Identifying the Controls on Flow and Contaminant Distribution in Siliciclastic Bedrock Aquifer Systems

This proposed research will advance characterization of groundwater flow in aquifers and aquitards in siliciclastic bedrock systems. Improved understanding of flow and contaminant transport in these hydrogeologic systems will allow for better water supply well siting and design. The research includes investigation of heterogeneity within siliciclastic aquifers and aquitards, and examines how heterogeneity is related to the distribution of anthropogenic and naturally occurring contaminants. The selected study area for this research, the Cambrian sandstone aquifer in south-central Wisconsin, is traditionally described as a relatively homogeneous hydrogeologic unit with a notable lack of differentiation= within Cambrian formations. However, recent evidence suggests significant variability in hydraulic conductivity, including fracture flow pathways, within these units. We will use distributed temperature sensing (DTS) technology, straddle packers, borehole flow logging, and traditional pumping tests to measure variation in hydraulic properties with depth in the aquifer. Such measurements, including evaluation of transmissive fractures, will determine the relative influence of fracture flow and intergranular flow in sandstone aquifers. Aquitards play a significant role in groundwater flow and contaminant distribution and are often assumed to be perfect barriers to contaminant transport, yet are often poorly characterized. The proposed work includes improvements to in situ hydraulic conductivity tests in low-permeability strata. We will refine a shut-in pressure test that will allow for more rapid and accurate assessment of hydraulic properties of low hydraulic conductivity sedimentary layers. Finally, this project includes assessment of groundwater chemistry to link physical heterogeneity of the system with chemical heterogeneity. This will be accomplished by sampling groundwater from discrete intervals of interest for major ions, trace metals, radioactivity, and indicators of redox conditions. The geochemistry data will support improved conceptual models of the effects of physical heterogeneity on water chemistry.