Geological Control upon Groundwater Flow and Major Ion Chemistry with Influence on Basin Management in a Coastal Aquifer, South Australia

Abstract

Estimation of natural recharge and potential for seawater intrusion are critical considerations for management of coastal freshwater aquifers. We show hydrochemical signatures of groundwater to identify the influence of geological control on chemical processes in a coastal groundwater system. We used dominant hydrochemical facies, salinity and magnesium ions to determine two main groundwater flow paths with different origins and ages. Mixing of groundwater with different origins and ages results in unreliable recharge estimates using chlorofluorocarbon (CFC) and chloride mass balance (CMB) methods, thus limiting available methods for recharge assessment. Interpretation of hydrochemical data suggests that calcium carbonate dissolution, ion exchange processes and mixing with sea aerosol in coastal zones are the main influencing factors on groundwater chemistry. Restricted groundwater flows due to occurrence of a basement high at the southern side of the basin boundary influence the distance to the toe of the saline wedge. Thus, knowledge of geological control over groundwater systems forms an important part of characterising basins and contributes toward effective management of groundwater resources.

Characteristics of Point Recharge in Karst Aquifers

“…In Poocher Swamp, the majority of recharge is from a large volume of creek water discharge into two sinkholes, resulting in formation of a fresh water lens [29]. Groundwater basins Uley South [30,31], Robinson lens [31], and Blue Lake capture zone in Mount Gambier [28] are previously described and therefore essential hydrogeogical features of the basins are provided below.…”

“…Data gaps were filled by linear regression of TDS to chloride (R 2 = 0.98) for monitoring wells where TDS are available but no chloride measurements have been undertaken. Selected monitoring wells are away from brackish water upward leakage areas and salinity stratified wells [30], the swamp and coastal monitoring wells to avoid chloride contamination from other sources. For the Blue Lake capture zone, existing groundwater chloride data were supplemented with samples taken from unconfined aquifer monitoring wells within and outside the city [28].…”

Karst Aquifer Recharge: A Case History of over Simplification from the Uley South Basin, South Australia

“…The TS water quality has influenced salinity stratification, hydrochemical and isotope composition of QL aquifer. Harrington et al [11] and Somaratne and Frizenschaf [5] show that the high-salinity plume of groundwater in the TS aquifer enters Uley South at the central part of the landward boundary where Tertiary Clay is absent (Figures 1b and 5a). The plume originates from the Big Swamp (Figure 1b) area located 18 km north-east of Uley South as a result of downward leakage of Big Swamp's surface water through the clay aquitard [5].…”

“…Since a more detailed description of Uley South basin features is given in Somaratne [1], Somaratne et al [4] and Somaratne and Frizenschaf [5], only a succinct summary is provided here, focusing in features relevant to the points of contention [2]. The basin is topographically closed due to surrounding sand dunes and basement high areas except in the landward direction where dry limestone is found (Figure 1a).…”

“…The different recharge mechanisms and their influence on salinity stratification and hydrochemical changes, evident in Uley South QL aquifer is described below. The reader is directed to Somaratne and Frizenschaf [5] for more detailed description of the different water types and mixing in the Uley South basin.…”