Hydrologic Connectivity and Nutrient Transport within the Great Bend of the Wabash River

Abstract

In the midwestern United States, nitrogen (N) pollution of surface and groundwaters is a substantial threat to water quality because of its ecological and human health effects. Hypoxia in the Gulf of Mexico is primarily caused by N runoff within the Mississippi River basin, and nitrate in drinking water may negatively impact human health in both adults and children. Agricultural tile drainage is a common practice that facilitates the transport of N from fields to streams. While the impacts of tile drainage have been studied extensively at the field scale, the impacts on hydrology, nutrient transport, and groundwater recharge are still uncertain at the watershed and landscape scales. The overall goal of this thesis work is to assess how tile drainage affects landscape-scale connectivity, hydrologic travel times, and N transport across a large catchment in west-central Indiana using 10 years of bi-annual water chemistry and stable isotope data from a community science education event. Land use data and a previously developed travel time distribution (TTD) model were also incorporated to accomplish this goal. A secondary goal is to estimate seasonal differences in groundwater recharge in west-central Indiana using stable water isotope data from precipitation and groundwater samples. Qualitative travel times derived from δ2H and δ18O variability support the idea that short travel times have greater nitrate concentrations than long travel times. Greater N concentrations are also observed during wetter conditions with increased connectivity. The results of the GIS TTD model support the hypothesis that increasing drainage intensity reduces travel times. Groundwater recharge appears negligible in Tippecanoe County using a traditional water balance approach, but an isotope mass balance approach suggests that about 55-65% of annual recharge occurs during the summer and may be linked to intense precipitation events. This knowledge improves our understanding of N transport and hydrologic connectivity in tile drained landscapes. The results of this thesis also demonstrate the importance of drainage density for travel times and provide additional insight into the seasonality of groundwater recharge in west-central Indiana.