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Section 10: 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.


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