An official website of the United States government.

This is not the current EPA website. To navigate to the current EPA website, please go to This website is historical material reflecting the EPA website as it existed on January 19, 2021. This website is no longer updated and links to external websites and some internal pages may not work. More information »


Section 10: Ground Water - Waste Characteristics & Targets

This graphic shows where this section falls into the Hazard Ranking System (HRS) structure. With the likelihood of release factor value already calculated in Section 9, calculating the remaining two factor values -- waste characterics and targets -- will allow the pathway score to be calculated.

Ground Water Waste Characteristics and TargetsRemember: A "pathway" score is determined for each aquifer and the score associated with the highest scoring aquifer is the ground water pathway score. The scoresheet (HRS Rule Table 3-1) is formally completed only for the highest scoring pathway.

10.1 Waste Characteristics

The waste characteristics factor category in the ground water pathway is made up of two components: the toxicity/mobility of the most hazardous substance associated with the site and the hazardous waste quantity at the site. The most hazardous substance at a site is identified as the hazardous substance receiving the highest toxicity/mobility value factor. The hazardous waste quantity factor is evaluated as discussed in Section 5.

The Most Hazardous Substance at a Site

The most hazardous substance at a site is identified by calculating the toxicity/mobility factor all eligible hazardous substances associated with the site. Eligible hazardous substances consist of those hazardous substances available to migrate from the sources at the site to ground water and include:

  • All hazardous substances found in ground water observed releases at the site.
  • All hazardous substances found in a source with a non-zero ground water containment factor value.
  • All hazardous substances assigned to the "unallocated source."

As noted in Section 5, the determination of the toxicity value for a hazardous substance is complex, involving assessments of a substances relative propensity to cause cancer and/or non-cancer adverse health effects (such as liver damage or death). Toxicity values for nearly all hazardous substances of interest can be found in Superfund Chemical Data Matrix (SCDM). If a value is needed for a hazardous substance not found in SCDM, then EPA should be consulted to determine the appropriate course of action. Toxicity factor values should not be independently calculated.

Ground water mobility is a measure of the propensity of a substance to migrate through an aquifer and reach targets. The mobility factor value assigned to a hazardous substance is based on generic and site-specific considerations. The mobility factor for any substance found in any ground water observed release at the site is assigned a value of 1. Otherwise, the substance mobility factor value is assigned based on the water solubility and distribution coefficient of the substance.

As with the toxicity factor, the rules for determining the mobility factor values for a hazardous mobility factor values for nearly all hazardous substances of interest can be found in SCDM. If a value is needed for a hazardous substance not found in SCDM, then EPA should be consulted to determine the appropriate course of action. Mobility factor values should not be independently calculated.

The SCDM mobility factor values may not be directly applicable at a site because the HRS contains special, site-specific provisions to be applied in certain situations:

  • Substances found in ground water observed releases are assigned a mobility factor value of 1.
  • Hazardous substance deposited or currently present in the source as a liquid are essentially assigned the maximum water solubility value.
  • Hazardous substances are essentially assigned the minimum distribution coefficient value when evaluating karst aquifers.
  • A default value of 0.002 is used if none of the hazardous substances eligible to be evaluated can be assigned a mobility factor value.

SCDM contains water solubility and distribution coefficient values for many hazardous substances for use in these situations.

It is important to note the sequential effect that demonstrating an observed release has on the ground water pathway score. If an observed release is demonstrated then:

  • the maximum likelihood of release value of 550 is assigned (a value 10 percent higher than the maximum potential to release value)
  • all hazardous substances meeting the observed release criteria are assigned the maximum mobility factor value.

Further impacts of observed release demonstrations on target evaluations will be discussed later.

Toxicity/Mobility Factor

With toxicity and mobility values calculated, the toxicity/mobility factor is obtained by referring to Table 3-9 of the HRS Rule. The most hazardous substance for the ground water pathway is the one with the highest toxicity/mobility value. This is the value used to score the aquifer (the value is entered in line 4 of Table 3-1 of the HRS Rule).

Note that many substances that are highly toxic have low values for ground water mobility. For example, polychlorinated biphenyls (PCBs), which are highly toxic (toxicity value of 10,000), absorb easily and have a mobility value of 0.0001, even when in liquid state. This explains why observed releases of PCBs to ground water have been documented at only a few sites.

Hazardous Waste Quantity

The hazardous waste quantity (HWQ) — the second component of the waste characteristics factor value — is evaluated based only on those sources that have a ground water containment value greater than zero.

See Section 5 for calculating the HWQ. Once calculated, enter the value in line 5 of Table 3-1 of the HRS Rule.

Waste Characteristics Factor Category Value

The waste characteristics factor category value is determined in a straight-forward manner by first multiplying the toxicity/mobility factor value by the hazardous waste quantity value, subject to a maximum value of 108. The waste characteristics factor category value is then determined using this product as specified in HRS Rule Table 2-7. The maximum value achievable in the ground water pathway is 100.

In the absence of both an observed release and karst terrain, the waste characteristics factor category value remains the same for all aquifers underlying the site.

Waste Characteristics Concluded

The above diagram summarizes the steps taken to arrive at the waste characteristics value.

Consider the site shown above. As illustrated, the site records indicate that 13 drums containing waste chlordane and arsenical pesticides were deposited in an unlined pit. The SCDM toxicity values for chlordane and arsenic:

  • arsenic: 10,000
  • chlordane: 10,000

The SCDM ground water mobility values for chlordane and arsenic:

  • arsenic: 0.01
  • chlordane: 0.01

10.2 Targets

Once the likelihood of release and waste characteristics factor category values have been calculated, the only remaining factor category value needed to calculate the ground water pathway score is targets. The four components that make up the target factor value score are listed below:

  • Nearest well represents the "maximally exposed individual."
  • Population represents the people drinking from the wells that are subject to actual or potential contamination.
  • Resources reflects the non-potable purposes of ground water.
  • Wellhead Protection Area (WPA) is a "surface or subsurface area surrounding a well or wellfield, supplying a public water supply, through which contaminants are reasonably likely to move toward and reach such water well or well field" specifically designated as such by a State under Section 1428 of the Safe Drinking Water Act.

Just like with the other factors, target factors in each aquifer are scored separately. However, when evaluating the target factor values for an aquifer, the overlying aquifers are also included in the calculation of the nearest well and population components. This is because contamination travels downward, so if an aquifer lies below a contaminated aquifer the lower aquifer is likely to experience contamination as well. For this reason, the target score tends to increase with depth (the deeper the aquifer, the more aquifers lie above it, increasing the number of targets). The factor value also tends to increase with depth because big municipal wells tend to be deeper than small private wells (big municipal wells produce a larger population value).

In Table 3-1 of the HRS Rule, the relative importance assigned by the HRS to each component can be seen from their maximum values:

  • Nearest well = 50
  • Population = No maximum value
  • Resources = 5
  • WPA = 20

Notice that the population component has no maximum value. As a result in high density population areas, the population factor usually dominates the targets factor category value. In more rural areas, actual contamination of one or two private wells can result in relatively high target scores for both population and nearest well.

Nearest Well

Once determined, this value is entered in Table 3-1 of the HRS Rule.


The population target factors are evaluated based on the number of people served by the drinking water wells in the aquifer of concern (and all overlying aquifers) located within the target distance limit (TDL). Population values are assigned to each well based on available information according to the following rules:

  • include residents, students, and workers who regularly use the well
  • exclude transient populations such as customers and travelers passing through the area
  • include standby wells maintained on a regular basis so that water can be withdrawn
  • for independent systems, estimate residential population by multiplying each residence by the average number of persons per residence for the county in which the residence is located, in the absence of better information (e.g., well-specific data)
  • populations served by blended systems (i.e., systems served by multiple wells and surface water intakes) are apportioned to individual wells:
    • if no source of drinking water exceeds a 40% relative contribution, each well and intake is allocated an equal portion of the population served
    • otherwise, populations are apportioned proportional to pumpage and/or capacity data (as specified in the HRS Rule)
  • population may be apportioned to wells and intakes located outside the TDL but only those wells located within the TDL are used to determine the population factor values.

It is critical to assign a population value to each well subject to actual contamination. Wells subject to potential contamination and located within the same distance categories (HRS Rule Table 3-11) can be treated in the aggregate.

The following illustrates the procedures employed for blended systems.

Blended Water Supply

In this example, there is a blended system that supplies 10,000 people. The system consists of 9 ground water wells and 1 surface water intake. Five of the nine ground water wells are within the target distance limit of the source. Two wells are within the 2 to 3 mile distance ring and three wells are in the 3 to 4 mile distance ring. Two scenarios are presented in the graphic. The percentage figures represent the relative contribution of the wells. The percentages without parentheses are the first scenario. The percentages with parentheses are the second scenario.

Scenario 1: There are ten sources of drinking water in the system. No one source contributes greater than 40 percent. Therefore, it is assumed that each well and the intake contribute equally. Population apportionment:

  • 10,000 people/10 sources of drinking water = 1,000 people per water source
  • 2-3 miles: 2,000 people; 212 HRS distance-weighted target points (HRS Table 3-12)
  • 3-4 miles: 3,000 people; 131 HRS distance-weighted target points (HRS Table 3-12)

Scenario 2: There are ten sources of drinking water. However, the surface water source contributes greater than 40 percent. Population allocation:

  • 5 percent of 10,000 = 500. 500 people are allocated to each of the eight wells.
  • 15 percent of 10,000 = 1,500. 1,500 people are allocated to the large well.
  • 45 percent of 10,000 = 4,500. 4,500 people are allocated to the surface water intake.
  • 2-3 miles: 1,000 people; 68 HRS distance-weighted target points (HRS Table 3-12) .
  • 3-4 miles: 1,500 people; 131 HRS distance-weighted target points (HRS Table 3-12).

Once populations are assigned to each well, populations totals are calculated for the (1) wells subject to actual contamination—Level I concentrations, (2) wells subject to actual contamination—Level II concentrations, and (3) wells subject to potential contamination, by distance category. It is important to remember to include populations served by the aquifer of concern as well as by the overlying aquifers.

The Level I concentrations factor is assigned a value equal to ten times its summed total. The Level II concentrations factor is assigned a value equal to its summed total. In evaluating the potential contamination population factor, distance weighted target points are determined from Table 3-12 for each distance category based on the population served within that category. The resulting (up to 6) distance category values are summed and the potential contamination population factor assigned a value of one-tenth this total.

Note the differences between the karst and the non-karst portions of Table 3-12. Because karst can tranmit water quickly and with little dilution over considerable distances, the population values are higher and remain constant from 1/2 mile out to the target distance limit. Also note the distance rings down the left side of the table. The 1/4 mile ring is drawn so that every point on the ring is 1/4 mile from the nearest HRS source. The assigned values drop with distance.

The HRS Guidance Manual, Section 7.5 provides several useful hints to facilitate evaluating the populations factors.

  • Local water supply authorities should be the first points of contact concerning populations supplied by municipal wells (see HRS Guidance Manual, Highlight 7-32, p. 166).
  • At a minimum, obtain an estimates of the number of service connections from the local water supply authority.
  • Initially assume that all service connections are residential. Avoid time-consuming inquiries concerning student and business populations unless necessary if the (raw) ground water pathway scores in excess of 100 or if the estimates lie near the upper ends of ranges in HRS Table 3-12. In the later case, it may be worthwhile to document student and worker populations associated with non-residential wells as a higher score may result.
  • Carefully document the existence and usage of wells not included in municipal system. This is particularly important in rural areas and in evaluating the nearest well factor.

Highlight 7-33 of the HRS Guidance Manual, p. 168 illustrates the calculation of the population factors.


The third component of the targets factor value is resources. The resources component evaluates the use of ground water in the aquifer or overlying aquifers for commercial or recreational purposes (not including drinking water). To score this component, answer the following questions:

  • Is ground water used for irrigation (5-acre minimum) of commercial food crops or commercial forage crops?
  • Is ground water used for watering commercial livestock?
  • Is ground water used for supplying commercial aquaculture?
  • Is ground water used for supplying a major designated water recreation area, excluding drinking water use?
  • If there are no drinking water wells within the TDL, is the ground water usable for drinking purposes?
  • Is ground water used as an ingredient in commercial food preparation?

If the answer to one or more of the above questions is yes, assign a resource value of 5. Otherwise, assign a value of 0. Enter this value in Table 3-1 of the HRS Rule.

Section 7.8 of the HRS Guidance Manual provides guidance on evaluating the resources component. Highlight 7-47 in particular is useful in that it lists sources of information on resource use, which may be useful at a site inspection.

Wellhead Protection Area

To be scored under the HRS, a wellhead protection area (WPA) must have been formally established under Section 1428 of the Safe Drinking Water Act. Each state lists and publishes its WPAs, so finding any WPAs will require a request for information from the state environmental agency. Sole source aquifers are not considered under this factor, nor are informally defined protection areas.

Assign a value of 20 if either of the following criteria applies for the aquifer being evaluated or overlying aquifers:

  • A source with a ground water containment factor value greater than 0 lies within or above a WPA; or
  • Observed ground water contamination attributable to the sources at the site lies within a WPA.

Assign a value of 5 if there is a WPA within the TDL applicable to the aquifer being evaluated or overlying aquifers, and if neither of the two criteria above apply.

Assign a value of 0 if none of the above apply.

Enter this value in Table 3-1 of the HRS Rule.

See Section 7.8 of the HRS Guidance Manual for assistance if needed.

10.3 Questions and Answers

What would happen to the toxicity/mobility value if PCBs were detected in a ground water sample at observed release criteria?

Toxicity would still equal 10,000. Mobility would now equal 1. This results in a toxicity/mobility value of 10,000.

How many target points would result from finding that a private well used by a family of five is contaminated above a health­based benchmark? Do not forget to consider the nearest well factor.

50 for nearest well plus 50 (5 x 10) for population = 100 target points. Note that this does not yet include the other two component values for resources and WPA.

What population value would result from a municipal well 0.6 miles from the site that served a resident population of 950? Assume the well is not subject to actual contamination and is not located in karst terrain.

Using Table 3-12 of the HRS Rule, a population of 950 using a well 0.6 miles from the source gives a value of 167. The number is then multiplied by 0.1 for potential contamination. 167 x 0.1 = 16.7. This result is rounded 17 (see note a of Table 3-12). Add 9points for the Nearest Well value and you get 16.

What population value would be assigned if you added in the 350 children and teachers at the local elementary school?

The population would increase to 1,300 yielding a value of 52 (523 * 0.1).

Whom might you call to get information on ground water use?

  • Ground water purveyors (municipal or private).
  • County water authority (private well inventories).
  • Others, including local well drillers.

What questions would you ask the manager of a public water supply system to get the information needed to apportion targets?

  • What are the screen interval depths of each well?
  • Are they screened in a karst aquifer?
  • Is it a blended water supply system?
  • How many people or service connections does the system supply?
  • How many sources of water contribute to the system (ground and/or surface water)?
  • Does any one source supply more than 40 percent to the system?
  • If so, what is the percent contribution of each water source in the system?
  • Where are all the wells located?

What information do you need to obtain in order to document that a well meets Level I criteria?

  • Is this well used by people for drinking (or was it used for drinking before being closed due to site-related contamination)?
  • Do concentrations of hazardous substances detected in the well meet the criteria for observed release by chemical analysis?
  • Is the concentration above a health-based benchmark (MCL, RfD, or CR)? Or, by calculating the I or J indices, does the cumulative exposure of either exceed 1.0?

If answers to all three of the above are "yes," then you have documented Level I contamination for the people apportioned to the well.

Navigate to another section of the course:

        Table of Contents      
1 2 3 4 5 6 7 8 9 10
11 12 13 14 15 16 17 18 19 20