Questions and Answers
What areas of observed contamination should be included for the nearby population threat?
Do not count the three areas on residential properties and exclude the area that is fenced and guarded. You are left with two areas of observed contamination: the area around the pile and the area on the school property.
The area of observed contamination is about 200' by 500'. Part of the area lies within a fenced but abandoned industrial property and part extends onto the neighboring Little League ballpark. What is the value for likelihood of exposure?
125. The attractiveness/accessibility values are 10 and 100 unless you documented public recreation within the fenced area. Select the higher. The area value for 100,000 square feet is 20.
1. As you were already warned, the values assigned to nearby populations "have been reduced to better reflect the relative levels of exposure." Note in the footnote that if a value has been assigned for resident individual, none is assigned to nearby individual. This is parallel to the practice in the migration pathways of assigning only the highest value (Level I, Level II, or potential) for the MEI risk.
What is the estimated value for population within 1 mile at a rural site with a population density of 100 people per square mile? Assume that the areas within the three distance rings are 0.2, 0.6, and 2.6 square miles respectively.
The population value is 0.21.
: 100 x 0.2 = 20: value = 0.4
: 100 x 0.6 = 60: value = 0.7
: 100 x 2.6 = 260: value = 1.0
::: sum = 2.1 x 1/10 for potential = 0.21
EXERCISE: Lets complete the nearby population threat with some quick what-if calculations based on the following site information. The site is a small, bankrupt wood preserver with observed contamination documented in the process area and drip yard for a total of about 50,000 square feet. The maximum toxicity value for wood-treating substances found in the sampling is 10,000. The site is not secured from access. There are no resident targets.
Likelihood of exposure: The site can probably be considered to be moderately accessible. Lets assume a worst case that we will be able to document some evidence of public recreation (= 50). This combined with an area value of 20 gives a likelihood of exposure value of 25 out of a possible 500 points (Table 5-8). The true area of observed contamination is probably larger than what has been sampled but it would have to be 2.5 times larger (greater than 125,000 square feet) to raise the value for likelihood of exposure to 50. After you estimate the pathway score, decide whether this sampling effort is worthwhile.
Waste Characteristics: The value for hazardous waste quantity from Table 5-2 is less than 1 (50,000/34,000) but the minimum value for the soil exposure pathway is 10 (text from page 51592, bottom half of the left column). Enter 10 x 10,000 into Table 2-7 and find a value of 18 for waste characteristics.
Targets: Since there are no resident targets, we'll assume that someone lives within 1/4 mile and assign a tentative value of 1 for nearby individual and add this to the value of 0.21 for population that was calculated in response to the previous question.
Nearby threat score: 25 x 18 x 1.21 = 544.5. Note, in the pathway formula in Table 5-1, page 51646, that you do not divide by 82,500 until after you have summed the score for the resident and the nearby threats.
Since the resident threat is zero, the pathway score is 0.007.