The Impact of Ground-Water/Surface-Water Interactions on Contaminant Transport Contaminated Sites

Introduction It is recognized that physical and chemical interactions between adjacent ground water and surface water bodies are an important factor impacting water budget and nutrient/ contaminant transport within a watershed (Winter et al., 1998). This observation is also of importance for hazardous waste site cleanup within the United States, since about 75% of sites regulated under the Resource Conservation and Recovery Act (RCRA) and the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA or Superfund) are located within a half mile of a surface-water body (Tomassoni, 2000; Biksey and Gross, 2001). The boundary between adjacent groundwater and surface-water bodies 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 intervening transition zone between a stream and its adjacent aquifer has historically been referred to as the hyporheic zone (see Triska et al., 1993 for specific definition). The more general terminology, GW/SW transition zone, is used throughout this document to stress the importance that water and solute exchange is not limited only to streams. The purpose of this document is to provide a brief overview of the dynamics of chemical processes that govern contaminant transport and speciation during water exchange across the GW/SW transition zone and to present results from a field study examining the fate of arsenic during ground-water discharge into a shallow lake at a contaminated site. A conceptual model of the GW/SW transition zone is defi ned to serve as a starting point for prioritizing tasks carried out during site characterization to define contaminant mass flux across the GW/SW transition zone. This information is a critical component towards establishing site-specific risks and alternatives for remedial intervention to reduce or eliminate these risks. Developing a knowledge base for delineating the biogeochemical processes controlling subsurface transport of the contaminant is one component of the investigative effort to define human or ecological risk. The discussion that follows necessarily ignores specific risk receptors. Since risk is dependent on the degree of current and future contaminant exposure to the receptor, it is important to define contaminant mass distribution (aqueous, solid, gas) and the dynamics of mass re-distribution within the regulatory boundaries established for the site. However, receptor response to contaminant exposure may not vary proportionally to contaminant mass or media-specific concentration, so the overall risk characterization effort must be guided by knowledge of both contaminant and receptor(s) distribution within the investigative boundary.