Edwards Aquifer Urban Hydrology Network

Urban and Rural Nitrate Sources to the Edwards Aquifer

Summary of findings from Opsahl and others, 2018

Groundwater nitrate data from continuous water-quality monitors in combination with discrete water-quality samples from surface-water and groundwater sites throughout the San Antonio segment of the Edwards aquifer provide insight into nitrate contributions to the aquifer from rural and urban sources.

Hydrogeologic setting and site locations in the San Antonio segment of the Edwards aquifer, south-central Texas.

The Frio River site near Concan, Texas, was used to evaluate contributions of nitrate-nitrogen (nitrate-N) from surface-water recharge to the west of San Antonio.

Frio River at Concan
Graph of nitrate levels at Frio River at Concan

Instantaneous discharge, nitrate-nitrogen concentration, and specific conductance for a single stormwater-runoff event at Frio River (station 0819500), San Antonio segment of the Edwards aquifer, south-central Texas, September 20, 2016—October 20, 2016. The shaded gray area associated with nitrate-nitrogen concentrations represents the probability range in error (plus or minus 0.07 milligrams per liter) with a 95 percent confidence interval. Gaps in the continuous record indicate missing data.

  • Nitrate data are reported as nitrate-nitrogen (nitrate-N) in units of milligrams per liter (mg/L)
  • Relatively low concentrations (< 1 mg/L nitrate-N) occurred during low streamflow conditions (< 200 cubic feet per second), which seeps directly into the Edwards aquifer downstream from the monitoring site in the aquifer's recharge zone.
  • Concentrations increase during floods, though rarely exceed 1.5 mg/L nitrate-N, indicating moderately higher nitrate-N concentrations in recharge water during flood conditions.

The Seco Creek monitoring well was used to evaluate temporal variability in nitrate concentrations in rangeland (rural) areas of the Edwards aquifer recharge zone west of San Antonio.

Seco Creek Monitoring well gage house
Graph of nitrate concentrations at the Seco Creek well

Rainfall, daily median water-level altitude, and daily median nitrate-nitrogen for Seco well, July 2014—December 2016, San Antonio segment of the Edwards aquifer, south-central Texas. The shaded gray area associated with nitrate-nitrogen concentrations represents the probability range in error (plus or minus 0.07 milligrams per liter) with a 95 percent confidence interval. Gaps in the continuous record indicate missing data.

  • Background concentrations were approximately 1.0 mg/L nitrate-N, similar to historical concentrations of nitrate-nitrogen in the Edwards aquifer.
  • Small but measurable concentration increases occurred during periods of recharge but never exceeded 1.3 mg/L nitrate-N, indicating relatively low concentrations for western recharge water.

The Parkwood Park monitoring well, located on the north side of San Antonio and completed in the Edwards aquifer recharge zone, was used to evaluate temporal variability in nitrate concentrations in developed (urban) areas of the aquifer's shallow recharge zone in and around the City of San Antonio.

Parkwood Park monitoring well
Graph of nitrate levels at Parkland Park well

Rainfall, daily median water-level altitude, and daily median nitrate-nitrogen for Parkwood well, September 2014—December 2016, San Antonio segment of the Edwards aquifer, south-central Texas. The shaded gray area associated with nitrate-nitrogen concentrations represents the probability range in error (plus or minus 0.07 milligrams per liter) with a 95 percent confidence interval. Gaps in the continuous record indicate missing data.

  • Background concentrations were approximately 2.4 mg/L nitrate-N, which is more than double values reported in the rural monitoring well. These results illustrate that the Edwards aquifer is vulnerable to urban sources of contamination.
  • Large decreases in nitrate occurred during periods of recharge, with concentrations decreasing to <1.0 mg/L nitrate-N, similar to concentrations observed in urban streams. These results are evidence of urban stormwater runoff recharging the shallow aquifer.