Handbook of Solvents - George Wypych - ChemTech - Ventech!

1168 William R. Roy
in groundwater at the compliance well were plotted against their MPCs. The boundary
shown in Figure 17.2.4 represents the situation where the steady-state concentration (Css)
equals the MPC. Consequently, the predicted Css is less than its corresponding MPC when
the Css of a given compound plots in the lower-right side. In this situation, these organic
compounds could enter the aquifer at a constant mass loading rate without exceeding the attenuation
capacity of the site. The steady-state concentrations of twenty solvents exceeded
their corresponding MPCs. The continuous addition of these organic compounds (i.e., a
constant mass loading rate) would exceed the site�s ability to attenuate them to environmentally
acceptable levels in this worst-case scenario. There are two avenues for reducing the
steady-state concentrations downgradient from the trench: (1) reduce the mass loading rate,
and/or (2) reduce the mass of organic compound available to leach into the aquifer. Because,
the RCRA-required double liner was regarded as the state-of-the-art with respect to
liner systems, it was not technically feasible to reduce the volume of leachate seeping into
the aquifer under the conditions imposed. The worst-case conditions could be relaxed by assuming
a lower leachate head in the landfill or by providing a functional leachate-collection
system. Either condition would be reasonable and would reduce the mass loading rate. Another
alternative is to reduce the mass available for leaching. In the previous simulations,
the mass available to enter the aquifer was assumed to be infinite. Solute transport models
can be used to estimate threshold values for the amounts of wastes initially landfilled. 2 A
threshold mass (M t) can be derived so that the down-gradient, steady-state concentrations
will be less than the MPC of the specific compound, viz.,
M t = V(MPC x 1000) t [17.2.2]
where:
Mt the threshold mass in g/hectare
V the volume of leachate entering the aquifer in L/yr/hectare
MPC the maximum permissible concentration as g/L, and
t time in years; the amount of time between liner breakthrough and when the predicted
concentration of the compound in the compliance well equals its MPC.
Using this estimation technique, Roy et al. 1 estimated mass limitations for the compounds
that exceeded their MPCs in the simulations. They found that benzene, carbon tetrachloride,
dichloromethane, pyridine, tetrachloroethylene, 1,1,1-trichloroethylene,
trichloroethylene and all chlorinated fluorocarbons would require strict mass limitations
(