Determining natural groundwater influx to a tropical river using radon, chlorofluorocarbons and ionic environmental tracers

Measurements of 222Rn, CFC-11, CFC-12, major ions and temperature of river water and springs are used to quantify rates of groundwater inflow to a tropical lowland river in the Northern Territory of Australia. Groundwater inflow results in increases in 222Rn concentrations within the river, but decreases in concentrations of CFC-11 and CFC-12, because the inflowing groundwater is relatively old. 222Rn, CFC-11 and CFC-12 concentrations are affected by gas exchange with the atmosphere, while ion concentrations are not. Additionally, CFC concentrations in the river appear to have been increased by air entrapment and dissolution during turbulent flow at river rapids. Because the regional groundwater is old, CFC concentrations in groundwater inflow are zero. In contrast, 222Rn and ion concentrations in the river are very sensitive to concentrations of these tracers in groundwater inflow. Numerical simulation of 222Rn, CFC-11 and CFC-12 stream concentrations allows the groundwater inflow rate, gas transfer velocity and air entrapment coefficient to be reasonably accurately constrained.