CHARACTERIZING SURFACE AND GROUNDWATER INTERACTIONS OF DIFFERENT SUBSURFACE GEOLOGIC ENVIRONMENTS USING GEOCHEMICAL AND ISOTOPIC ANALYSES IN KANSAS

Shallow aquifers located near streams can be affected by groundwater contamination as a result of recharge from surface water; however, subsurface geology and stream stage variation can alter the magnitude of groundwater-surface water interactions. Quantifying the magnitude of these interactions will constrain possible water and contaminant flow paths for future water supply wells installed in similar alluvial environments. This research capitalized on previously collected physical data, including geology, water level measurements, and hydraulic conductivity, and new geochemical and isotopic data to assess the effects of hydrogeological and seasonal conditions on groundwater-surface water interactions at two geologically distinct field sites.

Groundwater levels were measured at both field sites during the growing and non-growing seasons. Water samples were collected from wells in shallow and deep sand and gravel aquifers and a clay aquitard in a river valley at the Larned Research Site (LRS). Water was sampled from a small stream and wells at different depths in alluvium resting on bedrock at the Rock Creek Site (RCS). Dissolved inorganic constituent concentrations and δ²H and δ¹⁸O were determined for all samples to indicate the influence of surface water on groundwater.

Water from the aquitard at the LRS had considerably lower dissolved solids concentration than in the shallow and deep aquifers. The isotopic compositions became lighter with depth from the shallow to deep aquifers, and were the lightest in the aquitard. The disparity between the aquifers and aquitard implied older, fresher water contained in a leaky aquitard system. At the RCS, total dissolved solids were higher in the surface water and water-table wells in the non-growing season and highest in the water-table wells during the growing season, indicative of riparian zone evapotranspiration. Specific dissolved constituent concentrations in the samples varied between sampling seasons and spatially. The isotopic compositions were the heaviest in the surface water and became lighter with depth. The low hydraulic conductivity of the LRS aquitard provided longer water storage and duration for geochemical reactions. Groundwater flow at the RCS included slow upward flow from the bedrock to the alluvium and lateral flow from and to the stream.