Drastic index map for the Fitzroy Basin

Full description

This dataset represents the intrinsic aquifer vulnerability to contamination evaluated for unconfined aquifers in Fitzroy, based on the DRASTIC method (Aller et al. 1987). This semi-quantitative but spatially distributed method is especially suited for regional investigations with limited data availability. The DRASTIC index is calculated based on seven components: Depth to groundwater, net recharge, Aquifer media, soil media, topography (slope), impact of vadose zone and hydraulic conductivity of the aquifer (Aller et al. 1987). Areas with unconfined aquifers or with shallow water tables receive a high index and, vice versa, area with fractured rock aquifers and deep water tables have a low index.

For the Fitzroy region, based on the spatial distribution of the index values, unconfined aquifer contamination due to a spill at the surface is considered not possible if the DRASTIC index is less than 70. In grid cells with a DRASTIC index between 70 and 100, unconfined aquifer contamination due to a spill at the surface is considered possible but not material. In areas with DRASTIC indices above 100, unconfined aquifer contamination due to a spill at the surface is considered material and avoidance and mitigation strategies are considered.

Further information about the DRASTIC map and the development of this map is described in Holland et al. (2024) [Securing water for an emerging Australian hydrogen industry - Methods report].
Lineage: The DRASTIC index represents the vulnerability to potential aquifer contamination, V (x), at any given location, calculated as the weighted sum of seven components:

V (x) = D (x) wD + R (x) wR + A (x) wA + S (x) wS + T (x) wT + I (x) wI + C (x) Wc

wK is the weighting applied uniformly to the classified spatially variable input dataset K (x).
(The attached document 'Variables and weights for the DRASTIC method' contains the wK weighting table)
The methods report (Holland et al., 2024) details the classification of the DRASTIC features. Depth to groundwater has seven classes, with shallower water tables receiving higher scores. The six classes of recharge assign higher ratings to higher recharge rates as higher recharge can promote rapid infiltration of contaminants. Aquifer lithology is classified in eight classes with more permeable lithologies assigned higher ratings. The original DRASTIC classification for the soil feature is based on soil type, with soils with higher clay content receiving lower ratings. In this study, soil clay content is used directly as that information is readily available in each study region. Areas with low slope indicate lower runoff and increased ponding, associated with higher risk of contamination. Similar to aquifer lithology, vadose zone lithology is classified such that more permeable lithologies are assigned higher values. Hydraulic conductivity is organised in six classes, where higher values correspond to higher ratings as they would permit higher rates of lateral contaminant transport within an aquifer.

Input datasets to DRASTIC modelling included a mixture of national scale and locally specific datasets. National scale datasets were obtained from the following sources:

R (x) Net recharge: nationwide map of chloride mass balance recharge estimates (Lee et al., 2023)
A (x) Aquifer lithology: Bureau of Meteorology’s National Groundwater Information System (Carrara et al., 2017)
S (x) Soil clay content: mapped clay content present at 1-2 metres depth below ground surface (Malone and Searle, 2022), part of the Soil and Landscape Grid of Australia (Grundy et al., 2015).
T (x) Topographic slope: 1-second hydrologically corrected Digital Elevation Model of Australia (GA, 2011; Gallant et al., 2011).

Locally specific datasets were obtained from databases maintained by the Queensland State government:

D (x) Depth to groundwater: based on measurements obtained from groundwater wells, typically during their construction.
I (x) Vadose zone lithology: surface geological mapping, at either 1:100K or 1:250K scale.
C (x) Aquifer hydraulic conductivity: from local pumping tests preferably; otherwise estimated from ranges published for known aquifer lithologies.
Prior to their inclusion in DRASTIC analyses, input datasets were pre-processed as follows:

reproject to GDA 1994 Australian Albers projection
resample and clip to match resolution and bounding extent of link evaluation grids
multiply by masked raster grid to limit extent to the relevant buffered project region