STORMWATER AND NON-POINT SOURCE CONTAMINANTS IN A SMALL URBAN WATERSHED

We evaluated transport (timing and amounts) of non-point source pollutants (NPS) from Sessom Creek into the Upper San Marcos River (San Marcos, Texas) using data from 11 storm events during 2018. Sessom Creek is a small, high gradient, and heavily urbanized tributary of the Upper San Marcos River. Runoff occurs within minutes of precipitation and there is no significant stormwater retention or detention in the watershed. Therefore, rapid transport and loading of contaminants from Sessom Creek into the 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. The Edwards Aquifer Habitat Conservation Plan requires that sediment and other NPS loads be removed or reduced to protect endangered species and their habitat.

We investigated quantitative relationships between contaminant loads and a variety of environmental conditions related to precipitation, discharge, and antecedent conditions. For each of the eleven storm events monitored, 24 water samples were collected and analyzed per event, including total/volatile/non-volatile suspended sediments, nutrients (nitrogen and phosphorous) and bacteria (E. coli). Transport and loading of these contaminants to the river is highly variable and appears to be dependent on antecedent and event-specific conditions. Increases in discharge can occur within 5 minutes of precipitation, and peak flows can occur within 15 minutes. Most of the load is transported during the first hour of a storm event, and peak concentrations of pollutants often occurs before the peak discharge for each event, suggesting that remediation efforts should focus on detention and retention to avoid transport during the first flush portion of the hydrograph.