IS BASEFLOW IN THE HEADWATERS OF THE WABASH RIVER, IN/OH SUPPORTED BY PLEISTOCENE RECHARGE?

A synoptic sampling campaign was conducted along the entire length of the Wabash River in the fall of 2015 to identify the sources of water that support baseflow in the watershed. Anomalously high chloride, sulfate, strontium, and sodium concentrations were observed in the headwaters of the Wabash River, along the reach from Fort Recovery, OH to Huntington, IN. This area coincides spatially with the presence of Silurian carbonate rocks (located beneath the glacial till) which host the Midwestern Basins and Arches Regional Aquifer System that was likely recharged during the Last Glacial Maximum. Our hypothesis is that the anomalous geochemical concentrations are evidence for the discharge of Pleistocene recharge from the regional carbonate aquifer to the headwaters of the Wabash River. We test the hypothesis using ³⁶Cl, ⁸⁷Sr/⁸⁶Sr, and stable isotopes of water (¹⁸O, ²H) measured at Wabash River sampling sites. Widespread flooding occurred in upper watershed during the summer and early fall of 2015. Consequently, many of the river samples collected in October and November of 2015 have ³⁶Cl/Cl ratios similar to precipitation collected over the summer and fall (40 to 80 x 10^-15). However, three headwater sampling sites have relatively low ³⁶Cl/Cl ratios and high Cl^-/Br^- ratios indicative of discharge from the regional carbonate aquifer. The headwater sites have distinct ³⁶Cl/Cl ratios from relatively recent recharge (within the last 60 years) and modern runoff (within the year). In addition, the headwater sampling sites have δ¹⁸O and δ²H compositions that are lighter than the sampling sites containing large fractions of modern runoff. Strontium isotope ratios are intriguing. The ⁸⁷Sr/⁸⁶Sr ratios of the three headwater sampling sites are similar to the ⁸⁷Sr/⁸⁶Sr ratio of the Silurian Brassfield Limestone, one of the carbonate rocks which hosts the regional aquifer. This multi-tracer study provides compelling evidence that Pleistocene recharge supports baseflow in the Wabash River. The fluxes of water and solutes from deep regional groundwater are not included in many models of the Wabash River, however these contributions are not insignificant and may impact a larger area than observed here.