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Section 7.5

The population factor in the ground water pathway evaluates the number of residents, students, and workers served by ground water wells (in the aquifer being evaluated and appropriate overlying aquifers) located within the TDL. The nearest well factor evaluates the threat to the maximally exposed individual and takes into account whether that individual is subject to actual or potential contamination. This section explains how to estimate the population (i.e., residents, students, and workers) that regularly uses ground water from wells within the TDL, how to score the ground water population factor, and how to score the nearest well factor.

The ground water population is the people served by wells located within the TDL, not the residents living within the TDL (see Highlight 7-31). People living within the TDL may obtain drinking water from wells outside the TDL or from surface water sources, and people living outside the TDL may obtain drinking water from wells located within the TDL.


RELEVANT HRS SECTIONS
  Section 3.0.1   General considerations
  Section 3.0.1.1   Target distance limit
  Section 3.3.1   Nearest well
  Section 3.3.2   Population

DEFINITIONS

Nearest Well Factor: Factor for evaluating the maximally exposed well. This factor is based on the presence of actual contamination or, for aquifers where no drinking water well is subject to actual contamination, the presence of karst and distance to nearest drinking water well.

Population for the Ground Water Pathway: Number of residents, students, and workers regularly served by wells that are located within the TDL for the aquifer being evaluated (and appropriate overlying aquifers). This population does not include transient populations, such as hotel and restaurant patrons, but may include seasonal populations (e.g., a resort area).

Students: Full- or part-time attendees of an educational institution or day care that is served by a well located within the TDL.

Target Distance Categories: Concentric rings (not necessarily circular) with radii 1/4, 1/2, 1, 2, 3, and 4 miles from the sources at the site. These distance categories are used to group the wells subject to potential contamination for distance weighting.

Target Distance Limit for the Ground Water Migration Pathway: The distance over which targets are evaluated. The TDL is generally a 4-mile radius from sources at the site, except:

  • Include any drinking water well with an observed release attributed to the site, regardless of its distance from the source.

  • Exclude wells completed in portions of an aquifer that are beyond an aquifer discontinuity (see Section 7.1).

Target Wells for Aquifer Being Evaluated: Wells that are located within the TDL, and drawing water from the aquifer being evaluated or an overlying aquifer through which hazardous substances would migrate.

Workers: Permanent employees (part-time or full-time) of a facility or business that is served by a well located within the TDL.


EVALUATING THE GROUND WATER POPULATION FACTOR

The steps below describe an approach to estimating the population served by target wells for the aquifer being evaluated. First, contact water authorities that have wells within the TDL to determine or estimate the population served by municipal water systems. (See Highlight 7-32 for data needs that the water authority may be able to fulfill.) If the water authority provides an estimate of the population served by the system, use that number for your ground water target calculations. The water authority should know if the population served includes workers and/or students in addition to residents. If the population estimate does not include workers and/or students, it may be possible to modify the following methodology. The assumptions used should be clearly presented in the documentation record.

If the water authority provides just the total number of connections, then estimate the population served by multiplying the number of connections by the county average number of persons per household. After making an initial estimate of residential population served, estimate any student and worker populations served by the municipal system, and adjust the total. Next, evaluate residential populations served by private wells within the TDL. At each stage, evaluate whether documenting additional population will be important to the site score.

Depending on site circumstances, the scorer may conduct these steps in a different order. For example, if many people within the TDL use private wells or if private wells are subject to actual contamination, it may be more efficient to consider residential populations served by private wells before considering student or worker populations served by municipal connections.

  1. Draw target distance categories. Draw concentric rings with radii 1/4, 1/2, 1, 2, 3, and 4 miles on a topographic map from the edges of the source. If there is an aquifer discontinuity, exclude any areas beyond the discontinuity. Remember that any well with a documented observed release attributable to the site is evaluated regardless of its distance from sources.

  2. Identify all municipal systems with target wells for the aquifer being evaluated. Repeat Steps (3) through (5) for each system if more than one municipal system has wells within the TDL. If no municipal system has a well within the TDL, proceed to Step (7).

  3. Identify all system water supply units in the aquifer being evaluated or an overlying aquifer. These units may include drinking water wells and standby wells. If the municipal system is a blended system, identify all wells inside and outside the TDL. Also identify all surface water intakes and standby intakes contributing to a blended system.

  4. Evaluate the population served by the municipal water system, assuming all service connections are residential. Because connections to schools or businesses generally serve more individuals than those in a typical household, this assumption may result in a lower estimate of the target population. If this assumption yields a high score, however, time-consuming inquiries to document student or worker populations may be avoided.

    • Locate target municipal wells. Mark all municipal wells located within the TDL and completed in the aquifer being evaluated (or an overlying aquifer) on the map.

    • Estimate population served by municipal wells, assuming all residential connections.

      • Independent systems. If a single well serves a particular group of residences and is not blended with water from other wells or surface water intakes, determine the number of service connections for that well. Multiply the number of connections by the county average number of persons per residence from, for example, U.S. Bureau of Census reports.

      • Blended systems. If the wells are part of a blended system, obtain information about the entire system in order to apportion the total population served to each well or intake. The necessary data include

        • total number of people served or service connections for the blended system;

        • number of wells inside the TDL;

        • number of wells outside the TDL;

        • number of surface water intakes in the system;

        • whether any individual well or intake provides more than 40 percent of the water to the system; and

        • whether any wells or intakes are standby wells or intakes.

      • If any one well or intake provides more than 40 percent of the water to the system, collect data on the annual average pumpage or capacity for each well or intake (see Section 7.6, which provides additional information on apportioning population in blended systems). Multiply the number of service connections assigned to each well within the TDL by the average number of persons per residence.

    • Identify any municipal wells subject to Level I or Level II concentrations for the aquifer being evaluated. (See Section 7.4.) Keep a separate count of persons served by wells contaminated at Level I or Level II; do not count them in the population subject to potential contamination for that aquifer. Tabulate data on number of persons served by level of contamination and, for wells subject to potential contamination, by karst/non-karst and target distance category.

  5. Calculate a population factor value, assuming all residential connections. Highlight 7-33 illustrates tabulating populations and calculating the population factor value.

    • Multiply the total number of individuals served by wells subject to Level I concentrations by 10.

    • Multiply the total number of individuals served by wells subject to Level II concentrations by 1.

    • Use HRS Table 3-12 to assign a distance-weighted population value for karst and non-karst for populations served by wells subject to potential contamination. For each target distance category, sum the karst and non-karst distance-weighted population values. Multiply the total distance-weighted population value by 1/10.

    • Sum the values calculated for Level I, Level II, and potential contamination to obtain the population factor value (for municipal wells, assuming residential connections only).

  6. Determine if documenting student or worker populations is cost effective. If it is, continue to Step (7). If not, proceed to Step (9). In making this decision, consider:

    • Ground water pathway score assuming all residential connections. If the ground water pathway scores well over 100 by assuming all residential connections, it may not be cost-effective to document the student or worker populations. However, note the presence of student or worker populations using wells within the TDL in the documentation record.

    • Position within ranges for determining distance-weighted population. If the population served by municipal wells located in a particular target distance category is in the lower part or middle of a broad range (HRS Table 3-12), documenting students and workers may not change the population factor value. However, if the population served by municipal wells is near the upper end of a range, a substantially higher population factor value might be achieved by documenting the students and workers. If the population is at the lower end of a range, evaluating the student or worker population may help solidify the score.

  7. Document student and/or worker populations.

    • Identify schools and businesses served by wells within the TDL. Obtain information from water authorities on schools and businesses served by the municipal system. Identify schools or businesses within the TDL that do not use municipal water (and thus may have a private well).

      • Document any schools or businesses served by wells subject to actual contamination.

      • For potential contamination, focus efforts generally on large schools (e.g., universities) or schools and businesses that are supplied by wells in the closer target distance categories.

      • For any newly identified private well, document that it is completed in the aquifer being evaluated or an overlying aquifer.

    • Document the number of students or workers for those schools or businesses identified.

      • Contact the school officials to document student population.

      • Contact the business in question to document worker population, or refer to business census data.

  8. Calculate a population factor value that includes the student/worker populations. Follow the procedure outlined in Step (5) above. Be sure to subtract any service connections to schools or businesses from the total number of service connections (i.e., no longer assume all service connections are residential).

  9. Evaluate population served by private/community wells within the TDL.

    • Delineate areas served by municipal and private/community wells.

      • Municipal wells. Some areas may be served by water systems with no wells within the TDL. Mark these areas on a topographic map(s). Generally exclude these areas from the evaluation of private/community wells.

      • Private/community wells. If some areas within the TDL are not supplied by a municipal water system, determine if they use private/ community wells (completed in the aquifer being evaluated or an overlying aquifer). Sources of this information include local agencies such as: water authority, public health department, or water commission. It may be helpful to mark areas that rely on private or community wells on a map.

    • Estimate population served by private/community wells. Refer to the areas served by private/community wells (perhaps using the reference map). Use the most accurate information available to document this population. Computerized census data for small areas (e.g., block-by-block) are likely to be most accurate. If such data are not available, count the number of houses within these areas for each target distance category as indicated on a topographic map and multiply this number by the county average number of persons per residence. If the USGS map is outdated due to recent population growth (e.g., a new residential development), consider supplementing this house count using land use maps, aerial photographs, field counts, or other methods.

  10. Revise the tabulation of ground water population from Step (5). Add the number of persons served by private wells to the appropriate category based on level of contamination and, for wells subject to potential contamination, karst/non-karst and target distance category. Use this revised tabulation to calculate a new population factor value.

  11. Calculate a population factor value that includes populations served by private wells. Follow the procedure outlined in Step (5) above.

Highlight 7-34 provides an example of scoring the ground water population factor.


EVALUATING THE NEAREST WELL FACTOR

In evaluating the nearest well factor, consider all target drinking water wells for the aquifer being evaluated used by residents, students, or workers. Do not consider wells other than drinking water wells, nor wells used exclusively by transient populations.

  1. Determine if any drinking water well is scored based on actual contamination for the aquifer being evaluated. If not, continue to Step (2). If so, score the nearest well factor as follows:

    • If any target drinking water well is subject to Level I concentrations, assign a factor value of 50.

    • If any target drinking water well is subject to Level II concentrations, but no well is subject to Level I concentrations, assign a factor value of 45.

  2. Determine if any target drinking water well for the aquifer being evaluated is in a karst aquifer that underlies any portion of the sources at the site. If not, continue to Step (3). If so, assign a nearest well factor value of 20.

  3. Determine the shortest distance to any target drinking water well for the aquifer being evaluated from any source at the site with a ground water containment factor value greater than 0. Use HRS Table 3-11 to assign a nearest well factor value based on this distance.


EVALUATING GROUND WATER PATHWAY WHEN MULTIPLE SOURCES ARE PRESENT

This section presents two methods that may be used to evaluate the potential contamination and nearest well factors when multiple sources are present at a site.

  1. In the first method (see Highlight 7-35), draw distance categories independently around every source, determine aggregate distance categories (e.g., make overlapping rings of the same distance category), and total the population subject to potential contamination from drinking water wells for each distance category. The total populations for each distance category are then used to determine the potential contamination factor value. Individuals are counted only once (except when an individual is a resident and a student or worker), in the distance category for the well nearest to a source and used by the individual. The distance to the nearest well is the shortest distance from any source with ground water containment greater than 0 to any target drinking water well for the aquifer being evaluated. At sites with a large number of sources, this method may be time-consuming and inefficient. Because factor values are assigned based on population range within distance categories, a simplified method may be used with little or no impact on the pathway score.

  2. In this method (see Highlight 7-36), the nearest well is measured from any eligible source (i.e., as in the first method). However, rather than calculate the population subject to potential contamination for all sources, the scorer determines which source or sources will give the most representative score for the site based on distances to wells from each source and populations served by each well. Distance categories are drawn only for this source (or sources). This method is most effective for sites with a large number of sources and for sites with large populations using wells within the TDL. Note, however, that this method may underestimate target scores.

TIPS AND REMINDERS

  • Determine if individuals are within the TDL by the location of their well, not the location of their residence, school, or workplace.

  • If a maximum score for the ground water pathway can be reached by evaluating only municipal wells, it may not be necessary to include the population served by private wells in the scoring. If people in the area use private wells, note this fact in the documentation record. One exception is that any well subject to Level I concentrations should be evaluated.

  • Remember that the distance-weighted population values for potential contamination are assigned based on population ranges. Documenting a few private wells subject to potential contamination will result in a different population factor value only if the original population estimate was at the higher end of the range.

  • The nearest well factor may have a significant effect on pathway score; therefore, evaluate this factor as accurately as possible. The nearest well factor can be scored based only on drinking water wells.

  • Include the population using wells that were closed because of site-related actual contamination in estimates of the ground water population. This population should reflect the number of people using the well at the time it was closed.

  • If a drinking water system being evaluated includes portions of more than one county and the specific number of residences supplied in each county is known, use county-specific estimates of persons-per-residence. Otherwise, use the lowest persons-per-residence figure to estimate the entire population served.

  • An individual may be counted as a resident and as a student or worker. If an individual lives and attends school at locations served by drinking water wells within the TDL, count that individual as a resident and as a student.

  • Well logs obtained from local drillers are a good data source for determining in which aquifer(s) private wells are completed. In areas with a large number of private wells, one way of documenting how many wells are completed in each of two aquifers is to obtain a representative sample of well logs and assume the same ratio for all private wells in the area.

 

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