Quantifying groundwater discharge to Cockburn River, southeastern Australia, using dissolved gas tracers ²²²Rn and SF₆

Abstract

Groundwater discharge to the Cockburn River, southeast Australia, has been estimated from comparison of natural 222Rn activities in groundwater and river water, interpreted using a numerical flow model that simulates longitudinal radon activities as a function of groundwater inflow, hyporheic exchange, evaporation, gas exchange with the atmosphere, and radioactive decay. An injection of SF6 into the river to estimate the gas transfer velocity assisted in constraining the model. Previous estimates of groundwater inflow using 222Rn activities have not considered possible input of radon due to exchange between river water and water in the hyporheic zone beneath the streambed. In this paper, radon input due to hyporheic exchange is estimated from measurements of radon production by hyporheic zone sediments and rates of water exchange between the river and the hyporheic zone. Total groundwater inflow to the Cockburn River is estimated to be 18500 m3/d, although failure to consider hyporheic exchange would cause overestimation of the volume of groundwater inflow by approximately 70%.

Characterisation of hyporheic exchange in a losing stream using radon-222

“…The streambed Effect of residence time distribution of water within the hyporheic zone on the radon flux from the hyporheic zone to the river. The exponential distribution is assumed by the OTIS model, and was also used by Cook et al (2006) and Lamontagne and Cook (2007). The piston flow model assumes that all water leaving the hyporheic zone has a residence time equal to the mean value.…”

“…The SF 6 gas was released by diffusion through nylon tubing connected to a gas cylinder supplying an outlet pressure of 90 kPa, following the procedure of Cook et al (2006). The gas was released continuously for 60 h until sampling was complete.…”

“…In gaining stream systems, groundwater inflow rates and hyporheic exchange have been estimated based on stream radon activities using a 1D mass balance model (Cook et al, 2003(Cook et al, , 2006. One of the major difficulties with this approach is separating the radon contribution through hyporheic exchange from the radon contribution of groundwater discharge.…”

“…This groundwater recharge flux is related to the infiltration rate, I, of Cook et al (2006) through the stream width: q gw = Iw.…”

“…Following Cook et al (2006) and Lamontagne and Cook (2007), we model the hyporheic zone as a one-layer, uniform, well-mixed hyporheic zone. The steady state solute mass balance can be written:…”