Sub-module 3A, Page 4

Here is a section view of the barrel dump as it was in August of 2000 when we did some soils investigations as part of the PA/SI phase and found the contaminant methyl-ethyl-killalot or MEK. We quickly cleaned up the barrels.

The drawing indicates the average concentration of MEK in a region. The concentration of MEK is not constant under the dump, but varies spatially. Are those concentrations the same today? That's unlikely. The MEK may be migrating towards the groundwater, it may be migrating into the air, it may be metabolized by soil microorganisms, and it may be changing chemically into something other than MEK. It is probably doing all this and more.

Our baseline risk assessment must evaluate the exposure to receptors if we do nothing. Perhaps in three years, when we get the budget to clean up the site, all the material will be gone via natural attenuation. Or perhaps the concentrations will be about the same. The RI/FS stage (by that name or another name) will ponder options. Here are some excavation options.

The brown rectangles indicate the limits of excavation and the label indicates the quantities of soil within those limits. If the excavation and disposal of MEK contaminated soil cost $5000/ cubic meter, there is a large difference in cost between only excavating the soil that is contaminated to 500 ppm (How many bucks is that ), versus excavating the soil to 20 ppm (and how many bucks is that ). Each of these cleanup options would result in different amounts of risk, so the risk assessments from the RI/FS would present the risk for each cleanup option.

Understanding the changes in MEK source concentrations in space and time is necessary to evaluating the risk associated with the various options. First we will consider changes with time.

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