Multiple Isotopes Reveal Spatial Complexities of Baseflow Sources in the Wabash River Watershed in Indiana and Ohio

Identifying the sources of water which contribute to baseflow in agriculturally-fragmented watersheds is critical to understanding spatial and temporal trends in solute and nutrient loading. Shallow phreatic aquifers and tile-drain waters commonly have elevated nutrient concentrations, but modern residence times. These waters can mask the geochemical signature and residence time of older sources of groundwater making it difficult to determine how they impact baseflow processes. This problem is addressed by sampling for general chemistry, ³⁶Cl, and ⁸⁷Sr/⁸⁶Sr at sites located longitudinally down the Wabash River. Many of the river samples collected in October and November of 2015, after widespread flooding during the summer of 2015, track the ³⁶Cl/Cl ratios of precipitation (40 to 80 x 10^-15). However, three sites had relatively low ³⁶Cl/Cl ratios and high Cl^-, SO₄^2-, Sr²⁺ and Cl^-/Br^- ratios indicative of discharge from a deep regional aquifer. Three sites had relatively high ³⁶Cl ratios and low Cl^-, SO₄^2-, Sr²⁺ and Cl^-/Br^- ratios indicative of mixing between recent runoff and discharge of bomb-pulse aged groundwater. Altogether, these data show the utility of incorporating cosmogenic isotopes in studies of agricultural watersheds where they provide a unique way to identify both recent, bomb-pulse, and very old recharge in agriculturally-fragmented watersheds.