Tracing sources of water to an alluvial aquifer using strontium isotopes and radon
Abstract
87Sr/86Sr and 222Rn were measured in groundwaters of an alluvial aquifer adjacent to an ephemeral stream to quantify the extent of surface water infiltration versus upward groundwater leakage from the brackish regional aquifer system, and their degree of mixing. These tracers were used to differentiate between the potential sources of water to the alluvial aquifer because hydraulic gradients between the regional groundwater, alluvial groundwater and surface water reservoirs were low. The pumping of near-stream alluvial groundwater in the Wollombi Brook Catchment (SE Australia) has compromised the health of the stream ecosystem, potentially affecting water quality and flow rates. It is crucial to understand stream water and groundwater interactions in the alluvial aquifer to balance both consumptive and environmental water requirements. The distinct 87Sr/86Sr and 222Rn signatures of the surface water and groundwater reservoirs were used to differentiate between the potential sources of water to the alluvial aquifer system. These were compared to the alluvial groundwater signatures sampled from a piezometer network that was installed at various depths beneath the water table and at various distances from the stream channel. 222Rn emanation rates from the alluvial sands were also measured to discriminate between the production of 222Rn within the alluvial aquifer and influxes from other water flow paths. Regional groundwater had low 87Sr/86Sr (0.7051 to 0.7069) and relatively high Sr concentrations, whereas surface waters had higher 87Sr/86Sr (0.7095 to 0.7141) and relatively low Sr concentrations. Alluvial groundwater 87Sr/86Sr (0.7056 to 0.7118) and Sr concentrations were more variable. 222Rn emanation rates from the alluvial aquifer ranged from 2 to 7 Bq/L, whereas surface water 222Rn activities were < 1 Bq/L and regional groundwaters were generally an order of magnitude higher. 87Sr/86Sr and 222Rn activities showed there was some regional groundwater leakage into the alluvial aquifer system. However, 87Sr/86Sr also indicated that regional groundwater had little influence on the near-stream alluvial groundwater quality. Infiltration of stream water into the alluvial aquifer, following periodic flooding, appeared to buffer the transfer of water and solutes from the regional groundwater into the near-stream zone. This was, however, a very narrow buffer zone that could easily be eroded by further surface water diversions or excessive groundwater pumping from the alluvial aquifer.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA.....3325P