Wastewaters in the Vadose Zone of Arid Regions: Hydrologic Interactions

In truly arid regions there is essentially no direct penetration of rainfall. Recharge to ground water is not only infrequent, but extremely localized, occurring only where surface runoff has been channelized or ponded. Over those vast desert areas covered by sparse xerophytic vegetation, the scant rainfall has little or no chance of becoming ground‐water recharge. Such water is quickly dissipated by capillarity‐assisted evaporation, or through rapid evapotranspiration by short‐lived annuals. Where perennial xerophytes cover the ground surface, the extensive shallow root systems quickly utilize all of the rainfall stored in the soil. Beneath the infrequently moistened soil zone is the lower part of the vadose zone, extending to water tables which are usually at depths of tens to hundreds of feet. Almost always these vadose zones have moisture contents well below field capacity. Regardless of the cause, these dry vadose zones are capable of holding additional water, at least up to field capacity. And no water‐carried pollutants can reach the water table from the ground surface until a pre‐wetted path has been formed for the entire vertical distance. A practical use of this water‐holding capacity can be made in the design of wastewater tailings ponds, with predictable safety and with great economic benefit. However, only in predictable geologic conditions, and in limited amounts can the use of this water‐holding capacity be recommended. On the other hand, to make no use whatsoever of these great natural dry sponges would be an economic waste.