Investigation of surface-water/groundwater interactions using environmental isotopes (2H, 18O, 14C and 3H) in the Maules Creek Catchment, NSW, Australia

Andersen, MS; McCallum, AM; Meredith, KT; Acworth, RI

Investigation of surface-water/groundwater interactions using environmental isotopes (2H, 18O, 14C and 3H) in the Maules Creek Catchment, NSW, Australia

Date

2009-12-03

Authors

Publisher

Australasian Environmental Isotope Conference

Abstract

The objectives of this study were to assess surface water/groundwater interactions and to estimate recharge for the Maules Creek Catchment (NSW), a sub-catchment of the Murray-Darling Basin. Surface water and groundwater were sampled for environmental isotopes 2H, 18O, 14C and 3H. Within the catchment groundwater abstraction, mainly for the irrigation of cotton, has been carried out since the mid 1980s. An average decline of groundwater levels of about 4 to 5 m has been observed over the last 30 years [1]. Flow in the Namoi River also appears to have become more intermittent over the same period [2]. The stable water isotope data from the catchment shows that there is a distinct contrast between the regional groundwater and the river water, with the river water having an evaporative signature [3]. Shallow groundwater (<20 m) in proximity of the river (0.1-1 km) generally shows a mixed stable isotope signature indicating river water recharging the aquifer and mixing with the regional groundwater. Although this data is useful in identifying end-member sources, it does not provide an indication of the groundwater residence time or rate of river recharge. The replenishment rates of the aquifer were investigated using 3H and 14C data. The uncorrected 14C results generally indicate increasing apparent groundwater ages with depth beneath the ground surface, with the deepest water having apparent radiocarbon ages of up to 21,000 yrs (Fig. 1). However, noticeable differences to this pattern are observed. Near the Namoi River, older groundwater is generally found at much shallower depths than anticipated (Fig. 1 – black squares). This indicates an upward discharge of groundwater into the river (gaining river conditions). It is uncertain whether this is a relict of past discharge patterns because presently there appears to be little or no discharge of groundwater to the river. Other variations in the 14C results observed with depth occur in some areas near the river, where groundwater abstraction is causing large seasonal drawdown, here the opposite pattern is observed with modern water found at depths of up to 60 m (Fig. 1 – white squares). It appears that the origin of this modern groundwater is recently infiltrated river water (losing river conditions) entering the aquifer due to the lowered groundwater levels. The diffuse (rain-fed) recharge to the aquifer has been estimated in this study by ignoring the samples close to the river which are considered to be either recharge or discharge zones. A simple exponential age-depth relationship (insert in Fig. 1) was obtained by assuming a homogeneous isotropic box shaped aquifer with uniform depth, recharge and porosity. Based on this, a long term diffuse recharge of 6 mm/yr was estimated. This is an initial first estimate of recharge conditions for the system and is subject to changes in the age distribution caused by deviations from the assumptions. The scatter observed in the data shows that the aquifer is most likely not homogeneous and isotropic. Further validation and assessment of these recharge rates will be presented based on 3H and corrected 14C results to verify this model. This study shows that the changes in the surface water/groundwater interactions, as implied by the isotopes, impacts on the catchment water balance and especially on the fluxes entering the river from the aquifer. The preliminary interpretation of the data suggests quite low recharge rates which will have implications for sustainable groundwater management in this part of the Murray-Darling Basin.

Keywords

Surface waters, Isotopes, New South Wales, Australia, Rivers, Ground water, Sampling, Aquifers, Carbon 14

Citation

Andersen, M. S., McCallum, A. M., Meredith, K. T., & Acworth, R. I. (2009). Investigation of surface-water/groundwater interactions using environmental isotopes (2H, 18O, 14C and 3H) in the Maules Creek Catchment, NSW, Australia. Presentation 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. 1).