Sorption and desorption processes affecting the enrichment of the fluorocarbons CCl₂F₂ and CCl₃F in ground water

Abstract

Field measurements of CCl₃F and CCl₂F₂ in ground water have shown that concentrations in recharge areas may be higher than those in water at equilibrium with the atmosphere. This research tested the hypothesis that sorption and desorption of fluorocarbons from grain surfaces in response to changes in soil moisture content are responsible for this enrichment. Soil slurry experiments were performed to determine whether aqueous fluorocarbon concentrations increased when water came into contact with dry soils. In all slurries, the aqueous CCl₃F concentration increased significantly, whereas the CCl₂F₂ concentrations increased in all but two of the slurries. The percent increase of both compounds correlated well with soil properties such as percent clay and percent organic matter. Column experiments were carried out to obtain distribution coefficients (Kd) for CCl₃F and CCl₂F₂ on dry soils. These values were 3.02 and 3.20 cm³/g, respectively, and the values for Kd on saturated sand were 0.67 and 0.360 cm 3 ig for CCl₃F and CCl₂F₂, respectively. The difference in magnitude of Kd for saturated and dry media suggests that significant amounts of both fluorocarbons desorb from grain surfaces when dry soil is wetted, causing enrichment of aqueous fluorocarbon concentrations.