A groundwater salinity hotspot and its connection to an intermittent stream identified by environmental tracers (Mt Lofty Ranges, South Australia)

Anderson, Thomas A.; Bestland, Erick A.; Soloninka, Lesja; Wallis, Ilka; Banks, Edward W.; Pichler, Markus

A groundwater salinity hotspot and its connection to an intermittent stream identified by environmental tracers (Mt Lofty Ranges, South Australia) | Un hotspot de salinité des eaux souterraines et sa connexion à un cours d’eau intermittent identifié par les traceurs environnementaux (Mt Lofty Ranges, Australie du Sud) Un hotspot de salinidad de agua subterránea y su conexión a una corriente intermitente identificada por trazadores ambientales (Mt Lofty Ranges, Australia del Sur) (澳大利亚Lofty Ranges山脉)依靠环境示踪剂确认地下水含盐度热点区及其与间歇河的联系 Ponto crítico de salinidade de águas subterrâneas a sua conexão com o fluxo intermitente identificado por traçadores ambientais (Cadeias do Monte Lofty, Sul da Austrália)

2017

High and variable levels of salinity were investigated in an intermittent stream in a high-rainfall area (∼800 mm/year) of the Mt. Lofty Ranges of South Australia. The groundwater system was found to have a local, upslope saline lens, referred to here as a groundwater salinity ‘hotspot’. Environmental tracer analyses (δ¹⁸O, δ²H, ⁸⁷/⁸⁶Sr, and major elements) of water from the intermittent stream, a nearby permanent stream, shallow and deep groundwater, and soil-water/runoff demonstrate seasonal groundwater input of very saline composition into the intermittent stream. This input results in large salinity increases of the stream water because the winter wet-season stream flow decreases during spring in this Mediterranean climate. Furthermore, strontium and water isotope analyses demonstrate: (1) the upslope-saline-groundwater zone (hotspot) mixes with the dominant groundwater system, (2) the intermittent-stream water is a mixture of soil-water/runoff and the upslope saline groundwater, and (3) the upslope-saline-groundwater zone results from the flushing of unsaturated-zone salts from the thick clayey regolith and soil which overlie the metamorphosed shale bedrock. The preferred theory on the origin of the upslope-saline-groundwater hotspot is land clearing of native deep-rooted woodland, followed by flushing of accumulated salts from the unsaturated zone due to increased recharge. This cause of elevated groundwater and surface-water salinity, if correct, could be widespread in Mt. Lofty Ranges areas, as well as other climatically and geologically similar areas with comparable hydrogeologic conditions.

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