Urban Runoff and the Edwards Aquifer: What Pesticides Reveal

By Stephen P. Opsahl, MaryLynn Musgrove and Keith E. Mecum. USGS Fact Sheet 2020–3028, August 2020, prepared with the City of San Antonio.

Why it matters

The San Antonio segment of the Edwards aquifer is a designated sole-source aquifer: the main water supply for more than 1.7 million people in San Antonio and around it. Homes and businesses keep spreading across its recharge zone in Bexar County, and every new roof, road and lawn can add contaminants to stormwater — water that may end up in the aquifer.

The Edwards is limestone karst. Where it lies open at the surface (the unconfined or recharge zone), water moves in quickly through fractures and cavities, so pollution at ground level has a short path down. Local agencies already protect land through measures such as conservation easements. What was harder to show was the link itself: how storm runoff connects to the groundwater below.

The study

From 2017 to 2019, the USGS and the City of San Antonio compared two pairs of sites, each a creek matched with a nearby well:

Figure 1. The study sites across the aquifer's zones. The Trinity aquifer lies beneath the whole San Antonio segment. Map: USGS.

Instruments recorded creek stage, well water levels, and the temperature and specific conductance of well water without a break. Crews also sampled the creeks and wells about every 8 weeks and after storms, testing for common chemicals and for a long list of pesticides and their breakdown products.

Rain on, water up

For the first 11 months, rain was below average, West Elm Creek rarely ran, and the Shavano well's water level fell steadily. Then heavy rain in September 2018 changed everything: West Elm Creek ran more often and higher, and the water level in the Encino Rio well rose nearly 70 feet.

Figure 2. Daily rainfall (A), the Encino Rio well's water level and specific conductance (B), and that well against West Elm Creek's stage (C), 2017–19. Graph: USGS.

A rise that fast is recharge — creek water reaching the aquifer almost directly. In karst that is expected at both pairs, urban or not. Well-water quality shifted at the same moments.

What the pesticides showed

Pesticides were the clearest signal of urbanization:

• In the creeks: urban West Elm Creek had 25 different compounds against Salado Creek's 7, and five times as many detections per sample.
• In the wells: each well had 13 different compounds, but the urban Encino Rio well had 74 detections against Shavano's 42 — consistent with more urban runoff in the water recharging it.
• Against the law's limits: not every pesticide has a drinking-water standard. Where one exists, no sample exceeded it under the 2020 federal rules.

The pump hoist truck at the Shavano Park at Fawn Drive site. Photo: USGS.

What it means, and what it doesn't

• Runoff is not the whole story. Well water changed in patterns that point to mixing: fresh surface recharge, older Edwards water, and perhaps inflow from other aquifers such as the underlying Trinity. So the effect of urbanization on groundwater cannot be read from runoff alone; local and regional hydrology matters too.
• The deep aquifer is not immune. The confined zone, downgradient, can still be reached by urban contaminants, though changes there would show up over longer timescales.
• Two pairs are a start. A study of two site pairs cannot settle the full effect of urbanization. It does support treating the Edwards' surface water and groundwater as one water resource.

USGS hydrographers collecting a groundwater sample at the Encino Rio site. Photo: USGS.

Sources

Based on Effects of Urbanization on Water Quality in the Edwards Aquifer, San Antonio and Bexar County, Texas, by Stephen P. Opsahl, MaryLynn Musgrove and Keith E. Mecum, USGS Fact Sheet 2020–3028, U.S. Geological Survey; a work of the United States government in the public domain. The map, the time-series figure, the pesticide table and the photographs are taken from the fact sheet's PDF. The full study is USGS Scientific Investigations Report 2020–5033.