Environ. Sci. Technol. 1993, 27, 2909-2913
COMMUNICATIONS Benzene Emissions from a Contaminated Air-Dry Soil with Fluctuations of Soil Temperature or Relative Humidity Davld R. Shonnard' and and Richard L. Bell+
Department of Chemical Engineering, University of California, Davis, Davis, California 956 16 Introduction Emission of volatile organic compounds (VOC) from the soil compartment to the atmosphere is one important process influencing the fate of these compounds at some industrial sites, at hazardous waste treatment and disposal facilities, and above shallow groundwater plumes. These emissions are a concern due to the potential for human exposure at these sites and also downwind from them. VOC emissions are controlled by transport in the vadose zone of the soil, which occurs mostly in the vapor phase by molecular diffusion (1-3). At the same time, diffusive transport of VOC is influenced strongly by sorption from the vapor to the soil phase. Recent studies on competitive sorption between VOC and water, for mineral sites in airdry soils, have shown dramatic changes in sorption with alterations in soil-bound water and vapor relative humidity (RH) (4-12).Other studies investigating the temperature dependence of VOC sorption in soils have found a decrease in sorption with increasing soil temperature (6, 12). Though significant advances in the fundamental understanding of sorption in soils has occurred, relatively few studies concerning the effects of soil temperature and relative humidity on emission rates from contaminated soils have appeared in the literature (3,13). Reported in this paper are preliminary experimental results which quantify the separate effects of either fluctuations in soil temperature or in air relative humidity on the changes in emission rates of benzene from an initially uniformly contaminated air-dry soil. In a later study, the combined effects will be investigated. Modeling results for the effects of soil temperature fluctuations on the predicted benzene emission rates are also presented and are compared with experimental data. Both the preliminary laboratory experimental and modeling results demonstrate the potential for large VOC emission changes in the field due to commonly-encountered fluctuations in soil temperature and atmospheric relative humidity. Experimental Section The experimental apparatus for benzene emission rate data and also for Brunauer-Emmett-Teller (BET) sorption parameters used in the mathematical model presented
in this report were described in detail during the course of a previous study (14). The soil was a sieved (
