Assessing soil permeability impacts from irrigation with marginal-quality waters.

Abstract

Abstract Increasing global freshwater scarcity and the demand for food from an expanding world population point to an urgent need to develop treated wastewater (WW) generated by farms and municipalities as an alternative water resource for irrigated agriculture. This approach, however, cannot be successful without an accurate assessment of its potential for causing a serious reduction in soil permeability, as numerous case studies have shown. In addition to sodium, treated WWs can have elevated concentrations of potassium, magnesium and organic carbon, which also may impact soil structure negatively. However, no irrigation water quality guidelines for these constituents exist in order to assess this possibility. This review documents field and laboratory evidence for potassium and magnesium as dispersive cations, as well as the impacts of dissolved organic carbon on exchangeable sodium and soil permeability. In light of the evidence presented, a recently-proposed irrigation water quality parameter, the Cation Ratio of Structural Stability, is evaluated as to its ability to predict potential soil permeability problems arising from exchangeable potassium and magnesium that are not accounted for in standard irrigation water quality criteria. The effect of dissolved organic carbon on this new diagnostic parameter, as well as its incorporation into irrigation water quality criteria, are discussed along with the role that transient-state models having both transport and geochemistry components will play in the continuing development of guidelines for using marginal-quality water in irrigated agriculture.