Use of stable isotopes to investigate groundwater-surface water interactions and tree water sources in the Macquarie Marshes, NSW

Hollins, SE; Meredith, KT; Twining, JR

Use of stable isotopes to investigate groundwater-surface water interactions and tree water sources in the Macquarie Marshes, NSW

Date

2009-12-03

Authors

Hollins, SE

Meredith, KT

Twining, JR

Publisher

Australasian Environmental Isotope Conference

Abstract

The Macquarie Marshes are important wetland habitats, and this is acknowledged with their listing as Wetlands of International Importance under the Ramsar Convention. They are in a critical state, and in some areas are at risk of irreversible decline, with the frequency and extent of the flooding required to sustain the marshes threatened by climate change, drought and the extraction of water from the Macquarie River upstream from the marshes. Integrated management of wetlands requires a detailed understanding of the degree of connectivity between the hydrological resources (groundwater and surface water) and their dependent ecosystems. The stable water isotope (SWIs: δ18O and δ2H) signatures of soil water can be used as an indicator of recharge conditions in an aquifer [1]. This study investigated where riparian vegetation, and in particular the iconic River Red Gum (Eucalyptus camaldulensis), is sourcing its water from and how the vegetation may respond to changes in the hydrology. We examined the SWIs of groundwater, soil water, precipitation and plant xylem to identify the most likely sources of water for the River Red Gums. In addition to the SWIs, physical and chemical parameters of the soil were used to infer the recharge mechanisms, and the degree of interaction between the surface and groundwater within the semiarid environment of the Macquarie Marshes. The SWI signatures of groundwaters were found to cluster to the right of local rainfall values (in the δ2H–δ18O space), and the Global Meteoric Water Line, a trend that is typical for water that has undergone evaporation before recharge. The SWI signatures and water chemistries of the surface and shallow groundwaters throughout the study site were found to be distinctly different. Typically, saline groundwaters dominate the shallow aquifer systems whereas overlying surface waters were found to be fresh, and with evaporated SWI signatures relative to the shallow groundwater. This trend suggests that in general, there is limited hydraulic connection between surface waters and the shallow aquifers in the Macquarie Marshes. However, at one site along the sampling transect, shallow groundwater was found to be fresh, with SWI signatures similar to the nearby surface water system. This implies that there are localised zones where preferential recharge to the shallow aquifer occurs. Top soils within the area were found to be exposed to evaporation; however, at depths below 1 to 2 m bgs the soil water SWI signatures were similar to the groundwater values (Fig 1). Inflections at various depths in the SWI soil profiles may be indicative of piston-flow vertical recharge, which would be expected during flooding of the marshes. The soil water Cl profiles indicate that the unsaturated soil zone does not contain a large salt load. It appears that Cl- is accumulating within the top 5 m of the soil zone, primarily through transpiration processes because the SWI signatures of the soil water are not enriched. Enrichment in SWIs would be expected as Cl- increases in the soil zone if evaporation was dominant (Fig 1). The River Red Gums, and other riparian vegetation in the Macquarie Marshes, are perched over a highly saline, shallow aquifer system. Periodic flushing by flood waters, or managed environmental flows, is required to prevent the development of adverse salinity levels in the soil and groundwater through evapotranspiration, and to maintain the health of the riparian zone. The data collected in this study underpins the effective management of this surface water by improving our understanding of the connection between surface and groundwaters in the Macquarie Marshes, and the dependence of the riparian zone on the soil and groundwater.

Keywords

Stable isotopes, Ground water, Surface waters, Marshes, New South Wales, Australia, Wetlands, Habitat, Rain, Soils, Evaporation

Citation

Hollins, S., Meredithm K., & Twining, J. (2009). Use of stable isotopes to investigate groundwater-surface water interactions and tree water sources in the Macquarie Marshes, NSW. Paper presented to The 10th Australasian Environmental Isotope Conference and 3rd Australasian Hydrogeology Research Conference, Resources and Chemistry Precinct, Curtin University Perth, Western Australia 1st – 3rd December 2009. In Grice, K. & Trinajstic, K. (eds), The 10th Australasian Environmental Isotope Conference and 3rd Australasian Hydrogeology Research Conference abstract volume : Resources and Chemistry Precinct, Curtin University, Perth, Western Australia, 1st-3rd December 2009, (pp. 20).