Effects of the Dissolved Organic Carbon of Treated Municipal Wastewater on Soil Infiltration as Related to Sodium Adsorption Ratio and pH

The increasing scarcity of fresh water in arid and semiarid regions means we must use alternative supplies for irrigation, such as treated municipal wastewaters. Generally, only their salinity level, the Na adsorption ratio (SAR), and the microbial content are evaluated. These waters contain higher concentrations of Na and thus higher SAR, and higher pH and dissolved organic C (DOC) than traditional supplies. We examined the effect of DOC in treated wastewater on soil physical properties, specifically the infiltration rate, aggregate stability, penetration resistance, and dispersibility of the soil. Wastewater and prepared waters (DOC-free) at SAR 4, 7, and 10 and at pH 7.0 and 8.0 were applied to a sandy loam soil outdoors as discrete events for 153 d, allowing soil to dry to -50 kPa before irrigation. Independent of Na level and pH, the treated wastewater reduced the infiltration rate, reduced aggregate stability, and increased dispersibility. The effect of DOC was equivalent to an increase in SAR of two to three units. An increase in the SAR (from 4 to 7) and pH (from 7 to 8) decreased infiltration and aggregate stability. The increased pH from 7.0 to 8.0 reduced infiltration and aggregate stability, with and without DOC, for all SAR values examined. We conclude that the SAR, DOC, and pH of wastewaters are high enough to cause a loss of infiltration and deterioration of soil physical properties. The pH and DOC of the water, plus salinity and SAR, should be considered when evaluating suitability for irrigation.