Groundwater residence time in a dissected and weathered sandstone plateau: Kulnura–Mangrove Mountain aquifer, NSW, Australia

Abstract

Groundwater residence time in the Kulnura–Mangrove Mountain aquifer was assessed during a multi-year sampling programme using general hydrogeochemistry and isotopic tracers (H₂O stable isotopes, δ¹³C_DIC, ³H, ¹⁴C and ⁸⁷Sr/⁸⁶Sr). The study included 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 were differentiated (shallow and deep). The shallow zone contains oxidising Na–Cl-type waters, low pH, low SC and containing ³H and ¹⁴C activities consistent with modern groundwater and bomb pulse signatures (up to 116.9 pMC). In this shallow zone, the original Hawkesbury Sandstone has been deeply weathered, enhancing its storage capacity down to ∼50 m below ground surface in most areas and ∼90 m in the Peats Ridge area. The deeper groundwater zone was also relatively oxidised with a tendency towards Ca–HCO₃-type waters, although with higher pH and SC, and no ³H and low ¹⁴C activities consistent with corrected residence times ranging from 11.8 to 0.9 ka BP. The original sandstone was found to be less weathered with depth, favouring the dissolution of dispersed carbonates and the transition from a semi-porous groundwater media flow in the shallow zone to fracture flow at depth, with both chemical and physical processes impacting on groundwater mean residence times.

Detailed temporal and spatial sampling of groundwater revealed important inter-annual variations driven by groundwater extraction showing a progressive influx of modern groundwater found at >100 m in the Peats Ridge area. The progressive modernisation has exposed deeper parts of the aquifer to increased NO₃⁻ concentrations and evaporated irrigation waters. The change in chemistry of the groundwater, particularly the lowering of groundwater pH, has accelerated the dissolution of mineral phases that would generally be inactive within this sandstone aquifer triggering the mobilisation of elements such as aluminium in the aqueous phase.

在一个为时多年的取样项目过程中，我们使用一般水文地球化学和同位素示踪剂（ H₂O稳定同位素、β¹³C_DIC、 ³H、¹⁴C 以及⁸⁷Sr/⁸⁶Sr），评估了Kulnura - Mangrove山含水层的地下水滞留时间。该研究包括了在四个地点建井过程中获得的样品的全岩分析，以更好地确定水 - 岩相互作用的特征。基于水文地球化学、同位素示踪剂和全岩XRD分析所得的矿物相分布，分辨出两个主要地下水区。浅水区具有氧化钠-氯型水、低pH值、低SC，并含有³H和¹⁴C活动，与现代地下水和弹脉冲Q1相一致（高达116.9pMC ）。在这种浅水区，原来的Hawkesbury砂岩已经高度风化，提高了其储水容量，在大部分地区可储水带下延到地面以下约50米，在泥炭岭地区约90米。较深的地下水区也相对氧化了，水倾向于Ca-HCO3型，虽然具有较高的pH值和SC ，没有³H，低¹⁴C活动，这与纠正后滞留时间范围从11.8到0.9 ka BP相一致。原砂岩随着深度增大而风化度减少，有利于分散碳酸盐的溶解以及从浅水区的半多孔介质地下水流至深水带裂隙水流之间的过渡，化学和物理作用影响着地下水平均滞留时间。地下水的详细时空采样显示由于抽取地下水而导致的重要的年际变化，展示出发现于Peat Ridge区域大于100米深处的现代地下水的逐渐涌入。逐步现代化使得含水层的深部受增加了的NO₃^-浓度和蒸发的灌溉用水的影响。地下水的化学变化，特别是地下水pH值的降低，加速了矿物相的分解，从而触发了水相中铝等元素的运移，而这种矿物相通常在砂岩含水层中是稳定不动的。

KEYWORDS:

• radiocarbon groundwater residence time
• dissected sandstone plateaus
• groundwater and weathering
• aquifer acidification
• modern groundwater input
• Kulnura–Mangrove Mountain aquifer

ACKNOWLEDGEMENTS

We would like to thank Dr David Stone, for developing the initial collaborations with the NSW Office of Water; Dr Peter Airey, for guidance during the initial stages of the work; Chris Dimovski from ANSTO, for his dedicated fieldwork and logistic support; Alan Williams, for providing stimulating conversations on ¹⁴C methods; Barbara Neklapilova, for help with ³H and stable isotope work; and Gordon Thorogood, for XRD support. Dr Colin Ward helped us come to terms with the basics of SiroQuant processing. Also, UNESCO IGCP-618 project (Paleoclimate information obtained from past-recharged groundwater) and in general the G@GPS network are acknowledged for financial support to the senior author.

Table S1 Groundwater wells with number corresponding to the NOW monitoring well number, sampling date, elevation, total well depth, screened interval and abbreviated water type.

Table S2 SIROQUANT calculated proportions (wt%) for the different whole-rock samples. The well location can be seen in Figure 1. Qtz = quartz, Clay = kaolinite + illite + interstratified illite/smectite, Goe = goethite, Rut + Ana = rutile + anatase, Sid = siderite, Mic = microcline, others depending on samples are Ank = ankerite, Mus = muscovite, Cal = calcite.

Table S3 Estimated average vertical hydraulic conductivity, K_v, between screens (or the ground surface) where SWLs, apparent ages and samples are available.