Worthington Groundwater
Carbonate aquifers
Borehole methods
Tracer tests
Spring monitoring
Tracer injection in well

Tracer tests

Tracer testing is the only reliable method of determining groundwater velocities in carbonate aquifers. At many sites we have demonstrated groundwater velocities of hundreds to thousands of meters per day. These velocities are far faster than would be predicted if these carbonate aquifers behaved like porous media (a common assumption which is rarely true in carbonate bedrock). We find the following three methods to be the most useful:

• forced gradient convergent flow well-to-well tracer tests to a pumping well

• natural gradient well-to-spring tracer tests

• natural gradient sinking stream to spring tracer tests

The image shows a tracer injection into the confined Edwards Aquifer in Texas. The well is artesian so an extension was added to the wellhead to facilitate tracer injection. The injection was into the open hole, at a depth of 630 - 950 feet below the surface. The tracer was detected at springs up to 3000 feet away within 48 hours.

One of the major challenges of tracer testing is in determining the mass of tracer to inject.

In Empirical determination of tracer mass for sink to spring tests in karst (published in Geotechnical Special Publication No. 122 by the American Society of Civil Engineers) Steve Worthington and Chris Smart derived two empirical equations to determine the tracer mass needed. One of these is shown in the graph. This uses the results from 185 tracer tests using fluorescent dyes (blue symbols) or salts (yellow symbols).