In Uley South, the occurrence of basement high at the basin boundary and discontinuity of aquitards and solution features constitute the most important controls upon salinity and water chemistry. Salinity measurements taken from the town water supply (TWS) wells indicate higher salinity in the north-central portion than elsewhere in the basin (Figure 1). Salinity distribution in the basin is influenced by two main geological controls: the occurrence of basement high in the southern half of the basin, and the area of Tertiary Clay absence in the central part (Figures 1 and 2).

Harrington et al. [17] and Somaratne [18] show that the high-salinity plume of groundwater in the TS aquifer originates from the Big Swamp area as a result of downward leakage of Big Swamp’s surface water through the clay aquitard. As the plume moves down gradient along its main flow path, the western edge of the Uley East basin, it is diluted from 5200 mg/L salinity at Big Swamp to 960 mg/L at the southern boundary of the Uley East basin. The area where aquitard is absent in Uley South, the TS aquifer receives direct recharge, resulting in further dilution of salinity to 645 mg/L (Figure 1). As a result, pronounced salinity stratification is found in QL aquifer monitoring well ULE197, down-gradient to the Tertiary Clay absence area (Figure 3(a)). In the ULE197 well, salinity of the QL base is identical to that in TS aquifer monitoring well ULE109, located at the same site. Harrington et al. [17] indicated that residence time in Big Swamp and Uley Wanilla groundwater age is >45 years. Mixing of old waters originating from Uley Wanilla and Big Swamp with younger water (<20 years) in Uley South has important ramifications on applicable recharge estimation methods. When mixing occurs of different groundwater systems with different ages and different chlorides concentrations, this results in unreliable recharge estimates using chlorofluorocarbon [21,22] and chloride mass balance methods [23,24], thus limiting the methods available for recharge assessment.

In contrast, the southern half of the QL aquifer has no link to the Big Swamp area through either QL or TS aquifers due to occurrence of the basement high at the Uley South boundary (Figure 1). Therefore, QL water in the southern part of Uley South is primarily derived from direct rainfall recharge within Uley South. ULE139 is a QL aquifer monitoring well, which shows no evidence of salinity stratification (Figure 3(b)).

The combination of aquifer heterogeneity, low hydraulic gradient, recharge through solution features in addition to diffuse recharge, mixing with different water types and modification along flow paths results in Uley South being hydrochemically complex. Hydrochemistry was assessed principally by major ion chemistry within the Uley Wanilla, Uley East and Uley South lenses, and how these are related to evolution of hydrochemical processes in the Uley South basin.

Groundwater can be classified into “water types” on the basis of the absolute and relative abundance of major ions [25]. However, due to the predominance of Na and Cl ions in both QL and TS aquifers, the major ion differences present are inconclusive in distinguishing between QL and TS waters. For all QL and TS groundwaters, Ordens et al. [20] has shown that Cl/Br in both rainfall and groundwater samples are close to the Cl/Br ratio of seawater indicating groundwater chloride are marine derived implying that both QL and TS waters may have similar chemical histories.

Discrimination based on relative major ion abundance is more successful where recharging freshwater is of continental (Ca-HCO₃ type) rather than marine (Na-Cl type) origin as in Uley South. QL and TS groundwater are analysed for pH, electrical conductivity, anions (, ,) and cations (Ca²⁺, Mg²⁺, Na⁺, K⁺). Although the majority of water has no dominant ion composition, Uley Wanilla water contains distinctively high Mg ions (Figure 4) as observed by Harrington et al. [17]. High chloride in TS waters is derived from the Big Swamp area and the Na-Cl type water is diluted towards Na-Cl-HCO₃ waters at Uley South. This represents either waters that have chemically evolved along flow paths or that result from mixing between Na-HCO₃ waters and Na-Cl waters.

Classification of groundwater into water types on the basis of major ion composition and other selected characteristics provides a framework for interpretation of groundwater pathways [26]. Eighty-five groundwater samples were used for QL aquifer. This includes the data of Evans [19] and town water supply wells’ water quality data. For the TS aquifer, twenty-seven water samples were taken from Evans [19].

Hydrochemical examination shows that carbonate dis-

solution and ion exchange are the two dominant chemical processes active in the QL aquifer. Water types are categorised into four types of groundwater, which represents a chemical evolution from Na-Mg-Cl-HCO₃ groundwater at Uley Wanilla lens to end member Na-Cl groundwater at Big Swamp (Figure 5).

Seawater intrusion is generally identified through chemical analysis of groundwater. No single analysis definitively identifies seawater intrusion [27]; however by examining various analysis methods, it is evident when fresh groundwater mixes with seawater. According to HydroMetric WRI [27], at low chloride concentrations, it is often difficult to identify incipient seawater intrusion. This is due to the natural variation in fresh water chemistry at chloride concentrations below 1000 mg/L.

Mixing trends between groundwater and seawater are

more easily defined when chloride concentrations exceed 1000 mg/L, as is evident in Figure 7 where mixing increases with depth. As shown in Figure 7, groundwater mixing takes place in coastal aquifer monitoring wells from Na-Ca-Cl-HCO₃ to Na-Cl water types at two different distances from the coast at the same elevation. On the northern side, TS groundwater is derived from downward leakage from the Uley Wanilla-Big Swamp system, which is approximately 22 km from the coastline. TS receives further direct recharge from areas where Tertiary Clay is absent.

Therefore, it is conceptualised that a large quantity of TS groundwater flows on the northern side with a higher hydraulic gradient (0.00458). This makes the distance to the toe of the saline wedge in TS smaller, less than 500 m on the northern side. In contrast, the TS aquifer on the southern side receives leakage from the QL aquifer within Uley South basin. The extent of the TS is limited only to the Uley South boundary, which is on average approximately 5 km from the coast, yielding an average hydraulic gradient of 0.00117. This results in comparatively small groundwater flows through TS along the southern side, creating a longer saline wedge for pressure balance at the same elevation. Distance to the toe of the saline wedge is about 900 m from the coastline on the southern side. In Uley South coastal aquifer monitoring wells, sea water is intercepted in Tertiary Sand aquifer in SLE069 at −23 m AHD and in ULE205 at −30 m AHD (Figure 7(b)) showing increasing salinity with depth. No QL aquifer coastal bores intercept saline waters indicating that the saline wedge in QL aquifer is less than 500 m.