Effects of concentration-dependent surface tension on vadose zone instrumentation

Abstract

Many of the organic compounds of environmental interest which are commonly found at contaminated sites have the effect of lowering the surface tension of water in proportion to their aqueous concentration. Laboratory studies were conducted to investigate the effects of reduced surface tension on instruments commonly used to measure pressure head in the vadose zone. Gypsum block electrical resistance sensors and heat dissipation probes were calibrated in tap water and in butanol solutions. A scaling relationship was used to correct pressure head measurements taken in liquids of reduced surface tension. The results indicate that the these measurements would be in error in that they would underestimate pressure head for solutions of lower surface tension to the same factor by which surface tension was reduced. This would result in pressure head gradient and flux being underestimated as well. Therefore, surface tension effects on these instruments should be considered when measuring pressure head in soils contaminated with organic compounds.