CHARACTERIZING GROUNDWATER-SURFACE WATER INTERACTIONS USING PHYSICAL DATA AND GEOCHEMICAL AND ISOTOPIC ANALYSES IN KANSAS

Water quality of shallow aquifers located near streams can be compromised by contamination from surface water recharge. Interactions between surface water and groundwater vary based on stream stage, subsurface geology, and seasonal influences. Insights into the effects these factors have on groundwater-surface water connections help constrain possible recharge and contaminant flow paths in alluvial environments. Groundwater-level measurements and surface and groundwater samples, analyzed for dissolved inorganic concentrations and δ²H and δ¹⁸O isotope compositions, were collected at two field sites in Kansas during growing and non-growing seasons.

The Larned Research Site has a river bed that is usually dry and an alluvial aquifer underlain by the High Plains aquifer. The river channel acts as a preferential flow path for surface water recharge to the shallow aquifer, evident by seasonal changes and variations during large precipitation events in the geochemical and isotopic data. Isotopic compositions of the aquifers followed the meteoric water line trend with no deviation from evaporation. This confirmed that groundwater loss from the riparian zone when the river was dry (indicated by diurnal water-level fluctuations) was mostly due to transpiration by phreatophytes. The isotopic compositions became lighter with depth in the aquifer units and were the lightest in an aquitard within the alluvial aquifer. The observed geochemical and isotopic trends indicate mixing within the aquifers past a leaky aquitard system containing older, fresher water.

The Rock Creek Site includes a stream and fine-grained alluvium overlying a shallow bedrock aquifer. The isotopic trend did not show evidence of evaporation, again indicating transpiration was a major source of water loss. Seasonal effects were observed in stream and water-table well samples, which give insight into the groundwater residence times. The stream and two water-table wells represent short-term water storage; isotopic compositions varied with each sampling event from seasonal changes, source of water, and spatial location. In contrast, the deeper alluvium base, bedrock, and remaining water-table well samples did not exhibit significant changes in isotopic composition due to precipitation events, and represent longer-term water storage.