Stormwater and Non-Point Source Pollutants in Sessom Creek, San Marcos, TX
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The primary goal of this project was to evaluate and model the transport (timing and amounts) of non-point source pollutants (NPS) from the Sessom Creek watershed into the Upper San Marcos River (San Marcos, Texas) during storm events. Sessom Creek is a small and heavily urbanized tributary of the Upper San Marcos River, a spring-fed river from the Edwards Aquifer. Runoff is extremely rapid in the high-gradient Sessom Creek watershed, and there are no significant stormwater retention or detention structures in the watershed. Therefore, rapid transport and loading of contaminants from Sessom Creek into the Upper San Marcos River occurs during storm events. This is a concern due to the presence of several federally endangered or threatened species in the river, and its heavy recreational use. Twelve storm events were sampled during 2018 with an ISCO automatic water sampler. NPS pollutants, including total/volatile/non-volatile suspended solids, nutrients (dissolved and total forms of nitrogen and phosphorous), and bacteria (E. coli) were analyzed in all samples using standard methods. Results indicate that transport and loading of stormwater pollutants to the river are highly variable and primarily dependent on peak discharge, maximum rain intensity, and runoff volume. Dissolved and total nutrients were significantly related to volatile and non-volatile suspended solids. Increases in discharge and peak of discharge can occur within 5 minutes of rain, and most of the NPS loads are transported during the first hour of a storm event. Peak concentrations of NPS pollutants often occur before the peak flow for each event, suggesting that remediation efforts should focus on detention and retention to avoid transport during the first flush portion of the hydrograph.