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.
Containment
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.
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:
- the depth to aquifer is 10 feet or less; or
- 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.
What would be the travel time factor value for the this site? ANSWER
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