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) 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.
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 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:
- 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.
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.
Questions to test the understanding of the population componenet
How many target points would result from finding that a private well used by a family of five is contaminated above a health-based benchmark? Do not forget to consider the nearest well factor. ANSWER
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. ANSWER
What population value would be assigned if you added in the 350 children and teachers at the local elementary school? ANSWER
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.
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.
The graphic above illustrates the evaluation of the wellhead protection area factor. In the situation on the left, the source lies partially within the WPA. Assuming that this source has a ground water containment value greater than zero, the WPA factor would be assigned a value of 20. In the situation on the right, the source lies wholly outside the WPA but within the 4-mile TDL for the source so a value of 5 is assigned. However, if an observed release from the source could be documented at a well within the WPA then a value of 20 would be assigned.
The targets factor category value is equal to the sum of the values for the target factors:
- nearest well
- population (sum of Level I concentrations population, Level II concentrations population, and potential contamination population)
- wellhead protection area
This sum is entered on Line 11 of the Ground Water Pathway Scoresheet.
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