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

Abstract

In order to evaluate groundwater discharge into small lakes we constructed a model that is based on the budget of 222 Rn (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 222 Rn source is via groundwater discharge; and the only losses are due to decay and atmospheric evasion. In order to evaluate the groundwater‐derived 222 Rn flux, we monitored the 222 Rn 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 222 Rn 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 × 10 4 m 3 d −1 in Mar 2008) and at Round Lake (no net groundwater seepage)