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Final Report: Assessment and Control of Arsenic Mobility in Contaminated Sediments

EPA Grant Number: R826202
Title: Assessment and Control of Arsenic Mobility in Contaminated Sediments
Investigators: Hering, Janet G.
Institution: California Institute of Technology
EPA Project Officer: Reese, David H.
Project Period: October 1, 1997 through September 30, 2000 (Extended to March 31, 2001)
Project Amount: $288,049
RFA: Contaminated Sediments (1997)
Research Category: Hazardous Waste/Remediation

Description:

Objective:

This project examined the fate and transport of naturally occurring arsenic in the Los Angeles Aqueduct (LAA) system and its tributaries. Elevated arsenic concentrations in the system are derived from geothermal sources. Because the geothermal input of arsenic is relatively localized, treatment to remove arsenic near the source may be feasible. The effectiveness of this strategy may be limited if arsenic stored in the reservoir sediments downstream of the potential near-source treatment site were to be remobilized. The research objectives were to determine whether:

Summary/Accomplishments (Outputs/Outcomes):

In the 3-year project period, work was conducted at three study sites within the LAA system and its tributaries: Hot Creek, Crowley Lake, and North Haiwee Reservoir. These study sites were chosen to highlight different aspects of the biogeochemical cycling of arsenic and, specifically, to examine the effects of arsenic inputs (Hot Creek), of stratification and the potential for uptake and deposition associated with primary production (Crowley Lake), and the impacts of in situ treatment for arsenic removal by ferric chloride addition (Haiwee Reservoir).

Site Descriptions. Hot Creek is a tributary of the Owens River and is subject to geothermal inputs of arsenic through hot springs located along the banks and in the streambed of the creek. Crowley Lake is the first reservoir in the LAA. It is highly productive due to elevated inputs of phosphorus (also derived from geothermal sources) and is stratified during the summer. The outlet of the reservoir is below the depth of the seasonal oxycline. North Haiwee Reservoir is below the Cottonwood Treatment Plant and patterns of sediment and arsenic deposition are dominated by the introduction of ferric chloride into the Aqueduct at Cottonwood.

Crowley Lake Study. Elevated arsenic concentrations in Crowley Lake derive from upstream geothermal inputs. We examined the water column of Crowley Lake under stratified and unstratified conditions, seeking evidence for algal uptake and transformation of arsenic and its deposition to and release from the sediments. Vertical profiles of other elements, which might either influence or track the cycling of arsenic, also were examined. Manganese and phosphorus concentrations increased with depth below the oxycline under stratified conditions, consistent with a sediment source of these elements. However, these elements did not accumulate in the hypolimnion during the period of stratification. This can be explained by accounting for the dynamics of reservoir operation in which water withdrawn from the hypolimnion is replaced from the epilimnion or surface inflows. Depletion of phosphorus in the surface water was incomplete during stratification suggesting that phosphorus is not a limiting nutrient. Vertical profiles of total arsenic during stratification did not provide evidence for release of arsenic from the sediment; concentrations were either uniform with depth or showed a mid-depth minimum at the oxycline attributable to internal recycling within the water column. There was neither depletion of arsenic nor evidence for methylated arsenic species in the productive surface water. Arsenic was present as arsenate in the epilimnion and as a mixture of arsenate and arsenite in the hypolimnion. In the absence of an efficient mechanism to transport arsenic from the water column to the sediments, the substantial mass flux of arsenic through Crowley Lake results in only a moderate accumulation of arsenic in the sediments. Arsenic in the sediments appears to be sequestered by sulfide phases; thus, release of arsenic from Crowley Lake sediments should not constitute a threat to the water quality of the LAA.

Haiwee Reservoir Study. Deposition of arsenic to the sediments of Haiwee Reservoir (Olancha, CA) has dramatically increased since March 1996, as a result of an interim strategy for arsenic management in the LAA water supply. Ferric chloride and cationic polymer are introduced into the Aqueduct at the Cottonwood treatment plant, 27 km north of Haiwee Reservoir. This treatment decreases the average arsenic concentration from 24.8 mg/L above Cottonwood to 8.3 mg/L below Haiwee. Iron- and arsenic-rich flocculated solids are removed by deposition to the reservoir sediments.

Analysis of sediments shows a pronounced signature of this deposition with elevated sediment concentrations of iron, arsenic, and manganese relative to a control site. Sediment concentrations of these elements remain elevated throughout the core length sampled (ca. 4 percent Fe, 600 ppm Mn, and 200 ppm As). A pore water profile revealed a strong redox gradient in the sediment. Manganese in the pore waters increased below 5 cm; iron and arsenic increased below 10 cm and were strongly correlated, consistent with reductive dissolution of iron oxyhydroxides and concurrent release of associated arsenic to solution. X-ray absorption spectroscopy (XAS) revealed an inorganic As(V) phase present only in the uppermost sediment (0-2.5 cm) in addition to an inorganic As(III) phase. In the deeper sediments (to 44 cm), only an As(III) oxide phase was detected. Analysis of the extended x-ray absorption fine structure spectrum indicates that the As(III) at depth remains associated with iron oxyhydroxide. We hypothesize that this phase persists in the recently deposited sediment despite its reducing conditions due to slow dissolution kinetics.

Other Findings. In addition to the completed studies on Crowley Lake and Haiwee Reservoir, other work performed as part of the project yielded important insights into the biogeochemical cycling of arsenic. This work is summarized below.

Hot Creek Study. Preliminary sediment and pore water sampling was conducted at Hot Creek (i.e., prior to work at Crowley Lake and Haiwee Reservoir). The advantage of performing this work at Hot Creek was ease of sample collection, but the disadvantage was that sediment deposition and erosion within Hot Creek is very dynamic. In particular, sediments are scoured from Hot Creek during high-flow periods that result from snow-melt. Thus, the sediments do not preserve an interpretable temporal record. Nonetheless, the Hot Creek studies were useful in testing sampling and sample analysis techniques (particularly, the gel probe sampler and XAS studies described below). In addition, accumulation of arsenic in Hot Creek sediments was found to be highly spatially variable and, in general, modest despite the geothermal inputs of arsenic at the site. The highest arsenic concentrations measured in the sediments was 100 ppm (dry-weight basis). There was some indication from XAS that arsenic in Hot Creek sediment was present in an organic form; arsenic content in the sediment of one core from Hot Creek was correlated with organic carbon in the sediment. Mass balance estimates of the transport of arsenic from Hot Creek associated with water and sediment indicated that the contribution of sediment-associated arsenic to the total arsenic transport is minor.

Gel Sampler for Pore Water Profiles. Laboratory testing was performed with a gel probe sampler adapted from the design of Krom, et al. (Limnology and Oceanography 1994;39:1967-1972). The modifications increase the ease of sample handling in the field. Vertical resolution was approximately 0.5 cm even though some resolution of the original design was compromised. Rapid equilibration was demonstrated in the laboratory. The sampler was deployed in the field at Hot Creek and Haiwee Reservoir. Vertical profiles of pore waters were self-consistent and arsenic concentrations determined above the sediment-water interface agreed well with values from grab samples of the overlying water.

X-ray Absorption Spectroscopy. XAS studies were performed at the Stanford Synchrotron Radiation Laboratory in collaboration with Dr. P. O'Day (Arizona State University). Although not part of the original scope of the project, these studies provided important information on the speciation of arsenic in the sediments. In particular, the oxidation state of arsenic could be distinguished from the x-ray absorption near edge (XANES) region, which also provided preliminary evidence for the type of solid present (by comparison with authentic standards). For example, XANES spectra of sediment collected from Hot Creek and Crowley Lake indicated that arsenic in the sediments was associated with an organic phase at Hot Creek and a sulfide phase at Crowley Lake. Further analysis of the extended x-ray absorption fine structure (EXAFS) spectra was used to examine the coordination environment of arsenic in the sediments. This analysis was limited to samples more enriched in arsenic than most collected in this study but, for the sediment collected from Haiwee Reservoir, convincingly demonstrated that As(III) was coordinated with oxygen and not sulfur throughout the sediment core (despite reducing conditions in the sediment).

Sediment Incubation Studies. Incubation studies were conducted with sediment from both Hot Creek and Haiwee Reservoir to examine the potential for either arsenic uptake onto or release from sediments. For the Hot Creek system, arsenic uptake onto low-arsenic sediments obtained above the area of geothermal inputs was examined as well as the equilibration of arsenic between sediments collected in the area of the geothermal inputs and solutions containing varying concentrations of dissolved arsenic. The results of the equilibration studies with the Hot Creek sediments were not straightforward; dynamic exchange between the sediment and water was observed with an apparent capacity for both uptake and release. These studies were not further pursued because mass balance considerations suggested that Hot Creek sediments were likely to be only a minor reservoir for arsenic (i.e., compared with the overlying water) in this system. In contrast, storage of arsenic in the sediments of Haiwee Reservoir is important. This storage is the result of the perturbation of the sediment and arsenic-deposition regimes by addition of ferric chloride to the LAA. Release of both arsenic and iron was observed in the sediment incubations. Arsenic release was depressed when the water overlying the sediment was aerated (compared with deaerated systems) but was not markedly affected by the addition of antibiotics.

Microbial Toxicity of Arsenic. One objective of this study was to examine microbial toxicity of sediment-associated arsenic and to determine whether toxicity and mobility were correlated. However, a related study in our laboratory on microbial arsenite oxidation in Hot Creek (Salmassi, et al., submitted to the Geomicrobiology Journal) indicated that at least some of the native microorganisms in Hot Creek are quite resistant to arsenic. The growth of one isolate, Agrobacterium albertimagni strain AOL15, was unaffected by arsenate at concentrations up to 50 mM (3750 mg/L); growth was slowed (but not entirely repressed) in the presence of 1 and 2.5 mM (75 and 188 mg/L) arsenite. Therefore, it seemed unlikely that a survey of microbial toxicity at ambient arsenic levels in Hot Creek would be particularly instructive and this aspect of the project was not pursued.

Conclusions:

The LAA and its tributaries provide an outstanding, large-scale laboratory for the study of arsenic geochemistry. In particular, the Aqueduct system presents distinct contrasts between sites that are subject to concentrated or diffuse inputs of arsenic (i.e., concentrated inputs at Hot Creek vs. diffuse inputs to Crowley Lake) and that are iron-poor (Hot Creek and Crowley Lake) as compared with iron-rich (Haiwee Reservoir) systems. The overwhelming importance of iron in controlling arsenic mobility is illustrated clearly by contrasting the conservative behavior of (total) arsenic in the iron-poor systems with the immobilization of arsenic (by its deposition to the sediments) in the iron-rich system.

Dynamic redox cycling of arsenic was observed along Hot Creek downstream of geothermal inputs, in the water column of Crowley Lake during stratified conditions, and in the sediments of Haiwee Reservoir. Redox speciation of arsenic in Crowley Lake appeared to be influenced by reservoir operations, specifically withdrawal of water from below the oxycline (during stratified conditions). A signal of the replacement of the withdrawn water with surface water (derived either from the epilimnion or surface inflows to the lake) was preserved in the arsenic redox speciation, specifically in the persistence of arsenate in the anoxic hypolimnion during the period of stratification. In the Haiwee reservoir sediments, reduction of As(V) to As(III) in the sediments was observed (by XAS) above the sediment depth at which manganese or iron and arsenic were released into the pore water. The continuing association of As(III) with iron oxyhydroxides throughout the sediment core suggests that reduction of As(III) occurs at the surface of the iron oxyhydroxides in the sediment.

The enhanced deposition of arsenic to the sediments of Haiwee Reservoir is a result of the addition of ferric chloride to the LAA at the Cottonwood Treatment Plant. Although the sediment sampling conducted at Haiwee cannot be considered representative of the reservoir as a whole, it does indicate some potential undesirable consequences of the Interim Arsenic Management Plan. Arsenic concentrations are substantially elevated (up to about 1 mg/L) in the pore waters of the Haiwee sediments. Physical disturbance of the sediments would result in the mixing of the pore water into the overlying water of the reservoir. Diffusive fluxes of arsenic to the overlying water are prevented by an oxic zone in the surficial sediments. If these conditions are maintained by contact between the surficial sediments and the overlying water, diffusion rates and the adsorptive capacity of the surface sediment will determine whether arsenic will be released from the sediment. If rapid deposition of particles helps to keep the surface sediment oxic, slowing sedimentation by halting the in situ treatment may cause migration of the redox boundary to the sediment-water interface, allowing release of arsenic to the overlying water. Extensive remineralization of organic carbon also could promote such a migration of the redox boundary. Removal of the arsenic-contaminated sediments (which, although a component of the Interim Arsenic Management Plan, has not yet been attempted) should receive immediate and careful consideration.


Journal Articles on this Report: 2 Displayed | Download in RIS Format

Other project views: All 9 publications 3 publications in selected types All 2 journal articles

Type Citation Project Document Sources
Journal Article Kneebone PE, Hering JG. Behavior of arsenic and other redox sensitive elements in Crowley Lake, CA: A reservoir in the Los Angeles Aqueduct system. Environmental Science & Technology 2000;34(20):4307-4312. R826202 (Final)
 not available
Journal Article Kneebone PE, O'Day PA, Jones N, Hering JG. Deposition and fate of arsenic in iron- and arsenic-enriched reservoir sediments. Environmental Science & Technology 2002;36(3):381-386. R826202 (Final)
 not available
Supplemental Keywords:

drinking water, water supply, reservoir, watersheds, , POLLUTANTS/TOXICS, Water, INTERNATIONAL COOPERATION, Geographic Area, Scientific Discipline, Waste, RFA, Arsenic, Drinking Water, Health Risk Assessment, Ecological Risk Assessment, Water Pollutants, Environmental Chemistry, Contaminated Sediments, Geochemistry, State, arsenic mobility, drinking water contaminants, treatment, water quality, California (CA), exposure and effects, arsenic exposure, risk management, geothermal water inputs, reservoir sediments, control technologies, ecology assessment models, monitoring, sediment quality survey, contaminated sediment, contaminant transport, arsenic remobilizatrion, sediment transport, microbial risk management, exposure, microbial pollution, water treatment
Relevant Websites:

http://www.its.caltech.edu/~ees/heringroup/ exit EPA

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The perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.

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