Impact of preferential flow paths on stream and alluvial groundwater interaction

A better understanding of stream-aquifer interactions is needed both for water policy and for quantifying its impact on stream chemistry and water quality. Assuming homogeneity in alluvial aquifers is convenient but limits our understanding of stream-aquifer interactions. Previous research in the Ozark ecoregion, which is characterized by cherty soils and gravel bed streams, identified subsoils with hydraulic conductivities of 140 to 230 m d^-1 and showed non-uniform groundwater flow. In a preferential flow path (PFP), even a sorbing contaminant, such as phosphorus, was found to be transported and not significantly attenuated through the subsurface. The objective of this research was to document the impact of flow heterogeneity (i.e., PFPs) on groundwater flow patterns, which will strengthen current understanding of contaminant interaction between streams and alluvial aquifers. Long term monitoring was performed adjacent to the Barren Fork Creek and Honey Creek in northeastern Oklahoma. Based on results from subsurface electrical resistivity mapping, observation wells were installed both in PFPs and in non-PFP subsoils. Water levels in the wells were recorded real-time using pressure transducers. Plots of the water table elevation, streamlines, and water level divergence were generated using six weeks of data including multiple high flow events. Divergence was used to quantify heterogeneity in hydraulic conductivity. The activity of PFPs depended on the elevation of the water table and the interaction between the stream and the groundwater. It appeared that PFPs acted as divergence zones, allowing stream water to quickly enter the groundwater system, or as flow convergence zones, draining a large groundwater area.