Why we investigated the Weaber Plain groundwater chemistry
The proposed Weaber Plain (Goomig) farmlands – 8000 hectare (ha) – are located north-east of the existing 14 000ha Ord River Irrigation Area (ORIA), 30 kilometres from Kununurra in the East Kimberley region of Western Australia (Figure 1). The existing ORIA covers parts of the Ivanhoe and Packsaddle plains.
In 2008, the Ord Irrigation Expansion Project was approved by the Western Australian Government to develop irrigated agriculture on the Weaber Plain. Construction of the M2 supply channel connecting the ORIA and the Weaber Plain, and the final period of irrigation design, environmental management and related approval processes, commenced later in 2009. This process followed a protracted period of public and private industry planning and environmental assessment (Kinhill 2000).
What we did
This report summarises an analysis of groundwater salinity trends on the Ivanhoe and Weaber plains and the preliminary results of an intensive water-quality sampling program carried out in 2010 as part of Phase 1 of the project. The purpose of this report was to provide interim results to inform groundwater management plans required as part of the approval process for the development of the Weaber Plain.
The specific aims of the analyses were to:
- forecast the salinity of groundwater that may have to be pumped for watertable control, by assessing past and present aquifer conditions on the ORIA (Ivanhoe Plain) and the Weaber Plain
- assess the potential for disposal of this groundwater by mixing it with water from the M2 irrigation supply channel
- make recommendations on long-term monitoring requirements for groundwater consistent with the ANZECC and ARMCANZ 2000 guidelines.
What we found
The analysis found:
- the Ivanhoe Plain groundwater salinity trends between 1984 and 2009 indicated that the development of the Weaber Plain for irrigated agriculture was unlikely to result in any significant change in groundwater salinity within the Ord palaeochannel
- where groundwater salinities in the palaeochannel were very high, as in the north-eastern portion of the Weaber Plain, they were likely to decrease due to dilution with fresh irrigation-supply water
- groundwater salinities were highest under Aquitaine soils on the Weaber Plain, and potential increases in groundwater salinity following development would be limited
- groundwater from the Ord River palaeochannel discharged into the irrigation supply channel was mostly unsuitable for direct irrigation, but when mixed with water from the M2 supply channel at calculated ratios, it could be suitable (based on USDA 1954 guidelines)
- that the mean total dissolved solids (TDS) of groundwater in the palaeochannel would be about 1162 milligrams per litre (mg/L) (Table 9), which agreed well with the 1200mg/L predicted by the solute transport model produced by KBR (2011)
- at the modelled pumping rates of 540 to 769 megalitres per day, that mixing that groundwater with supply-channel water at average and peak flow rates would result in water of 170–200mg/L TDS.
What this means
Blending groundwater into the main supply channel should provide a viable option for managing groundwater-related risks on the Weaber Plain if aquifer management is required.
A network of 58 groundwater monitoring bores at 44 sites was recommended for a water-quality monitoring program for the life of the development. The recommended network was to have bores monitored at a high intensity with dataloggers, low intensity bores, and eight reference bores. The published report includes a list of required analytes for comparison to the ANZECC and ARMCANZ (2000) water quality guidelines and a strategy to develop a baseline water quality data set to ensure that the environmental impacts of the development are within required limits. Trigger mechanisms and guidelines for the escalation of groundwater quality monitoring are recommended in the report for analytes that exceed the baseline by a set amount.
For a copy of this report
Download a copy of Groundwater chemistry of the Weaber Plain: preliminary results, Resource management technical report 368.