Evaluating groundwater/surfacewater transition zones in ecological risk assessments


Courtesy of US EPA - Environmental Protection Agency

Currently, there is a common perception that the discharge of contaminated groundwater to a surfacewater body does not pose an ecological risk if contaminant concentrations in surface-water samples are below analytical detection limits or at very low concentrations. The transition zone represents a unique and important ecosystem that exists between surface-water and the underlying ground-water, receiving water from both of these sources. Biota inhabiting, or otherwise dependent on, the transition zone may be adversely impacted by contaminated ground-water discharging through the transition zone into overlying surface-waters.

Ecological Risk Assessments (ERA) addressing contaminated ground-water discharge to surface-waters typically have not evaluated potential contaminant effects to biota in the transition zone. However, numerous hydrogeological and ecological methods and tools are available for delineating ground-water discharge areas in a rapid and cost-effective manner, and for evaluating the effects of contaminant exposure on transition zone biota. These tools and approaches, which are commonly used in hydrogeological and ecological investigations, can be readily employed within the existing EPA framework for conducting screening- and baseline-level ERAs in Superfund (U.S. EPA 1997) to identify and characterize the current and potential threats to the environment from a hazardous substance release.

This document was initially prepared as an ECO Update/Ground Water Forum Issue Paper to highlight the need to treat the discharge of groundwater to surface-water not as a two-dimensional area with static boundary conditions, but as three-dimensional volumes with dynamic transition zones. This ECO Update applies equally to recharge zones and can be used to evaluate advancing plumes that have not yet reached the transition zone. This document encourages project managers, ecological risk assessors, and hydrogeologists to expand their focus beyond shoreline wells and surface sediments and define and characterize the actual fate of contaminants as they move from a strictly ground-water environment (i.e., the commonly used “upland monitoring well nearest the shoreline”) through the transition zone and into a wholly surface-water environment. The approach is presented to help users identify and evaluate potential exposures and effects to relevant ecological receptors within the zone where ground-water and surface-water mix. The transition zone data collected for the ERA may also supplement data collected for the evaluation of potential human health risks associated with the discharge of contaminated ground-water.

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