INTERFLOW IN SEMIARID ENVIRONMENTS - AN OVERLOOKED PROCESS IN RISK ASSESSMENT

Risk assessment, both human and ecological, embodies fundamental assum ptions about hydrological processes, especially how they affect the mo vement of contaminants in the environment. The lateral movement of wat er through the soil, or interflow, is frequently a component of risk a ssessments for humid environments, but not of those for semiarid envir onments. Our research has shown that, contrary to what was previously thought, interflow can be important in semiarid landscapes and is, the refore, an essential consideration for risk assessment in these region s. To illustrate and assess the effect of interflow on estimates of ri sk, we (1) developed a simple conceptual model to describe the role th at interflow may have in the redistribution of surface and near-surfac e contamination, and (2) used RESRAD, an exposure model for assessing radionuclide doses to humans, to evaluate the effectiveness of landfil l covers in mitigating doses of three contaminants (H-3, U-238, and Pu -239/240) at a site in northern New Mexico at which interflow is known to be occurring. Only those calculations of the model that took inter flow into account yielded the result that the radionuclides would cont aminate groundwater - underscoring the potential importance of interfl ow as a mechanism for the transport of contaminants. We conclude that failure to take interflow into account can render risk assessments ina ccurate and remediation ineffective. Further, our work demonstrates th at a general understanding of hydrological processes is essential for accurate risk assessment, ecological as well as human.