Statistical and hydrochemical methods to compare basalt- and basement rock-hosted groundwaters: Atherton Tablelands, north-eastern Australia

Multivariate analysis of physico-chemical and chemical data has enabled differentiation among groundwaters sourced from different litho-logical formations in the Atherton Tablelands region of north-eastern Australia. The main water resource is stored in basalt, although basement rocks such as granite and metamorphics also contain variable amounts of water. Groundwater in the basalt is mostly Mg-Ca-Na, HCO3 type, with electrical conductivities less than 300 muS/cm and pH values from 6.5 to 8.5. Some of the other groundwater is quite Similar, making the identification of hydrochemical facies difficult. Groundwater samples were grouped based on the results of a principal component factor analysis of the major dissolved constituents H4SiO4, Na+, Ca2+, Mg2+ and HCO3, as well as pH and electrical conductivity. Based on this differentiation it was possible to identify the likely host rocks of groundwaters from unidentified lithological units, define the basalt thickness and provide a better understanding of the groundwater resource. Principal component factor analysis has also been useful in identifying the likely hydrochemical processes controlling the composition of these groundwaters, including the production of weak acids in the soil layers, silicate mineral weathering, ion-exchange reactions, evapotranspiration and the leaching of ions from organic matter. Supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s00254-002-0667-z.