Water repellency and its measurement by using intrinsic sorptivity

Abstract

Measurements of intrinsic sorptivity (S*) by using both ethanol and water were used to indicate the extent of water-repellency in soil. Experiments with initially dry, acid-purified sand verified that in a non-repellent, nonswelling porous medium S' was indeed intrinsic to the medium, and independent of the sorbing liquid. For initially water-moist, non-repellent sand, the measured S* was different when ethanol and water were the invading fluids. But a bound can be put on this difference. It was concluded that in structurally stable soils, the ratio of S* from ethanol to that for water may be used as an index of water-repellency. In the laboratory, disturbance of repellent soil by sieving or shaking effectively removed the repellency. Subsequent incubation restored it. Field measurements of sorptivity, at a scaled negative surface potential, into an initially moist, fine, sandy loam were made. While the soil appeared to absorb water normally, the sorptivity for water was an order of magnitude lower than expected from the ethanol data. This was due, we suggest, to repellency. Some possible implications of this unexpected result are discussed.

Application of minidisk infiltrometer to estimate water repellency in Mediterranean pine forest soils

“…The geometric mean values of RI were practically coincident with the mean RI values calculated according to Tillman et al (1989) as adjusted ratio of the mean of S e and S w at each of seven experimental conditions (Table 3). In other terms, conducting pairwise infiltration experiments with ethanol and water to determine mean values of S e and S w yields a mean RI value equal to that obtained applying the combination procedure suggested by Pekárová et al (2015) with the advantage that this last procedure allows a straightforward quantification of SWR variability.…”

“…In other terms, a higher ability of WRCT to detect weak SWR phenomena was observed, as compared to the more traditional WDPT test. Therefore, the WRCT, determined from a single water infiltration experiment conducted by the MDI, seems potentially more suitable to assess "sub-critical" repellency that occurs when the water-solid contact angle is less than 90° but not zero (Tillman et al, 1989). Under these circumstances, water infiltration rate is reduced but not prevented at all, as in the case of severe hydrophobicity.…”

“…According to Tillman et al (1989), the repellency index, RI, is the adjusted ratio of ethanol sorptivity, S e (L T -0.5 ), to water sorptivity, S w (L T -0.5 ) (i.e., RI = 1.95 S e / S w ). However, measurements of S e and S w cannot be performed sequentially at the same point since the initial conditions change between the two fluids whereas pairwise measurements of S e and S w may result in possible misestimating of RI due to soil heterogeneity.…”

“…The adjusted ratio between the soil-ethanol and soil-water sorptivities was proposed as a measure of the degree of hydrophobicity (Tillman et al, 1989). At the aim of estimating soil sorptivity, tension infiltration experiments are preferred to ponded ones to exclude the contribution of macropores that may overwhelm soil hydrophobicity (Cerdà, 1996;Ebel et al, 2012;Nyman et al, 2010).…”

Water vapour adsorption on water repellent sandy soils

“…Water repellency phenomenon results from the decrease in attraction between soil particles and water molecules which means that large-scale environmental effects may depend on processes that vary at microscopic scales. The hydraulic parameter, which appropriately describes the initial stage of infiltration, is sorptivity, S and it has been shown in many studies that this parameter can be strongly influenced by water repellency (Orfanus et al, 2008;Orfanus et al, 2014;Tillman et al, 1989). Sorptivity lumps together the combined effects of various liquid water and vapour redistribution mechanisms occurring in relatively dry soils (Rose, 1968).…”