Salama, R.B., Otto, C.J., Bartle, G.A. and Watson, G.D. 1994. Management of Saline Groundwater Discharge by Long-Term Windmill Pumping in the Wheatbelt, Western Australia. Journal of Applied Hydrogeology, Vol. 2, p.19 - 33.

This project investigates the potential of enhanced discharge by way of windmill pumping to reduce excess pressure heads of the deep semi-confined aquifers in discharge areas in order to lower water levels at shallow depths. Three catchments in the Western Australian wheatbelt are investigated, Cuballing, East Perenjori and Wimmera.

Land and stream salinisation in the Western Australian wheatbelt developed as a result of clearing of native vegetation. This caused a shift in the hydrological balance and a 10 to 30 fold increase in recharge. Rising water levels rejuvenated groundwater flow through paleao channels and increased baseflow to drainage lines. An increase in saline groundwater ascension (5,000 to 30,000 mg/L TDS) to the near surface, induced by rising pressure heads in the deeper and semi-confined aquifer system has advanced land and stream salinisation.

The three catchments are situated within the Western Gneiss Terraine and the Southern Cross Province of the Yilgarn Craton.
Cuballing is a first order catchment of an area of 1.7 km2, in the western margin of the wheatbelt, with an average rainfall of 462 mm/y. East to west trending lineaments, which have been interpreted as dolerite dykes divide the catchment into hydraulically connected compartments. The regolith is formed of sand, gravel, sandy clay and clay (alluvium) underlain by weathered bedrock. The aquifers present at Cuballing, are a shallow phreatic aquifer in the eastern flanks and valley floor, a semi-confined aquifer on the western flanks, and a deeper, confined aquifer within the channel and weathered bedrock in the western flanks. The stream acts as a focal discharge for the three aquifers and groundwater salinity is moderately saline (5,000 to 6,000 mg/L TDS).

East Perenjori is a first order catchment, with an area of 32 km2, situated in the northern wheatbelt area, with an average rainfall of 310 mm/y. The central part of the catchment is constricted by northeast and southwest trending basement highs, which cut across the relict channel. The windmill is located upstream of the basement high.
Wimmera is a small subcatchment of an area of 3 km2, in the western part of Wallatin Creek catchment, with an average rainfall of 360 mm/y. The pumping well is drilled in the highly weathered and altered granitic bedrock in the top of the hill, which is dissected by quartz veins.

Free energy pumps were installed in each of the three catchments. The windmills are Yellowtail 100mm pumps and the discharge rates varied between 15 and 30 m3/d, depending on wind velocities.

A piezometric network established in each site has monitored the changes in water levels at different depths beginning at June 1990.

At Cuballing, drawdowns measured in the four observation bores from June 1990 to June 1991, ranged from 2.3 to 2.5 metres, with an average rate of drawdown of approximately 6 mm / day. The long term average recession of wells in the area ranged from 0.1 to 1.0 mm / day. Based on the water level response to the pumping test, transmissivity and storativity values were calculated at 2 to 8 m2/d and 0.03 to 0.3, respectively. Prior to pumping, groundwater flow direction was interpreted to be towards the northeast and after one year of pumping, the hydraulic head distribution exhibits an oval shaped cone of depression. Although the present windmill is effective in reducing the water pressures and water levels in the southern compartment, it is thought that a second windmill pump upstream of the basement high is required to lower the potentiometric heads in the northern compartment of the catchment.

At East Perenjori, the pumping test began on August 10th 1990. The drawdowns measured in the three observation bores from June 1990 to October 1991 ranged from 1.4 to 5 metres with an average rate of drawdown of approximately 1.5 to 3 mm /day. Based on the water level response to the pumping test, transmissivity and storativity values were calculated at 2.3 m2/d and 0.01, respectively. After pumping, a cone of depression developed with steep hydraulic gradients developing towards the windmill. Modelling has shown that pumping for 10 years will extend the radius of drawdown beyond a distance of 1,000 metres. Currently the effluent is pumped into the main drainage line which drains into the saline Mongers Lake. As the drain was initially constructed to a depth of 1.8 metres to tap the unconfined aquifer, continuous discharge of the pumped water from the deep confined aquifer will reduce the pressures and limit the extent of salinisation.

At Wimmera, the pumping test began on August 8th 1990, however pumping ceased on several occasions due to mechanical difficulties with the windmill. The drawdowns measured in the four observation bores from June 1990 to December 1991 ranged from 0.5 to 3 metres, with an average rate of drawdown of approximately 5 to 6 mm/day. Based on the water level response to the pumping test, transmissivity and storativity values were calculated at 1 to 6 m2/day and 0.004 to 0.14. During pumping, hydraulic gradients exhibit a steeper gradient towards the windmill, upstream. Therefore the recharged groundwater is intercepted by the pumping well and a reduction of groundwater discharge in the salt affected area downstream can be expected, as a result of the pumping.

For each of the trial sites, long term field experiments and modelling demonstrate that windmill pumping (15-30 m3/d) in the wheatbelt can reduce water levels by 1 to 2 metres at a radial distance of more than 1 km after several years of pumping. Depending on the groundwater salinity, the pumped groundwater can be reused for irrigation (TDD < 3,000 mg/L) or feed stock (TDS < 10,000 mg/L). Alternatively, highly saline groundwater can be discharged into saline streams or salt lakes.

The authors conclude that the results of the study demonstrate that in a discharge area with a high potentiometric head, pumping reduces the pressure heads in the unconfined and confined aquifers. When the deeper aquifer is pumped at an optimal rate, the upwards vertical gradient is reversed and natural discharge ceases. In all three catchments, it is evident that pumping will reduce water levels by 2 metres in the vicinity of the pumping well and by approximately 0.2 metres in an area one hectare around the pumping well. The pumping will immediately focus the area of discharge to a single point in the landscape and will halt the trend of the expanding saline area through diffuse discharge within the area influenced by pumping.

Furthermore, this will give land managers the opportunity to apply well planned long term vegetation options. At the same time, the creation of a cone of depression around the pumping well will improve the surface infiltration and will enhance the leaching of salts from the salt affected area.
It is recommended that the enhanced discharge method be part of an integrated catchment management program for the restoration of saline land and streams. Furthermore, the technology is transferable and applicable to other irrigated and non-irrigated salt contaminated regions in Australia and the world.

This study has demonstrated that long term windmill pumping, can reduce excess pressure heads of deep semi-confined aquifers in discharge areas in order to lower the water levels at shallow depths and provide some degree of saline land restoration, in a sub-catchment or catchment perspective.

The reported technical success of the engineering option is highly site specific (ie. within groundwater discharge zones) and would require detailed hydrogeological investigations prior to considering implementation. Additionally, the technical success of the windmill pumping on a sub-catchment or catchment scale, is highly dependent upon collaborative salinity mitigation measures and as such, should be part of an integrated catchment management program.
Although long term windmill pumping has proven to be technically feasible at the trial sites, their economic viability and impact to crop / pasture productivity requires evaluation prior to considering implementation.

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