Application of pore water stable isotope method to characterise a wetland system
Authors: David Katarina , Timms Wendy , Hughes Cath E. , Crawford Jagoda , McGeeney Dayna 
Published: May 2018

Abstract. Three naturally intact wetland systems (swamps) were characterized based on sediment cores, analysis of surface water, groundwater and porewater stable isotopes. These swamps are classified as temperate highland peat swamps on sandstone (THPSS) and in Australia they are listed as threatened ecological communities. This study is the first application of the stable isotope direct vapour equilibration method in a wetland, enabling quantification of the contributions of evaporation, rainfall and groundwater to swamp water balance. This technique enables understanding of the depth of evaporative losses and the relative importance of groundwater flow within the swamp environment without the need for intrusive piezometer installation at multiple locations and depths. Additional advantages of the stable isotope direct vapour equilibration technique include detailed spatial and vertical depth profiles of δ18O and δ2H, with good accuracy comparable to the porewater compression technique. Depletion of δ18O and δ2H in porewater with increasing depth (to around 40–60 cm depth) was observed in two swamps, but remained uniform with depth in the third swamp. Within the upper surficial zone, the measurements respond to seasonal trends and are subject to evaporation in the capillary zone. Below this depth the pore water δ18O and δ2H signature approaches that of groundwater indicating lateral groundwater contribution. Significant differences were found in stable pore water isotopes for samples collected after dry weather period compared to wet periods where recharge of depleted rainfall was apparent. The organic rich soil in the upper 40–60 cm retains significant saturation following precipitation events and maintains moisture necessary for ecosystem functioning. An important finding for wetland and ecosystem response to changing groundwater conditions (and potential ground movement) are the observations that basal sands underlay the swamps, allowing relatively rapid drainage at the base of the swamp and interaction with lateral groundwater contribution. Based on the novel stable isotope direct vapour equilibration analysis of swamp sediment, our study identified the following important processes: rapid infiltration of rainfall to the water table with longer retention of moisture in the upper 40–60 cm and lateral groundwater flow contribution at the base. This study also found, that evaporation estimated using stable isotope direct vapour equilibration method is more realistic compared to reference evapotranspiration (ET). Importantly, if swamp discharge data were available in combination with pore water isotope profiles, an appropriate transpiration could be determined for these swamps. Based on the results, the groundwater contribution to the swamp is a significant component of the water balance during dry period. Our methods could complement other monitoring studies and numerical water balance models to improve prediction of the hydrological response of the swamp to changes in water conditions due to natural or anthropogenic influences.