SHORT-TERM AQUIFER RESIDENCE TIMES ESTIMATED FROM RN-222 DISEQUILIBRIUM IN ARTIFICIALLY-RECHARGED GROUND-WATER

Radioactive disequilibrium of Rn-222 permits age dating of very young ground water which has resided in the subsurface for up to 15 days. In herent in this technique is the assumption that transported recharge f lows through porous media with a relatively uniform Rn-222 emanation r ate. Changes in radon activity in response to rapid surface water infi ltration from seasonally operated spreading basins were used to estima te subsurface residence times along ground water recharge flow paths. Radon activity was relatively constant in deep monitoring wells isolat ed from the immediate influence of surface recharge, suggesting that r adon 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 re charge and allowed estimation of ground water ages ranging from 4 to 1 4 days, using disequilibrium theory. Shorter residence times correspon ded to wells most directly connected with the surface as indicated by seasonal changes in water temperature. Surface water recharge routes w ere confirmed in a related study with stable isotope fingerprinting, w ater level variations and agrichemical concentrations. Flow rates esti mated from residence times ranged from 1.1 to 4.6 m day(-1), and compa re favourably with average subsurface velocities up to 4.5 m day(-1) e stimated from infiltration rates at the site. Residence time estimates using radon disequilibrium were in general agreement with bromide tra cer breakthrough times determined in a related study. Differences betw een residence and breakthrough times indicate that changes in recharge flow routes, permeability and parent nuclide concentrations may produ ce erroneously high residence time estimates. Accumulation of fine sed iment, eventually resulting in basin clogging, may increase parent iso tope concentrations and radon emanation in the zone of infiltration. ( C) 1997 Elsevier Science Ltd.