Reuse and disposal of higher salinity subsurface drainage water — A review

Abstract

At present the “dilution by discharge” to usable water supplies is the most widely practiced “disposal” alternative. There is little incentive for the individual farmers to change and undertake direct reuse. Continuing water supply degradation in many areas however is likely to force changes in this practice but direction will need to come from basinwide policy makers. The most practical immediate step for reducing this degradation appears to be improved “on-farm” irrigation management to include better water distribution and use to reduce the volume of drainage water as recent results indicate that this is likely to reduce the pollution potential of the discharge. Gaining a better understanding of the crop use of shallow water tables will also help in reducing drainage water volumes. Several researchers are finding significant crop water use even with poor quality shallow groundwater. Expansion of this work to irrigation scheduling would greatly increase our understanding of drainage flow reductions. Recent research by Rhoades (1988a, 1988b) and others show that several options for drainage water reuse for crop production are available but that much more work is needed to make it widely applicable to commercial production practices. There is a need to increase research on reuse of drainage water in areas underlain by shallow brackish and saline ground water tables. Expansion of this work will greatly increase our options for reuse. Eventually use and reuse of subsurface drainage water will produce a water so brackish or saline that its further use for crop production, wetlands enhancement or agroforestry is nil. At this point, using the common practice of blending with low salinity water will not add to the total usable supply available for evapotranspiration and may even reduce it (Rhoades, 1988a). The good quality supply thus becomes the delivery and disposal system and the responsibility for coping with the added salt from the drainage water then becomes the responsibility of the downstream water user. This may or may not be acceptable. Isolating the unusable brackish or saline drainage requires a disposal point. Evaporation basins are beginning to be used but salt and trace element buildup in these disposal systems may make them only an interim or temporary solution. Eventually the salt must be transported and discharged to an acceptable disposal site. Historically, this has been the ocean. The amount of salt discharged to the ocean each year by drainage systems is not likely to exceed that which would normally be discharged by the natural river system which are providing the irrigation supply water. The need however, will be to ensure that the concentrated brines at the point of discharge do not cause localized problems with the marine life-cycle system, especially fishery resources.