Deep meteoric leaching and its implications for groundwater residence time in a dissected Hawkesbury sandstone plateau (Kulnura-Mangrove Mountain Aquifier, NSW, Australia)

Abstract

In the Kulnura-Mangrove region, groundwater extraction for potable water supply and for industrial activities such as farming and mining, can co-exist provided the main recharge areas are protected, pumping does not exceed recharge, and knowledge of the basic parameters within the aquifer are known through appropriate studies. In this study, groundwater residence time in the Kulnura-Mangrove Mountain aquifers was assessed over multiple years using environmental tracers (H2O stable isotopes, 13CDIC, 3H, 14C and 87Sr/86Sr) and general hydrogeochemistry. The Kulnura-Mangrove Mountain aquifer is mostly hosted in its upper part by the Hawkesbury Sandstone, where intense and deep sandstone weathering profiles have resulted in enhanced groundwater storage. Weathering reactions favoured by the local geological setting has transformed the original Hawkesbury Sandstone quartz arenite into a semisolid or friable sandstone with variable weathering depths where most of the original carbonate cements have been leached, resulting in higher porosity and permeability. XRD analyses show an upper zone down to ~50 m and even 90 m in some areas where all carbonates and probably feldspars have been dissolved and the derived products goethite and kaolinite have formed. With depth, carbonates, mostly siderite, are present representing fresher or less-weathered sandstone. Isotopic analysis of dispersed carbonates shows consistent values with their depositional environment and devoid of 14C. The study incorporated whole rock analysis from samples recovered during well construction at four sites to better characterise water–rock interactions. Based on hydrogeochemistry, isotopic tracers and mineral phase distribution from whole rock XRD analysis, two main groundwater zones are differentiated in areas not disturbed by groundwater extraction. A shallow zone where oxidising Na–Cl-type waters with low pH and EC contain 3H and 14C activities consistent with very modern groundwater affected by bomb pulse signatures (up to 116.9 pMC). In this shallow zone the original Hawkesbury Sandstone has been deeply weathered, enhancing storage capacity for groundwater down to ~50 m in most areas and up to ~90 m in the Peats Ridge zone. The deeper groundwater zone is also relatively oxidising with a tendency towards Ca–HCO3 type waters, higher pH and EC, no 3H and 14C activities consistent with residence times from 0.9 to 11.8 ka BP, depending on the specific areas. The original sandstone is less weathered with depth, favouring the dissolution of dispersed carbonates and a transition to a fractured-rock flow type aquifer, both impacting on groundwater mean residence times.