Transport and Fate of Selected Organic Pollutants in a Sandy Soil

Ground waters are increasingly vulnerable to pollution by organic chemicals that migrate through the soil mantle. To assess the minimal protection that soil can be expected to provide ground water, the transport and fates of 13 organic pollutants were studied in a sandy soil with low organic matter content (0.087% organic C). Glass columns were packed with soil to a depth of 140 cm in a manner that preserved to the maximum extent possible the characteristics of the original soil profile. In separate experiments, the columns received 14 cm/day of water containing a mixture of approximately 1.0 or 0.2 mg/liter of each of the organic compounds. Quantities of compounds that volatilize from the soil surface were measured, as well as concentrations in the column effluent. Chloroform (trichloromethane), 1,2-dibromo-3-chloropropane, dichlorobromomethane, 1,2-dichloroethane, tetrachloroethene, 1,1,2-trichloroethane, and trichloroethene were not degraded in this soil. These compounds percolated rapidly through the soil; their retardation factors (velocity of tritiated water through soil divided by apparent velocity of pollutant through soil) were 2.5 or less. Between 19 and 65% of the material applied to the surface percolated to a depth of 140 cm, and the remainder volatilized. Chlorobenzene, 1,4-dichlorobenzene, and 1,2,4-trichlorobenzene also percolated through the soil; retardation factors were 1.7, 3.4, and 9.4, respectively. Between 26 and 49% of the material applied reached 140 cm. Toluene and nitrobenzene degraded in some of the columns but not in others. In the absence of degradation, 60 and 80% of nitrobenzene and 13% of toluene applied to the soil surface reached 140 cm. The retardation factors were 2.3 or less. Bis(2-chloroethyl)ether did not degrade; 86% of the material applied reached 140 cm, and the compound's retardation factor was <1.5. A simple mathematical model based on water solubility of the compound and the organic C content of the soil predicted the retardation factors of the pollutants within a factor of three.