Aquifer heterogeneity: Hydrogeological and hydrochemical properties of the Botany Sands aquifer and their impact on contaminant transport

Abstract

Detailed studies in the Botany Sands aquifer have shown that minor changes in lithology and mineralogy are responsible for significant changes in the hydraulic properties and chemical composition of groundwaters. The aquifer is homogeneous and isotropic on a macroscopic or regional scale, while on a microscopic or local scale it is heterogeneous and anisotropic. Detailed geological, hydrogeological and hydrochemical studies carried out at the Eastlakes Experimental Site, Daceyville, New South Wales, have shown that both hydraulic conductivity and groundwater chemistry vary due to changes in the depositional environment. Distinct hydraulic boundaries between different geological units produce variations in hydraulic conductivities from 2.1 × 10−5 to 5.8 × 10−4 m/s. These geological units appear as layers or lenses in the sandy aquifer. Variations in hydraulic properties are closely related to lithological units including quartz sand, silty/peaty sand and sandy peaty/clay. All these lithological units on a regional scale are described as sand. Chemical reactions resulting from water‐sediment interaction produce significant changes in groundwater chemistry, which is consequently a function of mineralogy, lithology and residence time in the aquifer. The aquifer is Na–Cl dominant in the shallowest part, Ca–HCO3 dominant in the middle and again Na–Cl dominant at the bottom. Groundwater analyses at depth 3.5 m show high concentrations of HCO3 −, Ca2+ and Sr2+ due to dissolution of shell debris. The same zone contains a significant proportion of clay material and both components are considered to be aeolian. Evaporative concentration in the unsaturated zone of the aquifer increases the concentration of Na+ and Cl−. Recharging rainwater mobilises this saline pore water, which acts as a source of Na+ in cation‐exchange reactions with the clay, thus releasing Ca2+. Changes in chemical composition cause large variation in saturation indices with respect to calcite, dolomite and strontianite. Mass‐balance modelling demonstrates that dissolution of CaCO3, ion exchange and reverse ion exchange, the flux of CO2 from the soil zone, and oxidation of pyrite are all important chemical reactions in this 4 m‐thick section of the aquifer. A small‐scale natural gradient tracer experiment was carried out to examine the effect of heterogeneity on solute transport. Variation in hydraulic conductivity resulted in extensive spreading and splitting of the plume along the flow path. As a result, a complex plume shape with several zones of high tracer concentrations was observed.