______ ______ ______ ______ ______ ______ ______ ______. Mx10-5. Mx10-5. Mx10-5. Mx10-5. Mx10-5. Mx10-5. Mx10-5. Mx10-5. Composition of freshest ...
Processes affecting groundwater quality in a basalt aquifer system in southern Australia D.A. Bennetts & J.A. Webb
Department of Earth Sciences, La Trobe University, Bundoora, Victoria, Australia
ABSTRACT: 36Cl and major element chemistry indicate that solutes from a basalt aquifer system located near Hamilton, southern Australia, are predominantly of cyclic origin. 36Cl 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. 1 INTRODUCTION Poor quality, saline surface and groundwater commonly occur across the Plio-Pleistocene Newer Volcanics Province of southwestern Victoria, Australia, threatening agricultural land, infrastructure and water resources. This study investigates the controls on groundwater quality in the province, in particular in a fractured basalt aquifer system near Hamilton, using hydrogeological, hydrochemical and isotopic methods. 1.1 Geological and hydrogeological framework The Newer Volcanics Province is dominated by basalt lavas, which within the study area are tholeiitic to transitional in composition (Price et al. 1997), and typically consist of 50-60% plagioclase (Na0.49Ca0.50Al1.34Si2.62O8), 20-25% clinopyroxene (augite; Ca0.85Mg0.82Al0.18Fe0.25Si1.89O6), and 15-20% olivine (Mg1.6Fe0.4SiO4), as determined using electron microprobe analysis during the present study. Accessory minerals include oxides such as ilmenite (Fe0.92Ti0.32O3.12). The basalts are typically 30-40 m in total thickness, although at eruption points they may be 140 m thick (Mann et al. 1992), and are underlain by Tertiary marine sediments throughout much of the study area. There were three major phases of volcanicity, at ~4 Ma, ~2 Ma and 0.5, the level at which precipitation usually commences (Herman & Lorah, 1987). Calcite is commonly found coating fractures and vesicles within the basalts (Mann et al. 1992).
log a(Na+/H+)
Composition of freshest groundwater sample Standardized composition of local rainfall Total contribution from silicate weathering Weathering of 13.9 plagioclase to 9.3 kaolinite Weathering of 17.4 augite to 1.6 kaolinite Dissolution of 10.3 olivine Deficit
+ Na _______ Mx10-5 169.6 162.8 6.8 6.8 0.0 0.0 0.0
-6
-5
-4 -3 -2 log a(H4SiO4) Figure 2. Stability diagram for minerals in Na20-Al2O3SiO2-H2O system (after Drever 1997). Samples are grouped according to Cl- content.
Cl-,
