Analysis of environmental isotopes in groundwater to understand the physical and chemical responses of a coastal aquifer to pumping.

Currell, MJ; Cendón, DI; Cheng, X

Analysis of environmental isotopes in groundwater to understand the physical and chemical responses of a coastal aquifer to pumping.

Date

2013-09-20

Authors

Currell, MJ

Cendón, DI

Cheng, X

Publisher

International Association of Hydrogeologists

Abstract

The response of a multi-layered coastal aquifer in southeast Australia to decades of groundwater pumping, and the groundwater age, flow paths and salinisation processes were examined using isotopic tracer (δ18O, δ2H , δ13C, 3H and 14C). Groundwater radiocarbon (0.91 To 77.8 pMC) and tritium (below detection to O.23TU) contents decline with distance and depth away from basin margins; however, in the main zone of pumping, radiocarbon activities are generally homogeneous within a given depth horizon. A lack of tritium and low radiocarbon activities (<25 pMC) in groundwater in and around the pumping areas indicate that seasonal recovery of water levels (observed each year since c.1970) is related to capture of water with low tracer activities, rather than arrival of water recharged in modern times. Possible sources of water facilitating the seasonal recovery include release from storage in low permeability layers and/or horizontal transfer of water from more distant, undeveloped parts of the basin. Groundwater δ18O, δ2H and chloride contents indicate mixing between fresh meteoric water that is slightly depleted in stable isotopes relative to the local weighted mean, and marine water. On the basis of mixing calculations performed with Cl and δ18O, the most saline groundwater approximates an 80:20 mixture of fresh to oceanic water. Leakage of marine water into the Westernport Group sediments through an incised tidal channel, and dilution of this water by fresh groundwater released from other sources (e.g. aquitards or offshore sediments) can explain the observed Cl-δ180 values. Overall stability in seasonally recovered water levels and groundwater salinities over the past 3 decades indicate that the system has likely reached a state of dynamic equilibrium with respect to water balance and solute compositions, following a major change in the ow system and solute distributions that occurred in the early stages following development of the aquifer. Complex geological structure and variable degrees of flushing of saline water (largely controlled by proximity to recharge sources), result in heterogeneous groundwater salinities around the coastline.

Keywords

Environment, Isotopes, Ground water, Aquifers, Pumping, Tritium, Chlorides, Water, Saline aquifers, Seasons

Citation

Currell, M., Cendon, D. I., Cheng, X. (2013). Analysis of environmental isotopes in groundwater to understand the physical and chemical responses of a coastal aquifer to pumping. Paper presented to the IAH 2013, Perth, Australia : "Solving the groundwater challenges of the 21st century", International Association of Hydrogeologists 40th International Conference, Perth, Western Australia, 15-20 September 2013. (pp. 203).