Application of radon-222 to investigate groundwater discharge into small shallow lakes

The objective of this work was to assess the groundwater discharge component of the hydrological budgets of several small lakes in north and central Florida using 222Rn (radon, t1/2 = 3.8 days) as a quantitative groundwater tracer. Methane gas and conductivity were also used as secondary natural tracers to help locate active sites of seepage. We present results based on a steady-state radon mass balance model developed to assess groundwater discharge into shallow unstratified lakes. Model assumptions include a constant 222Rn input flux over relatively short (days–weeks) periods and a well-mixed water column. Detailed surveys in several Florida lakes using continuous 222Rn measurements supported these assumptions. In five of the seven studied lakes a high to moderate groundwater inflow was detected using this technique, while in two of them the discharge was very low to not detectable. Based on these results and the size of the lakes the calculated water-renewal times based on groundwater inflows were in a range between 3 and 40 months. Close 2-year examination of the groundwater dynamics at two of the lakes that showed substantial discharge, did not show great seasonal groundwater discharge variations. These radon-derived groundwater seepage fluxes agreed well with seepage meters and water budget calculations performed independently for some of the lakes. The approach proved to be very efficient, relatively inexpensive, and should be able to be applied as a routine procedure for estimating groundwater discharge for similar lakes elsewhere. The ultimate purpose of our efforts is to use this method in obtaining important information for the hydrological budget of these lakes that will allow evaluating nutrient loadings and TMDLs (Total Maximum Daily Loads) calculations.