Abstract

The presence of iron in potable water supply aquifers poses a major water quality problem and may render the supply unsuitable for domestic or industrial uses, unless appropriate treatments is undertaken. Evaluation of redox conditions in groundwater is a prerequisite for studies involving Fe(III)/Fe(II) transformation in aquifers. This study characterizes the redox conditions, geochemical evolution and distribution of iron in a small karstic freshwater aquifer, the Poocher Swamp freshwater lens, located in south-east of South Australia. The freshwater lens is formed as a result of flows from Tatiara Creek that enter Poocher Swamp and recharge the aquifer through two sinkholes located in the northwest section of the swamp. Stable isotopes of water molecules, δ²H and δ¹⁸O, indicate that both Tatiara Creek and Poocher Swamp waters are isotopically more enriched due to evaporation than the pristine water of the aquifer. The Na⁺/Cl^- ion ratio >0.85 for Tatiara Creek, Poocher Swamp and wells within the lens suggest additional sodium source(s) are involved in the evolution of groundwater chemistry. This study use redox potential, dissolved oxygen (DO) and dissolved organic carbon (DOC) contents to describe Fe(III)/Fe(II) transformation in the groundwater, and relate this to iron distribution within the lens. Three distinct redox zones are present in the freshwater lens; anoxic zone up to one kilometre from Poocher Swamp where dissolved oxygen (DO)  is <1 mg.L^-1, redox potential (Eh) <100 mV, and iron (Fe) in groundwater 2.3-9.3 mgL^-1. Following the anoxic zone is the suboxic zone (from 1to1.5 km from Poocher Swamp), where DO is <1 mg.L^-1, Eh is about 110-275 mV and Fe is 0.4-0.6 mg.L^-1. The oxic zone is 1.5 to 2.3 km from Poocher Swamp and is characterized by high DO (2-4 mg.L^-1), higher Eh (>300 mV) and low in Fe (<0.015 mg.L^-1). Only 0.4 km² of the 13 km² area of the freshwater lens is oxygenated, thus limiting further developments of the lens as potable water supply source.

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