Further studies on tritium tracing of soil moisture for rainwater infiltration measurements in Gatton, Queensland, Australia

Further to preliminary results presented at the last SPERA 96 conference in Darwin, final results based on two years of measurements are summarised here. The tritium tracer was injected in April 1995 at 10 sites scattered in Gatton experimental area (70 km2) and first sampled in April 1996. The second soil sampling was carried out in May 1997, after a major flood event that took place in May 1996. The sites were named as G 1-10 and the site G-8 was located in Forest Hill to the south of the study area. The vertical tracer distribution was essentially Gaussian in shape indicating piston-type moisture movement. Within the study area the tracer peak movement during 1995-97 was 5-30 cm from the initial depth of injection at 70 cm. The total infiltration ranged from 21-177 mm within two years. The site G 8 located in Forest Hill showed 201 mm of infiltration, with a skewed tracer distribution. The errors of measurement are large due to limitation in depth resolution (10 cm and 5 cm in 1996 and 1997 respectively), rendering the lower infiltration value insignificant. Yet, very small tracer movement in two years clearly indicated the problem of poor recharge through top soil layers to the alluvial aquifer. There are however other sources of recharge from creeks and sandstone outcrops as identified using stable isotopes. The Crowley Vale irrigation area (7 km2) has already exhausted its groundwater by the middle of 1997, having little or no impact after the major flood in May 1996