THE USE OF STRONTIUM ISOTOPES IN DETERMINING GROUNDWATER MIXING AND BRINE FINGERING IN A PLAYA SPRING ZONE, LAKE TYRRELL, AUSTRALIA

Playas in arid and semi-arid regions can be important regional groundwater discharge zones. Located in the northwestern portion of Victoria, Australia, Lake Tyrrell is a large playa that serves as a major discharge zone of regional groundwater. The chemistry of the regional groundwater is unique, having salinities of approximately 30 g l(-1) and pH values as low as 2.8. As the regional groundwater is discharged along the western edge of the playa in the 'spring zone', it is evapo-concentrated into a dense brine which sinks through the playa sediments, Macumber's pioneering work on the geohydrology of this region established that this denser, 'reflux' brine below the playa surface has evolved from the evaporation of regional groundwaters. We have measured the Sr-87/Sr-86 ratio of a number of groundwater samples taken at shallow depths along the spring zone at the playa edge and compared them with the regional groundwaters and the reflux brine. Our data reveal a rather complex pattern of mixing of the various water masses associated with the playa system, most probably the result of density driven convective plumes. Two types of regional groundwater can be identified based on their Sr-87/Sr-86 values, with groundwater from the south being less radiogenic than that flowing from the north (i.e. 0.7108 vs. 0.7142). Vertical variations in Sr-87/Sr-86 in the regional groundwaters observed in wells adjacent to the spring zone can also be identified along a vertical gradient in the spring zone, Besides the hydrogeological information provided by these measurements, the strontium isotope variations allow us to evaluate the source of strontium to these waters. The regional groundwaters south of the lake are very similar in Sr-87/Sr-86 ratio to the reflux brines, whereas the northern regional groundwaters are comparatively enriched in Sr-87. This enhanced Sr-87 signature indicates the addition of Sr along the flow path via the weathering of the aquifer material, the Miocene Parilla Sand. The southern waters have Sr-87/Sr-86 values closer to that of present-day seawater and/or Miocene seawater, indicating that the major source of Sr to the system is from one or both of these sources. Our results demonstrate that strontium isotopes can be extremely useful in studying the dynamics as well as the geochemistry of surface-water-groundwater interactions.