Interflow in semiarid environments: An overlooked process in risk assessment

Risk assessment, both human and ecological, embodies fundamental assumptions about hydrological processes, especially how they affect the movement of contaminants in the environment. The lateral movement of water through the soil, or interflow, is frequently a component of risk assessments for humid environments, but not of those for semiarid environments. Our research has shown that, contrary to what was previously thought, interflow can be important in semiarid landscapes and is, therefore, an essential consideration for risk assessment in these regions. To illustrate and assess the effect of interflow on estimates of risk, we (1) developed a simple conceptual model to describe the role that interflow may have in the redistribution of surface and near‐surface contamination, and (2) used RESRAD, an exposure model for assessing radionuclide doses to humans, to evaluate the effectiveness of landfill covers in mitigating doses of three contaminants (³H, ²³⁸U, and ²³⁹/²⁴⁰Pu) at a site in northern New Mexico at which interflow is known to be occurring. Only those calculations of the model that took interflow into account yielded the result that the radionuclides would contaminate groundwater‐underscoring the potential importance of interflow as a mechanism for the transport of contaminants. We conclude that failure to take interflow into account can render risk assessments inaccurate and remediation ineffective. Further, our work demonstrates that a general understanding of hydrological processes is essential for accurate risk assessment, ecological as well as human.