Edwards Aquifer Urban Hydrology Network

Groundwater Age Tracers, Residence Time, and Aquifer Vulnerability for the Edwards Aquifer

Summary of findings from Musgrove and others, 2023

Understanding groundwater contamination risks is crucial for best resource management practices. Karst groundwater supplies, such as the Edwards aquifer of central Texas, are formed by the dissolution of soluble limestone rocks, and are an invaluable part of the world’s drinking water. Yet karst processes, such as rapid recharge and conduit flow, make these aquifers highly complex and vulnerable to contamination by human activities.

Estimates of groundwater age provide direct insight into the vulnerability of groundwater to contamination. Groundwater that is relatively young, or contains a component of recent recharge is more vulnerable to potential contamination from the land surface. Although groundwater age cannot be directly measured, environmental tracers associated with recharge water are used with analytical methods of idealized tracer transport to interpret the age of a groundwater sample. When calibrated to tracer data, these analytical models estimate the age distribution of water comprising a groundwater sample, as well as the mean age; all groundwater samples are a mixture of water of different ages. As tools and techniques for estimating groundwater age have become both more sophisticated and available, their application has demonstrated unique value in understanding aquifer systems, and in particular, for assessing aquifer vulnerability.

This study uses groundwater ages in combination with other geochemical tracers for the Edwards aquifer to assess how vulnerable the aquifer likely is to contamination from human activities on the land surface. Samples from 60 wells across the San Antonio segment of the aquifer were collected between 2017 and 2018 and analyzed for a suite of age tracers; sampled wells were a combination of monitoring wells, domestic supply wells, and public supply wells.

Some key findings are:

• Mean groundwater ages for groundwater sampled from the Edwards aquifer cover a large range from 4 years to about 17,000 years.
• Because of the hydrogeology of the aquifer –its structure and flow patterns –the youngest age groundwater tends to be in the relatively shallow and unconfined part of the aquifer. This younger groundwater, which is mostly less than 10 years in age and nearly all less than 30 years in age (based on mean ages), is most vulnerable to land-surface contaminants.
• Groundwater in confined and deeper parts of the aquifer is mostly older than 30 years and some is hundreds or thousands of years old (based on mean ages); while less vulnerable, even the oldest groundwater is mixed with some recent recharge. As a result, it is still vulnerable to land-surface contamination and water-quality changes in recent recharge.
• Measurements of common land-surface contaminants, such as nitrate concentrations and the detection of pesticide compounds, along with measurements of some natural tracers that provide insight into how groundwater chemically evolves with time, support the interpretation and understanding of age tracers and associated vulnerability of the aquifer.

A map of the San Antonio segment of the Edwards aquifer showing:

• the hydrogeologic setting (the aquifer’s contributing zone, recharge [unconfined] zone, and confined zone)
• groundwater sample locations and their well type (monitoring wells, domestic supply wells, and public supply wells)
• mean groundwater ages (grouped by ranges)