Technical Support Document for U.S. EPA’s Proposed Rule for Addition of a 

Subsurface Intrusion Component to the Hazard Ranking System 

Appendix C: Conceptual Site Modeling

Table C-1: Initial Conceptual Site Scenario Evaluation

Table C-2: Sample HRS Site Scores Based on Potential Exposures Only and Actual Hazardous Waste 

Quantity Values

Table C-3: Sample HRS Site Scores Based on Potential Exposures Only and Minimum Hazardous Waste 

Quantity Values

Table C-4: Sample HRS Site Scores Based on Potential Exposures Only with Consideration of Subslab 

Sampling Data

Table C-5: Sample HRS Site Scores Based on Different Population Densities and Actual and Potential 

Exposures 

Table C-6: Sample HRS Site Scores Based on a Combination of the Ground Water Migration Pathway 

and the Subsurface Intrusion Component of the Soil Exposure and Subsurface Intrusion 

Pathway

Table C-7a: Proposed Conceptual Site Scenario Evaluation 

Table C-7b: Sample HRS Site Scores Based on Potential Exposures Only with Consideration of Different 

Population Densities and Varying Types of SsI Sampling Data 

Table C-7c: Sample HRS Site Scores Based on Different Population Densities, a Mix of Actual and 

Potential Exposures and Different Types of SsI Sampling Data

Table C-7d: Minimum Populations Needed to Achieve an HRS Site Score of 28.50 with Different 

Likelihood of Exposure, Waste Characteristics and Different Types of SsI Samples

Table C-7e: Minimum Actually Exposed Population Needed to Achieve an HRS Site Score of 28.50 Based 

on Varying Waste Characteristic Factor Values

Table C-7f: Minimum Potentially Exposed Population Needed to Achieve an HRS Site Score of 28.50 

Based on Varying Waste Characteristic Factor Values

Table C-7g: Minimum Population Needed to Achieve an HRS Site Score of 28.50 Based on 

Varying Likelihood of Exposure and Waste Characteristics Factor Values and no ASC

Table C-7h: Minimum Populations needed Based on a Combination of the Evaluation of a Second HRS 

Pathway and the Subsurface Intrusion Component of the Soil Exposure and Subsurface 

Intrusion Pathway

Table C-7i: Minimum Residents and Workers Needed to Achieve an HRS Site Score of 28.50 Based 

on Weighting by Level of Exposure and Sample Type

Page C-1 
Conceptual Site Modeling

A sensitivity analysis was performed during development of the proposed Hazard Ranking System (HRS) 

addition to test the proposed HRS addition, identify and establish the relative magnitude of the factors that have 

the greatest impact on the HRS site score, and ensure that the factors driving the score are similar to those of 

existing pathways and components. The analysis illustrates the types of sites that may qualify for the National 

Priorities List (NPL) considering vapor intrusion (VI) contamination under various scenarios based on different 

site characteristics and different factor value weightings. These scoring models are presented in Tables C-2 

through C-6 of this appendix. Table C-1 presents the initial conceptual site scenario evaluation with varying 

likelihood of intrusion levels, zone of contamination, waste characteristics and levels of contamination. The tables 

included below were used in the development of the proposed HRS Subsurface Intrusion (SsI) addition, and as a 

result of changes to the rule, weighting factors and values may not be representative of the current proposed HRS 

addition. Table C-7 presents scoring simulations using the proposed HRS addition methodology and identifies

various factors impacting the HRS site score. Additional information regarding conceptual site modeling 

completed as a part of the development of the proposed HRS addition is included in Section 8.1.b of the Technical 

Support Document (TSD).

Page C-2 
Table C-1 

Initial Conceptual Site Scenario Evaluation 

Factor Categories Impacting Potential Site Scores

Potential Number 

of VI Sites Eligible 

for Listing on the 

NPL

Population Density/Individual 

Risk Balance

Minimum Level of Impact Required to Score a Site 

(i.e., Greater than EPA Removal action) 

(Assumes 3 people per residence) 

Likelihood of Intrusion

Scenario 1 – Consider only releases above health based 

benchmarks (Score only Level I Exposed People)

• Observed Release Score = 550

• Need 23 Level I exposed people to reach 28.5

• Assumes Waste Characteristics Value of 32

Low

Places emphasis on populations 

with high individual risk; 

minimizes impact on population 

density

Minimum population needed to qualify for the NPL

• 23 People exposed to contamination above a health 

based benchmark, or

• 8 Homes with documented contamination above a health 

based benchmark, or

• 1 Multi-story building with 8 lower-floor contaminated 

apartments, or

• 1 Daycare facility with 23 attendees

Scenario 2 – Consider all releases greater than background 

(Score Level I and II Exposed People)

• Observed Release Score = 550

• Need 23 Level I exposed people; or

• Need 231 Level II exposed people; or

• Combination of Level I / II exposed people to reach 28.50

• Assumes Waste Characteristics Value of 32

Medium

Places emphasis on populations 

with high and medium individual 

risk; equal balance on population 

and risk

Minimum number of people exposed to contamination, but 

below a health based benchmark, to qualify for the NPL

• 231 People exposed to contamination, but below a 

health based benchmark

• 1 Ten story building with 8 apartments per floor , or

• 1 Daycare facility with 231 attendees

Scenario 3 – Consider all releases or potential releases (Score 

Level I, II and Potential Exposed People)

• Potential Intrusion Score = 500

• Need 2,850 potential targets; or

• Combination of Level I / II and Potential exposed people to 

reach 28.50

• Assumes Waste Characteristics Value of 32

High

Places emphasis on populations 

with high, medium and low 

individual risk; maximizes impact 

of population density

Minimum number of potentially contaminated people to 

qualify for the NPL

• 2,850 Potential exposed people, or

• 950 Potentially contaminated homes, or

• 4 Twenty story buildings with 12 apartments per floor 

and documented contamination only in the unoccupied 

basement

Page C-3 
Table C-1 (cont.)

Initial Conceptual Site Scenario Evaluation

Factor Categories Impacting Potential Site Scores

Potential Number 

of VI Sites Eligible 

for Listing on the 

NPL

Population Density/Individual 

Risk Balance

Minimum Level of Impact Required to Score a Site 

(i.e., Greater than EPA Removal action) 

(Assumes 3 people per residence) 

Waste Characteristics / Waste Quantity

Scenario 1 – Use a minimum waste quantity value of 10 

(Consistent with existing soil exposure pathway)

• Need 41 Level I exposed people; or

• Need 446 Level II exposed people; or

• Need 5,210 Potential exposed people; or

• Combination of Level I / II and Potential exposed people to 

reach 28.50

• Assumes Toxicity Value of 10,000 (e.g., Vinyl Chloride)

• Assumes Waste Characteristics Value of 18

Low

Places same emphasis on 

population and individual risk as 

the soil exposure pathway

Increases the number of exposed people required for a site 

to qualify for the NPL

• 14 Homes with documented contamination above a 

health based benchmark, or

• 149 Homes with documented contamination, but below 

a health based benchmark, or

• 1,737 Potentially contaminated homes

Scenario 2 – Use a minimum waste quantity value of 100 

(Consistent with existing ground water, surface water and air 

migration pathways)

• Need 23 Level I exposed people; or

• Need 231 Level II exposed people; or

• Need 2,850 Potential exposed people; or

• Combination of Level I / II and Potential exposed people to 

reach 28.50

• Assumes Toxicity Value of 10,000 (e.g., Vinyl Chloride)

• Assumes Waste Characteristics Value of 32

Medium

Places same emphasis on 

population and individual risk as 

the ground water, surface water 

and air pathways

Decreases the number of exposed people required to score 

28.50 relative to the existing soil exposure pathway; 

however, maintains the number of exposed people required 

relative to the other existing pathways

• 8 Homes with documented contamination above a health 

based benchmark, or

• 77 Homes with documented contamination, but below a 

health based benchmark, or

• 950 Potentially contaminated homes

Page C-4 
Table C-1 (cont.)

Initial Conceptual Site Scenario Evaluation

Factor Categories Impacting Potential Site Scores

Potential Number of 

VI Sites Eligible for 

Listing on the NPL

Population 

Density/Individual Risk 

Balance

Minimum Level of Impact Required to Score a Site (i.e., Greater 

than EPA Removal action) 

(Assumes 3 people per residence) 

Zones of Contamination

Scenario 1 – Consider only people within the Area of 

Contamination (AOC)

• Use weighting factors of 10 and 1

Low

Places emphasis on 

populations with high 

individual risk; minimizes 

impact on population density

Minimum population needed to qualify for the NPL

• 23 People exposed to contamination above a health based 

benchmark, or

• 8 Homes with documented contamination above a health based 

benchmark, or

• 1 Multi-story building with 8 lower-floor contaminated apartments, 

or

• 1 Daycare facility with 23 attendees

Scenario 2 – Consider people within the AOC and 

Zone of Subsurface Contamination (ZOC)

• Use weighting factors of 10, 1, 0.1

Medium

Places emphasis on 

populations with high and 

medium individual risk; 

equal balance on population 

and risk

Minimum number of people exposed to contamination, but below a 

health based benchmark, to qualify for the NPL

• 231 People exposed to contamination, but below a health based 

benchmark

• 77 Contaminated homes, but below a health based benchmark, or

• 1 Ten story building with 8 apartments per floor , or

• 1 Daycare facility with 231 attendees

Scenario 3 – Consider people within the AOC, ZOC 

and Potential Exposure Zone (PEZ)

• Use weighting factors 10, 1, 0.1 and Dispersion 

Weighted / 10

High

Places emphasis on 

populations with high, 

medium and low individual 

risk; maximizes impact of 

population density

Minimum number of potentially contaminated people to qualify for the 

NPL

• 950 Potentially contaminated homes, or

• 2,850 Potential exposed people, or

• 4 Twenty story buildings with 12 apartments per floor and 

documented contamination only in the unoccupied basement

• Requires greater amounts of sampling to identify to qualify for the 

NPL

Scenario 4 – Do not infer contamination Low

Increases emphasis on 

population with measured 

high individual risk

Page C-5 
Table C-1 (cont.)

Initial Conceptual Site Scenario Evaluation

Factor Categories Impacting Potential Site Scores

Potential Number of 

VI Sites Eligible for 

Listing on the NPL

Population 

Density/Individual Risk 

Balance

Minimum Level of Impact Required to Score a Site (i.e., Greater 

than EPA Removal action) 

(Assumes 3 people per residence) 

Levels of Contamination

Scenario 1 – Only count Level I exposed people Low

Places emphasis on 

populations with high 

individual risk; minimizes 

impact on population density

Minimum population needed to qualify for the NPL

• 23 People exposed to contamination above a health based 

benchmark, or

• 8 Homes with documented contamination above a health based 

benchmark, or

• 1 Multi-story building with 8 lower-floor contaminated apartments, 

or

• 1 Daycare facility with 23 attendees

Scenario 2 – Only Count Level I and II exposed 

people Medium

Places emphasis on 

populations with high and 

medium individual risk; 

equal balance on population 

and risk

Minimum number of people exposed to contamination, but below a 

health based benchmark, to qualify for the NPL

• 231 People exposed to contamination, but below a health based 

benchmark

• 77 Contaminated homes, but below a health based benchmark, or

• 1 Ten story building with 8 apartments per floor , or

• 1 Daycare facility with 231 attendees

Scenario 3 – Count Level I / II and Potential exposed 

people High

Places emphasis on 

populations with high, 

medium and low individual 

risk; maximizes impact of 

population density

Minimum number of potentially contaminated people to qualify for the 

NPL

• 2,850 Potential exposed people, or

• 950 Potentially contaminated homes, or

• 4 Twenty story buildings with 12 apartments per floor and 

documented contamination only in the unoccupied basement

Assumptions/ Explanations

1. Several of the weighting factors presented in Table C-1 were preliminary considerations only, and were revised as part of the second phase of conceptual site testing as discussed in 

Section 8.0 of the TSD. 

Page C-6 
Page C-7 

Table C-2 

Sample HRS Site Scores Based on Potential Exposures Only and Actual Hazardous Waste Quantity Values

Assumptions / Explanations

1. People per square mile

2. Number of Homes = Population density / 2.6 (average people per household; Source: US Census 2005-2009 http://quickfacts.census.gov/qfd/states/00000.html);

rounded up to nearest whole integer

3. Total Volume = [Number of homes x 1300sf (average basal footprint) x 8ft (average ceiling height)] / 27 (cubic feet to cubic yard conversion)

4. Source Waste Quantity = Total Volume / 2.5

5. Pathway Waste Quantity: Referenced from HRS Table 2-6

6. Contaminant: Sample contaminants chosen based on varying volatility/mobility factors

7. Toxicity: Referenced from HRS Section 2.4.1.1

8. Mobility: Referenced from Proposed Rule Table 5-17 (Based on Contaminant Vapor Pressure)

9. Waste Characteristics Factor Category Value: Referenced from HRS Table 2-7 (Subject to a Maximum of 100)

10. Exposed Individual: Assumes all targets are located within the ASC (i.e., no Level I or II targets)

11. Target Weighting Factor - Assumes entire population within the ASC (1 square mile) are subject to the same contamination concentration and therefore the same weighting factor

12. Population Score = Population Density x 1 square mile x Target Weighting Factor

13. Total Target Points = Exposed Individual + Population Score (Assumes no worker targets and no resources present)

14. Site Score - Assumes only SsI component is being scored

• Italicized indicates subject to a maximum score of 50 (Based on a pathway score of 100)

• Boldface indicates a site score greater than 28.50

ASC = area of subsurface contamination
Page C-8 

Table C-3 

Sample HRS Site Scores Based on Potential Exposures Only and Minimum Hazardous Waste Quantity Values
Page C-9 

Table C-3 (cont.) 

Sample HRS Site Scores Based on Potential Exposures Only and Minimum Hazardous Waste Quantity Values

Assumptions / Explanations

1. People per square mile

2. Number of Homes = Population density / 2.6 (average people per household; Source: US Census 2005-2009 http://quickfacts.census.gov/qfd/states/00000.html); rounded up to nearest 

whole integer

3. Pathway Waste Quantity: Subject to a minimum of 10 or 100

4. Contaminant: Sample contaminants chosen based on varying volatility/mobility factors

5. Toxicity: Referenced from HRS Section 2.4.1.1

6. Mobility: Referenced from Proposed Rule Table 5-17 (Based on Contaminant Vapor Pressure)

7. Waste Characteristics Factor Category Value: Referenced from HRS Table 2-7 (Subject to a Maximum of 100)

8. Exposed Individual: Assumes all targets are located within the ASC (i.e., no Level I or II targets)

9. Target Weighting Factor - Assumes entire population within the ASC (1 square mile) are subject to the same contamination concentration and therefore the same weighting factor

10. Population Score = Population Density x 1 square mile x Target Weighting Factor

11. Total Target Points = Exposed Individual + Population Score (Assumes no worker targets and no resources present)

12. Site Score - Assumes only SsI component is being scored (i.e., soil exposure component, ground water migration pathway, surface water migration pathway and air migration pathway 

are not scored)

• Italicized indicates subject to a maximum score of 50 (Based on a pathway score of 100) 

• Boldface indicates a site score greater than 28.50

ASC = area of subsurface contamination
Page C-10

Table C-4 

Sample HRS Site Scores Based on Potential Exposures Only with Consideration of Subslab Sampling Data 

Assumptions / Explanations

1. People per square mile, assuming one square mile

2. Number of Homes = Population density / 2.6 (average people per household; Source: US Census 2005-2009 http://quickfacts.census.gov/qfd/states/00000.html); rounded up to nearest whole integer

3. Pathway Waste Quantity: Subject to a minimum of 10 or 100

4. Toxicity: Referenced from HRS Section 2.4.1.1

5. Mobility: Referenced from Proposed Rule Table 5-17 (Based on Contaminant Vapor Pressure)

6. Waste Characteristics Factor Category Value: Referenced from HRS Table 2-7 (Subject to a Maximum of 100)

7. Exposed Individual: Assumes all targets are located within the area of subsurface contamination (ASC) (i.e., no Level I or II targets)

8. Level I Target Points: Number of people subject to Level I concentrations x 10

9. Level II Target Points: Number of people subject to Level II concentrations x 1

10. Subslab Target Points: Number of people in the ASC with subslab concentrations 10x great than an appropriate benchmark x 0.75

11. Non-Subslab Target Points: All other individuals in the ASC x 0.1

12. Total Target Points = Exposed Individual + Population Score (Assumes no worker targets and no resources present)

13. Site Score - Assumes only SsI component is being scored (i.e., soil exposure component, ground water migration pathway, surface water migration pathway and air migration pathway are not scored)

• Italicized indicates subject to a maximum score of 50 (Based on a pathway score of 100)

• Boldface indicates a site score greater than 28.50
Table C-5 

Sample HRS Site Scores Based on Different Population Densities and Actual and Potential Exposures 

Subslab Target 

Points 10 

(Weighting 

Factor = 0.75)

Non-Subslab 

Target Points 11 

(Weighting 

Factor = 0.1)

500 192 100 10,000 1 1.00E+06 32 50 40 0 0 50 140 14.89

1,000 385 100 10,000 1 1.00E+06 32 50 40 0 0 100 190 20.22

5,000 1,923 100 10,000 1 1.00E+06 32 50 40 0 0 500 590 50 (62.89)

500 192 100 10,000 1 1.00E+06 32 50 40 0 74 40 204 21.77

1,000 385 100 10,000 1 1.00E+06 32 50 40 0 149 80 319 34.04

5,000 1,923 100 10,000 1 1.00E+06 32 50 40 0 749 400 1,239 50 (132.17)

500 192 100 10,000 1 1.00E+06 32 50 40 0 50 198 338 36.05

1,000 385 100 10,000 1 1.00E+06 32 50 40 0 100 398 588 50 (62.72)

5,000 1,923 100 10,000 1 1.00E+06 32 50 40 0 500 1,998 2,588 50 (276.05)

Scenario

Population 

Density1

Targets

Site Score (SsI 

Only)13 

(Likelihood of 

Exposure = 550)

Pathway 

Waste 

Quantity 3 Toxicity 4 Mobility 5

Pathway WQ x 

Toxicity x 

Mobility

Waste 

Characteristics 

Factor Value 6

Exposed 

Individual 7

2. Four Level I homes within the AOE with the remaining homes located in the

ASC where 20% have subslab contaminant concentrations 10x greater than an

appropriate benchmark.

3. Four Level I homes within the AOE with the remaining homes located in the

ASC where 50% have subslab contaminant concentrations 10x greater than an

appropriate benchmark.

Population

Total Target 

Points 12

Level I 

Target 

Points 8 

(Weighting 

Factor = 10)

Level II 

Target 

Points 9 

(Weighting 

Factor = 1)

Population on ASC

1. Four Level I homes within the AOE with the remaining homes located in the

ASC where no subslab data is available.

Number of 

Homes2

Waste Characteristics (WC)

Assumptions / Explanations

1. People per square mile, assuming one square mile

2. Number of Homes = Population density / 2.6 (average people per household; Source: US Census 2005-2009 http://quickfacts.census.gov/qfd/states/00000.html); rounded up to nearest whole integer

3. Pathway Waste Quantity: Subject to a minimum of 10 or 100

4. Toxicity: Referenced from HRS Section 2.4.1.1

5. Mobility: Referenced from Proposed Rule Table 5-17 (Based on Contaminant Vapor Pressure)

6. Waste Characteristics Factor Category Value: Referenced from HRS Table 2-7 (Subject to a Maximum of 100)

7. Exposed Individual: Assumes all targets are located within the ASC (i.e., no Level I or II targets)

8. Level I Target Points: Number of people subject to Level I concentrations x 10

9. Level II Target Points: Number of people subject to Level II concentrations x 1

10. Subslab Target Points: Number of people in the ASC with subslab concentrations 10x great than an appropriate benchmark x 0.75

11. Non-Subslab Target Points: All other individuals in the ASC x 0.1

12. Total Target Points = Exposed Individual + Population Score (Assumes no worker targets and no resources present)

13. Site Score - Assumes only SsI component is being scored (i.e., soil exposure component, ground water migration pathway, surface water migration pathway and air migration pathway are not scored)

• Italicized indicates subject to a maximum score of 50 (Based on a pathway score of 100)

• Boldface indicates a site score greater than 28.50

ASC = area of subsurface contamination

Page C-11
Table C-6 

Sample HRS Site Scores Based on a Combination of the Ground Water Migration Pathway and the Subsurface Intrusion Component of the Soil Exposure and Subsurface Intrusion Pathway

Nearest 

Individual

Level I Target 

Points

Level II 

Target 

Points

Potential 

Target 

Points

Pathway 

Waste 

Quantity6

WC Factor 

Value6

Subslab Target 

Points10 

(Weighting 

Factor = 0.75)

Non-Subslab 

Target Points11 

(Weighting 

Factor = 0.1)

50 - - 550 32 50 104 0 36 40.53 20.26 0 0 0 0 0 0 0 0 0 0 0 20.26

100 - - 550 32 50 104 0 53 44.07 22.03 0 0 0 0 0 0 0 0 0 0 0 22.03

250 - - 550 32 50 104 0 171 69.35 34.67 0 0 0 0 0 0 0 0 0 0 0 34.67

500 - - 550 32 50 104 0 360 100 (110) 50 .00 (55.00) 0 0 0 0 0 0 0 0 0 0 0 50.00 (55.00)

50 - - 550 32 45 0 10.4 36 19.49 9.74 0 0 0 0 0 0 0 0 0 0 0 9.74

100 - - 550 32 45 0 10.4 53 23.04 11.52 0 0 0 0 0 0 0 0 0 0 0 11.52

250 - - 550 32 45 0 10.4 171 48.32 24.16 0 0 0 0 0 0 0 0 0 0 0 24.16

500 - - 550 32 45 0 10.4 360 88.64 44.32 0 0 0 0 0 0 0 0 0 0 0 44.32

50 19 50 550 32 50 104 0 36 40.53 20.26 500 10 18 20 0 0 0 5 25 2.72 1.36 20.31

100 38 100 550 32 50 104 0 53 44.07 22.03 500 10 18 20 0 0 0 10 30 3.27 1.63 22.09

250 96 250 550 32 50 104 0 171 69.35 34.67 500 10 18 20 0 0 0 25 45 4.90 2.45 34.76

500 192 500 550 32 50 104 0 360 100 (110) 50 .00 (55.00) 500 10 18 20 0 0 0 50 70 7.63 3.81 50.15 (55.13)

50 19 50 550 32 45 0 10.4 36 19.49 9.74 500 10 18 20 0 0 0 5 25 2.72 1.36 9.83

100 38 100 550 32 45 0 10.4 53 23.04 11.52 500 10 18 20 0 0 0 10 30 3.27 1.63 11.63

250 96 250 550 32 45 0 10.4 171 48.32 24.16 500 10 18 20 0 0 0 25 45 4.90 2.45 24.28

500 192 500 550 32 45 0 10.4 360 88.64 44.32 500 10 18 20 0 0 0 50 70 7.63 3.81 44.48

50 19 50 550 32 50 104 0 36 40.53 20.26 500 100 32 20 0 0 8 4 32 6.10 3.05 20.49

100 38 100 550 32 50 104 0 53 44.07 22.03 500 100 32 20 0 0 15 8 43 8.33 4.16 22.42

250 96 250 550 32 50 104 0 171 69.35 34.67 500 100 32 20 0 0 38 20 78 15.03 7.51 35.48

500 192 500 550 32 50 104 0 360 100 (110) 50 .00 (55.00) 500 100 32 20 0 0 75 40 135 26.18 13.09 51.69 (56.54)

50 19 50 550 32 50 104 0 36 40.53 20.26 500 100 32 20 0 0 19 3 41 8.00 4.00 20.65

100 38 100 550 32 50 104 0 53 44.07 22.03 500 100 32 20 0 0 38 5 63 12.12 6.06 22.85

250 96 250 550 32 50 104 0 171 69.35 34.67 500 100 32 20 0 0 94 13 126 24.48 12.24 36.77

500 192 500 550 32 50 104 0 360 100 (110) 50 .00 (55.00) 500 100 32 20 0 0 188 25 233 45.09 22.54 54.85 (59.44)

50 19 50 550 32 50 104 0 36 40.53 20.26 550 100 32 50 104 0 0 0 154 32.85 16.42 26.08

100 38 100 550 32 50 104 0 53 44.07 22.03 550 100 32 50 104 0 0 0 154 32.85 16.42 27.48

250 96 250 550 32 50 104 0 171 69.35 34.67 550 100 32 50 104 0 0 0 154 32.85 16.42 38.36

500 192 500 550 32 50 104 0 360 100 (110) 50 .00 (55.00) 550 100 32 50 104 0 0 0 154 32.85 16.42 52.63 (57.40)

50 19 50 550 32 50 104 0 36 40.53 20.26 550 100 32 50 104 0 0 5 159 33.92 16.96 26.42

100 38 100 550 32 50 104 0 53 44.07 22.03 550 100 32 50 104 0 0 10 164 34.98 17.49 28.13

250 96 250 550 32 50 104 0 171 69.35 34.67 550 100 32 50 104 0 0 25 179 38.18 19.09 39.58

500 192 500 550 32 50 104 0 360 100 (110) 50 .00 (55.00) 550 100 32 50 104 0 0 50 204 43.52 21.76 54.53 (59.15)

50 19 50 550 32 50 104 0 36 40.53 20.26 550 100 32 50 104 0 8 4 166 35.30 17.65 26.87

100 38 100 550 32 50 104 0 53 44.07 22.03 550 100 32 50 104 0 15 8 177 37.76 18.88 29.01

250 96 250 550 32 50 104 0 171 69.35 34.67 550 100 32 50 104 0 38 20 212 45.12 22.56 41.36

500 192 500 550 32 50 104 0 360 100 (110) 50 .00 (55.00) 550 100 32 50 104 0 75 40 269 57.38 28.69 57.65 (62.03)

50 19 50 550 32 50 104 0 36 40.53 20.26 550 100 32 50 104 0 19 3 175 37.38 18.69 27.56

100 38 100 550 32 50 104 0 53 44.07 22.03 550 100 32 50 104 0 38 5 197 41.92 20.96 30.41

250 96 250 550 32 50 104 0 171 69.35 34.67 550 100 32 50 104 0 94 13 260 55.52 27.76 44.41

500 192 500 550 32 50 104 0 360 100 (110) 50 .00 (55.00) 550 100 32 50 104 0 188 25 367 78.18 39.09 63.47 (67.48)

Population on ASC

1. Four homes subject to Level I ground water concentrations, potential ground 

water targets and NO SsI component evaluated.

2. Four homes subject to Level II ground water concentrations, potential ground 

water targets and NO SsI component evaluated.

3. Four homes subject to Level I ground water concentrations, potential ground 

water targets and SsI component evaluated for ASC only. Assumes none of the 

ASC homes have subslab contaminant concentrations 10x greater than an 

appropriate benchmark and WQ minimum of 10 for SESsI pathway.

4. Four homes subject to Level II ground water concentrations, potential ground 

water targets and SsI component evaluated for ASC only. Assumes none of the 

ASC homes have subslab contaminant concentrations 10x greater than an 

appropriate benchmark and WQ minimum of 10 for SESsI pathway.

5. Four homes subject to Level I ground water concentrations, potential ground 

water targets and SsI component evaluated for ASC only. Assumes 20% of ASC 

homes have subslab contaminant concentrations 10x greater than an appropriate 

benchmark and WQ minimum of 100 for SESsI pathway.

Targets

7. Four homes subject to Level I ground water concentrations, potential ground 

water targets, four homes subject to Level I SsI concentrations and no ASC 

evaluated.

8. Four homes subject to Level I ground water concentrations, potential ground 

water targets, four homes subject to Level I SsI concentrations and ASC 

evaluated assuming none of the ASC homes have subslab contaminant 

concentrations 10x greater than an appropriate benchmark and WQ minimum of 

100 for SESsI pathway.

9. Four homes subject to Level I ground water concentrations, potential ground 

water targets, four homes subject to Level I SsI concentrations and ASC 

evaluated assuming 20% of ASC homes have subslab contaminant 

concentrations 10x greater than an appropriate benchmark and WQ minimum of 

100 for SESsI pathway.

10. Four homes subject to Level I ground water concentrations, potential ground 

water targets, four homes subject to Level I SsI concentrations and ASC 

evaluated assuming 50% of ASC homes have subslab contaminant 

concentrations 10x greater than an appropriate benchmark and WQ minimum of 

100 for SESsI pathway.

Number of Homes in ASC2

6. Four homes subject to Level I ground water concentrations, potential ground 

water targets and SsI component evaluated for ASC only. Assumes 50% of ASC 

homes have subslab contaminant concentrations 10x greater than an appropriate 

benchmark and WQ minimum of 100 for SESsI pathway.

Ground Water Pathway13

Site Score (GW Only)

Total Target 

Points12 Scenario Population Density

Site Score (Combined GW and SESsI)

1,1A

Likelihood of ReleaseWaste Characteristics4Number of People in ASC3GW Pathway ScoreSite Score (SESsI Only)

Soil Exposure and Subsurface Intrusion Pathway Score14

Likelihood of Exposure5

Waste Characteristics 

(WC)

SESsI Pathway Score

Level II 

Target 

Points9 

(Weighting 

Factor = 1)

Targets

Level I 

Target 

Points8 

(Weighting 

Factor = 10)

Exposed 

Individual7

Population

Page C-12
Assumptions / Explanations

1. People per square mile

1A. Assumes ground water (GW) pathway potential population density is uniform for entire TDL and all individuals are on private wells

2. Number of Homes = Population density / 2.6 (average people per household; Source: US Census 2005-2009 http://quickfacts.census.gov/qfd/states/00000.html); rounded up to nearest whole integer

3. Population in ASC assuming one square mile

4. GW Waste Characteristics: Assumed to be 32 (i.e., Toxicity = 10,000)

5. SsI Likelihood of Exposure: Assumed to be 500 for scenarios where SESsI Pathway is evaluated except where Level I concentrations are present when WC = 550 

6. SsI Waste Characteristics Factor Value: Referenced from HRS Table 2-7 (Subject to a Maximum of 100) Assumes Waste Quantity of 10 or 100)

7. Exposed Individual: Assumes all targets are located within the ASC (i.e., no Level I or II targets)

8. Level I Target Points: Number of people subject to Level I concentrations x 10

9. Level II Target Points: Number of people subject to Level II concentrations x 1

10. Subslab Target Points: Number of people in the ASC with subslab concentrations 10x great than an appropriate benchmark x 0.75

11. Non-Subslab Target Points: All other individuals in the ASC x 0.1

12. Total Target Points = Exposed Individual + Population Score (Assumes no worker targets and no resources present)

13. Ground Water Pathway = Assumes only drinking water threat is evaluated

14. SESsI Pathway Score: Assumes only SsI Component is being scored

• Italicized indicates subject to a maximum

• Boldface indicates a site score greater than 28.50

ASC = area of subsurface contamination

Page C-13
Table C-7a 

Proposed Conceptual Site Scenario Evaluation 

Factor Categories Impacting Potential Site Scores

Potential Number 

of VI Sites Eligible 

for Listing on the 

NPL

Population Density/Individual 

Risk Balance

Minimum Level of Impact Required to Score a Site 

(i.e., Greater than EPA Removal action) 

(Assumes 3 people per residence) 

Likelihood of Exposure

Proposed Option

Scoring all Actual and Potential Exposures at site 

Actually Exposed People

• Level I (actually exposed people at levels above healthbased benchmarks)

• Level II (actually exposed people but sampling does not

show levels above health-based benchmarks)

Potentially Exposed People

• People living or working above a soil gas or ground water

plume

High

Places emphasis on populations 

with high, medium and low 

individual risk; maximizes impact 

of population density

Population needed to qualify for the NPL

• 23 Level I exposed people, (e.g., 8 homes, a daycare

center with 23 students and staff, 69 full-time workers,

or 138 part-time workers), or

• 230 Level II exposed people, or

• 371 people living or working above a soil gas plume, or

various combinations of Level I exposed people, Level

II exposed people, and potentially exposed people

• (Assumes a toxicity of 10,000 and a Hazardous Waste

Quantity score of 100 for actually exposed people and

10,000 for potentially exposed people)

Waste Characteristics / Waste Quantity

Proposed Option

• Use a minimum waste quantity value of 100 and an

estimate of 1,740 square feet per regularly occupied

structure or subunit unless actual square footage available Medium

Places similar emphasis on 

population and individual risk as 

the ground water, surface water 

and air pathways

• Decreases the number of exposed people required to

score 28.50 relative to the existing soil exposure

pathway; however, maintains the number of exposed

people required relative to the other existing pathways.

• Based on an area of 1,740 square feet per residence, need

1 home to reach a Hazardous Waste Quantity score of

100 and 75 regularly occupied structures or subunits to

reach a Hazardous Waste Quantity score of 10,000

Zones of Exposure

Proposed Option

Consider people within the AOE and ASC

• AOE: area of observed exposure (area delineated by regularly

occupied structures with actual contamination)

• ASC: area of subsurface contamination (area delineated by

regularly occupied structures above soil gas or ground water

contamination)

• (Uses weighting factors of 10, 1 and between 0.1 and 0.9 for

Level I, Level II and potentially exposed people)

Medium

Places emphasis on populations 

with high and medium individual 

risk; equal balance on population 

and risk

• Includes all people exposed to contamination, above or

below a health-based benchmark, and those where

intrusion is considered to be presently possible

(potentially exposed)

Levels of Exposure

Proposed Option

Count Level I, Level II and Potentially Exposed people

Actually Exposed People:

• Level I (actually exposed people at levels above health-based

benchmarks),

• Level II (actually exposed people but sampling does not show

levels above health-based benchmarks)

Potentially Exposed People 

• People living or working in regularly occupied structures

above a soil gas or ground water plume

High

Places emphasis on populations 

with high, medium and low 

individual risk; maximizes impact 

of population density

• Maximum number of people needed in each Level to

reach a score of 28.50

• 23 People exposed to contamination above a healthbased benchmark (Level I)

• 223 People exposed to contamination but sampled levels

not above a health-based benchmark (Level II)

• 225 Potentially Exposed People in structures with crawl

space contamination or subslab contamination

• 371 Potentially Exposed People in structures above

shallow soil gas contamination

• 741 Potentially Exposed People in structures above a

ground water plume

Page C-14
Table C-7b 

Sample HRS Site Scores Based on Potential Exposures Only with Consideration of Different Population Densities and Varying Types of SsI Sampling Data

Data 

Scenario Population Density1 Number of Homes2

Waste Characteristics (WC) Targets Site Score (SsI Only)11

Pathway Waste Quantity3Toxicity4

Biodegradation5

Pathway WQ x Toxicity x BiodegradationWaste Characteristics Factor Value6Exposed Individual7

Population

Total Target Points10Level I Target PointsIf Likelihood of Exposure = 180If Likelihood of Exposure = 320If Likelihood of Exposure = 500

8

(Weighting Factor = 10)Level II Target Points9 (Weighting Factor = 1)

Population on ASC

Crawl Space samples (Weighting Factor = 0.09)Subslab samples (Weighting Factor = 0.4)Soil Gas samples (Weighting factor = 0.2)Ground Water Samples (Weighting Factor = 0.1)

1. All targets located within the ASC (No Level I or II

Concentrations). All crawl space samples (Assumes WQ of 10,000)

100 38 100 1,000 1 1.00E+05 18 20 0 0 90 0 0 0 110 2.16 3.84 6.00

500 192 10,000 1,000 1 1.00E+07 56 20 0 0 450 0 0 0 470 28.71 51.04 79.76

1,000 385 10,000 1,000 1 1.00E+07 56 20 0 0 900 0 0 0 920 56.20 99.92 156.12

5,000 1,923 10,000 1,000 1 1.00E+07 56 20 0 0 4500 0 0 0 4,520 276.13 490.90 767.03

2. All targets located within the ASC (No Level I or II

Concentrations). All subslab samples (Assumes WQ of 10,000)

100 38 100 1,000 1 1.00E+05 18 20 0 0 0 40 0 0 60 1.18 2.09 3.27

500 192 10,000 1,000 1 1.00E+07 56 20 0 0 0 200 0 0 220 13.44 23.89 37.33

1,000 385 10,000 1,000 1 1.00E+07 56 20 0 0 0 400 0 0 420 25.66 45.61 71.27

5,000 1,923 10,000 1,000 1 1.00E+07 56 20 0 0 0 2000 0 0 2,020 123.40 219.38 342.79

3. All targets located within the ASC (No Level I or II

Concentrations). All soil gas samples (Assumes WQ of 10,000)

100 38 100 1,000 1 1.00E+05 18 20 0 0 0 0 20 0 40 0.79 1.40 2.18

500 192 10,000 1,000 1 1.00E+07 56 20 0 0 0 0 100 0 120 7.33 13.03 20.36

1,000 385 10,000 1,000 1 1.00E+07 56 20 0 0 0 0 200 0 220 13.44 23.89 37.33

5,000 1,923 10,000 1,000 1 1.00E+07 56 20 0 0 0 0 1000 0 1,020 62.31 110.78 173.09

4. All targets located within the ASC (No Level I or II

Concentrations). All ground water samples (Assumes WQ of

10,000)

100 38 100 1,000 1 1.00E+05 18 20 0 0 0 0 0 10 30 0.59 1.05 1.64

500 192 10,000 1,000 1 1.00E+07 56 20 0 0 0 0 0 50 70 4.28 7.60 11.88

1,000 385 10,000 1,000 1 1.00E+07 56 20 0 0 0 0 0 100 120 7.33 13.03 20.36

5,000 1,923 10,000 1,000 1 1.00E+07 56 20 0 0 0 0 0 500 520 31.77 56.48 88.24

Assumptions / Explanations

1. People per square mile, assuming one square mile

2. Number of Homes = Population density / 2.6 (average people per household; Source: US Census 2005-2009 http://quickfacts.census.gov/qfd/states/00000.html); rounded up to nearest whole integer

3. Pathway Waste Quantity: 10,000 based on number of homes and default tier D square footage

4. Toxicity: Referenced from HRS Section 2.4.1.1 for TCE

5. Biodegradation for non-readily biodegradable substance

6. Waste Characteristics Factor Category Value: Referenced from HRS Table 2-7 (Subject to a Maximum of 100)

7. Exposed Individual: Assumes all targets are located within the ASC (i.e., no Level I or II targets)

8. Level I Target Points: Number of people subject to Level I concentrations x 10

9. Level II Target Points: Number of people subject to Level II concentrations x 1

10. Total Target Points = Exposed Individual + Population Score (Assumes no worker targets and no resources present)

11. Site Score - Assumes only SsI component is being scored (i.e., soil exposure component, ground water migration pathway, surface water migration pathway and air migration pathway are not scored)

ASC = area of subsurface contamination

Page C-15
Table C-7c

Sample HRS Site Scores Based on Different Population Densities, a Mix of Actual and Potential Exposures and Different Types of SsI Sampling Data

Scenario

Population 

Density1

Number of 

Homes2

Waste Characteristics (WC) Targets

Site Score 

(SsI Only)11 

(Likelihood 

of Exposure = 

550)

Pathway 

Waste 

Quantity3 Toxicity4

Pathway WQ x 

Toxicity

Waste 

Characteristics 

Factor Value6

Exposed Individual7

Population

Total Target 

Points10

Level I Target Points8(Weighting Factor = 10)Level II Target Points9 (Weighting Factor = 1)

Population on ASC

Biodegradation5

Crawl Space Samples(Weighting Factor = 0.9)Subslab Samples(Weighting Factor = 0.4)Soil Gas Samples (Weighting = 0.2)Ground Water Samples (Weighting = 0.1)

1. Three Level I homes within the AOE with the remaining

homes located in the ASC with crawl space samples

100 38 100 1,000 1 1.00E+05 18 50 78 0 87.30 0.00 0.00 0.00 215 12.92

250 96 10,000 1,000 1 1.00E+07 56 50 78 0 222.30 0.00 0.00 0.00 350 65.39

500 192 10,000 1,000 1 1.00E+07 56 50 78 0 447.30 0.00 0.00 0.00 575 107.39

2.Three Level I homes within the AOE with the remaining

homes located in the ASC with subslab samples.

100 38 100 1,000 1 1.00E+05 18 50 78 0 0.00 38.80 0.00 0.00 167 10.01

250 96 10,000 1,000 1 1.00E+07 56 50 78 0 0.00 98.80 0.00 0.00 227 42.34

500 192 10,000 1,000 1 1.00E+07 56 50 78 0 0.00 198.80 0.00 0.00 327 61.00

3.Three Level I homes within the AOE with the remaining

homes located in the ASC with soil gas samples.

100 38 100 1,000 1 1.00E+05 18 50 78 0 0.00 0.00 19.40 0.00 147 8.84

250 96 10,000 1,000 1 1.00E+07 56 50 78 0 0.00 0.00 49.40 0.00 177 33.11

500 192 10,000 1,000 1 1.00E+07 56 50 78 0 0.00 0.00 99.40 0.00 227 42.45

3. Three Level I homes within the AOE with the remaining

homes located in the ASC with ground water samples

100 38 100 1,000 1 1.00E+05 18 50 78 0 0.00 0.00 0.00 9.70 138 8.26

250 96 10,000 1,000 1 1.00E+07 56 50 78 0 0.00 0.00 0.00 24.70 153 28.50

500 192 10,000 1,000 1 1.00E+07 56 50 78 0 0.00 0.00 0.00 49.70 178 33.17

Assumptions / Explanations

1. People per square mile, assuming one square mile

2. Number of Homes = Population density / 2.6 (average people per household; Source: US Census 2005-2009 http://quickfacts.census.gov/qfd/states/00000.html); rounded up to nearest whole integer

3. Pathway Waste Quantity: 10,000

4. Toxicity: Referenced from HRS Section 2.4.1.1

5. Biodegradation for a non-readily biodegradable substance.

6. Waste Characteristics Factor Category Value: Referenced from HRS Table 2-7 (Subject to a Maximum of 100)

7. Exposed Individual: Assumes all targets are located within the ASC (i.e., no Level I or II targets)

8. Level I Target Points: Number of people subject to Level I concentrations x 10

9. Level II Target Points: Number of people subject to Level II concentrations x 1

10. Total Target Points = Exposed Individual + Population Score (Assumes no worker targets and no resources present)

11. Site Score - Assumes only SsI component is being scored (i.e., soil exposure component, ground water migration pathway, surface water migration pathway and air migration pathway are not scored)

AOE = area of observed exposure

ASC = area of subsurface contamination

Page C-16
Table C-7d 

Minimum Populations Needed to Achieve an HRS Site Score of 28.50 with Different Likelihood of Exposure, Waste Characteristics and Different Types of SsI Samples

Scenario

Likelihood of 

Exposure

Waste Characteristics (WC)

Targets

Pathway 

Score

Site 

Score

Exposed 

Individual

AOE ASC

Target 

Score

Waste 

Quantity Toxicity Biodegradation

WC 

Product

WC 

Factor 

Value

# Level I 

Targets

# Level II 

Targets

# Crawl 

Space 

Targets

# 

Subslab 

Targets

# Shallow 

Soil Gas 

Targets

# 

GW/Other 

Targets

1 550 100 1000 1 1.00E+05 18 50 43 0 0 0 0 0 480.00 57.60 28.80

2 550 10000 1000 1 1.00E+07 56 50 11 0 0 0 0 0 160.00 59.73 29.87

3 550 10000 10000 1 1.00E+08 100 50 4 0 0 0 0 0 90.00 60.00 30.00

4 550 10000 1000 1 1.00E+07 56 45 0 108 0 0 0 0 153.00 57.12 28.56

5 550 10000 10000 1 1.00E+08 100 45 0 41 0 0 0 0 86.00 57.33 28.67

6 500 10000 1000 1 1.00E+07 56 20 0 0 165 0 0 0 168.50 57.19 28.59

7 500 10000 1000 1 1.00E+07 56 20 0 0 0 370 0 0 168.00 57.02 28.51

8 500 10000 1000 1 1.00E+07 56 20 0 0 0 0 740 0 168.00 57.02 28.51

9 500 10000 1000 1 1.00E+07 56 20 0 0 0 0 0 1480 168.00 57.02 28.51

10 550 10000 1000 1 1.00E+07 56 50 10 0 0 0 100 0 170.00 63.47 31.73

Assumptions/Explanations

1. Hazardous waste quantity: Assigns a value of 100 or 10000

2. Toxicity: Assigns a value of 100, 1000 or 10000

3. Biodegradation: Assumes that a biodegradation factor value of 1 is assigned

4. Waste Characteristics Factor Category Value: Reference HRS Table 2-7 (Subject to a Maximum of 100)

5. Exposed Individual: Reference HRS Section 5.2.1.3.1

6. Level I Targets: Reference HRS Section 5.2.1.3.2.1 (multiply by 10)

7. Level II Targets: Reference HRS Section 5.2.1.3.2.2 (multiply by 1)

8. ASC Targets: Reference HRS Table 5-20 (multiply by a weighting factor assigned from Table 5-20)

9. Target Score = Exposed Individual + Level I targets + Level II targets + Crawl Space targets + Subslab targets + Shallow Soil Gas targets + GW/Other targets (multiplied by appropriate

weighting factors)

10. S ite Score: Assumes only SsI component is being scored (i.e., soil exposure component, ground water migration pathway, surface water migration pathway and air migration pathway are not scored)

11. AOE = area of observed exposure, ASC = area of subsurface contamination, GW = ground water

12. Subslab samples are assumed to be at or within 5 feet of a structure’s foundation

13. Shallow soil gas are assumed to be greater than 5 feet from a structure’s foundation and at a depth less than or equal to 30 feet below ground surface

14. Ground water samples are assumed to be greater than 5 feet from a structure’s foundation

Page C-17
Table C-7e 

Minimum Actually Exposed Population Needed to Achieve an HRS Site Score of 28.50 Based on Varying Waste Characteristic Factor Values

Scenario

Likelihood 

of 

Exposure

Waste Characteristics (WC)

Targets

Pathway 

Score

Site 

Score

Exposed 

Individual

 AOE ASC 

Total 

Target 

Value

Waste 

Quantity Toxicity Biodegradation

WC 

Factor 

Value

# Level I 

Targets

# Level II 

Targets

# Targets 

(Crawl Space 

Samples)

# 

Targets 

(Samples 

≤5 ft)

# 

Targets 

(Samples 

≤30 ft)

# Targets 

(GW/Other 

Samples) 

1 550 100 100 1 10 50 81 0 0 0 0 0 860.00 57.33 28.67

2 550 100 1000 1 18 50 43 0 0 0 0 0 480.00 57.60 28.80

3 550 100 10000 1 32 50 22 0 0 0 0 0 270.00 57.60 28.80

4 550 10000 100 1 32 50 22 0 0 0 0 0 270.00 57.60 28.80

5 550 10000 1000 1 56 50 11 0 0 0 0 0 160.00 59.73 29.87

6 550 10000 10000 1 100 50 4 0 0 0 0 0 90.00 60.00 30.00

7 550 10000 100 1 32 45 0 223 0 0 0 0 268.00 57.17 28.59

8 550 10000 1000 1 56 45 0 225 0 0 0 0 270.00 100.80 50.40

9 550 10000 10000 1 100 45 0 225 0 0 0 0 270.00 180.00 90.00

Assumptions / Explanations

1. Likelihood of exposure: Assigns a value of 550

2. Hazardous waste quantity: Assigns a value of 100 or 10000

3. Biodegradation: Assumes that a biodegradation factor value of 1 is assigned

4. Toxicity: Assigns a value of 100, 1000 or 10000

5 . Waste Characteristics Factor Category Value: Reference HRS Table 2-7 (Subject to a Maximum of 100)

6. Exposed Individual: Reference HRS Section 5.2.1.3.1

7. ASC targets: Assumes only actually exposed populations are present

8. AOE = area of observed exposure, ASC = area of subsurface contamination, GW = ground water

9. Site Score: Assumes only SsI component is being scored (i.e., soil exposure component, ground water migration pathway, surface water migration

pathway and air migration pathway are not scored)

Page C-18
Table C-7f 

Minimum Potentially Exposed Population Needed to Achieve an HRS Site Score of 28.50 Based on Varying Waste Characteristic Factor Values 

Scenario

Likelihood 

of 

Exposure

Waste Characteristics (WC)

Targets

Pathway 

Score

Site 

Score

Exposed 

Individual

 AOE ASC 

Total 

Target 

Value

Waste 

Quantity Toxicity Biodegradation

WC 

Factor 

Value

# Level 

I 

Targets

# Level 

II 

Targets

# Targets 

(Crawl 

Space 

Samples)

# Targets 

(Samples 

≤5 ft)

# Targets 

(Samples 

≤30 ft 

bgs)

# Targets 

(GW/Other 

Samples) 

1 500 10000 100 1 32 20 0 0 305 294.50 57.12 28.56

2 500 10000 1000 1 56 20 0 0 225 222.50 75.52 37.76

3 500 10000 10000 1 100 20 0 0 225 222.50 134.85 67.42

4 500 10000 100 1 32 20 0 0 685 294.00 57.02 28.51

5 500 10000 1000 1 56 20 0 0 370 168.00 57.02 28.51

6 500 10000 10000 1 100 20 0 0 225 110.00 66.67 33.33

7 500 10000 100 1 32 20 0 0 1370 294.00 57.02 28.51

8 500 10000 1000 1 56 20 0 0 740 168.00 57.02 28.51

9 500 10000 10000 1 100 20 0 0 371 94.20 57.09 28.55

10 500 10000 10000 1 100 20 0 0 741 94.10 57.03 28.52

Assumptions / Explanations

1. Assumes the maximum potential for exposure factor value of 500

2. Assumes a hazardous waste quantity factor value of 10000 and a biodegradation factor value of 1

3. Toxicity: Assigns a value of 100, 1000 or 10000

4. Waste Characteristics Factor Category Value: Reference HRS Table 2-7 (Subject to a Maximum of 100)

5. Exposed Individual: Assumes all targets are located within the ASC (i.e., no Level I or II targets); see HRS Section 5.2.1.3.1

6. ASC targets: Reference HRS Table 5-20 (multiply by a weighting factor assigned from Table 5-20)

7. # Targets Samples ≤5 ft: references subsurface samples located at or within 5 feet vertically or horizontally from a regularly occupied structure's foundation

8. # Targets Samples ≤30 ft: references subsurface samples located less than or equal to a depth of 30 feet below ground surface

9. AOE = area of observed exposure, ASC = area of subsurface contamination, GW = ground water

10. Site Score: Assumes only SsI component is being scored (i.e., soil exposure component, ground water migration pathway, surface water migration pathway and air migration pathway are not scored)

Page C-19
Table C-7g 

Minimum Population Needed to Achieve an HRS Site Score of 28.50 Based on Varying Likelihood of Exposure and Waste Characteristics Factor Values and no ASC

Scenario

Likelihood 

of Exposure

Waste Characteristics (WC)

Targets

Pathway 

Score Site Score

Exposed 

Individual

 AOE ASC 

Total 

Target 

Value

Waste 

Quantity Toxicity Biodegradation

WC Factor 

Value

# Level I 

Targets

# Level II 

Targets

# Targets 

(Crawl Space 

Samples)

# Targets 

(Samples ≤5 

ft)

# Targets 

(Samples 

≤30 ft)

# Targets 

(GW/Other 

Samples) 

1 550 100 1000 1 18 50 43 0 0 0 0 0 480.00 57.60 28.80

2 550 10000 1000 1 56 45 0 225 0 0 0 0 270.00 100.80 50.40

3 500 10000 1000 1 56 20 0 0 225 0 0 0 222.50 75.52 37.76

4 400 10000 1000 1 56 20 0 0 225 0 0 0 222.50 60.41 30.21

5 300 10000 1000 1 56 20 0 0 289 0 0 0 280.10 57.04 28.52

6 200 10000 1000 1 56 20 0 0 450 0 0 0 425.00 57.70 28.85

7 100 10000 1000 1 56 20 0 0 911 0 0 0 839.90 57.01 28.51

8 500 10000 10000 1 100 20 0 0 225 0 0 0 222.50 134.85 67.42

9 400 10000 10000 1 100 20 0 0 225 0 0 0 222.50 107.88 53.94

10 200 10000 10000 1 100 20 0 0 239 0 0 0 235.10 56.99 28.50

Assumptions/Explanations

1. Waste quantity is assigned based on the following assumptions: 3 people per regularly occupied structure, an area of 1740 square feet per structure

2. Toxicity: Assigns a value of 1000 or 10000

3. Biodegradation: Assumes a biodegradation factor value of 1

4. Waste Characteristics Factor Category Value: Reference HRS Table 2-7 (Subject to a Maximum of 100)

5. Exposed Individual: Reference HRS Section 5.2.1.3.1

6. Level I Targets: Reference HRS Section 5.2.1.3.2.1 (multiply by 10)

7. Level II Targets: Reference HRS Section 5.2.1.3.2.2 (multiply by 1)

8. ASC Targets: Reference HRS Table 5-20 (multiply by a weighting factor assigned from Table 5-20)

9. Target Score = Exposed Individual + Level I targets + Level II targets + ASC targets (each multiplied by appropriate weighting factors)

10. S ite Score - Assumes only SsI component is being scored (i.e., soil exposure component, ground water migration pathway, surface water migration pathway and air migration pathway are not scored)

11. Assumes there are no workers or resources present

12. AOE = area of observed exposure, ASC = area of subsurface contamination, GW = ground water

13. Site Score: Assumes only SsI component is being scored (i.e., soil exposure component, ground water migration pathway, surface water migration pathway and air migration pathway are not scored)

Page C-20
Table C-7h 

Minimum Populations needed Based on a Combination of the Evaluation of a Second HRS Pathway and the Subsurface Intrusion Component of the Soil Exposure and Subsurface Intrusion Pathway

Scenario

Likelihood

of 

Exposure

Waste Characteristics (WC)

Targets

SsI 

Component 

Score

2nd 

Pathway 

Score 

SsI 

Component 

Score 

Site 

Score

Exposed 

Individual

AOE ASC

Target 

Score

Waste 

Quantity Toxicity Biodegradation

WC 

Product

WC 

Factor 

Value

# Level I 

Targets

# Level 

II 

Targets

# Crawl 

Space 

Targets

# 

Subslab 

Targets

# 

Shallow 

Soil Gas 

Targets

# GW 

/Other 

Targets

1 550 100 1000 1 1.00E+05 18 50 43 480.00 57.60 0 57.60 28.80

2 550 100 1000 1 1.00E+05 18 50 41 460.00 55.20 15 55.20 28.60

3 550 100 1000 1 1.00E+05 18 50 38 430.00 51.60 25 51.60 28.67

4 550 100 1000 1 1.00E+05 18 50 18 230.00 27.60 50 27.60 28.56

5 550 10000 1000 1 1.00E+07 56 50 11 160.00 59.73 0 59.73 29.87

6 550 10000 1000 1 1.00E+07 56 50 10 150.00 56.00 15 56.00 28.99

7 550 10000 1000 1 1.00E+07 56 50 9 140.00 52.27 25 52.27 28.97

8 550 10000 1000 1 1.00E+07 56 50 3 80.00 29.87 50 29.87 29.12

9 550 10000 10000 1 1.00E+08 100 50 4 90.00 60.00 0 60.00 30.00

10 550 10000 10000 1 1.00E+08 100 50 1 60.00 40.00 50 40.00 32.02

Assumptions / Explanations

1. Waste quantity is assigned based on the following assumptions: 3 people per regularly occupied structure, an area of 1740 square feet per structure

2. Toxicity: Assigns a value of 1000 or 10000

3. Biodegradation: Assumes a biodegradation factor value of 1

4. Waste Characteristics Factor Category Value: Reference HRS Table 2-7 (Subject to a Maximum of 100)

5. Exposed Individual: Reference HRS Section 5.2.1.3.1

6. Level I Targets: Reference HRS Section 5.2.1.3.2.1 (multiply by 10)

7. Assumes there are no workers or resources present

8. AOE = area of observed exposure, ASC = area of subsurface contamination, GW = ground water

9. 2nd pathway scores were chosen because they represent a range from little or no significant threat to almost enough to score the site based on the 2nd pathway alone

Page C-21
Table C-7i

Minimum Residents and Workers Needed to Achieve an HRS Site Score of 28.50 Based on Weighting by Level of Exposure and Sample Type

Scenario

Likelihood

of 

Exposure

Waste Characteristics (WC)

Targets

Pathway 

Score

Site 

Score

Exposed 

Individual

 AOE ASC

Target 

Score

Waste 

Quantity Toxicity Degradation

WC 

Product

WC 

Factor 

Value

# Level I 

Residents

# Level 

I 

Workers

Level I 

Value

#Level II 

Residents

# Level 

II 

Workers

Level II 

Value

#Crawl 

Space 

Residents

# Crawl 

Space 

Workers

Crawl 

Space 

Value

# 

Shallow 

Soil Gas 

Workers

Shallow 

Soil Gas 

Value

1 550 100 1000 1 1.00E+05 18 50 20 68 426.67 0 0 0.00 0.00 476.67 57.20 28.60

2 550 100 1000 1 1.00E+05 18 50 35 23 426.67 0 0.00 0.00 476.67 57.20 28.60

3 550 100 1000 1 1.00E+05 18 50 40 8 426.67 0 0.00 0.00 476.67 57.20 28.60

4 550 10000 1000 1 1.00E+07 56 45 0 40 205 108.33 0.00 0.00 153.33 57.24 28.62

5 550 10000 1000 1 1.00E+07 56 45 0 50 175 108.33 0.00 0.00 153.33 57.24 28.62

6 550 10000 1000 1 1.00E+07 56 45 0 60 145 108.33 0.00 0.00 153.33 57.24 28.62

7 550 10000 10000 1 1.00E+08 100 45 0 25 47 40.67 0.00 0.00 85.67 57.11 28.56

8 500 10000 1000 1 1.00E+07 56 20 0 0 100 195 148.50 0.00 168.50 57.19 28.59

9 500 10000 1000 1 1.00E+07 56 20 0 0 0.00 0.00 168.00 57.02 28.51

10 500 10000 1000 1 1.00E+07 56 20 0 0 0.00 1925 148.33 168.33 57.13 28.57

Assumptions / Explanations

1. Waste quantity is assigned based on the following assumptions: 3 people per regularly occupied structure, an area of 1740 square feet per structure

2. Toxicity: Assigns a value of 1000 or 10000

3. Degradation: Assumes a degradation factor value of 1

4. Waste Characteristics Factor Category Value: Reference HRS Table 2-7 (Subject to a Maximum of 100)

5. Exposed Individual: Reference HRS Section 5.2.1.3.1

6. Level I Targets: Reference HRS Section 5.2.1.3.2.1 (number of workers divided by 3 before being added to number of residents and the sum multiplied by 10)

7. Assumes there are no resources present

8. AOE = area of observed exposure, ASC = area of subsurface contamination, GW = ground water 

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