Urban growth changes how rainwater flows across the land and affects the quality of water that soaks into the ground. In the Edwards Aquifer region of south-central Texas, stormwater runoff from cities carries nitrate, a common pollutant, into streams that recharge the underground aquifer. High nitrate levels can harm ecosystems and contaminate drinking water.

This study investigated nitrate in five urban streams during storm events from 2019 to 2021. We collected water samples and used computer models to understand where nitrate comes from and how it enters the aquifer. We found that most nitrate originates from city runoff—especially after long dry periods followed by rainfall. This “first-flush” effect causes pollutants to build up on surfaces and then rapidly wash into streams and the aquifer.

Current stormwater controls often focus on removing sediments but may not reduce nitrate pollution effectively. Our findings suggest that managing stormwater with attention to rainfall timing and dry conditions could better protect groundwater. This research supports more effective water management strategies to keep drinking water safe and maintain healthy ecosystems.