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Summary of Example Two

The modeling work performed in this example suffers from two fundamental problems:

Suggestion icon The site characterization was inadequate. As we saw, there were no wells in the direction of ground water flow, and the extent of contamination was unknown. Without this data we can't calibrate the model, nor can we judge the model results. There is no observed plume to compare model results against. A modeled plume of 100 feet, 200 feet or 500 feet in extent are all equally plausible given this lack of data and each could easily be produced from the model by changing input parameters.

Suggestion icon Most of the parameter values were undocumented, with no explanation given for their sources. These could be measured values, literature values, the results of calibration, or estimates of some sort. We simply don't know which they are. Further, no calibration data nor results were shown. The report should be rejected if only for this reason.

The calculators were used to second-guess four parameters used in the model. Here's a summary

Summary of Revised Parameter Values
Parameter Report value Revised value
Raises a red flag bulk density 2.65 g/cm3 1.72 g/cm3
Really raises a red flag retardation factor for benzene 10 1.04 to 1.4
Beginning to raise a red flag source benzene concentration 40 mg/l 17 to 25 mg/l
Beginning to raise a red flag longitudinal dispersivity 18.7 ft 10.2 ft

The estimates of seepage velocity, travel time and concentration at a receptor across the street from the station indicate that the benzene would have likely made it that far. These results further reinforce the idea that the aquifer should be sampled in this location.

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