Why we tested the Weaber Plain aquifer
Our department was requested by the Western Australian Government to lead a project supporting a second phase of modelling for the Weaber Plain (Goomig) farmlands.
The state government approved the Ord Irrigation Expansion Project in 2008, to develop irrigated agriculture on the proposed 8000-hectare Weaber Plain (Goomig) farmlands, which is located north-east of the existing 14 000-hectare Ord River Irrigation Area (ORIA), 30 kilometres from Kununurra (see Figure 1).
The project began in 2009, with construction of a new irrigation water supply channel connecting the Weaber Plain to the existing ORIA, the final period of irrigation design, and environmental management and related approval processes.
As part of the environmental planning and approvals process, the state government was required to prepare a groundwater management plan and a hydrodynamic plan. These plans were to investigate potential risks of increasing salinity and changed water quality that could result from developing the proposed farmlands, and to specifically use new hazard mapping derived from SkyTEM airborne electromagnetics data.
Previous studies identified that the existing groundwater data was inadequate for evaluating risk and quantifying the impact of options to manage shallow watertables and salinity; that soil and subsoil data was limited; and that downstream impacts required further evaluation.
What we did for the assessment
- tested the hydraulic properties of the palaeochannel and aquifers to the north
- tested the yields that could be expected from pumping from these aquifers.
Production bores 10WP35PB and 10WP36PB were drilled in the palaeochannel on sites selected from the interpretation of airborne geophysics. Production bore 10WP35PB was drilled into 23 metres of mainly coarse sand and 10WP36PB was drilled into 18 metres of coarse sand and gravel.
What we found
The calculated transmissivity for production bore 10WP35PB was in the range 3480–6240 square metres per day (m2/d) and the resultant hydraulic conductivity range was 150–270 metres per day (m/d).
The 16 metres of screen in production bore 10WP35PB was optimally set against coarse sands and gravels.
The calculated transmissivity for production bore 10WP36PB was in the range 4340–8790m2/d and the resultant hydraulic conductivity range was 240–390m/d.
Production bore 10WP36PB has the screens set against 6 metres of the upper section of gravel. The gravel in the section below the screen was coarse and could not be lifted from the borehole with the mud rotary system used. It is probable that the lower gravel aquifer will have a higher transmissivity, hydraulic conductivity and potential yield.
The transmissivity values calculated for production bores 10WP35PB and 10WP36PB are similar to values obtained from previously tested production bores in the ORIA that intersect the thickest sand and gravel sections of the Ord palaeochannel. These production bore estimates are more reliable estimates of the palaeochannel conductivity than the value obtained from a series of slug tests performed at bore 10WP31.
Observation of water levels during pumping from production bores 10WP35PB and 10WP36PB at 25 litres per second indicated drawdown up to 1 kilometre away from the bores.
Hydraulic conductivity values for aquifer materials on the northern portion of the Weaber Plain were derived from slug tests. These values are at least an order of magnitude less than those determined for the palaeochannel.
For more information
Full details are in 'Weaber Plain aquifer test results', Resource management technical report 367.