Using multiple isotopic and geochemical tracers to disentangle the sources of baseflow and salinity in the headwaters of a large agricultural watershed

Subsurface drainage modifications are a common practice in agricultural watersheds, especially in poorly drained soils of the American Midwest, yet our understanding of their impact on baseflow generation processes is incomplete. By extension, these same practices can make baseflow vulnerable to salinization. To address these knowledge gaps, a combined geochemical and multiple isotope approach was used to identify sources of base-flow and salinity in the Wabash River, draining Indiana, Ohio, and Illinois. Anomalously high salinity and high concentrations of F-, SO42-, Na+, Sr2+, and K+ were measured in baseflow along the headwaters reach of the Wabash River from Fort Recovery, OH to Huntington, IN. Three river sites in the headwaters reach have low Cl-36/Cl ranging from 27.5 (x 10(-15)) to 33.4 (x 10(-15)) that do not fall within the Cl-36/Cl range for monthly precipitation. Their Cl-36/Cl are consistent with Cl-36 dilution most likely due to the presence of old Cl- found in potash (KCl) fertilizers. However, the Sr-87/Sr-86 ratios of the headwaters reach range from 0.708530 to 0.708904 (+/-0.000025) and are similar to Silurian bedrock samples (0.708292 +/-0.000047) indicating a geologic source for some solutes. The delta O-18 of the headwaters reach are isotopically light and similar to cold-season recharge, whereas the delta O-18 of the downstream river samples are similar to summer precipitation. The Cl-, NO3-, Cl-/Br- , and Cl-36/Cl data point to an anthropogenic source of salinity, namely KCl and road salt, whereas the F-, Sr2+, SO42-, and Sr-87/Sr-86 data point to a geologic source of solutes. We infer that shallow groundwater that is Cl- and NO3- rich mixes in the river with regional groundwater that is Cl- and NO3- poor. The contri-bution of regional groundwater to baseflow in the headwaters reach is not insignificant (>58 percent) and these interactions should be considered in future hydrologic models of the Upper Wabash River. Without the aid of multiple isotopic and geochemical tracers, baseflow generation processes and sources of salinity in the head-waters reach would remain masked by the elevated solute concentrations associated with the shallow aquifers. Further research is needed to better understand the impact of KCl fertilizers on the overall salinity budgets of agricultural watersheds.

Automated summary

Subsurface drainage modifications are commonly used in agricultural watersheds, but their impact on baseflow generation processes and salinity sources is not well understood. This study used geochemical and multiple isotope methods to investigate the sources of baseflow and salinity in the headwaters reach of the Wabash River. The results suggest that some solutes have a geologic source, while salinity may result from both anthropogenic and geologic factors.