2.9 SURFACE IMPOUNDMENTS


        EPA's (1988, 1990) model for computing volatilization rates from surface impoundments is implemented in the user-interface (AG-VCASE = 6) of MEPAS 3.n versions. Surface impoundments include ponds (solar evaporation, disposal, or holding), holding tanks, treatment tanks, or any other containment storing or treating of liquid waste. Surface impoundments may hold aqueous type waste or a waste consisting of a mixture of organic substances. The surface impoundment emission model presented in this section is applicable when there is a pool of the liquid containment on the surface of a structure. This model is no longer applicable when volatilization of the contaminant has developed a dry zone (e.g., a layer without any liquid-phase contaminant at the surface). When the dry zone is developed on the entire surface of the contamination, the physical condition relating to volatilization resembles contaminated soil and the emission rate can be estimated with one of the contaminated soil models.

        MEPAS uses a model for estimating the emission rates from surface impoundments with undisturbed surfaces because disposal ponds at hazardous waste sites do not normally provide forced agitation on the surface. However, the model provided can be readily adapted to the surface impoundments with turbulent surfaces by simply modifying the values for mass transfer coefficients.

        The model for predicting the emission rate is based on the two-resistance theory, which assumes that the resistances to volatilization from the bulk of the liquid to the atmosphere are mainly in the liquid phase and the gas phase as indicated in textbooks and publications (Thibodeaux 1979; Hwang 1982; Mackay and Leinonen 1975). The equation for estimating the emission rate is

(28)



where

       Equation 28 is a steady-state emission equation and applies when the amount of the constituent i does not change with respect to time. When the amount of the contaminant is finite in the surface impoundment, the emission rate would decrease as the contaminant concentration in the surface impoundment decreases. Equation 28 can be solved as a transient problem for the time-dependent emission rate (Mackay and Leinonen 1975):

(29)



where
The average emission rate (Ei) over a time period, T seconds, can be obtained by integrating Equation 29 over time:

(30)



        The overall mass-transfer coefficient can be expressed in either liquid- phase or gas-phase concentration. Equation 28 uses the liquid-phase concentration. When the overall mass-transfer coefficient based on the gas-phase concentration is used in Equation 28, the gas-phase concentration of constituent i in equilibrium with the liquid phase concentration in the surface impoundment should be used. The overall mass-transfer coefficient relates the volatilization rate from liquid to air. The overall liquid-phase mass transfer coefficient of the constituent can be estimated using a method presented by Hwang (1982):

(31)


where
In Equation 31, the conversion factor Cg/CL = 7.4 x 10-4 is necessary in the right-hand side to convert gas-phase units to liquid-phase units.

        For constituent i, the individual mass transfer coefficients can be estimated by reference to constituents whose base values are known. Experiments by Owens et al. (1964), Smith et al. (1979), and Thibodeaux (1978) used oxygen as a reference compound for liquid-phase mass transfer and water vapor as a reference for gas-phase mass transfer. Hwang (1982) used equations derived by Cohen et al. (1978), Mackay and Matsugu (1973), Owens et al. (1964), Thibodeaux (1978), and Reinhart (1977) to obtain equations for individual liquid- and gas-phase mass-transfer coefficients for a given constituent.

The equation for computing individual liquid-phase mass-transfer coefficients for natural surfaces is
 

(32)


where
     Hwang (1982) evaluated the individual liquid-phase mass-transfer coefficient for oxygen at 25° C for natural and turbulent surfaces; the coefficient values are 2.2 x 10-3 cm/s for natural surfaces and 2.3 cm/s for turbulent surfaces.
The equation for individual gas-phase mass-transfer coefficients for natural surfaces is

(33)



where
Hwang (1982) computed individual gas-phase mass-transfer coefficients for water at 25° C of 1.4 cm/s for natural surfaces and 23.2 cm/s for turbulent surfaces.