TRACING SOURCES AND FATE OF NITRATE USING ENVIRONMENTAL TRACERS AND NUMERICAL MODELS IN KARST WATERSHEDS AND SPRINGS, CENTRAL TEXAS

Assessing the fate and transport of contaminants in karst aquifer is challenging because it is not possible to identify the location and geologic characteristics of all karst features within an aquifer. Soil & Water Assessment Tool (SWAT) models were developed to identify the predominant source of nitrate entering surface waters in karst watersheds in central Texas. The analysis of watershed nitrogen inputs identified livestock, atmospheric deposition, wildlife, and fertilizer use as primary sources of nitrogen. The SWAT model showed that the largest contributors to nitrate stream yield were atmospheric deposition, livestock, and a Wastewater Treatment Plant (WWTP). A contaminant transport model was developed using CFPv2, an experimental version of the Conduit Flow Process (CFP) module of MODFLOW, in conjunction with the Three-Dimensional Underground Mine Transport Model (UMT3D). Nitrate concentration and aquifer recharge estimated from the SWAT model were used as input to the CFPv2 and UMT3D models. The hydrologic calibration of the CFPv2 model demonstrated that it outperformed a previously developed Equivalent Porous Medium (EPM) model for the Edwards Aquifer. The UMT3D model was successfully calibrated for nitrate concentrations and the results obtained were consistent with the assessment of nitrate sources from the SWAT model. Preliminary results of multiple radiogenic and stable isotopes on surface water and groundwater demonstrated that multi-isotope tracers can be used to characterize residence time and sources of nitrate in several streams and springs in central Texas.