GROUNDWATER RESIDENCE TIME AND PALEOHYDROLOGY IN THE OTWAY BASIN, SOUTH AUSTRALIA - H-2, O-18 AND C-14 DATA

Stable isotopes (deltaH-2, deltaO-18 and deltaC-13) and radiocarbon (C -14) have been used in conjunction with chemical data to evaluate rech arge mechanisms, groundwater residence time and palaeohydrology within the confined Dilwyn sand aquifer in the Gambier Embayment of the Otwa y Basin. This aquifer does not receive recharge down-gradient of the h ydraulic hinge-line and data have been interpreted along two discrete flow lines. The mean residence time of groundwater (determined by C-14 ) in the confined aquifer from the hydraulic hinge position to the sea (a distance of about 50 km) along an inferred flow path is approximat ely 12800 years. The corresponding hydraulic travel time calculated fr om Darcy's law is approximately 49 000 years. The apparent discrepancy may be a result of eustatic sea-level lowering during the last glacia l. Because the groundwater system is hydraulically connected to the se a, lower sea-level would result in increased gradients and a decrease in groundwater residence time. Variations in stable isotopic compositi on along flow lines suggest a number of recharge mechanisms. Stable is otope data indicate progressive depletion of H-2 and O-18 in the groun dwaters over the past 30000 years. Groundwaters older than about 10000 years B.P. were recharged either during a cooler climate climatic reg ime (lower precipitation/evapotranspiration and temperature) and/or th e vapour source(s) had travelled over greater continental mass than th ose recharged over the past 10 000 years. Lower chloride concentration s in some of the older groundwaters indicate lower evapotranspiration rates in the recharge areas before 10 000 years ago.