Short-term aquifer residence times estimated from 222Rn disequilibrium in artificially-recharged ground water

Abstract

Radioactive disequilibrium of 222Rn permits age dating of very young ground water which has resided in the subsurface for up to 15 days. Inherent in this technique is the assumption that transported recharge flows through porous media with a relatively uniform 222Rn emanation rate. Changes in radon activity in response to rapid surface water infiltration from seasonally operated spreading basins were used to estimate subsurface residence times along ground water recharge flow paths. Radon activity was relatively constant in deep monitoring wells isolated from the immediate influence of surface recharge, suggesting that radon remained in secular equilibrium and deeper ground water residence times were in excess of the 15-day dating method limit. In contrast, radon activity in shallower ground water was highly variable during recharge and allowed estimation of ground water ages ranging from 4 to 14 days, using disequilibrium theory. Shorter residence times corresponded to wells most directly connected with the surface as indicated by seasonal changes in water temperature. Surface water recharge routes were confirmed in a related study with stable isotope fingerprinting, water level variations and agrichemical concentrations. Flow rates estimated from residence times ranged from 1.1 to 4.6 m day −1, and compare favourably with average subsurface velocities up to 4.5 m day −1 estimated from infiltration rates at the site. Residence time estimates using radon disequilibrium were in general agreement with bromide tracer breakthrough times determined in a related study. Differences between residence and breakthrough times indicate that changes in recharge flow routes, permeability and parent nuclide concentrations may produce erroneously high residence time estimates. Accumulation of fine sediment, eventually resulting in basin clogging, may increase parent isotope concentrations and radon emanation in the zone of infiltration.