Changes in the hydraulic properties of a clay soil under long-term irrigation with treated wastewater

Abstract

Treated wastewater (TWW) is an important water resource, especially in semiarid and arid regions. However, there are concerns that irrigation with TWW could lead to degradation of soil physical and hydraulic properties. The objective of our study was to determine the effects of long-term (≥15years) irrigation with secondary TWW on some basic and hydraulic soil properties of a clay soil. Undisturbed soil samples (cores) were taken to a depth of 4.5m (in sections of 0.5m) over a diagonal cross section of a five year old orchard irrigated with TWW. Samples were taken from five sites within the tree rows (i.e., representing soil directly affected by TWW; referred to as “within rows”) and four sites between the rows of trees (i.e., the control treatment representing soil that was not directly subjected to the irrigation water; referred to as “between rows”). Soil analyses included an array of basic properties, determination of a continuous particle size distribution and measurement of the saturated hydraulic conductivity (HC). The latter two were used for the computation of soil characteristic curve, Θ(ψ), and the unsaturated HC curve, K(ψ). Similar bulk density, moisture content, cation exchange capacity, pH and exchangeable sodium percentage (ESP) levels were observed for the TWW irrigated samples and the control ones. However, irrigation with TWW caused a significant reduction in the saturated HC, Ks. The computed Θ(ψ) curve at a given soil depth, averaged over the different sites, was similar for the TWW-irrigated samples and the control ones. On the contrary, the computed K(ψ) curve at a given soil depth, averaged over the different sites, for the TWW-irrigated samples were lower than those for the control samples at matric potential ≳−100cm (=pF≲2); similar K(ψ) values were noted at pF>2 for the two treatments. The observed differences in the hydraulic properties between the TWW-irrigated samples and the control ones in this specific matric potential range, albeit the similarity in their ESP, suggest that long term irrigation with TWW affected structural porosity via narrowing macro- and mesopores (>70 and 30–70μm, respectively). It is further postulated that this adverse impact of irrigation with TWW on structural porosity might be associated with previously reported effects of TWW on the composition of the dissolved organic matter in the soil solution.