Use of automated radon measurements for rapid assessment of groundwater flow into Florida streams

Naturally occurring 222Rn (radon; t1/2 = 3.8 days) is a good natural tracer of groundwater discharge because it is conservative and typically 2–3 orders of magnitude higher in groundwater than surface waters. In addition, new technology has allowed rapid and inexpensive field measurements of radon-in-water. Results from the C-25 Canal, a man-made canal in east-central Florida thought to be dominated by groundwater inflows, display how one can quickly assess a water body for locations of groundwater inputs. Although only the eastern portion of the canal was surveyed, use of a few assumptions together with some continuous radon measurements allowed reasonable estimates of the groundwater inflows to be made. Groundwater discharge estimates of 327,000 m3/day and 331,000 m3/day were measured for two stations based on determining the groundwater fraction of the total stream flow. This fraction in each case was calculated by correcting radon concentrations for decay over transit times determined from concentration differences between the apparent focal point of groundwater discharge (with a concentration of 520 ± 80 dpm/L) estimated to be ∼17.7 km upstream from the downstream sample locations. During the same period, an average flow of 312,000 ± 70,000 m3/day was determined from time-series measurements of radon at a fixed downstream location. Coincident current meter readings and a measured cross-section area allowed an independent assessment of the total stream discharge of 336,000 m3/day. The radon-derived estimates thus indicate that >90% of the total flow is groundwater derived, consistent with the known characteristics of this waterway.