Simplified Method for Estimating Ground Water Discharge to Surface Water for the purpose of Total Maximum Daily Loads (TMDL) Allocation

Virtually all of Florida is underlain by karst geology dominated, in some parts of the state, by conduits that cause ground water to flow under multi-porosity conditions fostering heavy interaction with surface waters. Quantifying this interaction has been a challenging problem to regulators involved in allocating TMDL to surface waters located in such settings. Difficulties in accurately quantifying ground water contribution to surface waters have forced those involved in implementing the TMDL program to estimate such contribution as percentages of total flow or to ignore it altogether. However, ground water contribution to surface waters has been shown by the USGS to range from as little as 10% to over 90% across the US (in some spring-fed streams it approaches 100% of base flow). Overlooking such contribution could obviously result in significant miscalculations in allocating waste loads to surface waters; thus in listing or delisting of water bodies respectively as “impaired” or “recovered” by the regulatory agencies.

Mass Balance approach: measure what's there and what's leaving - calculate benthic flux to support observations - convert to water flux.

To address this dilemma; the Hydrogeology Section outsourced a research task, in 2004/2005, to faculty members (Drs Burnett and Chanton) of the Department of Oceanography at FSU to develop a scientifically based yet simplified and economical method of estimating ground water contribution to surface waters. The method is based on using radon (typically found in high concentrations in ground water) as a natural tracer of such water as it is discharged to streams, lakes and estuarine systems (normally containing very low concentrations of radon). Schematic diagrams on this web page illustrate the theoretical basis of the method.

In order to allow for easy use in the field by regulatory personnel; a user-friendly interface for calculating flow through permeable sediments, based on radon inventories in the overlaying waters, was also developed. Calculations are based on a radon mass balance and assume steady-state conditions. A simplified Graphical User Interface (GUI) using MS Excel 2002 or higher version with “macros enabled” is employed to simplify calculation of advective flux estimates while fully executing the necessary mass balance calculations. The purpose of the GUI is to facilitate the calculation of the velocity of ground water movement into surface waters through an incremental approximation method and an iteration process using the bisection method. The report submitted by the researchers discusses the type of data (wind speed, radon concentrations etc) entered into specific forms and exported to allow hand-held or note-book computers to perform calculations that convert advective radon flux estimates in (dpm/m2/day) into ground water discharge volumes in (L/m2/day). Currently the FGS and FSU scientists are cooperating to test the method in bodies of water that have been identified as “impaired” under the Florida TMDL program.

A continuous radon monitor integrates a commercial radon-in-air monitor (Durridge RAD7) with an air-water exchanger. Water is continuously pumped from a desired depth with a submersible pump to the exchanger. A closed air path circulates between the Rn monitor and the exchanger. The Rn in the air is measured and recorded electronically – a separate measurement of the water temperature allows conversion to radon-in-water via a calculated solubility coefficient.

It might be worth mentioning that a presentation of this method by Dr. DeHan of the FGS, at a national conference in Portland, Oregon in September of 2005; attracted the attention of the US EPA who wanted to investigate the possibility of developing the radon method for national application. The EPA and its consultants are currently in discussions with Dr. DeHan about ways to advance that idea. For more details about that effort or about the method itself or applications thereof; Dr. DeHan can be contacted at Rodney.dehan@dep.state.fl.us

Selected References

1. Burnett, W.C., G. Kim, and D. Lane-Smith, 2001. A continuous radon monitor for assessment of radon in coastal ocean waters. Journal of Radioanalytical and Nuclear Chemistry, 249, 167-172.
2. Burnett, W.C., J. Chanton, J. Christoff, E. Kontar, S. Krupa, M. Lambert, W. Moore, D. O’Rourke, R. Paulsen, C. Smith, L. Smith, and M. Taniguchi, 2002. Assessing methodologies for measuring groundwater discharge to the ocean. EOS, 83, 117-123