Hydraulic Characterization of Karst Aquifers through Quantitative Groundwater Tracing

Quantitative groundwater tracing involves the addition of a known quantity of conservative artificial tracer to a groundwater flow system and then measuring the mass of tracer recovered over time at down-gradient sampling locations. Generally accepted benefits of this characterization approach include a defensible delineation of groundwater flow pathways and groundwater velocities. Perhaps of even greater significance however is an emerging understanding that tracer recovery curves provide important insights into the organization and structure of karstic conduits and their impact on both conduit and matrix flow.  

This paper will discuss the results of three quantitative tracer tests performed in the Woodville Karst Plain of North Florida, which is home to North America’s largest underwater cave system and three of the four largest springs in Florida. One of the tests connected a group of swallets that receive ~60% of the City of Tallahassee’s storm water runoff to Wakulla Spring. Another connected wells and a swallet located on the City’s wastewater spray field to Wakulla Spring. And, the third test connected the Lost Creek swallet to both Wakulla Spring and Spring Creek thereby indicating the presence of a north-south migrating groundwater divide that effects size and shape of the springsheds and water quality at both springs.

A comparison of the shape and timing of the tracer concentration recovery curves to groundwater levels and swallet stage during the tested periods, revealed that the aquifer is composed of conduits with varying capacities to convey water and that those capacities establish controls on local hydraulic gradients in the aquifer. More broadly, the results of these tracer studies indicate that tracer recovery curves can reveal significant and quantifiable insights about the hydraulic dynamics of the aquifer when interpreted relative to continuously measured hydraulic data such as heads and flows within the region being tested.