Using Radon Isotopes for Studying Hydrological Processes in Marine and Aquatic Systems

This study is focused on the application of 222Rn (radon, t1/2=3.8 d) as a geochemical tracer for evaluation groundwater fluxes in different aquatic systems including submarine springs and lakes. For this purpose improvements of some established methods for detection of 222Rn in natural waters and investigating the possibilities of concurrent measurements of 220Rn (thoron, t1/2=56 s) for detection of groundwater point sources were developed. The 222Rn groundwater tracer technique was applied at study sites in Spring Creek Springs system (Florida Panhandle) and several small lakes in central Florida. The fresh water groundwater fluxes of Spring Creek Springs were evaluating using salinity and 222Rn as geochemical tracers. Two different approaches were applied. The first model is based on time series measurements of either of the tracers (salinity or 222Rn) and data for stream velocity recorded downstream in the spring area. To verify the first approach we developed a simple two-box model that is independent of stream-point measurements and use concurrent salinity data from both upstream and the spring area. Our results from almost two year of monthly based deployments at the study site indicate that the dynamics of the springs' flow is governed most probably by fluctuations of the local water table. The total spring discharge fluctuated between zero to up to ~3.0x106 m3/day (March 2008). The 222Rn approach for assessing groundwater discharge was tested in seven relatively small lakes in North and Central Florida (Lake Newnans, Lake Butler, Clear Lake, Lake Haines, Lake Shipp, Lake Hunter, and Lake Josephine). A mass balance advection-diffusion model a well mixed non-stratified water body showed to be adequate for evaluating the groundwater fluxes in these systems. Comparison of the estimates of some of the lakes with independent seepage meters and water balance studies showed very good agreement. A special investigation on the groundwater end-member for the model evaluations resulted in a decision of using a sediment equilibration approach for determining this parameter. Finally, we used 222Rn-time-series to asses the groundwater discharge in small shallow lakes. We monitored the 222Rn concentration in lake water over time for a period long enough (usually 1-3 days) to observe changes likely caused by variations in atmospheric exchange (primarily a function of wind speed and temperature). We then attempted to reproduce the observed record by accounting for decay and atmospheric losses and by estimating the total 222Rn input flux using an iterative approach. Once a quasi steady-state 222Rn flux was evaluated by balancing the calculated outputs, we divided this flux by the measured or assumed groundwater radon concentration to determine the groundwater discharge