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Likelihood of Release

Only now that we have defined the hazardous substance migration path we can proceed to obtain scoring values. We start with likelihood of release. Go to the HRS rule, table 4-1, page 51608.

  • Evaluate the likelihood of release factor for each watershed in terms of either an observed release factor or a potential to release factor.
  • Evaluate potential to release based in two parts: potential to release by overland flow and potential to release by flood. As in GW pathway, LR = OR or PR.

OR = DO or CA, like before. If an OR can be documented, assign 550 for LR and do not evaluate PR!

  • Although the surface water pathway has three threats (drinking water, human food chain, and environmental) the likelihood of release is assessed only once and applies to all three threats.
  • Likelihood of release is evaluated once and is the same for all three threats in the overland/flood migration component.

Note: Look at lines 5, 14, and 24. Notice that the same score for LR is used for all three threats.

There are two mechanisms by which hazardous substances may potentially reach surface water: overland flow and flooding. Both are evaluated under potential to release and the HRS values for the two components are summed to obtain the potential to release factor value.

Two PRs are evaluated: PR by overland flow and PR by flood. The two values are summed (line 4). First, let's look at PR by overland flow. The factors are:

Next, let's look at PR by flood. There are only two factors.

  • Containment (flood): either you have it (10) or you don't (0).
  • Flood frequency: based on flood zones.

At an SI, gather information to support the assessment of potential to release even if sampling for an observed release. At the HRS stage, document potential to release only if an observed release failed to be documented.

Observed Release by Direct Observation

The first bullet of HRS Section (Page 51609) has three inserts which describe how an observed release by direct observation can be documented.

  1. material containing a hazardous substance is documented to have entered surface water either by migration (e.g., wastestream) or by deposition. If a wastestream is seen entering surface water, document the presence of a hazardous substance by a sample taken close to the point of entry (e.g., field log, photograph). If the wastestream is a permitted outfall, either document that it is out of compliance with the terms of the permit or contact EPA for a policy decision (see Federally Permitted Releases).
  2. A source area has been flooded such that hazardous substances have been in contact with surface water.
  3. Documentation of "adverse effects" must be tightly linked to an inferred release. What, for instance, is the evidence that the fish were killed by the release and not by some other cause? Contact EPA for site-specific guidance.

Observed Release by Chemical Analysis

Three types of samples are accepted: water, sediment, and tissue from an essentially sessile, benthic organism.

  • Background and release samples must be from the same medium and should be as similar as possible.
  • Sediment samples are more apt than water samples to capture historical releases, releases in high volume streams, and materials in releases that sorb. SI-GM 67
  • Sediment samples that meet observed release criteria can document actual contamination for the drinking water and environmental threats and may document actual contamination for the human food chain threat if a further threat-specific criterion is met. Aqueous samples are required to document Level I concentration for the drinking water and environmental threats and tissue samples are required for the human food chain threat.
    • The trade-off between likelihood of results, impact on HRS score, and cost should be considered in an SI sampling plan when deciding between sediment samples, aqueous samples, or both.
  • Although the HRS allows background concentration to be established by methods other than sampling, a background sample may be the preferred, particularly for establishing attribution of the release to the site.

When the site is contaminated surface water sediments with no known source, no documentation of attribution is required.

What are the criteria for OR by CA?

  • Click here for answer.

How can you document OR to SW by DO?

  • Click here for answer.


Go to the HRS rule, table 4-2, pages 51609-51610. Once again, the source type names and containment descriptions on this table should guide the way you describe sources in your field logbook. Take photographs of salient features whenever possible.

Look at the first line of "All Sources." What would you consider "evidence of hazardous substance migration"?

Go to Section 8.4 of the HRS Guidance Manual (HRSGM), page 244. Look at the definition of evidence of hazardous substance migration near the top of the page. Refer to the HRSGM whenever the meaning of an HRS term is not clear.

As in the ground water pathway, do not assess containment for any source that fails to meet the minimum size requirement. (HRSGM, page 246. If no source meets the minimum size requirement, ignore the minimum size requirement entirely.


Runoff is an estimate of the volume of water that might run over, around, or through the sources and carry either entrained particulate or dissolved hazardous substances toward surface water. The larger the drainage area responsible for this volume of water, the less permeable the predominant soil group within the drainage area, and the heavier the 2-year, 24-hour rainfall event, the larger the value for runoff.

The first step in estimating runoff is to define the size of the HRS "drainage area." If a drop of rain falls in the drainage area, it will migrate over or through the sources at the site.

Drainage Area

Determine the areas up-gradient of each source using topographical contours. If a raindrop landed uphill from a source and flowed downhill over the source, it would have fallen within the source drainage area. Frequently, the drainage area is only the area of the source itself.

Go to Highlight 8-19 of the HRS Guidance Manual, page 226. Notice how man-made structures can reduce the drainage area by diverting or capturing runoff.

Go to the HRS Rule, table 4-3, page 51611. Assign a drainage area value (1-4) from table 4-3. Notice the wide ranges in drainage area size for assigned values.

Soil Group

Go to the HRS Rule, table 4-4, page 51611. Assign a soil group designation (A-D) from table 4-4, based on the description of your soil. Obtain a description of the predominant soil group in the drainage area from the Soil Conservation Service's county soil survey. The Soil Conservation Service uses these same designations (A-D) for each soil group, in an appendix to the county soil survey (designation D includes pavement and other impermeable surfaces).

Note: Be aware that site-disturbed soil or "fill-dirt" may not have the characteristics listed in the survey. Use site-specific information whenever possible.


Rainfall must be 2-year, 24-hour. The best source for this is the out-of-print Climactic Atlas of the United States, U.S. Dept. of Commerce, 1983. Often, it can be computed from local weather station data, but that is generally not necessary.

In HRS Rule, table 4-5, combine the soil group designation, which is an indicator of impermeability or runoff, with a measure of a heavy rainfall event. The result is a number that indicates how much runoff to expect from a unit area during a heavy rainfall. In HRS Rule, table 4-6, this measure of unit runoff is combined with a measure of the size of the drainage area. The result is a measure of the runoff over or through the sources during a heavy rainfall. This measure, called runoff, is the force that drives the migration of dissolved and sorbed hazardous substances.

Distance to Surface Water

Go to the HRS Rule, table 4-7, page 51611. This factor takes into account the possible loss mechanisms for hazardous substances as they make their way from sources to the PPE. Persons planning to sample for contamination within overland drainage paths should be aware of the break-points in HRS Rule, table 4-7.

For multiple PPEs, select the shortest to score each watershed.

Go to the HRS Rule, table 4-1, page 51608.  The distance to surface water factor value should be entered on line 2c. Score PR by overland flow.

Go to the HRS Rule, table 4-1, page 51608.   Now evaluate PR by flood. PR by overland flow and PR by flood will be added to derive the PR by overland flow/flood (because a single source may both leak and be periodically flooded).

Potential to Release by Flood

Containment (flood)

This evaluation requires that you determine which flood plains the sources at the site lie within, then, for each flood frequency a source is exposed, to decide whether the source is "designed, constructed, operated, and maintained" to prevent washout of hazardous substances by that level of flood.

  • Remember that "washout" does not require that a source be washed away or eroded. Rather, water that enters a source during a flood event may carry out hazardous substances as the flood waters recede.
  • The yes/no judgment of whether the source is "designed, constructed, operated, and maintained" to prevent washout requires an estimate of the site inspection team, supported by observations or other data. If the PRP believes the judgment to be incorrect, further information can be supplied for EPA review during public comment on the proposed NPL listing.

Flood Frequency

When you think of floodplains, you have to think of water rising. If a source is within the 10 year floodplain, it is also within the deeper water of the 100 year and the 500 year floods. But it is not within the lower water levels of the annual flood. Training Graphic

  • Sources of information on floodplain categories are insurance maps, Corp of Engineers, and Federal Emergency Management Administration (FEMA). If mapped information is unavailable, engineers may be able to calculate the flood levels from rainfall and topographic information.

Multiply the containment and flood frequency factor values for each source in each floodplain. Choose the highest-scoring source to score PR by flood.

Go to the HRS rule, table 4-1, page 51608. Review the potential to release portion (lines 2 to 4).

PR for the overland flow/flood migration component (line 4) = the sum of PR by overland flow (line 2d) + PR by flood (line 3c).

LR for the overland flow/flood migration component (line 5) = the higher of PR (line 4) or OR (line 1). OR will always be higher. If you document OR, do not bother with PR.

Line 5 is LR for the drinking water threat. But, LR will be the same for the other two threats in this component (look at lines 14 and 22, "same as line 5").

WC and T, however, are different for each of the three threats. After an exercise on LR for surface water, we'll evaluate WC and T for each threat.

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