1. Arsenic Mobilization Mechanisms at a Landfill Site in Southern New Hampshire and an Evaluation of the Effectiveness of Remedial Actions on Dissolved Arsenic Concentrations White, C A  and Sevee, J E
2. Arsenic in GroundwaterK Stollenwerk, USGS, Denver; J Colman, USGS, Marlborough
3. Arsenic in Contaminated Sediment and Groundwater: A Case Study in Central Brandon,W, Byrne, J McTigue, D, and  Stein, C
4. Arsenic in Southeastern Michigan Ground Water: Results of USGS Test Drilling Kolker, A Cannon, W F,  Woodruff, L G,  Westjohn, D B,  Haack, S K,  Kim, M
5. Arsenic in Ground Water of the United States: Processes Leading to Widespread High Concentrations Welch, A H
6. Severe Arsenic Contamination of Ground Water in Bangladesh: Its Sourceand Speciation Islam, M R and Rojstaczer, S
7. Relation of Arsenic, Iron, and Manganese Concentrations in Ground Water to Bedrock Geology and Land Use in Eastern New EnglandAyotte, J D, Nielsen, M G, and Robinson, G R
8. Arsenic Contamination in New Hampshire Drinking Water From Pegmatite Hosted Sulfides and OxidesPeters, S C and Blum, J D
9. Evidence for Predominantly Adsorbed Arsenic in a Wetland Sediment Keon, N E, Swartz, C H, Brabander, D J, Harvey, C, and Hemond, H F
10. Using Bioindicators to Monitor Past and Present Arsenic Contamination in Groundwater Near Two Superfund Sites in Woburn, Massachusetts Gawel, J E, Brabander, D J, Morel, F M, and Hemond, H F
11. Microbial Arsenic Reduction: A New, Ubiquitous, Yet Still Mysterious Factor in Aquatic Arsenic MobilityAhmann, D
12. Sources and Geochemical Associations of Arsenic in Leachate Plumes From a Landfill in Saco, MaineColman, J A and Lyford, F P
13. Natural Remediation of Arsenic-Contaminated Groundwater: Solute-Transport Model Predictions Stollenwerk, K G and Colman, J A
14. An Evaluation of Potential Arsenic Sources to the Groundwater at a Landfill Site in Eastern MaineWhite, C A, Scully, M V
15. Natural Immobilization of Arsenic in the Shallow Groundwater of a Tidal Marsh, San Francisco BayVlassopoulos, D, Andrews, C B, Hennet, R J, and Macko, S A

 Elevated levels of arsenic are present in the groundwater downgradient of the three former  waste disposal sites located at a Superfund site in southern New Hampshire. Arsenic concentrations range from approximately 50 to 500 ug/L in a vertically discrete zone in the deep overburden and shallow bedrock.  Arsenic is below detectable levels in the groundwater that is outside the downgradient flow field of the landfills at the site. Detailed geochemical studies determined that the zone of elevated arsenic is also characterized by elevated zones of iron, manganese, alkalinity, ammonia, total organic carbon and methane and low concentrations of dissolved oxygen, nitrate and sulfate. The landfills themselves are not a direct source of dissolved arsenic to the groundwater.  However, the landfills contribute to the mobilization of arsenic by generating total organic carbon, which creates and sustains the reductive dissolution mechanism responsible for the release of arsenic to the groundwater. The source of dissolved arsenic in the groundwater is not known, but laboratory-leaching studies suggest that arsenic is released from iron and manganese oxides, which occur naturally in the site soils and rocks.  Remedial actions at the site were completed in 1994 and consisted of capping the landfills and surface water drainage improvements. Long-term groundwater monitoring has continued using a limited number of monitoring wells to evaluate the effectiveness of the remedial actions. A review of the post-remediation monitoring data from 1994 -1999 indicates that arsenic concentrations have decreased slightly in two monitoring wells near the landfills, while concentrations in monitoring wells downgradient have remained relatively stable.
 

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Two public water-supply wells in north-central Massachusetts have exhibited elevated levels of arsenic (As) historically. An adjacent pond is known to have As in bottom sediment at concentrations on the order of 100 $\mu$g/g, most likely due to historic disposal of tannery wastes. A field and analytical program is under way to quantify induced infiltration from the pond and to evaluate the potential for mobilization of As from pond sediments.  The wells are screened at approximately 12 to 18 m below ground surface in glacial outwash sands and gravels.  Dense till and/or bedrock is encountered approximately 8 m below the well screens. The production wells and five adjacent observation wells were monitored following redevelopment and startup.  Arsenic concentrations up to 47 $\mu$g/L were detected in the pumping wells initially and declined to $\sim$20 $\mu$g/L within a few days. Wells on the upgradient and pondward sides of the production wells consistently show As at or below the detection limit of 1 $\mu$g/L.  Major-element chemical compositions of the pond water and upgradient groundwater were used to quantify mixing at the production wells.

The early-time {\it decrease} in As is concurrent with the breakthrough of pond water at the production wells, and suggests that the pond is not the As source. Pond sediment-pore water partitioning of As, based on available data, is also consistent with retention of As in the sediments.  These results indicate that the As observed in the production wells is derived elsewhere, possibly from bedrock or overburden aquifer materials. Previous reports of elevated levels of As in groundwater in central Massachusetts and the presence of arsenopyrite in bedrock in the vicinity of the site support this scenario.
 
 

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Nine counties in eastern and southeastern Michigan are participating with the USGS in a collaborative study of domestic wells that exceed the EPA drinking water standard for arsenic (50 $\mu$g/L).  Most of the problem wells are completed in the Mississippian Marshall Sandstone, the major bedrock aquifer in the region. As part of this study, water, well cuttings, and drill core samples from 2 USGS test wells, LP-1 in Lapeer County, and H-15D, in Huron County were analyzed, together with cuttings from existing wells in Lapeer and Tuscola Counties.  Arsenic contents determined for 1.2 m intervals of H-15D core range from less than 4 mg/kg to 140 mg/kg for sandstones and $\sim$40 to 310 mg/kg for shale-dominated sections.  Electron microprobe/SEM studies of the sandstones show that pyrite locally forms pore-filling cement and/or fossil replacements, in which arsenic-poor framboids or framboid masses are succeeded by arsenic-rich (up to 6.5 wt. percent) overgrowths.  In some cases a third pyrite generation is present that is low in arsenic.
 

The arsenic content of waters approaches 50 and 125 $mu$g/L nic content of waters shows no co-variation with arsenic in the cores, indicating that pyrite is not reacting, consistent with preliminary hydrogeochemical modeling.  Arsenic speciation results show that 85-90 percent of the arsenic in H-15D waters occurs as As (III).  These results suggest several possibilities for how arsenic enters the wells.  The vertical distribution of arsenic indicates that oxidation occurred near the top of the aquifer, perhaps due to fluctuations of the water table.  Alternatively, oxidation of till containing detrital Marshall Sandstone could contribute arsenic to glacial aquifers that directly overlie the sampled intervals. We are currently examining core and till samples for oxidation products of the arsenic-bearing pyrite.
 
 

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Although only about 10\% of ground water samples in the conterminous United States exceed 10 micrograms per liter, ground water with these high concentrations are found in most parts of the Nation. Widespread high concentrations generally result from natural processes, although human activities can increase arsenic concentrations. The most prevalent causes of widespread high concentrations are release from iron oxide and sulfide mineral oxidation. Upflow of geothermal water and evaporative concentration also can produce high arsenic concentrations in ground water. More than one of these processes can be operative at a given locale.

Arsenic can be released to ground water by desorption from, and dissolution of, iron oxide. Aquifers with oxic ground water commonly contain iron oxide with arsenic as an impurity. Desorption of arsenic can be promoted by either an increase in pH or the concentration of a competing ion, such as phosphorous. Arsenic also can be released from iron oxide because of chemical reduction of the oxide. Deposition of Fe-coated sediment along with organic matter can lead to the dissolution of the oxide coating with consequent release of arsenic to ground water. Introduction of synthetic organic compounds into aquifers also can lead to reductive dissolution of iron oxide and arsenic release.

Pyrite commonly contains arsenic in trace amounts, although arsenic concentration can exceed five percent. The rate of sulfide-mineral oxidation is limited by the supply of an oxidizing agent, most commonly molecular oxygen. High nitrate concentrations from agricultural activities also can oxidize sulfide minerals. Human activities that increase the supply of oxygen, or another oxidizing agent such as nitrate, to ground water can lead to increased mineral oxidation and, consequently, high arsenic concentrations. Irrigation in arid and semi-arid regions can increase evaporative concentration, which can lead to high arsenic concentrations.
 
 

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Motivated by the desire to decrease water born disease, the World Health Organization (WHO) began to promote the use of small diameter "tube" wells in Bangladesh in the 1960s.  As a result, groundwater is now the main source of drinking water in Bangladesh, a country of 125 million people.  While the incidence of water born diseases, such as cholera, have been reduced as a result of groundwater use, over 70 million people in Bangladesh are potentially being exposed to groundwater-based arsenic in their drinking water supplies.  The actual magnitude of the health impact of arsenic laden groundwater in the country has yet to be  determined.

We analyzed 10 groundwater, 7 surface water, and 31 soil samples from arsenic-affected areas in Bangladesh.  Mean As concentrations in groundwater in the Char Ruppur (253 ppb), Rajarampur (1955 ppb), and Shamta (996 ppb) areas dramatically exceed the WHO recommended drinking water standard of 10 ppb. Relatively arsenic free areas show a balance in As between groundwater, surface water, and soil. In contrast, concentrations of As in groundwater are much higher than those in surface water and soils in the areas with As related health problems.  This imbalance in concentration suggests that As in groundwater is principally derived from upward transport and its source is in the  bedrock.  Relatively high concentrations of As in surface water in the Rajarampur area likely are derived from anthropogenic sources such as agriculture.  In the soil, elevated concentrations of As, Cr, Cu, Ni, Pb, and Zn are due to their strong affinity to organic matter, hydrous oxides of Fe and Mn, and clay minerals. As (V) is generally much more prevalent than As (III) in the samples, representing 47-100\% of total arsenic concentration.
 
 

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Data from 804 public-supply wells in bedrock of eastern New England, analyzed as part of a USGS National Water Quality Assessment study, indicate that arsenic concentrations in water exceeded 0.005 mg/L in 20 percent of the wells and exceeded 0.01 mg/L in water from 13 percent. This has implications for Safe Drinking Water Act compliance under the lower arsenic standard proposals being examined by USEPA. Data for the study are compliance monitoring analyses obtained from the Maine Department of Human Services, New Hampshire Department of Environmental Services, Massachusetts Department of Environmental Protection and Rhode Island Department of Health. The data are limited in that concentrations of arsenic must meet the drinking water standard of 0.05 mg/L.

Despite this limitation, the concentration of arsenic in bedrock well water is significantly different among six major bedrock lithochemical groups, which are calcareous metasedimentary, sulfidic metasedimentary, undifferentiated metasedimentary, clastic sediments, mafic igneous, and felsic igneous. Multiple comparison tests showed the mean rank concentration of arsenic was significantly higher in water from the calcareous metasedimentary group than in water from the other bedrock groups (p=0.0001). The sulfidic metasedimentary group had the lowest mean rank arsenic concentrations. Iron and manganese concentrations were significantly higher in water from the sulfidic metasedimentary group than in water from the other metasedimentary groups. Spearman's rank correlation coefficients computed for arsenic and iron in water from all lithochemical groups are weak and are only slightly stronger between arsenic and manganese; neither is significant. Iron and manganese correlations are strong, however, for water from all three metasedimentary groups, and these correlations  are significant (p=0.01) and positive. Thus, the iron and manganese data were not strong predictors of arsenic concentration. These relations suggest that simple dissolution of arsenic-bearing iron phases (sulfides, hydroxides, arsenates) may not control arsenic in bedrock groundwater, although interactions between precipitation and sorption processes could decouple arsenic and iron. Statistical correlations linking the calcareous metasedimentary group with arsenic may reflect correlations with arsenic sources, solubility controls, land-use characteristics, or vulnerability to contamination.

Relations between arsenic concentration and three land uses (urban, agricultural, and forest, derived from early 1970's high-altitude photography) were tested to determine if an anthropogenic source of arsenic was apparent. The mean rank arsenic concentration was significantly higher in water from wells in agricultural land than in water from wells in forested areas; results for wells in urban land were not significantly different from those from wells in the other two land uses (p=0.013). Relations are, however, weaker for the land-use association than for the lithochemical association. The relation between arsenic concentration and agricultural land may result, in part, from agricultural land being present predominantly on particular geologic substrates. The data examined to date indicate that geology is an apparent predictor of arsenic occurrence and distribution in bedrock ground water.
 
 

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In a previous study, we measured arsenic concentrations in 992 drinking water samples collected from New Hampshire households (Peters et al. {\it in press}).  From this research, we determined that elevated ($>$50 $\mu$g/L) arsenic in drinking water was generally restricted to drilled bedrock wells.  We identified four clusters with elevated arsenic concentrations and have launched an investigation characterizing the source(s), fate, and transport of arsenic in each cluster.

Here, we report on the most prominent cluster, located  $\sim$5 km southwest of Concord, NH.  In this cluster, the spatial distribution of elevated arsenic concentrations correlates with the contact of the Concord Granite and surrounding schists.  Field observations along this contact revealed abundant pegmatite dikes radiating into the country rock from the main granite stock.  Analysis of rock digests yielded arsenic concen-trations up to 60 mg/kg in these pegmatites, with much lower values in the parent granite, and associated schists.  Weak acid leaches show that approximately half of the total arsenic in the pegmatites is labile and therefore can be mobilized during rock-water interaction.   SEM/EDS analysis of the pegmatites shows the predominant arsenic phase to be arsenopyrite, with secondary scorodite reaction rims.  Arsenic concentrations measured in water samples from local bedrock wells range from 2 to 453 $\mu$g/l, well above the statewide mean.  In the same water samples, we also measured arsenic speciation, and found As(III)/As(tot) ratios ranging from 8\% to 90\%.  We are in the process of characterizing the redox parameters of these groundwaters in order to gain insight into the cause(s) of the wide variability in arsenic speciation.
 
 

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Geochemical processes that determine the distribution of arsenic among sediment, dissolved and colloidal phases ultimately control arsenic mobility in groundwater.  Our research focuses on identifying associations of arsenic with various amorphous and mineral sediment phases in a wetland sediment to elucidate controls on transport.  Conventional consideration of inorganic arsenic geochemistry has included two primary arsenic associations: (1) in oxic environments, arsenate (As(V)) strongly complexes or co-precipitates with iron oxides and hydroxides, similarly to phosphate, and (2) in sulfide rich sediments, stability diagrams may predict that arsenic is predominantly as arsenopyrite (FeAsS) and orpiment (As$_{2}$S$_{3}$), depending on concentrations, pH, and redox.

We have compared these thermodynamic predictions to experimental findings of arsenic distribution in an urban wetland in Woburn, Massachusetts, to identify implications for arsenic mobility including conditions favoring arsenic remobilization.  To determine arsenic apportionment among specific pools, we have developed a selective extraction protocol that combines several established procedures with a phosphate extraction step. Extractions of sediment cores with 1M PO$_{4}$ indicate that roughly 1/2 to 2/3 of the total arsenic is adsorbed on constituent surfaces.  Another 1/3 of the total arsenic is dissolved along with amorphous iron-bearing phases by 1N HCl and 0.2M oxalate/ oxalic acid. Adsorbed arsenic and HCl- and oxalic acid- extractable arsenic are presumably associated with amorphous iron oxyhydroxide phases; yet, these iron oxyhydroxides must co-exist with porewater characterized by negative platinum electrode potential and high Fe(II) concentrations. We hypothesize that wetland plant oxygenation produces oxidized microzones, which promote the precipitation of root-associated iron oxyhydroxide phases followed by arsenic adsorption in the sediments.
 
 

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Monitoring metal contamination (e.g. As) in groundwater traditionally requires the installation of sampling wells, an expensive, time-consuming, and logistically-complicated process.  We report here the first results to establish the efficacy of using phytochelatins (metal-sequestering polypeptides) measured in tree leaves as an inexpensive, non-intrusive alternative that effectively negates sample contamination concerns.  Unlike bulk measurements of metals in groundwater and soils, phytochelatins analyses are a direct means for assessing the bioavailabilty of the toxic metals.  Glossy buckthorn (Rhamnus frangula) trees growing in situ above the shallow groundwater aquifer underlying the metal-contaminated Industri-Plex 128 Superfund site in Woburn, Massachusetts, reveal a pattern of phytochelatin production consistent with known groundwater contamination.  Phytochelatin levels are significantly higher downgradient of this site than in the remainder of the watershed, mirroring measured groundwater metal concentrations.

Contaminated groundwater flowing from the Industri-Plex site is discharged to the Aberjona River.  Downstream of this contaminated discharge zone, as expected, trees growing along the Aberjona River do not exhibit elevated levels of phytochelatins as they draw water from the relatively clean surrounding aquifer.  However, 2 km downstream from the Industri-Plex site, an exception to this pattern is observed at the Wells G \& H Superfund site.  Here phytochelatin concentrations are again as high as those observed near Industri-Plex.  By combining metal analysis from present day red oak (Quercus rubra) leaves with metal uptake histories in growth rings as revealed by secondary ion mass spectroscopy (SIMS) we are investigating the hypothesis that the aquifer underlying the Wells G \& H site was contaminated by metal-laden water pumped from the adjacent Aberjona River through highly contaminated (As concentrations 1,000 - 5,000 µg/gm) wetland sediments.
 
 

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Microbial arsenic reduction is emerging as a potentially important factor in aquatic arsenic mobility. Investigations of numerous arsenic-contaminated aquatic systems have revealed microbial transformation of As(V), a form that adsorbs strongly to sediment solids, to As(III), a form that is often much more mobile. Because this process generates energy for the microbes involved, it has the potential to modify the arsenic speciation of a system both rapidly and extensively. Laboratory experiments have shown significant arsenic mobilization from contaminated sediments by microbial arsenic reduction; indications exist, however, that nitrate, phosphate, and oxygen concentrations may limit arsenic-transforming microbial activity. Current efforts to quantify the significance of microbial arsenic mobilization in natural environments will be described.
 
 

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Concentrations of arsenic in leachate-plume water at the municipal landfill, Saco, Maine, varied with the character of geologic materials that underlie separate landfill piles. Comparisons were made of arsenic concentrations and arsenic-to-iron ratios in ground water, whole rock, and hydroxylamine-hydrochloride leaches of aquifer solid materials between leachate plumes that pass through (1) glaciomarine fine sands not in contact with bedrock, and (2) gravel and diamict deposits, consisting of an upper and a lower unit, over a fine-grain hornfels bedrock. Low arsenic concentrations and ratios were measured in plume water associated with the fine-sand deposits (average As = 0.033 mg/L, As:Fe = 0.031 w/w percent). High concentrations and ratios were associated with the upper gravel and diamict deposits (average As = 0.326 mg/L, As:Fe = 0.64 w/w percent). These corresponded with differences in arsenic concentration and arsenic-to-iron ratio measured between the solid materials associated with the plumes. The results were compatible with a hypothesized source of arsenic in the iron- and manganese-hydroxide minerals coating the aquifer solid materials. These minerals could be dissolved under reducing conditions in anaerobic leachate plumes. In the ground-water samples from the lower gravel and diamict aquifer and bedrock wells, an additional bedrock source was required to explain high arsenic concentrations and arsenic-to-iron ratios measured in water samples. Speciation analysis of arsenic in the leachate plumes indicated that high-level concentrations were primarily inorganic arsenic(III) associated with anaerobic conditions and with high concentrations of organic carbon and dissolved iron and manganese (0.1-micron filter). Equilibrium-speciation modeling indicated that no saturation indices were approached for arsenic-containing minerals; sorption reactions of arsenic onto iron-hydroxide minerals coating aquifer sediments, however, could control concentrations of arsenic in ground water.
 
 

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Leachate from a municipal landfill in Saco, Maine has resulted in a plume of anoxic groundwater containing high concentrations of arsenic(III), ferrous iron, manganese, and dissolved organic carbon. The source of arsenic appears to be reductive dissolution by dissolved organic carbon of arsenic-containing iron oxides in the bedrock and alluvium downgradient from the landfill. The landfill was covered with an impermeable membrane in 1997 to eliminate the source of anoxic groundwater and allow natural flushing of the aquifer to decrease arsenic concentrations to acceptable levels. A one-dimensional reaction-transport model was used to predict the evolution of the plume for the first 60 years after the landfill was covered. The modeled flow path extends from the landfill to a stream where the plume discharges. Groundwater analyses from wells along the 112 meter flow path were used to initialize the model. Selection of chemical reactions that are proposed to describe the evolution of the plume are based on interpretation of chemical data from samples of water and sediments in the aquifer, and laboratory experiments with contaminated cores leached with uncontaminated groundwater. The dominant biogeochemical reactions in the model were oxidation of organic carbon by dissolved oxygen, manganese oxides, and iron oxyhydroxides. These reactions were simulated using a modified form of Monod kinetics. Transport of arsenic was controlled by equilibrium sorption. Model parameters for these reactions were adjusted to obtain the best fit between the model and observed breakthrough curves of constituents in the laboratory experiments. Simulation results indicated that concentrations of most constituents in the landfill plume would rapidy decrease to near background levels within the next few years; however, the sorbed organic carbon in the aquifer was predicted to consume oxygen and maintain anoxic conditions for at least 60 years. Arsenic concentrations were predicted to slowly decrease from a high of 650 micrograms per liter to the current drinking water standard of 50 micrograms per liter in about 30 years. After 60 years, arsenic concentrations were predicted to be greater than 10 micrograms per liter, which is the drinking water standard currently under consideration by the Environmental Protection Agency.
 
 

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At an unlined ash landfill in eastern Maine, arsenic concentrations in the groundwater beneath and down gradient of the landfill range from 200 to 580 ug/L.  Elevated levels of arsenic are associated with depressed Eh readings and low concentrations of dissolved oxygen suggesting that the arsenic is present in a reduced groundwater environment.  Elevated levels of total organic carbon in the landfill leachate are the suspected energy source for microbial activity that produces reduced groundwater conditions at the site. Field and laboratory studies indicate that the bedrock is the most significant source of arsenic to the groundwater, rather than the landfill wastes.  The bedrock at the site consists of metasedimentary rocks comprised of a metasiltstone-sandstone and pelite unit, and a sulfidic spotted schist unit.   The distribution of arsenic in the groundwater shows a spatial correlation with elevated levels of arsenic in the bedrock determined from whole rock chemical analysis.  Detailed lithologic logging, laboratory leaching tests, thin section analysis and microprobe studies on rock core samples from the site indicate that the source of arsenic to the groundwater is likely the sulfidic minerals present in the bedrock.  Sulfide minerals identified in bedrock from the site include pyrite, pyrrhotite and arsenopyrite.  Microprobe studies also identified loellingite, an iron arsenate mineral, associated the sulfide minerals.  Determination of the source of arsenic to the groundwater at the site is important in the evaluation of water quality impacts and consideration of any future remedial actions or closure activities at the site.
 
 

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A former manufacturing facility in East Palo Alto, California, was used for the production and formulation of sodium arsenite-based herbicides and pesticides from 1926 to 1971.  Soil and groundwater in the vicinity of the site, which is adjacent to a tidal salt marsh, were found to contain elevated levels of arsenic that can exceed 1000 mg/kg and 100 mg/l, respectively.  A plume of arsenic-contaminated groundwater was previously delineated and source removal measures implemented. Currently, emplacement of a slurry wall is being considered for mitigating further arsenic migration.  Groundwater velocities at the site are on the order of 100 feet per year towards San Francisco Bay to the east, with some variation due to tidal influence.  Monitoring of arsenic concentrations in wells within 100 feet down-gradient of the plume boundary (defined by the 0.050 mg/l isopleth) over the last ten years has consistently shown arsenic levels to be less than 0.010 mg/l, indicating that the plume boundary has not moved appreciably during this time, and suggesting that natural processes in the tidal marsh mixing-zone are effectively limiting the subsurface migration of arsenic.  These observations have prompted more detailed investigations aimed at understanding the geochemical and mineralogical controls on arsenic transport and behavior and a reassessment of the appropriate future corrective action.

Geochemical conditions in the upper shallow groundwater zone change markedly from the site to the tidal marsh where groundwater discharges.  Groundwater beneath the main site is relatively fresh and oxic, whereas groundwater beneath the tidal marsh is saline and reducing.  Dissolved iron and chloride concentrations increase and arsenic decreases along the flow path.  Mixing of bay water and groundwater beneath the tidal marsh causes a reduction in dissolved arsenic due to dilution.  Mixing calculations indicate, however, that dissolved arsenic does not behave conservativiely in the groundwater mixing zone and is quantitatively removed relative to chloride over a short distance.  Processes responsible for the observed reduction in dissolved arsenic concentrations include sorption and precipitation/coprecipitation in sulfide minerals beneath the tidal marsh.  Sulfur isotope ratios of sulfate indicate that sulfate reduction is occurring within the tidal marsh.  Dissolved iron concentrations in groundwater at the fringes of the arsenic plume and in the tidal marsh appear to be controlled by precipitation of an iron sulfide mineral and, locally, iron (hydr)oxides as well.  Mass balance calculations indicate that dissolved sulfate originating from San Francisco Bay is sufficient to account for all the sulfur required to form iron sulfides.  The geochemical modeling calculations further indicate that As concentrations are not at equilibrium with any arsenate minerals, but are likely controlled by formation of an arsenic sulfide phase, such as orpiment.  This result is questionable, however, and it seems more likely that arsenic is coprecipitated with iron sulfide.  Sequential extraction and mineralogical data will be presented in support of this hypothesis.

Arsenic immobilization beneath the tidal marsh appears to be an irreversible process.  This is largely due to buffering of redox conditions by iron sulfide-iron (hydr)oxide-groundwater equilibria.  In the unlikely future event that iron sulfide begins to oxidize, the formation of iron (hydr)oxides would likely mitigate arsenic migration in groundwater through coprecipitation and sorption.
 

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