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Calculating Likelihood of Release

Section 9 -- Slide 9:16

This graphic shows where likelihood of release for the ground water pathway fits into the overall HRS structure.

Paralleling the other two migration pathways, the ground water pathway likelihood of release factor category reflects the likelihood that there has been, or will be, a release of hazardous substances in any of the aquifers underlying the site. If an observed release can be demonstrated to an aquifer, the LR factor category is assigned a value of 550. Otherwise, the potential to release factor are evaluated for the aquifer and the LR factor category assigned that value (up to a maximum of 500).

It is important to remember that interconnected aquifers are treated as a single hydrologic unit. Thus, if three interconnected aquifers are stacked one above the other, the potential to release is evaluated based on the characteristics of the uppermost aquifer. Since two of the four potential to release factors incorporate depth as a consideration, the interconnection of the aquifers may significantly increase the potential to release values for deeper aquifers.

An observed release can result from either direct observation of the deposition of hazardous substances into an aquifer or chemical analysis of ground water samples from an aquifer. Ground water sampling is distinct from other types of sampling (e.g., suface water or air) in that there is a high cost of installing monitoring wells and there is uncertainty about flow direction and, hence, uncertainty in both background and attribution.

The two most common cases of observed releases to ground water by direct observation are:

  • Burial of material below water table (including evidence that there are hazardous substances present in the material and the water table has risen); and

  • Injection wells to deeper aquifers (including evidence that hazardous substances are injected).

The following diagram illustrates the former scenario.

Section 9 -- Slide 9:20

In this scenario, a pit has been excavated to a depth below the normal water table. This may have taken place during a drought, for example, or prior to a change in the water table. Drums known to contain hazardous substances were then placed into the pit and the pit was filled-in with soil. Because at least some of the drums placed in the pit are now below the water table, the requirements for an observed release by direct observation have been met. Evidence indicating that the drums are leaking is not required.

An observed release by chemical analysis is documented when analysis of ground water samples from the aquifer indicates that the concentration of hazardous substance(s) has increased significantly above the background concentration. Some portion of the signficant increase must be attributable to the site to establish the observed release.

A few things to keep in mind when trying to document an observed release through chemical analysis:

  • Be sure to take all ground water samples from aquifers, not from the saturated material above an aquifer, which is a common mistake made at site inspections.
  • At the focused site inspection, obtain ground water samples from any available wells, especially residential wells that may be subject to actual contamination.

Often the screening intervals of residential wells are not known, making comparisons to background difficult. Nevertheless, always be sure that the background and release wells are in similar locations within the same aquifer. Highlights 5-5, 5-6, 5-7 of the HRS Guidance Manual illustrate the importance of well screening intervals when comparing background and release wells, particularly when evaluating LNAPLS (floaters) and DNAPLS (sinkers).

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