Determining the Influence of Surface Water on Ground Water in An Alluvial Aquifer

Many municipal water systems in the northeastern United States pump ground water from shallow unconfined alluvial aquifers. The wells are commonly located close to existing streams, making the wells candidates for Ground Water Under the Direct Influence of surface water (GWUDI) classification, a designation that carries substantial regulatory and financial consequences. Because all ground water ultimately is sourced from the surface, all water is GWUDI at some temporal and spatial scale.

We are studying the interaction between ground water and surface water in wells in an alluvial aquifer that has not been classified as GWUDI, with the goal of determining if different methods yield different conclusions about the degree of connection between stream and wells and to better determine what GWUDI and non-GWUDI mean in the real world. We are using measurements of head, stream discharge, temperature, electrical conductivity, and major ion and isotope chemistry to determine the temporal and spatial relationship of interaction between water from the two sources. Preliminary results show that pumping is inducing infiltration from the stream in the vicinity of the well field. Preliminary major ion analyses support these results, but the timing and extent of the interaction between stream and wells is unknown.