Monitoring and Modeling Nitrogen Loading within Streams Crossing the Recharge Zone of the Edwards Aquifer, South-Central Texas

McMahan, Michael Scott
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Rapid development on the Recharge and Contributing Zones of the Edwards Aquifer has led to increased risk of non-point source contamination in stormwater runoff. A growing concern is the increased levels of nitrates in surface waters that directly recharge the aquifer. Nitrate levels in the Edwards Aquifer have been observed to be higher than national background concentrations in groundwater (Sullivan & Gao, 2016). The purpose of this study was to examine and characterize existing nitrogen loading conditions within stream systems that cross the Recharge Zone of the Edwards Aquifer. 238 discrete water samples were analyzed for nitrate concentrations from five streams (Huesta Creek, Leon Creek, UTSA Tributary to Leon Creek (LCT), Maverick Creek and Helotes Creek) during twenty-six (26) storm events from Spring 2019 to Winter 2021. Observed flow and water quality data were used to calibrate and validate a SWAT hydrologic model to estimate nitrogen loads that could potentially enter the Edwards Aquifer from the losing drainage system. The SWAT model identified approximately 82-95% of nitrate originated from urban runoff. Pairwise correlation analysis and multiple linear regression methods were used to identify significant explanatory variables to estimate event mean nitrate concentrations and seasonal aquifer contamination risk. Other than Helotes Creek, all sub-watersheds exhibited strong positive correlations between nitrate concentrations and number of incipient dry days (r-value of 0.9776 to 0.6516; p-value<0.0013). Seasonal nitrogen loading was shown to occur within the study area for 3 of the 5 sub-watersheds (r-value of -0.9144 to -0.1693; p-value<0.0335). Nitrate was significantly correlated with antecedent dry periods for the Upper Leon Creek Watershed (r = 0.5778, p<0.001), suggesting that long dry periods followed by the first rain events of the wet season, resulted in the strongest predictors for increased nitrate concentrations in runoff. Total Suspended Solids (TSS) did not appear to be the dominant predictor of NO3 concentrations for this study. Antecedent dry days, drought conditions, duration of storms and seasonality showed stronger correlations with elevated nitrate concentrations. This suggests current stormwater best management practices (BMPs) for the study area that mitigate TSS alone, may not effectively target variables potentially more influential on nitrogen loading. This research shows BMP maintenance should be prioritized for times following long dry periods and prior to the first storms of the season in August, September and October.

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Edwards Aquifer, Flush, Loading, Nitrate, Recharge, SWAT