The Impact of Irrigation on Ground Water Quality

Most soils in arid regions contain significant quantities of water soluble salts. When these soils are brought under irrigation, soluble salts are leached into the ground water. The salt leaching rate and the annual salt outflow depends upon the quantity of water applied in excess of evapotranspiration each irrigation season. The total quantity of salt leached from a Portneuf silt loam, 5 m deep, was 70 metric tons/ha. The first 14 cm of water passing out the bottom of the soil carried 38 metric tons of soluble salt/ha of soil into the ground water. Essentially all of the residual soluble salt was leached into the ground water after 30 cm of water/m of soil depth had passed from the soil as leachate, regardless of the number of irrigation seasons required for that amount of leaching. After the residual salts were removed, the salt concentration in the newly irrigated soil was the same as in Portneuf silt loam that had been irrigated for 70 years with a high leaching fraction. Subsequent salt outflow from the soil into the groundwater was directly related to the quantity of water leaching through the soil, indicating that more minerals dissolved with more leaching, but the salt concentration in the soil did not change significantly with the leaching fraction. Salt outflows from both newly irrigated and old irrigated lands can be predicted based upon prevailing laNd conditions and given irrigation practices. The impact of these outflows upon ground water quality can be estimated