Application of the pore water stable isotope method and hydrogeological approaches to characterise a wetland system
Authors: David Katarina , Timms Wendy , Hughes Catherine E. , Crawford Jagoda , McGeeney Dayna 
Published: November 2018

Abstract. Three naturally intact wetland systems (swamps) were
characterised based on sediment cores, analysis of surface water, swamp
groundwater, regional groundwater and pore water stable isotopes. These
swamps are classified as temperate highland peat swamps on sandstone (THPSS)
and in Australia they are listed as threatened endangered ecological
communities under state and federal legislation. This study applies the stable isotope direct vapour equilibration method in
a wetland, aiming at quantification of the contributions of evaporation,
rainfall and groundwater to swamp water balance. This technique potentially
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 other
physical and chemical extraction methods. Depletion of δ18O and δ2H in pore water 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
regional groundwater, indicating lateral groundwater contribution.
Significant differences were found in stable pore water isotope samples
collected after the dry weather period compared to wet periods where recharge
of depleted rainfall (with low δ18O and δ2H
values) was apparent. The organic-rich soil in the upper 40 to 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 swamp groundwater conditions (and potential ground
movement) is that basal sands are observed to underlay these swamps, allowing
relatively rapid drainage at the base of the swamp and 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 the 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 rate could be determined for these swamps. Based on the
results, the groundwater contribution to the swamp is a significant and
perhaps dominant component of the water balance. 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.