Desorption behavior of carbon tetrachloride and chloroform in contaminated low organic carbon aquifer sediments

Slow release behavior of carbon tetrachloride (CCl4) and chloroform (CHCl3) in low organic carbon (<0.1%) deep aquifer sediments was quantified by 1-D column desorption studies with intact cores. The compounds had been in contact with the sediments for 30 years. Comparison of the CCl4 distribution coefficient (Kd) from this study with those from short contact time experiments suggested that CCl4Kd’s calculated from site contaminated sediments of long contact time are likely a factor of 10 or more higher than those calculated from short contact-time lab experiments. A significant portion of the CHCl3 mass (55% to more than 90%) was resistant to aqueous desorption in sediments with clay contents ranging from 2.0% to 36.7% and organic carbon content ranging from 0.017% to 0.088%. In contrast, CCl4 showed greatest mass retention (31% or more) only in the highest clay and organic carbon content sediment. Relatively easy solvent extraction of the residual masses of CCl4 and CHCl3 from the sediments indicated the compounds were not permanently sequestered. Tracer breakthrough in columns was well behaved, indicating interparticle diffusion was not causing the slow release behavior. Diffusion out of intraparticle pores is suggested to be the main process governing the observed behavior although, diffusion out of natural organic matter cannot be ruled out as a potential contributing factor. The half-life for release of the slow fraction of CHCl3 mass from sediments was estimated to be in the range of weeks (100 h) to months (1100 h). Neither CCl4 or CHCl3 were detected at measurable levels in the column effluent of one of the sediments even though a significant mass fraction of CHCl3 was found present on the sediment following desorption suggesting that our estimate of hundreds to thousands of hours for complete release of CHCl3 masses from such sediment is conservative.