Using high-resolution electrical resistivity to estimate hydraulic conductivity and improve characterization of alluvial aquifers

Shallow alluvial aquifers are important local and regional water resources but obtaining sufficient hydrogeologic data to properly characterize their spatial extent and hydraulic conditions can often be difficult, time-consuming, and expensive. In this study, we used over 40,000 measurements of electrical resistivity tomography (ERT) data and hydraulic conductivity (K) data from 33 pump tests across eight shallow alluvial and glacial outwash aquifer field sites in Iowa to: (1) assess the relation between hydraulic K tests and electrical resistivity (ER) in alluvial and glacial outwash systems; and (2) evaluate the application of the ER-K relationship as input data for local-scale groundwater flow modeling. Average field-measured K across the eight sites was 95 m/day and ranged from 29 to 246 m/day, whereas aquifer resistivity ranged from 27 to 896 (ohm-m). Both K and ER were greater at the glacial outwash aquifer sites compared to the alluvial aquifer sites and a linear relationship (R-2 = 0.90) was observed between field-measured K and ER when incorporating all field sites. We evaluated how K derived from the ER-K relationship compared to K estimated from pump tests and calibrated K in a local-scale groundwater flow model. The K estimated by the ER-K relationship was comparable to the K estimated from the pump tests in terms of providing reasonable values for model development. Utilizing the ER-K relationship to estimate K would save significant time compared to conducting pump tests. Hence, estimating K from electrical resistivity can provide a non-intrusive, cost and time efficient way to evaluate and model alluvial aquifers.