Mapping regional groundwater chemistry zones in the Fitzroy Basin, using statistical and conceptual methods

• Published in: Proceedings of the Royal Society of Queensland Vol. 118; pp. 37 - 61
• Main Authors: McNeil, Vivienne H, Raymond, Myriam A A
• Format: Journal Article
• Language: English
• Published: 2013
• Subjects: Analysis; Environmental aspects; Groundwater; Sustainable development; Water chemistry
• ISSN: 0080-469X
• DOI: 10.5962/p.357778

In Australia the precautionary principle is one of the guiding tenets of ecologically sustainable development, and implies the need to define acceptable limits for water chemistry, if necessary on the basis of current scientific information. Groundwater chemistry is difficult to characterise with certainty because of the many interacting controls, which may contribute to the typically high variability. A desktop methodology is presented, which partitions a large, diverse catchment, the Fitzroy River Basin, into reasonably homogeneous groundwater chemistry zones on the basis of an extensive historical dataset, collected opportunistically at irregular intervals. A dual approach was adopted for assessment, with multivariate analysis supported conceptually by a range of environmental factors and presented as GIS background layers. Surface water samples were included in the multivariate analysis to define areas of interaction. Assessment was based on salinity and the major ions, which are reasonably insensitive to sampling and storage protocols, and the data were checked for reliability through chemical balance and independent salinity measures. The data were sorted into water types using cluster and principal component analysis, and then bores were classified with their predominant water type and plotted as a map layer for visual comparison with conceptual criteria of geology, climate and land use. Spatial bias in the sampling necessitated subjective judgement in delineation of zonal boundaries, decisions on degree of subdivision, and explanation of uncertainty across the basin. This enabled the definition of 44 discrete groundwater chemistry zones in the Fitzroy Basin. Although the study is too broad scale to evaluate processes, it indicated that the groundwater contains two major chemical sequences, each evolving through a wide range of salinity. One is consistent with chemical weathering of Palaeozoic or basaltic rocks or derived alluvium in humid areas, and the other is a more sodic sequence associated with sub-coastal, quartz rich terrains such as sandstones and granites.