Impact of long-term irrigation with treated wastewater on soil-structure stability – The Israeli experience

Abstract

The differences between the characteristics of treated wastewater (TWW) and those of its fresh water of origin, coupled with the increased necessity to use TWW for irrigation, particularly in arid and semiarid regions, present the farming community with unique and unfamiliar problems, among which is the possible degradation of soil structure and stability. Probable risks for adverse changes in the structure and stability of soils and their hydraulic properties following irrigation with TWW may stem from the higher levels of dissolved organic matter, suspended solids, sodium adsorption ratio (SAR), and salinity in the TWW compared with its fresh water of origin. Laboratory studies with specimen clays have indicated that irrigation with TWW can lead to conditions in the soil that enhance clay swelling and dispersion. These phenomena can, in turn, initiate and/or increase clay depletion from the upper soil layer and the deterioration in aggregate stability; decrease soil hydraulic conductivity; and increase soil susceptibility to seal formation, runoff, and soil erosion. These possible scenarios are expected to occur mostly in winter when the soil is exposed to rainwater (i.e., water without electrolytes), which tends to enhance the sensitivity of the soil clays to swelling and dispersion. The current review is limited to Israeli studies from the past 15 years, because in studies prior to this period most of the TWW used for irrigation was of extremely poor quality. The impact of irrigation with TWW on a number of soil determinants that are closely related to soil-structure stability (e.g., clay movement and illuviation at deeper soil layers, aggregate stability, saturated hydraulic conductivity, and the generation of runoff and soil loss) is presented. Results from the examined studies suggest that the effects of irrigation with TWW are inconsistent and complex. The results seem to depend, beyond variation in the quality of the TWW, on soil properties (e.g., texture, lime content) and conditions prevailing in the field (e.g., type of tillage, rate of wetting, etc.). It is recommended that caution be exercised when TWW is used for irrigation of cultivated land and that indices representing soil-structure stability be closely monitored to ensure sustainability of that structure.

Keywords:

• treated wastewater
• soil-structure stability
• aggregate stability
• hydraulic conductivity