# EPA On-line Tools for Site Assessment Calculation

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#### Investigative Techniques & Tools

In an investigation of this type it is useful to collect groundwater samples from temporary probe points, using push-probe equipment, in transects perpendicular to the long axis of the plume. The distance between the sampling points and the transects will vary depending on site conditions and accessibility. In this case two transects were completed at distances of about 250 and 450 feet from the source with a point-to-point spacing of approximately 25 to 30 feet. A portable GC was used to analyze the probe samples in the field to allow real time decisions and accelerate the delination process.

The results from the on-site analysis of the first transect indicated high (>10,000 ppb) levels of BTEX and MTBE in the samples collected at the water table. This changed to very low levels (<100 ppb) of both BTEX and MTBE in the second transect.

• Is this what you would expect?
• Is it likely that we have reached the extent of the dissolved component plumes?

 Although the release date is unknown, the spill is believed to be fairly recent having occurred sometime within the past three to five years. Depending on the actual release date and the combined rates of dissolution, transport velocity and attenuation, it is quite possible that the plumes are limited to less than 450 feet in length. Still, with the potential exposure to residences involved, we must be sure. Let's use the some of the calculators to help us determine if this assumption is reasonable or not. First, let's see what we can expect in terms of relative concentration based on the percent of each component in the fuel and their respective solubility. Try running the effective solubility calculator for each of the built-in MTBE fuels to determine the maximum concentration range of total BTEX and MTBE in the groundwater.

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Contact Jim Weaver to ask a technical question on this material.