Otto, C. and Salama, R. 1994. Linked Enhanced Discharge - Evaporation Disposal Systems for the Control of Dryland Salinity. Water DownUnder '94, Vol. 2, Part A, Adelaide, South Australia, 21-25 November 1994.

This study was commissioned as part of the Land and Water Care Program project "Discharge Enhancement Techniques for the Control of Salinity", to investigate the feasibility of on-farm evaporation basins for the disposal of pumped saline groundwater. Based on the study results and existing information, guidelines are provided on the optimal location and design of evaporation basins, hydraulically linked to an enhanced discharge scheme.
The trial site is located south of Quairading, Western Australia, within the upstream part of the Salt River system which forms a series of salt lakes to the south known as Yenyening Lakes.

Within the Western Australia wheatbelt, land clearing and replacement of native vegetation with lower water using agricultural systems have caused changes to the hydrology of landscape, increasing run-off and recharge of groundwater. The increased recharge has caused watertables to rise and mobilise stored salts. As groundwater comes close to the surface capillary action takes the water to the surface where it evaporates, leaving the salt behind to accumulate in the soil profile.

The area is located within a Palaeochannel system in which there are coarse grained aquifer systems up to 70m thick. Aquifer hydrauloic condctivity is in the order of 5m/day. The aquifer receives recharge from direct connection to the Salt River.

The upper soil profile (or topsoil) at the trial site is saline, grey in colour and is composed of sandy clay with intermittent gravel. Underlying this upper profile is an alluvial sedimentary succession of approximately 49 metres thickness followed by weathered bedrock. The alluvium is composed of coarser sands, with gravelly weathered bedrock material within a sequence of clays and sandy clays.
It was generally assumed that the regional groundwater flow is from the northwest to the southeast with a hydraulic gradient of 0.001.

The option considered is to pump groundwater from the coarse alluvial deposits to lower water levels and achieve salinity protection, while discharging the highly saline water into an adjacent evaporation basin located within the cone of depression of the pumping well.
The pumping well, situated 30 metres to the southwest of the basin, is 49 metres in depth to the top of the basement. The well is screened along 2 intervals at 48 metres and 27 metres. A submersible pump, installed at 17 metres in depth, discharges saline groundwater (Cl- content of ~ 45,000 mg/L) at a constant rate of 115 m3/d.

The evaporation basin, with a nominal surface area of 3,080 m2 was excavated on the western flank of the Salt River palaeochannel system, 20 metres west of the Salt River. The basin is 35 metres in width and 88 metres in length with walls 1.5 metres in height and slopes of approximately 40°. The walls and floor of the basin were not subjected to compaction and no liner has been applied.

A comprehensive water balance calculation and suitable monitoring around the production well and the evaporation basin, demonstrated that aquifer leakage from the evaporation basin was largely captured and recycled by the pumping well. A positive linear relationship was found to exist between hydraulic head difference (between the stage height in the evaporation basin and the pumped aquifer) and leakage rate of the evaporation basin.
The leakage to the aquifer was found to be captured by the pumping well. A groundwater mound was found beneath the basin, but lateral spreading waas confined within the radius of drawdown.

The authors concluded that on-farm evaporation basins, which are hydraulically linked to an enhanced discharge scheme, are a practical solution to the disposal problem of saline effluent generated by pumping to control land and stream salinisation. For the system to be effective, the following hydrogeological criteria are required: shallow depth to watertable, increasing hydraulic head with depth, low regional hydraulic gradient, hydraulic continuity between aquifer systems and a degree of aquifer confinement. Additionally, the basin must be positioned within the radius of influence of the enhanced discharge scheme. Suitable sites identified include depressions in the downstream end of a catchment, upstream of dykes and basement highs. Conversely, unsuitable sites for the engineering scheme include areas prone to flooding in the winter months, topographic highs, downgradient of hydraulic barriers, at break slopes in the upper parts of a catchment and near streams.

The design of the basin may be optimised by applying a linear relationship between hydraulic head difference and aquifer leakage rate. A small hydraulic head difference between stage height in the basin and hydraulic head in the aquifer system implies small aquifer leakages. Further optimisation may be achieved by increasing the basin size (lower stage height, higher evaporation) and / or by positioning the basin at a greater distance from the pumping well.

This study demonstrates that evaporation basins are capable of receiving pumped saline groundwater for disposal, with minimal impact from leakage through the basin. A successful implementation of the engineering option is likely to be restricted to certain sites as its success is highly dependent upon appropriate hydrogeological and geological conditions. However, the economic viability and ability to practically meet the stringent design requirements should be evaluated prior to implementation.

The following are key determining factors for the successful implementation of evaporative disposal systems to dispose pumped saline groundwater in dryland areas:

• suitable hydrogeologic criteria as outlined in the study;
• a suitable disposal site as outlined in the study;
• a level of economic return from the land subject to the influence of the groundwater pump; and / or
• a perceived or actual benefit from the protection of areas other than dryland.