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We now have a hypothesis to explain the apparent truncation of the MTBE plume. The next step is to test it in the field and revise our conceptual model if needed to account for vertical migration. All of these things, developing a working hypothesis, collecting more data to test the hypothesis and revising the conceptual model should be performed in real-time as the investigation proceeds. In our case when the shallow samples came up with an unexpected low concentration, the project manager revised his sampling plan on the spot and directed the crew to take samples at depths 10 and 20 feet below the first sampling depth to see if the plume had been missed vertically. The results of these samples indicated that the highest concentrations were present at a depth of about 13 feet below the water table.
How would you revise the conceptual model to assist in delineating the rest of the plume?
After proceeding in this manner; sampling in cross-gradient transects every 200 to 500 feet to define the plume's width, centerline and vertical position, the Benzene plume was found to be approximately 900 feet long, the MTBE plume was found to be approximately 3,200 feet long.
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