|Field Study of the Fate of Arsenic, Lead, and Zinc at the Ground Water/Surface Water Interface (EPA/600/R-05/161) December 2005
Physical and chemical interactions between adjacent bodies of ground water and surface water are important factors that affect water budget and contaminant transport within a watershed. These interactions are also of importance for hazardous waste site cleanup within the United States; about 75 percent of sites regulated under the Resource Conservation and Recovery Act and the Comprehensive Environmental Response, Compensation, and Liability Act are located within a half mile of a surface water body.
The boundary between adjacent bodies of ground water and surface water is referred to as the ground water/surface water (GW/SW) transition zone. The transition zone plays a critical role in governing contaminant exchange and transformation during water exchange between the two water bodies. The transition zone is host to a wide diversity of aquatic organisms, and it can serve as a sink for contaminants transported in surface water or ground water. Ultimately, the potential for human exposure within a watershed and the health of the ecosystem inhabiting the transition zone will depend on the bioavailability of accumulated contaminants. The extent of contaminant bioavailability will, in part, be dictated by partitioning reactions that control the distribution and speciation of contaminants within water and sediments in the GW/SW transition zone.
The purpose of this document is to illustrate some of the chemical processes that govern contaminant transport and speciation during water exchange across the GW/SW transition zone. The focus is on the assessment of metal speciation transformations in contaminated sediments. A field investigation was conducted of the fate of arsenic, lead, and zinc transported across the GW/SW transition zone at a contaminated site. Results are presented in order to illustrate the importance of using a site conceptual model and to provide an example of approaches that may be used to characterize the spatial and temporal distributions of inorganic contaminant speciation.
The field site is located immediately downgradient from the Industri-Plex Superfund site in Woburn, Massachusetts, and is characterized by a ground water contaminant plume that discharges into the Halls Brook Holding Area (HBHA) pond, resulting in contamination of surface water and sediments. The results from this field investigation provide insight into the source of inorganic contaminants within the pond and present a conceptual framework relative to the design of strategies to mitigate human exposure to site-derived contaminants.
Spatial and temporal trends in ground water data indicate that arsenic and zinc observed within surface water and sediments of the pond are primarily derived from continuing ground water discharge. In contrast, lead observed in sediments appears either to be derived from historical discharges or is currently derived from sediment transport or soil erosion from upgradient source areas.
In addition, the vertical distribution and temporal patterns in arsenic concentrations within the water column of the pond indicate that sediment dissolution and desorption processes contribute to the overall dissolved concentration of this contaminant. The fate of these inorganic contaminants is coupled to the fate of iron and sulfate derived from ground water discharge.
Iron (hydr)oxides are actively produced in oxic portions of the HBHA pond, while iron sulfides are produced in suboxic/anoxic portions of the pond. The generation of iron (hydr)oxides is a result of oxidation and precipitation of ferrous iron upon contact with oxygen within oxic portions of the pond. The generation of iron sulfides is tied to microbial sulfate reduction coupled with degradation of anthropogenic and naturally occurring dissolved organic compounds in discharging ground water within suboxic/anoxic portions of the pond. These newly formed precipitate phases possess significant sorption capacity for arsenic, lead, and zinc. The retention of these solids within deeper portions of the pond water column or sediments helps to mitigate downgradient transport of these freshly deposited contaminants through the watershed.
Analytical data are presented to define the spatial distribution and chemical speciation of arsenic, lead, and zinc within pond sediments relative to the spatial distribution of predominant redox processes throughout the pond. While the observed distributions of arsenic, lead, and zinc in ground water, sediments, and surface water described in this report are ultimately site-specific, some observed patterns in contaminant geochemistry are relevant to other contaminated sites. Specifically, the results of this study may have application to sites where an anoxic iron-rich ground water plume encounters an oxygenated environment (e.g., anoxic landfill leachates or organic contaminant plumes).
The results of this intensive field investigation provide useful information on technical approaches to characterize inorganic contaminant transport through subsurface redox gradients that are frequently established at sites with co-occurring organic and inorganic contaminant plumes. As demonstrated for this particular site, the ultimate fate of inorganic contaminants will depend on both site-specific characteristics (e.g., hydrology and soil/sediment mineralogy) and the chemical properties of the contaminant in question.
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