Hydrogeochemistry and hydrology of a basalt aquifer system, the Atherton Tablelands, North Queensland

Abstract

The Atherton Tablelands basalt aquifer is a major source of groundwater supply for irrigation and other agricultural use. The Tertiary to Quaternary age basaltic aquifer can be regarded as a generally unconfined, layered system, comprising numerous basalt flows separated by palaeo-weathering surfaces and minor alluvial gravels of palaeo-drainage channels. Layers of massive basalt and clay-rich weathered zones act as local aquitards, with some local perched aquifers also present. The aquifer is regarded as a system in which several factors interact to produce the overall characteristics of the hydrogeochemistry of the groundwaters. They include the mineralogical composition of both the basalt aquifer and the thick overlying weathered zone, the porosity and permeability of the basalt aquifer, its thickness, bedrock composition, and climate and topography. The hydrogeochemical processes operating in this aquifer system have been investigated though the analysis of 90 groundwater samples collected from October 1998 to October 1999, groundwater chemistry data provided by the Queensland Department of Natural Resources & Mines for more than 800 groundwater samples, rain water samples collected during 1999 by CSIRO, stream chemistry data provided by CSIRO and James Cook University, and mineralogical and whole rock geochemistry data of drill chip samples. The methods used in this research study include the assessment of groundwater major ion chemistry data and field physico-chemical parameters using hydrochemical facies and statistical approaches, investigation of the mineralogical composition of the aquifer, assessment of concentrations and activities of the ions in solution, the degree of saturation with respect to both primary and secondary minerals, and hydrogeochemical modelling to determine the likely controls on the chemical evolution of these groundwaters. The basaltic groundwaters are mostly Mg-Ca-Na, HCO3 type waters, with electrical conductivities generally less than 250 μS/cm and pH values from 6.5 to 8.5. Dissolved silica (H4SiO4) comprises a large proportion of the total dissolved load, with average concentrations of around 140 mg/L. Concentrations of potassium, chloride and sulphate are low, that is, generally less than 3 mg/L, 15 mg/L and 10 mg/L, respectively. Despite the very low salinity of the Atherton Tablelands basalt groundwaters, the relative concentrations of the major ions are comparable to groundwaters from other basaltic regions, and are consistent with expected waterrock interactions. A variety of multivariate statistical techniques may be used to aid in the analysis of hydrochemical data, including for example, principal component analysis, factor analysis and cluster analysis. Principal component factor analyses undertaken using the hydrochemical data for the Atherton groundwaters has enabled the differentiation of groundwaters from various lithological formations, the underlying geochemical processes controlling groundwater composition in the basalt aquifer to be inferred, relative groundwater residence and flow directions to be inferred and mapping of the estimated thickness of the basalt aquifer. The limitations of multivariate statistical methods have been examined, with emphasis on the issues pertinent to hydrochemical data, that is, data that are compositional and typically, non-normally distributed. The need to validate, normalize and standardize hydrochemical data prior to the application of multivariate statistical methods is demonstrated. Assessment of the saturation states of the Atherton basalt groundwaters with respect to some of the primary minerals present indicate that the groundwaters are mostly at equilibrium or saturated with respect to K-feldspar, and approach equilibrium with respect to the plagioclase feldspars (albite and anorthite) with increasing pH. These groundwaters are at equilibrium or saturated with respect to the major secondary minerals, kaolinite, smectite (Ca-montmorillonite) and gibbsite. They also tend to be saturated with respect to the oxidation products, goethite and hematite, common accessory minerals in the Atherton Tablelands basalt sequence. Silicate mineral weathering processes are the predominant influence on the composition of these basalt groundwaters. These weathering processes include the weathering of pyroxenes, feldspars and other primary minerals to clays, aluminium and iron oxides, amorphous or crystalline silica, carbonates and zeolites, releasing ions to solution. The contribution of substantial organic carbon dioxide to the groundwater is an important factor in the extent to which silicate mineral weathering occurs in this aquifer system. Evaporative enrichment of recharging waters, oxidation and ion-exchange reactions and the uptake of ions from, and decomposition of, organic matter, are processes that have a minor influence on the composition of the basalt groundwaters. The relationships observed between mineralogical compositions, basalt character and groundwater occurrence in the Atherton Tablelands region improved the understanding how groundwater is stored and transmitted in this basalt aquifer system. Groundwater is mostly stored in vesicular basalt that may be fresh to highly weathered, and movement of this water is facilitated by pathways through both vesicular and fractured basalt. Related work undertaken as part of this research project showed that the groundwater flow patterns defined by the hydrogeochemical interpretations correspond well with the spatial trends in water level fluctuations, and response to recharge events in particular. Groundwater baseflow to streams and discharge to topographic lows in the Atherton Tablelands region is indicated by the relationships between the major cations and anions in the stream waters. Fracture zones are likely to be preferred pathways of groundwater movement. Recharge estimates, based on a chloride mass balance, range from 310 mm/yr in the north-western part of the study area (north of Atherton) to 600 mm/yr in the wetter southern and eastern parts of the study area. These recharge estimates should be treated with caution however, due to the low groundwater chloride concentrations and the high variability in rainfall chloride concentrations. The findings of this research project have improved the understanding of the hydrogeochemical processes controlling the composition of the low salinity basalt groundwaters in the Atherton Tablelands region, and are applicable to other basalt groundwater systems, particularly those in high rainfall environments