SALINITY AND DRAINAGE STRATEGY

The Murray Darling Basin is a vital resource for the economic wealth and cultural heritage of the nation.  All of its natural resources must be protected.

 The River Murray is South Australia’s life blood, and the State’s development depends on a guaranteed supply of good quality water from the River Murray.   On average, about half of the State irrigation, stock, domestic and industrial water supply comes from the River, and this increased during dry periods. During the drought of 1982-83, the River supplied Adelaide with 90% of its water.  Many pipelines carry water to Adelaide, the ‘Iron Triangle’ as far north as Woomera and as far as Keith in the south-east.

 The Murray Darling Ministerial Council manages the natural resources of the Basin by identifying problems, developing solutions and undertaking necessary works and measures.  All of this is jointly funded by the Commonwealth and State Governments of New South Wales, South Australia and Victoria.

 The Woolpunda Salt Interception Scheme is one part of an entire Strategy. The Salinity and Drainage Strategy provides a framework for the four Governments to manage the pressing problems of salinity, waterlogging and land salinisation in the Murray and Murrumbidgee valleys.  These problems are a major threat to the human, economic and natural resources of the southern part of the Murray-Darling Basin.

 Annual production losses from land salinisation and waterlogging in the major irrigation areas of the Murray Darling Basin are estimated at $65m.  Additional losses occur in the Mallee and Upland zones.  The cost of salinity levels on urban, industrial and agricultural water users is $37m per year.

 Without intervention, the area affected by high watertables could increase by 500 000 hectares, with lost production increasing to $95 per year by the year 2015.  Costs to water users would increase to %57m per year by that year.

 The strategy provides an opportunity for tackling these problems by reducing salinity and, at the same time, providing an opportunity to reduce waterlogging and land salinisation, through a range of land management schemes.

 An initial program of works to reduce salinity and establish initial salinity credits for the upper State will be undertaken.  Elements of this program include:

 ·       River salinity reduction schemes (of which Woolpunda is one), which will reduce average salinity at Morgan by 80 EC units;

·       New operating rules for major storages, which will achieve another 35 EC units reduction;

·       Land management schemes, some of which will reduce salinity, while others increase salinity.

 The net effect on River salinity at Morgan will be a long-term average decrease of 113 EC units.

 THE PROBLEM

Salinity upstream of Echuca is minimal but the quality deteriorates as saline waters flow into the river further downstream.  A substantial salinity increase occurs in South Australia and by far the most concentrated point of increase is in the Woolpunda reach between Overland Corner and Waikerie.  The natural inflow of saline groundwater in this reach contributes 170 tonnes of salt daily to the Murray.  This  represents approximately 8% of the total salt load in the River.  The groundwater with a salt content of 20,000 mg per litre (two-thirds of the salt content of sea water) seeps into the River in this reach because the river cuts through an area of high groundwater, and in effect, drains it.

 The EWS Department in South Australia began addressing salinity problems of the River in the late 1960s.  Intensive investigations in the Woolpunda reach began in the early Eighties.

 THE SOLUTION

The Woolpunda Salt Interception Scheme is designed to prevent saline groundwater reaching the River.  Groundwater inflow is intercepted as it moves toward the River by 49 bores set in 2 lines, each side of the River and 600 m back from the valley cliffs.  The groundwater flows to these bores rather than the River.  The water is then pumped away from the River.  To appreciate this phenomenon it is necessary to realise how slowly this underground water is flowing, at approximately 10 m per year, and that bores create “cones of depression” beneath the ground into which the groundwater flows and is captured.

 Protection of a River by such large scale groundwater interception has not been attempted anywhere else in the world.  Extensive investigations were required to ensure its feasibility.

 CONSTRUCTION/OPERATIONS

 Once approved by the four Governments of the Ministerial Council, construction of the $25 million scheme began in March 1989.  Construction was completed in mid 1991. Half of the bores were put into production in July 1990 with the remainder being brought on-line progressively until the end of 1992.

 Major elements of the scheme are:

 ·        Boreholes

Each 250 mm diameter bore is about 110 m deep with a fibreglass slotted casing.  The standing water level in the bores is generally 40 m below the surface.  A stainless steel electric submersible pump is installed at a depth of 70 m and will pump water at the rate of 4 litres per second.

 The pumps will operate non-stop, delivering 15 million litres per day to the disposal basin.

 ·        Pipelines

A total of 85 km of below-ground pipeline will be laid.  The routes have been selected for economy, ease of laying and minimal environmental disturbance. 

 The saline water is pumped out of the bores and into the interception mains via 100 mm diameter UPVC spur mains.  The interception mains increase from 150 to 375 mm in diameter as they carry the water from more bores.  The saline water is pumped from the interception manifold pipe into the disposal pipeline which ranges from 450 to 550 mm in diameter and carries the saline water to the disposal basin at Stockyard Plain.

 Pipes used within this scheme are UPVC pressure pipes with rubber ring joints, up to 300 mm in diameter, and ‘Hobas’ fibreglass, rubber ring jointed  pipes, above 300 mm.  The pipes are laid at a minimum of 750 mm below ground.

 ·        Disposal Basin

The saline water is discharged from the disposal pipeline to a natural basin about 15 km south-west of Waikerie, and is disposed of by a combination of evaporation and infiltration.

 Intense geological investigations were made on 26 possible locations before the site of the disposal basin was finally selected.

 The site is within a natural depression, is well defined and on minor embankments and interconnecting  channels were required to form the whole basin.  The underlying soils are relatively permeable so that planned leakage would constitute a large proportion of the water disposal.

 The disposal basin site covers approximately 2.5 square km.  It began filling in July 1990 and is expected to take more than five years to reach its designed capacity.  Land around the basin is degraded, and is not suitable from cropping.  There is little natural vegetation and hence the site will have minimal environmental impact.  The basin will attract and support a healthy bird population, and there may be other recreational uses to which the basin could be put.

 ENVIRONMENTAL CONSIDERATIONS

 The environmental impact of the scheme was considered during the planning stage and an environmental assessment involving public participation began early in 1987.

 The overall scheme has minimal adverse effects on the environment and by intercepting the saline groundwater entering the River Murray has a net overall benefit.

 At the basin the saline water is disposed of by evaporation and infiltration with the salts remaining in solution and infiltrating to the ground.  Consequently the basin has a finite life, but the impact of the salt infiltration should not become significant until after 400 years of pumping.  If use of the basin is stopped after 100 years, the saline water will take approximately 50 000 years to return to the River.  On decommissioning, the water level at the basin will again fall and natural precipitation will commence flushing of the soil profile, after which regeneration will be possible.

 Monitoring

 The performance and effects of the scheme will be monitored to ensure that the reductions in salt in the River, and other factors, are as calculated.  Also, some fine tuning of its operation will be required from time to time.

 The following aspects are being monitored:

 ·        River Salinity

The salinity in the River is continuously monitored up and downstream of the scheme.  The residual salinity increase over the reach will indicate the amount of salt being prevented from entering the River by the scheme.

 ·        Borefield Groundwater Levels

Groundwater levels are being monitored throughout the Borefield to ensure the bores are performing as expected and that groundwater movement towards the River has been prevented.

 These results will allow the adjustment of any bores which are either under or over performing, in order to achieve additional salt interception or reduce the power consumption.

 ·        Basin Performance

The movement of groundwater adjacent to the basin is being monitored to ensure the infiltration of the saline water produces results expected and that the lateral spread does not influence areas outside the buffer zone.

 Sound management requirements for vermin, pest plant and soil conservation control will also be monitored.

 The rehabilitation of the natural environment will be recorded and encouraged where practical.