Treated Wastewater Reuse in Irrigated Agriculture: Implications for Soil Hydraulic Properties and Water Flow Regime.

In Israel, as in many arid and semi-arid regions, water demand has exceeded the reliable supply of surface water and renewable groundwater due to rapid growth in municipal and industrial use, and application in irrigated agriculture. To make up the shortfall, about 40% of the water used in Israel consists of treated wastewater, utilized mainly by the agricultural sector. Recently, we found that long-term irrigation with treated wastewater can cause soil water repellency, the main impacts of which on soil hydraulic properties are reduced infiltration capacity, development of fingered flow in structural or textural preferential flow pathways, and creation of unstable, irregular wetting fronts. Fingered flow can lead to significant variations in water content in the soil profile, resulting in poor seed germination and plant growth, and accelerated leaching of surface-applied agrochemicals and salts.

Repellency (contact angle CA > 90°) is identified when water does not spontaneously absorb into the soil. Repellent soils exhibit time-dependent wetting, but the contact angle does not necessarily approach 0°. Upon wetting, repellent soils may exhibit sub-critical repellency (0° < CA < 90°) whereby water is instantaneously absorbed, yet fingered flow and reduced infiltration capacity occur. This will be shown by flow chamber experiments using repellent soils (CA > 90°) and model coated sands (CA approx. 70°). In addition, water retention curves and capillary rise experiments for these media will be presented.

Such adverse effects of irrigation with treated wastewater on soil hydraulic properties need to be recognized, so as to avoid irreversible degradation of soil quality, improve agricultural water usage and conservation, and preserve quality of underlying groundwater resources.