Nitrate Sources, Vulnerability, Fate and Transport in the Cibolo and Dry Comal Creek Watersheds of the Edwards Aquifer, South Central Texas

Nitrate contamination of drinking waters is a global concern because it enters the environment through numerous natural and anthropogenic sources and is very mobile. Contamination of karst aquifers is of particular concern because karst features allow the rapid infiltration and transport of contaminants through the aquifer. Assessing the vulnerability of karst aquifers as well as the fate and transport of contaminants in them is challenging because it is not possible to identify the location and geologic characteristics of all karst features within an aquifer. The research presented here addressed this limitation through an analysis of nitrate sources, development of a new karst aquifer vulnerability index, and the development of a groundwater transport model using experimental modeling software. The study area consists of the Cibolo and Dry Comal Creek watersheds overlying the karstic Edwards aquifer in the San Antonio region of Texas, USA.

For the first phase of research, a Soil & Water Assessment Tool (SWAT) model was developed to identify the predominant source of nitrate entering surface waters in the study area. The analysis of watershed nitrogen inputs identified livestock (42.0 %), atmospheric deposition (26.8 %), deer (9.9 %) and farm fertilizer use (9.7 %) as the primary sources of nitrogen. The SWAT model showed that the largest contributors to nitrate stream yield were atmospheric deposition (56.3 % of nitrogen yield), livestock (25.9 %), and a Wastewater Treatment Plant (WWTP) (7.1 %).

During the second research phase, a new aquifer vulnerability assessment methodology was created to minimize exploration bias. The P3 method ( Probability, Protection, and Precipitation) minimizes explorational bias through a decision tree model generated from probability maps and nearest neighbor analysis to assign a reduction in aquifer protection based on the probability of encountering karst features. The P3 method was successfully validated by using spring hydrographs, tracer tests, nitrate samples, and output from the SWAT model. The nitrate inputs from the first research phase were combined with the P3 methods vulnerability map to develop a Risk Intensity Index (RII) map for the study area.

Finally, a contaminant transport model was developed using CFPv2, an experimental version of the Conduit Flow Process (CFP) module of MODFLOW, in connection with a Conduit Mass Transport Three-Dimensional (CMT3D) model. Nitrate and aquifer recharge estimates from the SWAT model were used as input to the CFPv2 and CMT3D models. The hydrologic calibration of the CFPv2 model demonstrated that it outperformed a previously developed Equivalent Porous Media (EPM) model for the Edwards. The CMT3D model was successfully calibrated for nitrate and the results obtained were consistent with the assessment of nitrate sources from phase one and the RII map developed during phase two. Together, the three research phases provide a comprehensive methodology for watershed managers to assess sources, fate, and transport of nitrate in karst aquifers.