 1994; U.S. Geological Survey [USGS], 1994). The subsequent discharge of dissolved metals into a receiving stream results in extensive formation of colloidal iron, aluminum, and (potentially) manganese oxyhydroxides (Schemel, et al., 2000). As pH increases downstream the oxyhydroxide colloids aggregate into larger particles, which then settle and deposit in the slower moving areas of the stream, where they become part of the bed sediment. Depending on pH, dissolved metals can co-precipitate and sorb to these aggregates as well as to naturally derived sediment particles (Smith, et al., 1998). Metal transport in mining influenced streams, therefore, could be significantly influenced by sediment physical characteristics. Additional effects on metal exposure to sediment-dwelling organisms will depend on sediment physicochemical characteristics (Förstner, 1987).
The characteristics of sediments from AMD- impacted streams were examined in order to further understand their role in metal toxicity. A number of sediment characteristics are commonly seen to depend on particle size. These include specific surface area and metal concentrations (Cu, Fe, and Zn). In this study, these two characteristics were investigated in sediments collected from mining- impacted streams exhibiting elevated metal concentrations. Two Colorado stream basins were examined in this study. The NFCC is the receiving water for the Central City – Blackhawk mining districts. California Gulch receives runoff from the Leadville mining district and discharges into the Arkansas River. Both stream systems are heavily impacted from historic mining activities and are listed as EPA superfund sites.
Sediments were collected using 500 mL polyethylene beakers. Sampling focused on fine-grained materials in depositional zones. The samples were composited into 20 L polyethylene containers and transported back to the laboratory. They were stored at 4°C until size fractionation and analysis. The samples were allowed to settle for several days and the overlying water was decanted and reserved for use in the wet sieving procedure. Sediment size fractionation was performed using a series of stainless steel sieves. Initially a 2 mm sieve was used to remove gravel and organic debris. Four size fractions were generated: a course sand fraction, 2 mm to 212 μm, a fine sand fraction, 212 – 53 μm, a coarse sift fraction, 53 – 25 μm, and a fine silt and colloids fraction, < 25 μm. The suspensions generated from the wet sieving procedure were allowed to settle for a week or more, after which time the overlying water was removed by siphoning. The remaining settled solids were then air dried at room temperature. After separation and drying, the total mass of each size fraction was recorded for each sampling site.
The specific surface areas were measured using a Micrometrics Flowsorb II 2300 instrument. Single point Brauner, Emmet and Teller (BET) nitrogen adsorption was performed. A silica alumina standard, having a specific surface area of 277 m2/g, was used to calibrate the instrument. Samples of 50 to 200 mg were placed in glass BET tubes and degassed at 300°C for 60 minutes. The samples were then submerged in liquid nitrogen and the adsorption of nitrogen measured.
Metal contents of the size-fractionated sediments were determined by ICP -atomic emission spectroscopy (ICP-AES) analysis of acid digests. Approximately 100 mg of sediment was placed in a 50 mL centrifuge tube and 10 mL of a 1 M nitric: hydrochloric acid mixture was added. The samples were allowed to digest for one week at room temperature. The digested samples were then centrifuged for 30 minutes at 3500 rpm using a Marathon 12 KBR centrifuge (Fisher Scientific). The supernate was transferred to a 15 mL centrifuge tube for ICP-AES analysis using a Perkin-Elmer Optima 2000 ICP-AES. Scandium was used as an internal standard. Metal concentrations in mg/g sediment were computed.
The observed relationship between particle size and metal content and specific surface area varied between two extremes. The size dependence of specific surface area is shown in Figure 1 for samples from (a) the Arkansas River, and (b) the NFCC. The size dependence of metal content for these two samples is shown in Figure 2. In the case of the Arkansas River, the commonly reported inverse relationship between particle size and both metal concentration and specific surface area was observed (Horowitz, 1991). This is consistent with the fact that the Arkansas River sediments were only slightly impacted by the AMD inputs and appeared to be typical of an alpine stream. The other extreme occurred in the NFCC, a more heavily mining-impacted stream, where both characteristics were nearly uniform with particle size. Other samples showed intermediate behavior. The NFCC observation may be explained by the dominance of aggregated iron oxyhydroxide particles that were observed by Scanning Electron Microscopy to occur in all size fractions of the sediment. The porous nature of such particles leads to elevated specific surface area and higher numbers of metal binding sites, relative to non-aggregated particles of similar size. The uniformity of metal content and specific surface area versus particle size was somewhat unexpected and suggests that large particles may represent a significant sink for metals in heavily impacted streams. This result is significant in that the established view that only the finer fractions of sediment are of concern with respect to metal contamination is not necessarily correct for streams impacted by AMD. The presence of a significant percentage of the total sediment-associated metals, in size fractions larger than is normally assumed to be ingestible by benthic organisms, may limit both metal bioavailability and sediment transport. Investigations are needed to determine if this observation is generally true in other AMD -impacted systems.
Figure 1. The Amount of Mass and Specific Surface Area Present in Size Fractions of (a) Arkansas River Sediments and (b) NFCC Sediments
Figure 2. The Concentration of Fe, Cu, and Zn in Size Fractions of (a) Arkansas River Sediments and (b) NFCC Sediments
Metal Uptake and Toxicity as Related to Particle Size
In general, sediment toxicity is assumed to be primarily a problem for benthic (sediment-dwelling) organisms. It is therefore not common to investigate the toxicity of sediments to water column organisms. However, such organisms can ingest resuspended sediments and therefore be directly impacted by sediment-associated metals. They are also subject to dissolved metal exposures that result from release of metals from sediment. In this study, direct measurements of aquatic toxicity were performed on both benthic and water column-dwelling species. Some of the tests were performed using modified EPA protocols for 5-day sediment toxicity tests with a benthic species (Chironomus tentans). Experiments were also performed on Daphnia magna, a species commonly used for testing of aquatic toxicity tests.
Experiments With Chironomus tentans. The organisms were exposed to three size fractions of sediment (< 53, 53-212, and > 212 μm) from the Cache la Poudre River (control sediment), the NFCC, or mixture of the two. These sediments were principally contaminated with zinc, however copper was present at significantly lower concentrations. The study focused on zinc body burdens, survival, and growth (as measured by individual dry weight) as toxicity endpoints. Procedures are outlined in Walski (2002).
In general, a size dependence on the biological effect of metal-contaminated sediments could be seen in some sediment. These effects were reflected primarily in organism growth. This occurred when size also affected metal content. However, we also observed that for some sediment, particularly the most contaminated samples, particle size did not influence metal content of the sediments. In these cases, particle size may play a more complicated role in metal bioavailability.
For these samples, the sediment zinc concentrations were highest in the smallest size fraction and lowest in the largest size. No mortality was seen in any of the experiments performed. Zinc C. tentans body burdens were similar across all contaminated treatments. Zn levels in C. tentans organisms exposed to contaminated sediments were usually greater than in organisms exposed to reference sediment fraction. The uniformity in body burden, despite significant variation in metal content with sediment particle size, suggests that the organisms could regulate the uptake of zinc when exposed to contaminated sediments.
C. tentans growth, expressed as individual dry weights, was most affected in the smallest size fraction of contaminated sediment and least affected in the smallest size fraction of reference sediment. There was a general trend of increasing C. tentans weight with increasing particle size of sediment in contaminated treatments. We explain this result as a direct metabolic cost to the organism for adjusting to the presence of high concentrations of metals in the system. So although mortality may not arise from exposure to metal-contaminated sediments, the health of the organism can be impaired. These sub-lethal affects, which appear to be related to the particle size of the sediments, can have an important effect on aquatic ecosystems in AMD-impacted streams.
Experiments with Daphnia magna. In addition to the experiments with C. tentans, we performed a collaborative study with Dr. Patricia Gillis, Dr. Pat Chow-Fraser, and Dr. Chris Wood at MacMaster University to investigate metal uptake by water-column organisms resulting from sediment exposures. D. magna sieve large quantities of water to collect suspended particles and, although their preferred food is algae, any suspended particles over 0.45 μm may be ingested (Brendlerger, 1985). D. magna will periodically browse on detritus and sediment particles if suspended food (i.e., algae) falls below a threshold (Horton, 1979). Therefore, although D. magna are planktonic organisms, feeding mainly on algae, they may ingest sediment and any associated contaminants, including metals, through both inadvertent sieving or through intentional resuspension of sediment.
Although there are numerous standard protocols (ASTM, EPA), for conducting toxicity tests with D. magna, there is little guidance on how to address the potential problem posed by the presence of contaminated particles in the digestive tract when whole body metal concentrations are used to assess contaminant bioavailability. Methodology to deal with this issue needs to be investigated with whole sediment prior to investigation of particle size effects.
The goals of this study were to determine the length of time required for D. magna to clear its gut after exposure to metal-contaminated sediment and to determine if the presence of food (algae) during gut clearance would alter gut passage time. Details of the experiments are presented by Gillis, et al., 2003.
Gut clearing patterns were determined for D. magna after exposure to both clean and metal-contaminated field-collected sediments. D. magna exposed to reference sediment had fuller guts than those exposed to metal-contaminated sediment (60 vs. 95% full). Neither reference-exposed nor metal-exposed D. magna could clear their gut of sediment particles when held in clean water for 24 hours. Changes in gut fullness and whole body tissue metal concentrations were also determined in D. magna that were transferred to either water alone or water containing 5x105 cells of Pseudokircheriella subcapita after they had been exposed to metal-contaminated sediment. For D. magna held in water, there was no significant difference in gut fullness even after 48 hours. Gut fullness in D. magna held in the presence of algae dropped from 56 (± 28 %) immediately after exposure to 17 (± 29 %) after 4 hours of gut clearance. Although there were no further significant declines in gut fullness after 2 hours, the data were much less variable after 8 hours of clearance 17 (± 12%). After accounting for background Cu levels (48 μg/g) in D. magna, whole body tissue Cu concentrations based on uncleared and water cleared D. magna would overestimate Cu accumulation by 11 and 7 fold, respectively, compared to D. magna that were fed algae (8 hours) during gut clearance. The results highlighted the need to clear the guts of these organisms prior to conducting measurements of metal body burdens by providing a food source.
Development of Field Flow Fractionation-Inductively Coupled Plasma-Mass Spectrometry as a Tool for Investigating the Size-Dependence of Metal Content in Sediments
Current methods of obtaining elemental composition as a function of particle size rely on chemical analysis of size fractions obtained by serial settling, centrifugation, or filtration. In order to increase the resolution of the size distribution of elements in sediments, we further developed the method of FFF- UV absorbance (UVA)-ICP-MS. FFF- UVA-ICP-MS is a high resolution size-separation and elemental analysis technique that can give elemental size distributions for colloids and particles smaller than about 10 microns. The technique involves coupling FFF, which separates particles based on size, with ICP-MS, which gives the elemental composition. Although we developed this technique using some well-characterized soil samples, future applications to sediments will help refine our understanding of trace metal distributions in AMD-contaminated sediments.
Numerous FFF separation methods have been developed for particle sizing, which differ principally in two respects: the nature of the applied field and the operating mode. Experimental details and theory for FFF are described in detail by Giddings, et al. (2000). Sedimentation FFF and flow FFF (FlFFF) are the most applicable for characterizing environmental colloids. In addition to using UV absorbance as a non-specific means of detecting particles eluting from the FFF, coupling FFF, which generates high-resolution size separations, with on-line element-specific detectors, such as ICP-MS or optical emission spectroscopy (OES), can be used to determine size-based changes in mineralogy. This combination of detectors makes it a valuable tool for characterization of environmental colloids (Ranville, et al., 1999). The FFF instruments can be operated under two conditions of particle elution, either normal-mode or steric-mode. Normal-mode separations are applicable to sub micron colloids and are based on particle diffusion. The greater the particle diffusion coefficient (i.e., the smaller the size), the more rapidly the particle elutes. Thus the time of elution from the FFF is positively correlated to particle size. In contrast, under steric-mode conditions the elution time is negatively correlated to size. Steric-mode separations are used for particles greater than approximately 1 mm and are based on hydrodynamic lift forces, which increase with increasing size. The consequence of this transition is that for samples that span the steric transition size, a prefractionation is required to eliminate the co-elution of particles traveling under both modes of elution.
Samples of a clay-loam soil were collected from the upper 5 cm of the soil surface at the Rocky Flats Environmental Technology Site (RFETS), a former nuclear weapons production facility located near Denver, Colorado. Four size fractions (< 0.2, 0.2-.08, 0.8-2, and 2-10 microns) were prepared by conventional means of bulk size fractionation using gravitational settling. The accuracy of the FFF technique is illustrated in Figure 3 where the relative mass of material (based on UV response) as a function of size is shown for size fractions of: 0.2, 0.2-.08, 0.8- 2, and 2-10 microns. The good agreement between the FFF- UVA-ICP-MS results and those obtained from the pre fractionation confirms the accuracy of the technique. Another example demonstrating the ability of the FFF-UVA- ICP-MS technique to examine the size dependence of elemental composition is shown in Figure 4. The plot shows good agreement between the total mass of particles (UVA) and the uranium content versus size. In contrast, the manganese content is shifted to larger particle sizes. The FFF-UVA-ICP-MS technique promises to be a significant advance in the characterization of metal-contaminated sediments.
Figure 3. FFF-UVA-ICP-MS Analysis of Four Size Fractions of Soil. The vertical axis represents the fraction of mass at any given particle size.
Figure 4. FFF-UVA-ICP-MS Analysis of the 0.2-0.8 Micron Fraction of Rocky Flats Soil
Future work should employ the FFF-ICP-MS technique to further examine the role of aggregation in minimizing size effects of metal content in AMD sediments. The presence of aggregated sediments may have significant impacts on how we think of metal transport and bioavailability in AMD systems. Further direct testing of metal uptake and biological affects in aggregated sediments also needs to be pursued.
References:
Brendelberger H. Filter mesh-size and retention efficiency for small particles: comparative studies with Cladocera. Archives für Hydrobiologie Bei heft Ergebnisse der Limnologie 1985;21:135-146.
Cunningham CG, Naeser CW, Marvin RF, Luedke RG, Wallace AR. Ages of selected intrusive rocks and associated ore deposits in the Colorado mineral belt. U.S. Geological Survey Bulletin, No. 2109, 1994.
Förstner U. Sediment-associated contaminants—an overview of scientific bases for developing remedial options. Hydrobiologia 1987;149: 211-246.
Giddings JC. Field flow fractionation techniques. In: Schimpf M, Caldwell K, Giddings JC, eds. Field Flow Fractionation Handbook. New York, NY: Wiley-lntersciences, 2000.
Horowitz AJ. A primer on sediment-trace element chemistry. Chelsea, MI: Lewis Publishers, 1991, p. 136.
Ranville JF, Chittleborough DJ, Shanks F, Morrison RJS, Harris T, Doss F, Beckett R. Development of sedimentation field-flow fractionation-inductively coupled plasma-mass spectrometry for the characterization of environmental colloids. Analytica Chimica Acta 1999;381(2-3):315-329.
Schemel LE, Kimball BA, Bencala KE. Colloid formation and metal transport through two mixing zones affected by acid mine drainage near Silverton, Colorado. Applied Geochemistry 2000;15:1003-1018.
Smith KS, Ranville JF, Plumlee GS, Macalady DL. Predictive double-layer modeling of metal sorption in mine-drainage systems. In: Jenne EA, ed. Metals in Geomedia: Sorption processes and model applications. Academic Press, 1998.
EPA. Technical document: acid mine drainage prediction. Washington, DC, U.S. Environmental Protection Agency, Office of Solid Waste, Special Waste Branch, 1994.
USGS. Guidebook on the geology, history, and surface-water contamination in the area from Denver to Idaho Springs, Colorado, United States Geological Survey Circular, 1994, 1097.
Wildeman TR, Cain D, Ramiriz AJR. The relation between water chemistry and mineral zonation in the Central City Mining District, Colorado. In: Water Resources Problems Related to Mining, Proceedings 18, American Water Resources Association, Minneapolis, 1974:219-229.
Journal Articles:No journal articles submitted with this report: View all 14 publications for this project
Supplemental Keywords:sediments, bioavailability, heavy metals, aquatic, environmental chemistry,
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Ecosystem Protection/Environmental Exposure & Risk, Water, Air, Geographic Area, Scientific Discipline, Waste, RFA, Ecosystem/Assessment/Indicators, Engineering, Chemistry, & Physics, Biology, Bioavailability, Chemistry, Ecological Indicators, EPA Region, Fate & Transport, Environmental Chemistry, Ecological Effects - Environmental Exposure & Risk, Ecosystem Protection, Contaminated Sediments, exposure assessment, heavy metal contamination, heavy metals, simultaneously extracted metals, fate and transport, acid volatile sulfide, flourescent bacteria, ecosystem modeling, chemical composition, ecological risk, sediment contaminant effects, sediment toxicity, ecological modeling, benthic macroinvertebrates, contaminated sediment, Region 8, ecological exposure, exposure, particle size, sediment
Progress and Final Reports:
2000 Progress Report
Original Abstract