SORPTION OF 1,2,4-TRICHLOROBENZENE AND TETRACHLOROETHENE WITHIN AN AUTHIGENIC SOIL-PROFILE - CHANGES IN K-OC WITH SOIL DEPTH

The sorption of 1,2,4-trichlorobenzene and tetrachloroethene was inves tigated in a series of well-controlled batch experiments, using authig enic soil materials from a profile extending to 2.5 m below ground sur face. Batch experiment techniques were verified by study with both pul verized and unpulverized soil at different times of equilibration, usi ng two widely different soil:water ratios, and at a wide range of aque ous concentration. Sorption isotherms were approximately Linear, with sorption distribution coefficients (K-d) found to decrease roughly 100 -fold down the soil profile. K-d decreased with depth to an extent gre ater than could be predicted on the basis of the only 10-fold decrease in natural solid organic matter (SOM) content and despite significant ly higher specific surface area in the lower horizons. All base-extrac table SOM in these deeper soil horizons was operationally defined as f ulvic acid (FA), although there was also a significant fraction that w as not extracted by the standard base technique. The lower K-d of the deeper soil horizons is believed to reflect a complex combination of ( 1) lower SOM content; (2) a more hydrophilic form of SOM; and (3) a mo re intimate association of the SOM with the mineral fraction, affectin g its accessibility, sorptivity, or both. For the deeper horizons, an increase in overall K-d by more than 1-fold was observed on solids tre ated by either base extraction or H2O2 treatment, demonstrating that s orption to remaining soil components could be dramatically increased b y fractional SOM removal and/or chemical alteration of the soil. A sim ple regression model that divides SOM into only two types (shallow and deep SOM) provides a reasonably good explanation of sorption in all s even horizons and suggests an order-of-magnitude variability in K(oc)a mong surface soil and deeper horizons. (C) 1998 Elsevier Science B.V.