Effect of water saturation on retardation of groundwater contaminants

This study is the first to report the effect of water saturation on the ret ardation of nonionic organic compounds under dynamic conditions. Three nona ggregated sandy soil samples, that varied in their organic carbon content b ut had similar grain size distributions, were utilized. Two nonionic organi c compounds were used: (1) dimethylphthalate, which served as a nonvolatile compound; and (2) benzene, which was volatile. Results showed that retarda tion coefficients for unsaturated soils are higher than those determined wh en the soil is saturated. The extent of deviation in retardation between th e saturated and unsaturated soils generally increased as the degree of wate r saturation was reduced. No functional relationship between the extent of deviation in the retardation coefficient and the amount of organic carbon o n the soil was found. When normalized to the saturated solid-to-water ratio , retardation coefficients for dimethylphthalate determined at different de grees of saturation were similar, leading to the conclusion that the distri bution coefficient did not increase as the media was desaturated. Volatiliz ation did not significantly affect the retardation of benzene under the emp loyed range of saturation. Theoretical analysis showed that the impact woul d be significant for volatile compounds with higher Henry's coefficients an d in aquifers with low organic matter content. Diffusive transport in the a ir phase had a significant impact on the spreading of benzene. Previously d eveloped empirical relations reasonably predicted this impact.