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Questions and Answers

What is the depth to aquifer at this site?

Depth from surface to top of aquifer = 32 feet. Depth from surface to lowest point of hazardsous substance = 12 feet. Depth to aquifer = 32-12 = 20 feet.

Suppose there is a bore hole with a hit in the plug at 25 to 28 feet below ground surface. What is the depth to aquifer now?

Depth from surface to top of aquifer = 32 feet. Depth from surface to lowest point of hazardsous substance = 25 feet. Depth to aquifer = 32-25 = 7 feet.

Now compare these two results using Table 3-5 of the HRS Rule. How would the factor value have changed between the depth to aquifer values of 20 feet and 7 feet?

In fact, it would not have changed at all, which illustrates that unless the measurement is near a breakpoint, it does not need to be very precise.

What is wrong with the example provided?

Depth was measured in terms of below ground surface instead of above mean sea level. This can create errors when the terrain is not flat.

What would be the travel time factor value for this site?

After ignoring the top 10 feet and the remaining two feet of the top layer (without the the layer's top 10 feet, only 2 feet would remain), the lowest hydrologic conductivity value is found to be that of silty clay (10E-6). Since there are two layers of silty clay, their thicknesses are added to derive a total thickness of 9 feet. The hydrologic conductivity value and the combined thickness are then applied to Table 3-7 to derive a travel time factor value of 15.

Name five references or sources of information you might use to get documentation for ground water likelihood of release values.

Geologic report of site and/or vicinity (with geologic cross-section).

  • Ground water use report (to establish whether ground water is an aquifer).
  • Site reports from nearby facilities.
  • Well logs.
  • Analytical results of ground water samples from wells (including flow direction).
  • Design/construction specifications for sources.
  • Field observations/photos of source characteristics.

Give at least three criteria that should be considered to show that a sample from an observed release well is "similar" to a sample from a background well.

Well depths and screen intervals.

  • Dates/times samples were collected (must be within reasonable period).
  • Well design/construction information.
  • Well sampling procedures.
  • Analytical procedures and handling of samples.
  • Environmental setting.

Can an observed release to an aquifer by chemical analysis be used to document a source containment value of 10 based on "evidence of hazardous substance migration"?

Only if the observed release is directly attributable (at least in part) to the specific source for which containment is being evaluated. Since containment is source-specific, it must be proved that the "evidence of hazardous substance migration" is from that source and not from another.


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