2.11 LAND TREATMENT FACILITIES

        EPA's (1988, 1990) model for computing volatilization rates from "land treatment facilities" or "old spill sites" is implemented in the user-interface (AG-VCASE = 4) of MEPAS 3.n versions. The use of microbial activity in the upper soil zone to biodegrade oily industrial sludge has been referred to as landfarming or land treatment. Petroleum industries often manage oily residuals by use of the land treatment process. Studies regarding potential air emissions from this process have been reviewed by Thibodeaux and Hwang (1982).

        A model developed by Thibodeaux and Hwang (1982) is used for estimating emission rates from land treatment of oily wastes. This model assumes that oily sludge spread on the soil surface forms lumps and films within the soil texture and that these lumps and films dry out slowly as a result of diffusion of vapors through the pores. The loss of contaminant to the air is computed as a function of time, assuming that the soil phase controls the vapor diffusion. Also, the concentration in the contaminant pool is assumed to remain constant until all the liquid-phase contaminant has been lost entirely to the atmosphere. A layering of the contaminant is assumed to occur in the soil consisting of a "dry" surface layer, with low-contaminant concentrations, located over a "wet" layer with high-contaminant concentrations. The terms wet and dry refer to the presence or absence, respectively, of significant quantities of the contaminant in liquid form. The contaminant is assumed to have pooled in the wet layer and, thus, has uniform concentrations within the layer. In addition to its use for land treatment facilities, this model is also applicable to estimating emissions from soils that contain a pool of waste liquid within the soil.

        The emission rate for the land treatment model (also referred to as the old spill model) is computed according to Thibodeaux and Hwang (1982) using

(35)


where

        Total diffusion from both liquid phases and gas phases is defined in terms of the flux of the constituent. The constituent first evaporates into voids in the soil and then diffuses into the atmosphere through the dry layer. This value can be estimated by the following equation:

(36)


 
where
Henry's law constant in concentration form, Hc, is defined as the ratio of near-surface air concentration to soil concentration of the constituent. This constant is computed using the following method, given by Lyman et al. (1982):

(37)


where
As noted above, this method of estimating the volatilization rate from land treatment/old spills assumes that a constant pool of concentration of the constituent exists in the soil until all of the constituent has been volatilized to the atmosphere. Based on this assumption, Hwang (1982) provides an equation for computing the dry-out time of a spill as

(38)



where