Catchment-scale groundwater-flow and recharge paradox revealed from base flow analysis during the Australian Millennium Drought (Mt Lofty Ranges, South Australia)

Catchment-scale recharge and water balance estimates are commonly made for the purposes of water resource management. Few catchments have had these estimates ground-truthed. One confounding aspect is that runoff and soil-water inputs commonly occur throughout the year; however, in climates with strong dry seasons, base flow can be directly sampled. In an experimental catchment in the Mt. Lofty Ranges of South Australia, run-of-stream hydrochemical parameters were monitored. In this Mediterranean climate during the Millennium Drought (2001–2009), the stream was reduced to disconnected groundwater-fed pools. Two groundwater types were identified: (1) high-salinity type from meta-shale bedrock with thick, clayey regolith and (2) low-salinity type from meta-sandstone bedrock with sandy regolith. End-member mixing using silica and chloride concentrations and robust 87Sr/86Sr ratios reveal an apparent groundwater-flow paradox as follows. According to chloride mass balance and spatial distribution of hydrogeological units, the low-salinity groundwater type has seven times more recharge than the high-salinity type. Over the 28-year record, low-salinity groundwater contributed 25% of stream water, whereas high-salinity groundwater contributed 2–5%. During the drought year, however, annual stream flow from the high-salinity groundwater contributed 50%, whereas low-salinity groundwater contributed 18%. High-salinity groundwater dominated dry-season base flow during all years. The paradox can be resolved as follows: The meta-sandstone terrane drains quickly following wet-season recharge and therefore contributes little to dry-season base flow. Conversely, the meta-shale terrane drains slowly and therefore provides stream flow during dry seasons and drought years. © 2021 Springer-Verlag GmbH Germany, part of Springer Nature.

Keywords

Watersheds, Groundwater recharge, Ground water, Water resources, Runoff, South Australia, Silica, Sandstones, Droughts, Salinity, Chlorine

Citation

Anderson, T. T., Bestland, E. A., Wallis, I., Kretschmer, P. J. C., Soloninka, L., Banks, E. W., Werner, A. D., Cendón, D. I., Pichler, M. M., & Guan, H. (2021). Catchment-scale groundwater-flow and recharge paradox revealed from base flow analysis during the Australian Millennium Drought (Mt Lofty Ranges, South Australia). Hydrogeology Journal, 29(3), 963-983. doi:10.1007/s10040-020-02281-0