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

Subsurface Intrusion Component to the Hazard Ranking System

Appendix F: Ground Water Contaminant Intrusion

Page F-2: The Ground Water Intrusion Threat 

Page F-2: Evaluation as Part of Development of the draft HRS SsI Addition 

Page F-3: Methodology Used to Evaluate Ground Water Intrusion Sites

Page F-4: Reviews of Possible Ground Water Intrusion Sites

Page F-6: Example of Ground Water Intrusion Site with Sufficient Data for HRS Scoring

Page F-1 
The Ground Water Intrusion Threat

Ground water contaminant intrusion is a type of subsurface intrusion where contaminants have entered a structure 

via infiltrating groundwater (e.g., metals precipitating from--or hazardous substances volatilizing out--of the 

ground water.) Ground water contaminant intrusion is an emerging issue that is being evaluated as part of the 

development of the Hazard Ranking System (HRS) Subsurface Intrusion (SsI) addition. The U.S. Environmental 

Protection Agency (EPA) regional site assessment programs have identified sites where intrusion of contaminated 

ground water is a potential issue and the related threat cannot be evaluated using the current HRS. The proposed 

SsI addition would make ground water intrusion into regularly occupied structures eligible for evaluation and 

inclusion in an HRS evaluation. As EPA further explores this emerging issue, the agency considers it likely that 

additional ground water intrusion sites requiring evaluation will be identified. 

Evaluation of Ground Water Contaminant Intrusion as Part of Development of the draft HRS SsI 

Addition 

The addition of a subsurface intrusion component to the HRS would expand the number of available options for 

EPA to evaluate hazardous substance releases that could endanger human health from ground water entering into

regularly occupied structures from the subsurface. There is no mechanism within the current structure of the HRS 

that allows for the evaluation, or consideration, of ground water contaminant intrusion into a site’s HRS score. 

Therefore, ground water contaminant intrusion cannot currently factor into a site’s potential placement on the 

National Priorities List (NPL) via the HRS, regardless of any threat to human health it may pose. 

During the development of the proposed SsI addition to the HRS EPA considered both vapor intrusion and ground 

water contaminant intrusion (both of which are types of subsurface intrusion). The draft SsI HRS addition 

includes the consideration of ground water contaminant intrusion in the evaluation criteria and, where needed, 

identifies any specific considerations for ground water contaminant intrusion because it occurs differently than the 

intrusion of vapors. It is proposed that subsurface intrusion be evaluated on the likelihood of exposure, waste 

characteristics and targets based on current site conditions. EPA considered including the evaluation of the 

potential for ground water contaminant intrusion in the proposed HRS SsI component, similar to the included 

evaluation of the potential for vapor intrusion. However, the available science and methods for predicting the 

potential for ground water contaminant intrusion would likely be beyond the scope of a site inspection. Therefore, 

ground water contaminant intrusion can only be evaluated in the proposed HRS SsI addition if it meets HRS 

observed exposure criteria. The evaluation of ground water intrusion in the draft HRS SsI addition is specified in 

detail in several sections: 

• Likelihood of Exposure (Draft HRS SsI Addition Section 5.2.1.1) 

• Waste Characteristics (Draft HRS SsI Addition Section 5.2.1.2) 

• Targets (Draft HRS SsI Addition Section 5.2.1.3) 

EPA evaluated multiple sites with documented and potential groundwater contaminant intrusion as part of the 

evaluation of including ground water contaminant intrusion in the draft HRS SsI. One of these sites, Garfield 

Groundwater Contamination, is currently on the NPL [based on an Agency for Toxic Substances and Disease 

Registry (ATSDR) evaluation, not on an HRS evaluation]. The other sites evaluated may have ground water 

contaminant intrusion issues, but the data currently available for these sites is not sufficient to support an HRS 

scoring evaluation. The sections below provide details about how the sites were evaluated given limited 

information, and include site descriptions and the results of the evaluation based on the information available. 

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EPA applied the draft HRS SsI addition to the Garfield site to further demonstrate the need to revise the current 

HRS by including a component for subsurface intrusion.

Methodology Used to Evaluate Ground Water Intrusion Sites

Several sites with possible ground water contaminant intrusion impacts were identified and reviewed using 

available information in order to assess the scope of the threat from ground water contaminant intrusion. A 

methodology for reviewing sites identified with possible ground water contaminant intrusion impacts was 

developed using the following parameters: 

• Evidence of ground water contaminant intrusion into a regularly occupied structure

• Probability of ground water contaminant intrusion into a regularly occupied structure, based on the depth 

from the structure foundation to ground water 

o < 10 feet (high) 

o > 10 feet (low) 

• Contaminants present in the ground water (e.g., chlorinated solvents, dissolved metals) 

• Number of Impacted Targets Including Residents and Workers

o Number of Residences Possibly Impacted

- >10 residences (high) 

- 5-10 residences (medium)

- < 5 homes (low) 

- Unknown 

o Number of Workers Possibly Impacted

- 1,000 workers (high) 

- 101 – 1000 workers (medium)

- 1-100 workers (low) 

Based on the parameters above, a tiering methodology was developed to evaluate possible ground water intrusion 

sites in a similar manner to which the sites tiered in the compiled inventory of sites (see Appendix A) were 

evaluated. The definitions of the tiers assigned to ground water intrusion sites are listed below: 

• Tier 1 – Evidence of ground water contaminant intrusion and a high number of targets; enough HRSrequired data to confirm that the site would not score sufficiently high to qualify for the NPL without the 

addition of a subsurface intrusion component to the HRS. 

• Tier 2 – Either a high probability of ground water contaminant intrusion and a high number or targets or 

evidence of ground water contaminant intrusion and medium number or targets. 

• Tier 3 – High probability of ground water contaminant intrusion and a low number of targets.

• Tier 4 – Low probability of ground water contaminant intrusion and a low or unknown number of targets. 

Page F-3 
Reviews of Possible Ground Water Intrusion Sites

The following sites were identified by EPA regional offices as having a possible ground water contaminant 

intrusion issues.

• Ground Water Intrusion Site 1 (Region 1)

o Located in a residential and commercial area; TCE documented in ground water samples 

o No evidence of actual ground water intrusion; however, high probability of occurrence based on 

depth to ground water of less than ten feet

o Low number of targets; four residential structures (including one multi-subunit residence)

o Tier 3; unlikely to score greater than or equal to 28.50 based on available information when 

evaluating ground water intrusion 

• Ground Water Intrusion Site 2 (Region 1)

o Located in urban area; PCE residue documented in building materials and in the soil of one 

elevator shaft 

o Data suggests documented contamination likely associated with an indoor source and may not 

represent subsurface intrusion 

o Low number of targets; one commercial tenant space located on the lowest floor of the building 

that is eligible for evaluation

o Tier 4; unlikely to score greater than or equal to 28.50 based on available information when 

evaluating ground water intrusion; building includes eight floors of residential tenant space 

• Ground Water Intrusion Site 3 (Region 1)

o Located in residential and commercial area; PCE documented in residential sumps.

o Evidence of contaminated ground water intrusion based on sample collected from residential 

sumps 

o Low number of targets; five commercial tenant spaces and one residential apartment 

o Tier 3; unlikely to score equal to or greater than 28.50 based on available information when 

evaluating ground water intrusion 

• Ground Water Intrusion Site 4 (Region 2)

o Located in rural area with homes and businesses within a half-mile of the site; chromium was 

documented in ground water samples 

o No evidence of actual ground water intrusion; however, high probability of occurrence based on 

depth to ground water of less than ten feet 

o Low number of targets; less than five residences and less than 100 estimated workers 

o Tier 3; unlikely to score greater than or equal to 28.50 based on available information when 

evaluating ground water intrusion 

• Ground Water Intrusion Site 5 (Region 2)

o Located in a residential and commercial area

o No evidence of actual ground water intrusion; however, insufficient information to completely 

tier this site

• Ground Water Intrusion Site 6 (Region 3)

o Located in a residential and commercial area; TCE and PCE documented in ground water 

samples 

o Insufficient information available to document whether ground water intrusion has occurred; to 

determine whether ground water intrusion has occurred additional research of the existing site file 

or further field work would need to be done 

o Sufficient targets present to achieve a score equal to or greater than 28.50 using the 1990 HRS 

Page F-4 
ground water migration pathway and the draft SsI component of the HRS addition 

• Ground Water Intrusion Site 7 (Region 5)

o Located in a residential and industrial area; arsenic and cyanide documented in surface and 

subsurface samples; blue waste material documented to contain cyanide was found in 

foundational cracks and a sump in two residences 

o Evidence of infiltration of blue waste material into basements 

o Low number of targets; two residential structures 

o Tier 3; unlikely to score greater than or equal to 28.50 based on available information when 

evaluating ground water intrusion 

Page F-5 
Example of Ground Water Intrusion Site with Sufficient Data for HRS Scoring – Garfield 

Ground Water Contamination Site

The Garfield Ground Water Contamination Site demonstrates the need to revise the current HRS with the addition 

of a component for subsurface intrusion. The Garfield site had shallow (surficial) ground water contaminated with 

chromic acid that intruded into residential basements. After the water receded, or evaporated, a precipitate of 

chromium remained in the basements as a residue. The presence of this residue posed a significant threat to public 

health; however, the site could not be evaluated using the current HRS due to the lack of a mechanism to evaluate 

human exposure resulting from intrusion of contaminated ground water (subsurface intrusion contamination). The 

only viable option to place the site on the NPL was to rely on ATSDR to make a determination that the exposure 

at the site posed a significant threat to public health. The decision to include sites on the NPL based on a 

determination by ATSDR is made infrequently because it requires ATSDR to issue a healthy advisory 

recommending dissociation of individuals from the release at a site. EPA then determines whether the release 

poses a threat. EPA will consider the cost-effectiveness to use remedial authority, rather than removal authority, 

to respond to the release. 

Garfield Ground Water Contamination (EPA ID NJN00020631)

Site Description

This site is located in a mixed area of commercial and residential structures in the state of New Jersey, across the 

Hudson River from New York City, and is the result of a historical release of chromic acid from a commercial 

operation’s storage tank that failed, releasing chromic acid to the environment. Approximately 29% of the 

material released was thought to have been recovered, but the unclaimed share entered the subsurface 

environment beneath the facility, eventually entering shallow ground water and migrating into the basements of 

nearby hydraulically downgradient properties. Past investigations have identified chromium and hexavalent 

chromium contaminated ground water and precipitates intruding into occupied commercial and residential 

structures. Since 2005 EPA has identified approximately 288 residences in the study area with possible ground 

water infiltration issues. The available analytical data has identified chromium or hexavalent chromium 

contamination in a basement or basement sump at 10 occupied structures. 

Key Scoring Limitations/Assumptions

• Available U.S. Census data is used to determine average number of persons per household residential targets 

in the AOE and ASC. Worker population data is assumed to be at least 1 full-time worker per workplace if 

more specific information is not available.

• It is assumed that the data presented within the available reports, including sample values collected between 

2000 and 2003 (basement and basement sump samples) and in April 2010 (ground water samples), are still 

representative of current conditions at the site. Additionally, the quality of analytical data is assumed to be 

sufficient and of known quality. 

• It is assumed that all ground water sampling data used in this analysis represent a single surficial ground 

water layer. 

• It is assumed that all contaminated samples considered as part of this evaluation are representative of a single 

contaminant plume associated with a single originating source and do not represent multiple, comingled 

contaminant plumes.

• Publicly available ariel imagery was used to identify and estimate types (e.g., single family residence, multitenant residence) of occupied structures for the purposes of this analysis. These images are assumed to 

Page F-6 
Page F-7 

represent current conditions and that all residences or commercial operations are occupied, unless evidence 

indicates otherwise. This information has not been validated.

• Sample locations presented are approximate and may not represent the actual extent of contamination at the 

site.

• An assigned area value of 1,740 square feet is used in determining a waste quantity estimate for all regularly 

occupied structures (including multi-subunit structures). 

• It is assumed that previous and current remedial or removal actions have not completely removed the threat to 

public health posed by this release under evaluation.

• A ground water migration pathway score of 50.77 was previously determined in historical site assessment 

reports and for purposes of this evaluation is assumed to be accurate and representative of current site 

condition. Supporting data for the ground water migration pathway evaluation was not available for review. 

It should be noted that these figures and site scores are presented for the sole purpose of testing the proposed 

subsurface intrusion component. The presented HRS site scores are based on limited information and that 

additional evaluation and sampling would be necessary to determine if the site would qualify for listing using the 

HRS. 
Garfield Ground Water Contamination (EPA ID NJN00020631)

Site Diagram

Areas of Observed Exposure (AOEs)

Delineated by Observed Exposure 

Indoor Basement/Sump Samples

Page F-8 
HRS TABLE 5-11 – SUBSURFACE INTRUSION COMPONENT SCORESHEET

Factor Categories and Factors

Maximum

Value

Value

Assigned

Subsurface Intrusion Component 

Likelihood of Exposure: 

1. Observed Exposure

Two areas of observed exposure have been delineated at the site, based on samples 

collected from the basements of several occupied structures (including sump samples). 

An observed exposure to chromium (total and/or hexavalent) by direct observation has 

been established at 10 regularly occupied structures.

550 550 

2. Potential for Exposure

Per the draft HRS, the potential for exposure factor was not evaluated based on the 

documentation of an observed exposure.

500 - 

2a. Structure Containment 10 - 

2b. Depth to Contamination 10 - 

2c. Vertical Migration 15 - 

2d. Vapor Migration Potential 25 - 

3. Potential for Exposure ( lines 2a * ( 2b + 2c + 2d ), subject to a maximum of 500) 500 - 

4. Likelihood of Exposure ( higher of lines 1 or 3 ) 550 550

Waste Characteristics: 

5. Toxicity/Degradation

The highest toxicity value for the substances documented in the area of subsurface 

contamination and/or the areas of observed exposure documentation is 10,000 for 

hexavalent chromium.

(a) 10,000 

6. Hazardous Waste Quantity

The footprint of the 43 regularly occupied structures located within the area of observed 

exposure is unknown. Therefore, as stated in the draft HRS, a value of 1,740 ft2 per 

regularly occupied structure was used in calculating a hazardous waste quantity, which 

was used to select a hazardous waste quantity factor value using HRS Tables 5-18 

(draft) and 2-6.

(a) 100 

7. Waste Characteristics (subject to a maximum of 100)

The toxicity/degradation and hazardous waste quantity factor values were multiplied and 

the resulting value of 1x105 was used to select a waste characteristics factor category 

value of 32 from the draft HRS Table 2-7.

100 32 

Targets: 

8. Exposed Individual

There is at least one individual present in a regularly occupied structure in the AOE that 

is exposed to Level II contaminant concentrations. Per the draft HRS, a value of 45 is 

assigned.

50 45 

9. Population:

9a. Level I Concentrations (b) 0 

Page F-9 
Factor Categories and Factors

Maximum

Value

Value

Assigned

Analytical data for indoor air samples were not included in the information provided.

9b. Level II Concentrations

In the two established AOEs, 34 single family homes, one duplex (i.e., two residences), 

one basement apartment and one commercial building were identified as exposed (or 

inferred to be exposed to) Level II concentrations. The number of people living or 

working within each regularly occupied structure is unknown, therefore, a county 

average of 2.69 was assumed for residential populations and at least one full-time 

worker is assumed (1 full-time worker divided by 3=.33). (U.S. Census Bureau). Based 

on this information, a Level II population of 99.86 is assigned.

(b) 99.86

9c. Population within Area(s) of Subsurface Contamination 

Population in the ASC was not evaluated for this site. (b) 0 

9d. Total Population ( lines 9a + 9b + 9c ) (b) 99.86

10. Resources

Based on the information available, one resource has been identified in the AOE. Per 

the draft HRS, a value of 5 is assigned.

5 5 

11. Targets ( lines 8 + 9d + 10 ) (b) 149.86

Subsurface Intrusion Component Score

12. Subsurface Intrusion Component ( lines 4 x 7 x 11) / 82,500c (subject to a

maximum of 100) 100 31.97

Soil Exposure and Subsurface Intrusion Pathway Score 

13. Soil Exposure Component + Subsurface Intrusion Component (subject to a

maximum of 100) 100 31.97

HRS Site Score 

14. √[ ( Sgw2 + Ssw2 + Ssessi2 + Sa

2 )/4] = √ [( 50.772

 + 02 + 31.972 + 02 ) /4] 100 29.99

a Maximum value applies to waste characteristics category.

b Maximum value not applicable.

c

Do not round to nearest integer.

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