Section 10: Questions and AnswersWhat 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.
The population would increase to 1,300 yielding a value of 52 (523 * 0.1).
- 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?
- 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.