Evaluation of the Fluoride Retardation Factor in Unsaturated and Undisturbed Soil Columns

Abstract

Fluorides deposited on the soil surface near aluminum smelters represent a risk for ground water contamination. This study was conducted to evaluate F retention in soils sampled near an aluminum smelter. The F retardation factor (R) was evaluated at different depths in four coarse‐textured soils by percolating a Br and F solution under steady‐state conditions through 26 unsaturated and undisturbed soil columns. The convective‐dispersive equation (CDE) and convective‐lognormal transfer function (CLT) models were used to evaluate R, using Br and F breakthrough curves (BTCs) and F accumulation profiles evaluated with oxalate‐extractable F (Fox). Following percolation of 4.0 to 13.7 pore volumes of F solution through the soil columns, F BTCs were obtained in 5 out of 26 columns. In the other columns, all F remained in the soil profiles, indicating higher F retention. When R was evaluated using F accumulation profiles, the CDE‐evaluated retardation factor (RCDE) was consistently higher than the one evaluated with the CLT (RCLT), by as much as 111%. This was explained by a difference in the models' behavior near the soil surface. The retardation factor varied among soils and depths by a 20‐fold factor, from 4.4 to 91. The highest retardation factors were obtained in slightly acid and neutral soils. The retardation factor was lower in strongly acid and alkaline soils. The large range of R values obtained and its influence on the velocity of F movement in the soil show that R is one of the critical factors controlling the risk of ground water contamination by F.