Effect of Water Saturation on Retardation of Ground-Water Contaminants

This study is the first to report the effect of water saturation on the retardation of nonionic organic compounds under dynamic conditions. Three nonaggregated sandy soil samples, that varied in their organic carbon content but had similar grain size distributions, were utilized. Two nonionic organic compounds were used: (1) dimethylphthalate, which served as a nonvolatile compound; and (2) benzene, which was volatile. Results showed that retardation coefficients for unsaturated soils are higher than those determined when the soil is saturated. The extent of deviation in retardation between the saturated and unsaturated soils generally increased as the degree of water saturation was reduced. No functional relationship between the extent of deviation in the retardation coefficient and the amount of organic carbon on the soil was found. When normalized to the saturated solid-to-water ratio, retardation coefficients for dimethylphthalate determined at different degrees of saturation were similar, leading to the conclusion that the distribution coefficient did not increase as the media was desaturated. Volatilization did not significantly affect the retardation of benzene under the employed range of saturation. Theoretical analysis showed that the impact would be significant for volatile compounds with higher Henry's coefficients and in aquifers with low organic matter content. Diffusive transport in the air phase had a significant impact on the spreading of benzene. Previously developed empirical relations reasonably predicted this impact.