3.6 Identifying Groundwater Discharge to Rivers
The chemical composition of groundwater will usually be distinct from surface water, and so groundwater flow into rivers and lakes can often be detected from water chemical surveys. A number of different environmental tracers have been used for estimating rates of groundwater discharge to lakes and rivers, including stable isotopes 2H and 18O (Meredith et al., 2009), helium (Gardner et al., 2011), chlorofluorocarbons (Cook et al., 2003) and radon (e.g., Ellins et al., 1990). Radon is particularly useful for this purpose, as its concentration in groundwater is typically much higher than in surface water. After groundwater containing radon discharges to surface water bodies, radon activities will decrease due to radioactive decay and gas loss to the atmosphere. If the groundwater concentration is constant, then measured radon concentrations essentially measure the time that has elapsed since groundwater discharge occurred, and high surface water concentrations only occur in the immediate vicinity of points of groundwater inflow.
Several studies have examined spatial patterns and volumes of groundwater inflow to lakes and rivers based on radon surveys. The Rio Grande de Manati, Puerto Rico (Figure 34), has an annual average discharge of 10.3 m3/s. Radon samples were collected from the river in May 1985, when discharge rates were lower than average at 1 to 4 m3/s (Figure 35; Ellins et al., 1990). Concentrations were mostly between 0.15 and 0.50 Bq/L, but ranged up to 1.5 Bq/L, compared to an average value of radon in the groundwater of 6.7 Bq/L. Assuming the groundwater concentration is constant, the location of highest concentration in surface water reflects the location with the greatest rate of groundwater inflow. For the Rio Grande de Manati, this occurs at river-km 17.5.
Quantification of the groundwater flux into surface water usually requires an estimate of the rate of gas loss to the atmosphere. This can sometimes be obtained from the rate of decrease of radon concentration in reaches receiving no groundwater inflow but can also be independently estimated from artificial tracer experiments with other gases.
