Catchment-Scale Flow-paths of Soil Water, Groundwater, and Surface Water Determined from Environmental Tracers: Scott Creek, Adelaide Hills, South Australia

A moderate sized catchment (27 km2) in the Adelaide Hills of South Australia was investigated by detailed hydrochemical analysis of soil water, groundwater, and stream water over a four year period. In this Mediterranean climate with strong dry season moisture deficits, several significant autumn and winter rainfalls are required to generate soil through flow and stream flow in the ephemeral sub-catchments. Both the permanent main stream and one of the ephemeral sub-catchments (3.4km2) are gauged. The permanent stream (Scott Creek), which is connected to the fractured rock aquifer, contrasts greatly in both flow rate and hydrochemistry between dry season low flow and wet season low flows. Strong hydrochemical contrast was identified between the fractured rock aquifer and the soil and regolith through flow. The groundwater was characterised hydrochemically by direct sampling of numerous bores as well as sampling of the permanent stream during dry season low flow. Strontium isotope ratios as well as elemental concentrations have identified several end-member mixing relationships. Dry season low flows as well as wet season low flows are a mix of fractured rock groundwater and soil water. Wet season high flows are dominated by soil water whereas the dry season low flows approach groundwater composition. Most of the catchment consists of ephemeral stream sub-catchments; when these systems flow, the waters are a mix of soil water and another end-member close to the composition of rainwater. Based on hydrochemical characteristics and flow data from both the permanent and ephemeral streams, the ephemeral component of the catchment, termed the soil-regolith flow pathway, can be quantified; it accounts for between 0.3% and 62% of the total stream flow. The huge variation is due to extreme drought years in which the ephemeral stream barely flows. During average rainfall-stream flow years, the ephemeral soil-regolith flow pathway accounts for about 17-52% of the total stream flow. In terms of dissolved load transport, the soilregolith flow pathway with its low concentrations and smaller flow volume per area accounts for on average only 2% of the total. Again, this system has large variation from as little as 0.1% to as much as 14% of the total dissolved load transport. Based on strontium isotope two end-member mixing calculations using groundwater and soil water as the end members, between 17% and 52% of the main stream is derived from the soil regolith flow path. The difference between the strontium isotope estimate of soilregolith input and gauged stream input is most likely due to shallow groundwater flow paths that bypass the ephemeral stream gauging station. This groundwater is hydrochemically distinct from the main fractured rock aquifer. Thus, an intermediate groundwater zone is envisaged that lies above the main portion of the fractured rock aquifer and includes alluvial aquifers and a mixed upper zone of the fractured rock aquifer.