Isotope studies and other hydrogeological techniques to estimate groundwater contributions to a swamp ecosystem

The Temperate highland peat swamps on sandstone (THPSS) in Eastern Australia are a threatened ecological community with endemic flora and fauna. The ecological health of the THPSS swamps are sensitive to changing swamp moisture content and saturation. There are multiple risks to many of these swamps from variable climate, wild fire, recreational vehicles and underground mining. At Newnes Plateau, some swamps are located above and near longwall coal mines that operate at a depth of several hundred metres below ground surface. Here we highlight findings of a recent study of pore water stable isotopes within selected swamps, and new groundwater monitoring data that is publically available. We used stable isotopes to evaluate groundwater contribution to three selected swamps (CC, GG, and GGSW) under different rainfall conditions, including analysis of pore water within swamp sediments using a direct vapour equilibration method. The October 2016 (cool weather) samples from CC swamp were typically depleted in δ18O and δ2H and we concluded that these values were within the range of winter rainfall isotope values. The isotope signature in the partially saturated zone in the warm period (May 2016) was a result of evaporation. Below 100 cm depth the values of δ18O remained uniform and consistent with groundwater values but also with surface water. The isotope results enabled key hydrological processes to be identified including rapid infiltration of rainfall to the water table and lateral groundwater flow at the base of the swamps. The method also enabled improved accuracy in quantification of evapotranspiration. It was found between 2016 and May 2017 that groundwater contribution to the swamps was significant. Groundwater levels near one of the swamps in this study (swamp GG) were subsequently affected by underground mining. At monitoring site SPR1104, installed in a sandstone aquifer on the ridge above the swamp, groundwater levels dropped rapidly below the base of the piezometer in July 2017. This drawdown occurred immediately after mining longwall 420 directly below the monitoring site and is attributed to bed separation effects and changes in sub-surface storage. However, some drawdown was also observed due to an earlier longwall mining through a fault structure that was at least ~600 m distant. Hydrological changes within the swamp that have occurred during this period are yet to be fully evaluated. Further work is required to quantify the importance of groundwater flow to the swamps from different sandstone aquifers below and above the swamps, particularly given natural variations between wet and dry periods. It is important to quantify complex changes to the hydrology of swamp ecosystems over the long term at both reference and mining impacted swamps.