Water quality and surfactant effects on the water repellency of a sandy soil

SummaryDifferences in irrigation water quality may affect the water repellency of soils treated or untreated with surfactants. Using simulated irrigations, we evaluated water quality and surfactant application rate effects upon the water repellency of a Quincy sand (Xeric Torripsamment). We used a split plot design with two irrigation water qualities, three surfactant application rates, two irrigations, and 12 sampling depths as fixed effects, with four replications. Each water quality × rate × irrigation combination was a main plot and depth was a repeated-measures subplot. A slightly water repellent Quincy soil (average water drop penetration time, WDPT, of 2.5 s) was packed in 25-mm lifts (or layers) to a bulk density of 1.6 Mg m−3 into 0.15-m-high × 0.105-m-diameter plastic columns. We studied a nonionic surfactant, a blend of an ethylene oxide/propylene oxide block copolymer and an alkyl polyglycoside. We sprayed the surfactant at rates of 0, 9.4, and 46.8 L ha−1, diluted with reverse osmosis water (RW) to apply 187 L ha−1 of solution, onto the soil surface of each packed column. About 1 and 5 days after surfactant application, columns were sprinkler irrigated with either RW or well water (WW). The WDPT was then measured with depth on soil air-dried after the first and after the second irrigation. After the first irrigation, WDPT at depths from 97 to 117 mm averaged across surfactant rates reached a maximum of 28 s, regardless of irrigation water quality. WDPT was greatest at 117 mm with RW but only at 97 mm with WW. After the second irrigation, maximum WDPT was 1202 s at 139 mm with RW but only 161 s at 117 mm with WW, nearly 7.5 fold less than with RW. WDPT was greatest near the wetting front, irrespective of water quality. We conclude that irrigation water containing modest amounts of electrolytes or salts, in this case mostly salts of Ca2+, reduces water repellency in the presence or absence of surfactant. Our experimental results may also help explain erratic surfactant performance under rainfed conditions where neither water quality nor depth of infiltration can be fully controlled.

Irrigating a Quincy sand with calcium-rich well water (WW) rather than calcium-poor reverse osmosis water (RW) decreased water drop penetration time (WDPT) by 86% or more below column depths of 97 mm after two irrigations.Figure optionsDownload as PowerPoint slideHighlights
► Irrigation and irrigation water quality but not surfactant affected water repellency.
► WDPT was 7.5 fold less after irrigating with calcium-rich rather than Ca-poor water.
► Irrigation water with Ca reduced water repellency in Quincy sand by 86% below 97 mm.