Evaluation of groundwater discharge into small lakes based on the temporal distribution of radon-222

In order to evaluate groundwater discharge into small lakes we constructed a model that is based on the budget of ²²²Rn (radon, t_1/2=3.8 d) as a tracer. The main assumptions in our model are that the lake's waters are well‐mixed horizontally and vertically; the only significant ²²²Rn source is via groundwater discharge; and the only losses are due to decay and atmospheric evasion. In order to evaluate the groundwater‐derived ²²²Rn flux, we monitored the ²²²Rn concentration in lake water over periods long enough (usually 1–3 d) to observe changes likely caused by variations in atmospheric exchange (primarily a function of wind speed and temperature). We then attempt to reproduce the observed record by accounting for decay and atmospheric losses and by estimating the total ²²²Rn input flux using an iterative approach. Our methodology was tested in two lakes in central Florida: one of which is thought to have significant groundwater inputs (Lake Haines) and another that is known not to have any groundwater inflows but requires daily groundwater augmentation from a deep aquifer (Round Lake). Model results were consistent with independent seepage meter data at both Lake Haines (positive seepage of ∼ 1.6 × 10⁴ m³ d⁻¹ in Mar 2008) and at Round Lake (no net groundwater seepage)