CLAY TRANSLOCATION WITHIN THE SOIL PROFILE AS AFFECTED BY INTENSIVE IRRIGATION WITH TREATED WASTEWATER

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Abstract

Intensive irrigation can lead to clay eluviation. This translocation process depletes the upper soil layers of clay, possibly resulting in degraded structural stability, and creates illuviated layers that may cause drainage problems. The presence of small amounts of dissolved organic carbon, especially under sodic conditions, circumstances likely to occur in soils irrigated intensively with treated wastewater (TWW) may enhance clay dispersion. Our objective was to compare changes in particle size distributions between soils after intense irrigation with either freshwater (FW) or TWW. We studied samples from the following: (i) loam and clay soils taken from a lysimeter experiment after irrigation with 6000 mm of either FW or TWW (equivalent to about 12 years of irrigation in commercial fields); and (ii) clay soils from an orchard that had been irrigated for more than 15 years with either FW or TWW. A laser particle size analyzer (Horiba, LA-910) was used to determine the particle size distribution of particles less than 250 μm. Generally, intensive irrigation alone resulted in significantly lower clay contents at all depths compared with the parent soil. In the loam, a statistically nonsignificant trend was noted: at all depths, clay contents were lower in TWW-irrigated samples than in FW-irrigated ones. In both clay soils irrigated with TWW, there were significant increases in clay depletion at certain subsurface depths, especially in the size fraction (0.2-0.5 μm). Our findings suggest that long-term intensive irrigation with TWW can, in certain soils, increase clay dispersion and eluviation from the upper soil layers, compared with irrigation with FW.

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