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Potential to Release

Potential to release is evaluated only when an observed release cannot be scored. Each aquifer is evaluated independently.

Potential to release or PR, is evaluated using the following four factors:

  • Containment. A value is assigned for each source and the highest value (least contained) is chosen. Containment is a critical factor in calculating potential to release. For example, if a source does not experience a loss of containment (i.e., the containment value is zero), no migration of hazardous substances will occur. Appropriately, if the containment value is zero (fully contained against release), then the potential to release value is zero, regardless of the values of the other factors (recall the equation PR = containment x [net precipitation + depth to aquifer + travel time]). (Normally, however, containment is usually 9 or 10 at CERCLA sites.) An increase in the containment value of only one unit has the effect of increasing the PR value by the sum of the net precipitation, depth to aquifer, and travel time values.
  • Net precipitation is determined from a map found in Figure 3-2 of the HRS Rule. In nature, net precipitation is the driving migration force for most substances, so wetter areas (e.g., the Pacific Northwest) will receive a higher value.
  • Depth to aquifer is measured for each aquifer scored. It ca receive a maximum of five points.
  • Travel time is estimated using the thickness and conductivity of the geological materials. Because travel time receives up to 35 points, it is also a critical factor in the PR value.

For each source affecting an aquifer, evaluate its containment value using Table 3-2 of the HRS Rule. Table 3-2 allows a score to be assigned to each source. Of all sources scored, the highest value is entered in Table 3-1, line 2a. However, do not include any source that would receive a source hazardous waste quantity value of less than 0.5 in determining the overall containment factor value. The containment minimum size requirement provision prevents a small spill area from swinging the score of a large, generally well-managed site.

Highlight 7-23 of Section 7.3 of the HRS Guidance Manual shows the sizes of various sources that satisfy the minimum size requirement.

Net Precipitation
In the absence of physical migration barriers, net precipitation is the driving force for migration to the aquifer. Figure 3-2 of the HRS Rule provides computed net precipitation factor values, based on site location. Notice that values are highest in the wettest areas (e.g., the Pacific Northwest).

For sites that cannot be assigned a value from Figure 3-2, a method for calculating net precipitation is provided in the HRS Rule, beginning on page 51600. From the equation provided, plug the product into Tables 3-3 and 3-4 are used to arrive at the factor value that is used in Table 3-1, line 2b.

Net precipitation value published in independent sources should not be used in HRS evaluations. The HRS definition of net precipitation addresses months in which evapotranspiration exceeds precipitation differently from the methods used in agricultural data (for example). Months with negative net precipitation do not affect the HRS net precipitation value so as to reflect the absence of upward substance migration during these months.

Depth to Aquifer
The depth to aquifer value measures the depth from the lowest known point of hazardous substances at a site to the top of the aquifer being evaluated. To calculate depth to aquifer, the depth from the surface to the lowest known point of hazardous substances at the site is subtracted from the depth from the surface to the top of the aquifer. Once the actual depth is calculated, the depth to aquifer factor value is obtained from Table 3-5 of the HRS Rule. This is the value then used in line 2c of Table 3-1, which calculates the score for the aquifer.

Additional information used in scoring the depth to aquifer factor value:

  • The depth measurements must be made within two miles of the sources at the site unless an observed release extends beyond two miles. Even with this range, however, a measurement should only be taken from the lowest point of hazardous substance dispostion at the site.
  • Because of the unique characteristics of karst terrain, intervening karst aquifers are assigned an effective thickness of zero in determining depth to an underlying aquifer. Thus, in calculating the depth to an aquifer lying below a karst aquifer, the true thickness of the karst aquifer should be deducted from the depth calculated as above, to determine the depth to the aquifer for HRS purposes.
  • The measurement of depth to aquifer should be in terms of elevation above sea level rather than in terms of depth from surface to avoid errors.

Below is an example of how to calulate depth to aquifer.

Section 9 -- Slide 9:28

What is the depth to aquifer at this site? ANSWER

Suppose there is a bore hole with contamination in the sample from 25 to 28 feet below ground surface. What is the depth to aquifer now? ANSWER

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? ANSWER

What is wrong with the example provided? ANSWER

Travel time
Travel time reflects the time it takes for substances to travel from the source through intervening layers to an aquifer. It is based on the thickness and the hydrologic conductivity of the least permeable layers between the lowest known location of hazardous substances and the top of the aquifer. Layers that are thick and/or have low hydrologic conductivities increase travel times.

  • The characteristics of the lowest conductivity layer(s) determine the travel time factor value: If more than one layer has the same lowest hydraulic conductivity, include all such layers and sum their thicknesses.
  • Hydraulic conductivities may be determined using HRS Table 3-5 based on the physical characteristics of the layers or from actual, representative, measured, hydraulic conductivity values. Actual values are preferred.
  • In the absence of an observed release to another aquifer, measurements of the layers must be made within two miles of the sources at the site and must lie in the interval between the lowest known point of hazardous substances at the site and the top of the aquifer being evaluated.
  • Consider only those layers at least 3 feet thick which do not lie within the first 10 feet of the depth to the aquifer.
  • For any given layer, determinations of thickness and hydraulic conductivity must be made at the same location. However, the locations used for the layers may differ from each other.
  • If information is available for several locations, evaluate the travel time factor at each location and use the location having the highest travel time factor value.

In evaluating the travel time factor, layers are grouped according to common hydraulic conductivity. The thicknesses of the layers lying in the lowest conductivity grouping are summed. This thickness and the hydraulic conductivity of the lowest conductivity group determine the travel time factor as provided in HRS Rule Table 3-7. The resulting value is entered in Table 3-1, line 24.

The maximum travel time factor value of 35 is assigned if either:

  1. the depth to aquifer is 10 feet or less; or

  2. all intervening layers under a source are karst (karst layers have unique features that can dramatically facilitate movement of substances).

Below is an illustrated example of scoring travel time.

Section 9 -- Slide 9:32

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

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