Carbon Tetrachloride Retention by Modern and Buried Soil A Horizons

Abstract

Carbon tetrachloride (CCl4) was commonly used as a fumigant for stored corn (Zea mays L.) and other grains. This practice and other industrial uses have resulted in instances of soil and groundwater contamination. To better understand the potential for downward CCl4 movement, retention by a buried soil A horizon was compared with that of the modern A horizon in that same location and its loess C parent material. Batch equilibration was used to determine sorption coefficients (Kd) to compare retention among horizons. Mean Kd values were 0.83 (standard deviation [SD] = 0.18) for the modern A, 0.36 (SD = 0.10) for the loess C and 0.41 (SD = 0.07) for the buried A. Mean organic carbon (OC) contents were 14.9 g kg−1 (SD = 2.6) for the modern A, 1.3 g kg−1 (SD = 0.50) for the loess C, and 5.3 g kg−1 (SD = 0.60) for the buried A. Sorption coefficients were most strongly related to percent OC (R = 0.91 across all soils). Log OC coefficients (log Koc) were 1.74 (SD = 0.04) for the modern A, 2.43 (SD = 0.18) for the loess C, and 1.89 (SD = 0.10) for the buried A. The OC coefficient varied with OC content of the horizons. This variation may be attributed to qualitative chemical differences in the organic matter fractions. This may also be caused by the increased importance of the clay mineral fraction at low OC contents, leading to falsely high Koc values. The presence of a buried A horizon may help retard movement of CCl4 to groundwater.