A basin-scale groundwater flow model of the Columbia Plateau Regional Aquifer System in the Palouse (USA). Insights for aquifer vulnerability assessment

Bedrock aquifers are vulnerable to contamination due to the preferential movement of pollutants via rock discontinuities and porous layers. In this research, we propose an approach to assess vulnerability in three dimensions by combining stable isotope values and particle tracking in a vertically anisotropic aquifer of basaltic-fluvial origin at the basin-scale. A steady-state flow and particle tracking model is presented for the Columbia River Basalt aquifer in the South Fork Palouse Basin. Backward particle analysis combined with the distribution of δ2H, δ18O and 4He values vs. depth shows how the aquifer is characterized by two separated zones. A shallow (< 150 mBGL) aquifer portion is characterized by low particle travel times from the pumping wells to the recharge boundaries. However, the deeper (> 150 mBGL) aquifer zone is characterized by much higher particle travel times as well as a distinctive isotopic fingerprint. At such depths, penetration of particles is partially impeded by the low hydraulic conductivity of the sedimentary layers and recharge preferentially occurs in correspondence of the basin margin. Along this margin, the vulnerability is higher for the contaminants to enter the aquifer system and reach the pumping wells. Thus, following this research, we envisage efforts to combine stable isotope techniques with particle tracking analysis in three dimensions to define areas exposed to contamination risk in fluvio-volcanic bedrock aquifers. These research efforts can represent an approach to integrate with two-dimensional GIS tools that are commonly used to assess aquifer vulnerability.