Processes affecting groundwater quality in a basalt aquifer system in southern Australia.

Cl and major element chemistry indicate that solutes from a basalt aquifer system located near Hamilton, southern Australia, are predominantly of cyclic origin. Cl signatures reflect that of local precipitation, showing no evidence of Cl derivation from connate sources or mineral weathering. Mass balance modeling shows that in the freshest groundwaters ( 1000 mg/L Cl) are recharged through thick clay soils, where significant evapotranspiration takes place. These groundwaters show no evidence of mineral weathering, the effects of which are masked by clay mineral reactions and evapotranspiration. This saline water is progressively added to the groundwater system downflow, causing an increase in salinity along flowpath. samples obtained during this study were analyzed for their Cl/Cl ratio. 2 RESULTS AND DISCUSSION 2.1 Source of solutes Groundwaters within the basalts are Na and Cl dominated, accounting for ~70% of the total dissolved solids (by weight). Cl analysis of 5 representative groundwaters gave a median of 19 x 10 Cl/Cl, consistent with atmospheric precipitation in southwest Victoria (Davie et al. 1989). Thus the groundwater Cl is probably sourced from cyclic sources (rainfall and/or windblown dust); contributions from connate water and/or basalt weathering are unlikely. Given the 301,000 year half-life of Cl, the Cl/Cl ratio of connate water in the underlying Tertiary marine sediments would have decayed to zero, or approached secular equilibrium with the aquifer sediments (~4 x 10 for sandstone aquifers; Bentley et al. 1986). In any case, carbon-14 and hydraulic estimates of groundwater residence time are much younger (in the order of thousands of years) than the Tertiary sediments (>4 Ma), indicating that any connate water would have long been removed. The Cl/Cl ratio of the 2 Ma and 4 Ma basalts would have approached secular equilibrium, likely to be close to zero given the low Th and U content of basalts in the area (Price et al. 1997). In the more recent basalts, the Cl/Cl ratio could potentially be higher, but the groundwater Cl/Cl ratios in both older and younger basalts are almost the same. Most solutes in the groundwaters (Na, Ca, Mg, K, HCO3, H4SiO4, SO4) are also of cyclic origin, with some additions from mineral weathering (see discussion below). Since Cl behaves conservatively, it is used here as a tracer to determine the relative influence of mineral weathering and evapotranspiration on the groundwater composition. 2.2 Influence of silicate weathering Three classes of water can be identified within the basalt aquifer based on their Cl composition: 1000 mg/L. Waters with <300 mg/L Cl show strong evidence of silicate mineral weathering, exhibiting ratios of Na/Cl, Ca/Cl, Mg/Cl, H4SiO4/Cl and HCO3 /Cl well above those in local rainfall (Hutton & Leslie, 1958). The pH of recharge rainfall is sufficiently low (~5.6; Drever, 1997) to attack silicate minerals; as the acidity is neutralized, the waters become less aggressive and mineral weathering all but ceases, as is evident in the ion ratios of higher Cl waters (see later discussion). Following the original method of Garrels & MacKenzie (1967), it is possible to calculate the contributions of weathering reactions to the groundwater composition (Table 1). The composition of the freshest groundwater sample (bore 111527) was used, so as to exclude the influence of clay mineral reactions (see later discussion). The Cl content in local rainfall (8.85 mg/L, 18month average at Heywood; Fig. 1; Hutton and Leslie, 1958) was normalized to that of this 650000mE 600000mE 58 40 00 0m N 550000mE 58 00 00 0m N 0.3Ma basalt aquifer 2Ma basalt aquifer 4Ma basalt aquifer Eruption point Groundwater flow direction kilometres 0 5 10 N Hamilton