Agricultural subsurface drainage water presents the single greatest threat to water quality. Careful consideration must be given to disposal of drainage water so that the water supplies downstream are not polluted.The disposal of highly saline drainage water into river courses may need to be controlled in order to meet certain minimum standards of water quality for irrigated agriculture in downstream areas. Changes in downstream agricultural practices may be necessary to adapt to the inferior water quality, or alternative schemes may need to be implemented where the drainage or other wastewater is isolated form the main water supply.

Agriculture is the major user of water and can accept lower quality water than domestic and industrial users. It is therefore inevitable that there will be a growing tendency to look toward irrigated agriculture for solutions to the overall effluent disposal problem. Because wastewater contains impurities, careful consideration must be given to the possible long-term effects on soils and plants from salinity, sodicity, nutrients and trace elements that occur normally manageable if associated problems with these impurities are understood and allowances made for them.

Marginal and poor quality water is being used in several places in the world. Its use requires careful management to prevent or cope with the potential problems related to the water. Often this water is the only supply available and while crop yields may not be at a maximum, they continue to provide an economical return. In other instances, agriculture may have to re-use wastewater from both agricultural and urban sources. Awareness is growing that this wastewater must be treated and returned to supplement the main water supplies. Most of this wastewater, while degraded, is still usable and its utilization often reduces the total volume of wastewater that must be disposed of ultimately. Many irrigation projects will be faced with this re-use problem as competition increases for existing supplies.