UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON. D.C. 20460 

"o\.B-EC-88-008 

January 14, 1988 

OFt=" ICE OF 

THE ADMI"'tSTr< ... TOR 

Honorable Lee M. Thoma·s 

Admi nis tr a tor 

u.s. Environmental Protection Agency 

401 M Street, s.w. 

Washington, D.C. 20460 

Dear Mr. Thomas: 

The Hazard Ranking System Review Subcommittee of the Science Advisory 

Board has completed its review of a number of issues related to the 

Superfund Hazard Ranking System (HRS). The Hazard Ranking System is the 

principal mechanism used by EPA to determine whether an uncontrolled 

waste site should be placed on the National Priorities List. 

Besides evaluating the questio~s referred by the Office of Emergency 

and Remedial Response (OERR), the Subcommittee has also chosen to address: 

the overall algorithm for the HRS, the inclusion of exposure in the HRS, 

how the HRS could be evaluated in the future, and work which could be 

done to provide better documentation for the next revision of the HRS. 

The Subcommittee has suggested changes that will allow the HRS to 

provide a more accurate and scientifically based estimate of the relative 

risk of candidate uncontrolled waste sites. Ideally, the HRS scores 

should accurately assess the relative degree of risk at a site. However, 

we recognize this is not always feasible due to scientific and data 

limitations and to value and policy decisions implicit when considering .· 

and balancing human health and environmental impacts, A revised HRS, 

better designed to evaluate sites by relative risk, will provide an 

improved mechanism for determining which sites should be included on the 

National Priorities List (NPL), and can potentially provide useful input 

to the subsequent prioritization of NPL si·tes. However, the Agency must 

continue to base this prioritization on many factors in addition to the 

HRS. Most of the changes needed to improve the current HRS are changes 

in the risk variables assessed and in the overall algorithm, not changes 

with vast new data requirements. 

The Office of Emergency and Remedial Response referred three issues 

to the Subcommittee: the types of toxicity the HRS shoMld address and 

how it should do so; distances from an uncontrolled hazardous waste 
It. • ... 
" • •• 


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site that are relevant when considering air pollutants from the site; 

the feasibility of including waste concentration in the HRS and whether 

large volume waste sites had been treated differently than other sites 

by the HRS. 

The Subcommittee finds that all the options OERR proposed for revising 

the toxicity factor are improvements over the current approach. The 

Subcommittee believes, given the difficulty of ambient air monitoring, 

that the inclusion of potential release via the air pathway is an important 

improvement to the HRS. The Subcommittee also believes that improved 

characterization of the source and of exposure/mobility will improve the 

functioning of the HRS for all sites, not just large volume waste sites. 

With respect to the three issues posed to the SAB,, the Subcommittee 

recommends the following: 

1. In order to improve the discriminating power of the HRS, the 

currently used Sax rating scale should be replaced by multiple measures 

of toxicity. In addition, exposure measures need to be improved. 

2. Modification of the HRS to include the potential for air release 

seems both appropriate and possible." A scoring system that weights 

populatio~ exposure in concentric rings is recommended. 

3. EPA's experience in applying the HR$ to mining sites has not 

proven biased against such sites, that is, it has not treated such sites 

with systematic error; however, it has that potential. Our recommendation 

for improvement includes incorporating hazardous-constituent concentration 

in a tiered system, modifying the toxicity factor to reflect metal speciation, 

incorporating a mobility factor, and adding additional transformation 

factors. 

The Subcommittee also recommends that more attempts be made to learn·· 

from subsequent experience so that when the HRS is next revised, a better 

basis for those revisions will be available. These recommendations are 

detailed in Appendix 6. 

The Subcommittee's evaluation of these and other issues, and its 

conclusions and recommendations, are discussed in greater detail in the 

attached report. Because of their cross cutting nature, certain issues, 

such as exposure and the overall algorithm, appear in more than one area. 

The Subcommittee would like to call to your attention the high 

q~ality efforts of the staff from the Office of Emergency and Remedial 

Response that briefed and worked with the Subcommittee. 'They were 

professional, well.:.prepared, and responsive to the Subcommittee's 

requests for information. 

-3­

We appreciate this opportunity to present our scientific views and 

look forward to an official written response fran the llgency concerning 

the comments and recommendations in the attached report. 

Sincerely, 

"A~ v\cJ~ Norton Nelson, Olairman 

Executive Committee 

..

SCience Advisory Board 

Hazard 

!x:::~ Ranking System Review Subcommittee 

Science Advisory Board 

cc: w. POrter 

H. f.Dngest 

T. Yosie 

United States 

Environmental Protection 

Agency 

Office of the Administrator 

Science Advisory 6oard 

Washington, DC 20460 

S.!.S~EC·S8-008 

January 1988 

Review of the Superfund 


Hazard Ranking System 


Review by the Hazard 

Ranking System Review ·" 

Subcommittee of the 

Science Advisory Board 

SAB-EC-88-008 

SCIENCE ADVISORY BOARD 

HAZARD RANKING SYSTEM REVIEW SUBCOMMITTEE 

REVIEW 


of the 


SUPERFUND HAZARD RANKING SYSTEM 


January 1988 

.. 

ii 

NOTICE 

This report has been written as a part of the activities of the 

Science Advisory Board, a public advisory group providing extramural 

scientific information and advice to the Administrator and other officials 

of the Environmental Protection Agency. The Board is structured to 

provide a balanced expert assessment of scientific issues related to 

problems facing the Agency. This report has not been reviewed for 

approval by the Agency and, hence, the contents of the report do not 

necessarily represent the views and policies of the Environmental 

Protection Agency, nor of other agencies in the Executive Branch of 

the Federal government, nor does mention of trade names or commercial 

products constitute endorsement or recommendation for use. 
iii 

U, S. ENVIRONMENTAL PROTECTION AGENcY 


SCIENCE ADVISORY BOARD 


HAZARD RANKING SYSTEM REVIEW SUBCOMMITTEE 


ROSTER 

CHAIRMAN 

Dr. 	 Raymond C. Loehr, Professor, Civil Engineering Department, 

8.614 ECJ Hall, University of Texas, Austin, Texas 78712 

VICE CHAIRMAN 

Dr. 	 Warner North, Principal, Decision Focus Inc., Los Altos 

Office Center, Suite 200, 4984 El Camino Real, Los Altos, 

California 94022 

MEMBERS AND CONSULTANTS 

Dr, Stephen L. Brown, Project Manager, Environ Corporation, 

1000 Potomac Street, N.W., Washington, DC 20007 

Dr. Thomas Burke, Deputy Commission·er, New Jersey Department 

of Health, Room 805, State Health, CN360, Trenton, 

New Jersey 08625 

Dr. Shepherd Burton, Systems Applications, 101 Lucas Valley Road, 

San Rafael, California 94903 

Mr. George F, Carpenter, Michigan Department of Natural Resources, 

Environmental Response Division, Post Office Box 30028, 

Lansing, Michigan 48909 

Mr. Richard Conway, Corporate Development Fellow, 

Union Carbide Corporation, Post Office Box 8361 (770/342), 

South Charle~ton, West Virginia 25303 

Dr. 	 Paul Deisler, 11215 Wilding Lane, Houston, Texas 77025 

Dr. John Doull, Professor of Pharmacology and Toxicology, 

University of Kansas Medical Center, Kansas City, Kansas 66103 

Dr. 	 Naihua Duan, Statistician, Rand Corporation, 1700 Main Street, 

Santa Monica, California 90406 
iv 

Dr, Keith Ferguson, Environmental Protection, Environment Canada, 

Kapilano 100, Park Royal, West Vancouver, British Columbia 

CANADA V7TlA2 

Dr, c. Daniel Kealy, U, S. Bureau of Mines, Spokane Research Center, 

East 315 Montgomery Avenue, Spokane, Washington 99207 

Dr. James 0. Leckie, Department of Civil Engineering, 

Stanford University, Stanford, California 94305-4020 

Dr, Donald Mackay, Department of Chemical Engineering and Applied Chemistry 

and Institute for Environmental Studies, University of Toronto, 

Toronto, Canada M5S 1A4 

Mr. Bruce Napier, Battelle NW, Post Office Box 999, 

Richland, Washington 99352 

Dr. David Pollock, U.S. Geological Survey, National Center, Mail Stop 411, 

Reston, Virginia 22092 

Dr, Joseph v. Rodricks, Environ Corporation, 1000 Potomac Street, N.W., 

Washington, DC 20007 

Dr. Ellen Silbergeld, Chief Toxics Scientist, Toxic Chemicals Program, 

Environmental Defense Fund, 1616 P Street, N,W,, Room 150, 

Washington, D.C. 20036 

Dr. Mitchel+ Small, Department of Civil Engineering, Carnegie Mellon 

University, Schenley Park, Pittsburgh, Pennsylvania 15213 

Dr, Rebecca T. Zagraniski, Division of Birth Defects and Developmental 

Disabilities, Center for Environmental Health and Injury Control, .• Centers for Disease Control, Koger Center Room 2008 (F-37) 

1600 Clifton Road, N.E., Atlanta, Georgia 30333 

Executive Secretary 

Mrs. Kathleen w. Conway, Deputy Director, Science Advisory Board (A-101F), 

u.s. Environmental protection Agency, 401 M Street, s.w., 


Washington, D.C. 20460 


Staff Secretary 

Mrs, Dorothy M. ·cl.ark, Secretary, Science Advisory Board (A-101F), 

U.S. Environmental Protection Agency, 401 M Street, S,W, 


Washington, D.C. 20460 

TABLE 	 OF CONTENTS 

PAGE 

EXECUTIVE SUMMARY 	 1 


Toxi.city Fa.ctor 1


Air Target Distance 2 


Large Volume Waste 3 


ExposUI:e 4 


Algorithm 5 


Recommendations To Evaluate and Improve the HRS 5 


INTRODUCTION 	 7 


The Current HRS: Purpose and Prior Reviews 7 


Required Revisions to the HRS 7 


S.cien.ce Advisory Board Review 8 


Structure and Practi.ce of the Current HRS 9 


SUMMARY OF THE ISSUES 	 11 


roxici ty Factor 11 


Air Target Distance 13 


Large Volume Waste 15 


Exposure 18 


Algorithm 19 


Recommendations to Evaluate and Improve the HRS 2 1 


APPENDICES 

1. Report of the Toxicity Factor Work Group 

2. Report of the Air Target Distance Work Group 

3. Report of the Large Volume .Waste Work Group 


4 Comments on Exposure 


s. Comments on Algorithm 

6. Recommendations to Evaluate and Improve the HRS 

7. 	 Requests from the Office of Emergency and Remedial 


Response (SUPERFUND) 


8. References Cited 
EXECUTIVE SUMMARY 

The Hazard Ranking System (HRS) is the principal mechanism used by 

EPA to determine whether an uncontrolled waste site should be placed on 

the National Priorities List (NPL), The Office of Emergency and Remedial 

Response (OERR) requested that the Science Advisory Board (SAB) review 

certain aspects of the technical basis for revising the HRS, The major 

issues identified by OERR.dealt with the toxicity factor, air target 

distance, and large volume wastes. (See Appendix 7 for the full text of 

this request). 

Overall, the Subcommittee is generally supportive of the changes 

OERR presented and found no fatal flaws in the approach to the HRS that 

OERR presented; however, the Subcommittee has made recommendations in 

this report that would further improve the HRS, 

Toxicity Factor 

Toxicity should be an important component of a site ranking scheme. 

Because the toxicity factor in the current HRS makes virtually no distinctions among sites listed or proposed to be on the NPL, and because EPA 

did not present evidence that it had examined the ability of the various 

factors to discriminate between sites· --including sites not on the NPL--the 

Subcommittee questions whether the existing toxicity factor has the power 

to discriminate "very toxic" from "not so toxic" sites, The Sax chemical 

toxicity ratings are a crude basis for setting priorities since they 

address only the acute toxicity of one constituent of the waste. The 

Subcommittee recommends replacing Sax ratings and using multiple measures 

of toxicity for ranking sites. All of the options presented by OERR are 

substantial improvements, These options are described in OERR's 

Discussion of Options for Revising the Hazard Ranking System (HRS) Toxicity 

Factor (12). 

Scientific techniques exist which would permit the HRS to consider 

several endpoints that are revelant to human health and the environment. 

Multiple measures of toxicity .should be considered, any one of which, if 

sufficiently seve·re, could, by pLicing the site on the NPL, trigger a 

more detailed evaluation. By multiple measures, the Subcommittee means 

acute human health effects, human cancer, non-cancer chronic disease in 

humans, and impact on the non-human natural environment, However, a 

system of weighting the severity of different health endpoints, as is 

done in the Reportable Quantities (RQ) approach, is not encouraged because 

a single composite score obscures the value judgments implicit in such an 

approach, 

It is possibie.that sites containing hazardous chemicals whose 

toxicities are not well or widely known might be omitted from the NPL, 

thus creating false negatives. Measures to counter this problem include 
- 2 ­

the continued development of toxicity profiles on additional chemicals and 

the keeping of good records so that sites can later be re-evaluated when 

additional toxicity data become available, 

Air Target Distance 

The Subcommittee supports efforts to modify the air pathway so that 

it considers the potential for release instead of relying only on a score 

for observed releases. Air emissions are often episodic and/or narrowly 

focused along a particular wind direction and, therefore, ~re difficult 

to detect. Furthermore, time and site activities (such as excavation in 

the course of site remediation) can markedly change the potential for air 

emi$sions ~ 

Methods are currently available which can be simplified to calculate 

approximate, order-of-magnitude emission rates for impoundments and 

landfills. Using them requires information on the chemical identities 

at the site. When such information is totally lacking, it is impossible 

to even estimate the quantity of the chemical present, and volatile air 

emissions can not be predicted. The subsequent consideration of air 

exposure routes is very weak, A minimum data requirement which mandates 

contaminant identification by record_review or direct sampling (either 

within containment structure or of emissions) would greatly improve the 

HRS' validity in all pathways. In the absence of an inventory, direct 

sampling methods such as soil gas analysis or contained surface sampling 

complement estimated emissions and provide a more effective estimate than 

the estimated emissions alone, Such methods normally will yield samples 

of much higher concentration than ambient air samples, and chemical 

analyses will' be more informative. 

The Subcommittee recommends that the Agency derive a scoring system 

which weights the number of exposed people in a "ring" according to the 

distance from the site at which they live, (ThiS approach is described in·· 

greater detail in Appendix 2 and illustrated by the figure on page A2-12.) 

The widths of the rings can be determined to account for the decrease in 

air concentrations with distance. 

This ring-weighting method does not account for risks to the 

maximally exposed individual (MEI), but it does explicitly account for the 

higher exposures that will occur closest to the site. The Subcommittee 

does not consider a separate or additional evaluation of MEl air risks 

as part of the HRS to be necessary. The recommended ring-weighting 

method will essentially set:ve the same purpose by weighting most heavily 

the nearest and 1nost exposed population. 
- 3 ­

The ring-weighting method should be calibrated on the basis of the 

variation of concentration with distance. The influence of pollutants 

borne on particles will likely be concentrated at lesser distances which 

would change the ring weights, but would not often lead to markedly 

different scores. 

Large Volume Waste 

The issues before the Subcommittee were the "applicability of the 

HRS in scoring mining waste sites" and the "feasibility of using waste 

concentration data in a revised HRS." 

The Subcommittee finds that the studies conducted thus far regarding 

the adequacy of the HRS listing process-for mining sites have been limited 

and inconclusive. Sufficient evidence based on experience has not been 

evaluated and presented to show an inadequacy of the HRS in regard to 

mining sites. However, the present scoring system has a potential to 

treat mining wastes with systematic error. 

The Subcommittee believes the present HRS is not well suited for 

scoring potential releases from mining site wastes because mobility is 

not included. Improved ways of considering the concentration (often 

low), toxicity release (mobility in various matrices), and information 

on the transport and transformation of chemicals would make the HRS more 

accurate. 

Mobility is a more discriminating concept for both inorganic and 

organic substances than is persistence in the subsurface, especially 

with respect to inorganic compounds. The Subcommittee recommends that, 

during the development of a structured-value representation of the mobility 

concept, a means of incorporating important matrix characteristics be 

explored. Significant matrix characteristics are extreme acidity or 

alkalinity (as expressed by both high and low pH), crystalline phase 

modifications of mining wastes that differ from native geologic materials, 

and the sorptive .capacity of surr~unding geologic materials (influencing 

migration tendency). 

tn regard to the surface water route, EPA is considering keeping 

persistence as the parameter to include with toxicity in the waste 

characteristics portion of the HRS, but would consider other transformation 

parameters along with the current use of biodegradation. Inclusion 

of these additional parameters would improve the existing HRS because 

it would more closely correspond with what happens in the real world. 

It appears from the presentations that OERR would also include 

mobility in the air route score. Mobility and waste quantity are important 

factors in determining exposure. Because of this importance, the basic 

approach o£ including mobility in the HRS should be pursued. (See also 

Appendix. 2.) 
- 4 ­

Modifying the current HRS to incorporate factors which capture some 

measure of both the physical-chemical characteristics of the hazardous 

constituents and the waste matrix, as well as those site characteristics 

responsible for risk, is clearly an improvement, 

EPA presented two approaches for incorporating concentration into 

the HRS. While the direct measurement approach provides the soundest 

scientific basis for the HRS, in most cases, it will not be practical-­

because of safety and cost considerations--to require waste constituent 

concentration data for every site at the site inspection stage. In 

contrast, the tiered approach used hazardous constituent concentration 

data where available and default values where they were not, The Subcommittee 

supports the use of the tiered approach because it encourages the gathering 

and use of concentration data and provides for the use of indirect estimates 

where data gathering is impractical. 

Both approaches translate waste constituent concentration data into 

an estimate of the total mass of a hazardous constituent at the site, 

which is then used to compute the waste quantity score for the HRS. The 

approach that uses the "total mass" of hazardous constituent, without 

relevant information on metal speciation and. mobility, would produce 

false positives for some large volume wastes and pathways. 

For the ground water pathway, the total concentration or mass 

of a constituent is of much less importance than the leachate concentration 

produced at the site. Consequently, more emphasis should be placed on 

the partitioning of hazardous constituents between the solid waste and 

the leachate •. Actual or estimated leachate concentration should then be 

factored into the waste quantity term in the HRS. For wastes that are 

in liquid form, total contaminant mass quantity should continue to be 

used. 

.. Exposure 

The Subcommittee believes that toxicity issues are integrally linked 

to dose, i.e., migration and exposure (see Appendix 4). 

Consideration of exposure in the evaluation of relative risk is 

vital. If there is no exposure, there is no risk and where there is 

exposure, risk varies with exposure. While the current HRS gives some 

consideration to exposure, often through the use of proxies such as 

persistence, distance to nearest well and target populations, the 

Subcommittee concludes that adnitional chemical-specific consideration 

of. routes of exposure, particularly with respect to the Toxicity Factor, 

would strengthen the HRS. 

The Subcommittee is mindful of the difficulties in characterizing 

exposures to chemicals at waste sites that have undergone only preliminary 

inv@Stigation. It does not propose the implemention of expensive and 
- 5 ­

time-consuming sampling programs to measure actual concentrations in 

ambient media where pollutant levels may be near or below detection 

limits. However, additional surrogates for eKposure--e.g., a calculation of 

quantity estimates with mobility' considerations--will provide better 

rankings than does the current system, Without appropriate estimates 

of mobility, consideration of toKicity in the algorithm is tantamount to 

assuming that the eKposures to every detected chemical at every waste 

site are equal. The Subcommittee recommends adoption of a tiered approach 

for exposure similar to Option W3 EPA developed for concentration 

(discussed in Appendix 3 of this report). 

Algorithm 

Revisions to the HRS should begin with the development of a chain of 

logic, without regard for the ease or difficulty of collecting data, that 

would lead to a risk assessment for each site. This framework, but not 

the underlying logic, would be simplified to account for the very real 

difficulties of data collection. 

This chain of logic, which is termed the algorithm in this report, 

should lead to a situation in which an increased score reflects an increased 

risk presented by a site. Without this consistency in the meaning of a 

score it is impossible to assess the relative degree of risk posed by a 

site: .. 

The Subcommittee places special emphasis on the algorithm issue 

because it is, impossible to review the components of the HRS without 

considering how the components fit together. If the HRS is to evaluate 

sites by relative risk, then the HRS must be consistent with the underlying, 

quantitative relationships of the factors involved, Specific recommendations 

concerning this issue are presented in Appendix S. .­

Consistency in terms of relative risk and score does not exist 

between sites in the current HRS, although for a single site a larger 

score represents" a larger risk, ·changes in the HRS algorithm mean that a 

score of 28.5 in a revised HRS will almost certainly mean something 

different than a score of 28,5 in the current HRS, 

Recommendations To Evaluate and Improve the HRS 

Grappling with a difficult problem like the HRS clarifies the need 

for additional data, studies, and evaluations. The Subcommittee assumes 

that the HRS will again be revised, Results from the following would be 

most helpful at that time. Peer review of the plans for ·such studies is 

recommended,., 

o 	 The evaluation of the relative risk of sites as estimated by a 

HRS score should be compared with the results of risk assessments 

based on the Remedial Investigation/ Feasibility Studies (RI/FS). 
- 6 ­

Advance planning is needed so that additional RI/FS-like data 

is gathered on some sites with scores below 28.5; these can be 

selected using an a priori sampling scheme. Similarly, since 

the success of the-HRS at ranking by relative risk depends, in 

part, on the consistency and completeness of the scoring, at 

least some sites scnring above 28.5 may require additional 

data gathering. Such work would provide the basis for a meaningful 

retrospective study of the HRS and a better understanding of 

the basic parameters that are important in the use of the HRS 

scoring model, 

o 	 EPA should determine, based on results of RI/FS studies, whether 

the health risk associated with a site was likely to be dominated 

by one or a few chemicals (for each pathway) or by a larger 

number of chemicals. Such a review would provide an objective 

basis for recommending how many chemicals should be selected 

for toxicity (or site) ranking. The two Agency studies of this 

issue presented to the Subcommittee were limited in the number 

and types of sites evaluated, and the sites studied were not 

selected for representativeness, but for convenience. A better 

evaluation is warranted. 

" 	 EPA should consider, how the scores through the various pathways 

relate to relative risk. With this information, the algorithm 

can be calibrated appropriately. 

o 	 A vigorous effort should be made to improve the overall quality of 

analytical data collected at sites, Standardized collection and 

laboratory methods currently exist for only a small fraction of 

substances potentially present, Expanded chemical characterization 

of all media, coupled with a strong laboratory certification program 

will improve not only the HRS but all aspects of the Superfund .. 

process. 

o 	 Screening models like the HRS must be simple. They do not have 

much resolving power and therefore, some false positives and 

false negatives are inevitable. Because of this limitation, 

HRS scores should not be overemphasized. A process should be 

established either to review sites subject to scoring or to 

review HRS scores in an attempt to spot false positives and 

negatives, Such a scientific review process could involve the 

use of additional models; in many cases, however, obvious false 

negatives or positives might be best handled by a "manual" review. 

The important point is that the system be flexible enough to 

allow fo~ a variety of approaches to be used, as needed, during 

the scientific review process. 
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INTRODUCTION 

The Current Hazard Ranking System: Purpose and Prior Revie~s 

The Hazard Ranking System (HRS) is the principal mechanism used by 

EPA to determine ~hether to place sites on the National Priorities List 

(NPL), The HRS ~as made available for public comment on t~o occassions. 

On March 12, 1982 a draft of the current version ~as placed in the Superfund 

Docket and a notice was published in the Federal Register. EPA summarized 

the public comments and addressed them in the preamble to the final HRS 

rule on July 16, 1982. On April 9, 1987 EPA published an Advance Notice 

of Proposed Rulemaking in the Federal Register requesting comments and 

information related to revising the current HRS. The Agency subsequently 

held a t~o-day public meeting to solicit public testimony. The issues 

referred to the Science Advisory Board (toxicity factor, air target 

distance, and concentration) are ones that the public had previously 

identified, 

Required Revisions to the Hazard Ranking System 

Section l05(c)(1) of the Superfund Amendments and Reauthorization 

Act of 1986 (SARA) requires that the-Agency modify the Hazard Ranking 

System so that, "to the maximum extent feasible, it accurately assesses 

the relative degree of risk to human health and the environment posed by 

sites and facilities subject to review." The amendments require an: 

o 	Assessment of the human health risks associated with contamination 

or potential contamination of surface waters, either directly or as 

the result of run-off from a site. 

o 	Evaluation of the damage to natural resources which may effect 

the human food chain and which is associated with any release or 

threatened release, 

o 	Assessment of the contamination or potential contamination 


of the ambient air which is associated with a release or 


threatened release. 


Section 125 of SARA also requires that for certain wastes generated 

primarily from combustion of coal, EPA consider the following: 

"(l) 	 The quantity, toxicity, and concentrations of 


hazardous constituents which are present in such 


waste and a comparison with other wastes; 


(2) 	 The extent of, and potential for, release of such 


hazardous constituents into the environment; and 


(3) 	 The degree of risk to human health and the 


environment posed by such constituents," 

- 8 ­

Section 1187 requires that EPA give a high priority to facilities where 


the release of hazardous substances has resulted in the closing of drinking 


water wells or has contaminated a principal drinking water supply. 


Science Advisory Board Review 

Three memoranda from the Office of Emergency and Remedial Response 


constitute the request for the review, (Appendix 7 contains copies 


·of these memoranda,) In response to the first memorandum requesting an 

independent outside scientific review of the technical basis supporting 

revisions of the HRS the Executive Committee of the Science Advisory 

Board formed the Hazard Ranking System Review Subcommittee, (The Subcommittee 

roster appears at the beginning of this report.) The members were chosen 

for their special expertise and experience in the hazardous waste area. 

The Subcommittee first met ~ay 19-20, 1987 in Washington, DC for 


background briefings on the Superfund program and on the current HRS. 


OERR provided the Subcommittee and \lark Group with briefings on the 


toxicity factor and air target distance issues, issue papers, and key 


references. At the July 16-17, 1987 Subcommittee meeting in Washington 


the Office of Policy Planning and Evaluation (OPPE) orally presented its 


Site Ranking Panel Study, four ranking systems and expert panel to rank 


20 hazardous waste sites. 


The Subcommittee formed three work groups to address issues 


referred by the OERR. These were the Toxicity Factor Work Group, the 


~ir Target Distance Work Group and the Large Volume Waste Work Group. 


(Rosters for these Work Groups can be found in the front of Appendices 1, 


2, and 3, respectively). Each work group reviewed documents on that 


issue prepared by OERR and reported to the full Subcommittee. (Review 


and important background documents are listed in Appendix 8.) 


' The Toxicity Factor Work Group met June 29-30, 1987 in Washington, DC 


to review Discussion of Options for Revising the Hazard Ranking System 


(HRS) Toxicity Factor (12). The Work Group reported its conclusions 


and recommendations to the full Subcommittee July 16-17, 1987. 


The Air Target Distance Work Group met July 27-28, 1987 in Washington 


to review Analysis of the Air Target Distance Limit in the Hazard Ranking 


System (9). 


The Large Volume Waste Group met August 20-21, 1987 in Denver 


to review two documents, The Superfund Hazard Ranking System (HRS): 


Applicability to Mining Waste Sites, and The Superfund Hazard Ranking 


System (HRS): Feasibility of Using Concentration Data in a Revised HRS (13, 


14). OERR briefed the Large Volume Waste Work Group, Two members of the 


public were invited to give presentations to the Work Group. Dr. Ishwar 


Murarka of the Electric Power Research Institute presented EPRI's utility 

- 9 ­

waste research program (1, 2, 3, 4, 5, 6, 7). Mr. William Bluke of CH2M 

Hill spoke on his experiences at Superfund mining waste sites. Two 

members of the public, Dr. Brian Murphy of Gradient Corporation and Ms. 

Susan Sawtelle representing the Edison Electric Institute and other 

utility organizations, provided oral comment. 

The latter two work groups reported their conclusions and 

recommendations at the final. full Subcommittee meeting held on September 

14-15, 1987 in Washington, DC. 

Materials given to the Subcommittee were made available to the 

public through the Superfund Docket. All meetings were announced in the 

Federal Register and opportunity for both written and oral public comment 

was provided at each meeting. 

The Subcommittee's report was approved by the Executive Committee of 

the Science Advisory Board on January 14, 1988 prior to transmittal to 

the Administrator. 

Structure and Practice of the Current HRS 

The current HRS uses a technique called structured value analysis 

(or scoring) to rank sites. In the HRS structured value analysis, a set 

of rules is developed which parallel what is thought to be occurring in 

the real world. The rules address what factors should be considered, and 

how they, should be scored and combined. The combination rules for the 

current HRS have been adjusted on the basis of professional judgment--that 

is, weights have been introduced to make the HRS output match a subjectively 

ranked test set of sites. 

The current HRS, described in Uncontrolled Hazardous Waste Site 

Ranking System: A Users Manual (8), assigns three scores to a site. Two 

are used primarily to identify facilities requiring emergency attention. ~ 

These are the score for Fire and Explosion and for Direct Contact. The 

.third, or Migration score, is. the primary basis for inclusion on the 

NPL. 

Three pathways (air, surface water, and ground water) are considered 

in developing the Migration score. For each pathway, various factors are 

considered. These fall into three broad categories: likelihood of release, 

waste characteristics, and targets. The actual factors considered can 

vary with the pathway. For example, distance to the nearest well applies to 

the ground water pathway but not to the air pathway. 
- 10 ­

After a numerical value is assigned to each factor, it is multiplied 

by a weight to obtain a factor score. Factor scores within the same 

category are added. Scores for the categories are multiplied together. 

This procedure yields a score for the pathway, The pathways are then 

combined through a method called quadratic averaging. Each step in this 

process affects the final score and, therefore, how well the HRS discriminates 

between sites of greater and lesser risk to human health and the environment. 

EPA and the states use the HRS to calculate a site score, from 0 to 

100, based on the actual or potential release of hazardous substances 

from a site through air, surface water or ground water that may affect 

people. This score is the primary technical factor used to decide if a 

hazardous waste site should be placed on the National Priorities List. 

EPA, the states and other public agencies have used the HRS to 

evaluate several thousand sites. The scores and supporting documentation 

of these sites are submitted to EPA Headquarters where they are reviewed 

to ensure correct and consistent application of the system. Sites having 

scores above 28.5 are proposed in the Federal Register for the National 

Priorities List. Based" on public comment and all available information 

EPA develops a final score for each site and places it on the NPL if the 

score is still above 28.5. 
- 11 ­

SUMMARY OF THE ISSUES 

Toxic::!ty Factor 

The Subcommittee believes that toxicity to human health and the 

environment is an important consideration in site ranking. To be useful, 

the toxicity factor should disc:rimate between "very toxic" and "not so 

toxic:" sites. If the objective of the !IRS is to generate estimates of 

relative risk, it must .·address both toxicity and exposure. Although 

toxicity is part of the current HRS, the toxicity factor makes distinctions 

among only a few of the 951 sites listed or proposed for the NPL. 

The Sax chemical toxicity ratings have provided the current HRS 

with a crude basis for prioritizing sites. The Sax system is insufficient 

because (a) it focuses on acute adverse outcomes instead of the range of 

toxicologic endpoints that need to be addressed in the HRS, and (b) it 

does not provide citations so that toxicologic: ratings can be reevaluated 

and verified for the purposes of the HRS. All of the options (El, E2, E3) 

discussed in Discussion of Options for Revising the Hazard Ranking System 

(HRS) Toxicity Factor (12) represent improvements over the Sax rating 

method. 

The Subcommittee recommends the Sax rating method be replaced with 

a toxicity factor which addresses multiple measures of toxicity because 

most known toxic chemicals are associated with a range of health and 

environmental effects. Therefore, to protect human health and the 

environment, the HRS needs to incorporate the potential for a wide range 

of adverse effects. A high score for any of these endpoints could mot·ivate 

a more detailed evaluation of a site at the RI/FS stage. 

To the extent that scientifically legitimate techniques exist for 

the consideration of various toxic: effects in humans and in nature, the 

decision of which endpoints to address is more of a policy rather than ~ 

a scientific choice. Nevertheless, because there are so many endpoints 

relevant to human health and the environment for which good methods exist, 

the Subcommittee encourages EPA to address multiple endpoints as part: of 

the toxicity factor. The multiple measures of human toxicity EPA should 

consider addressing with the revised toxicity factor include carcinogenicity, 

other chronic health endpoints used to establish reference doses, and 

acute toxicity. 

The three options presented in EPA-s Discussion of Options for Revising 

the l!azard Ranking System (HRS) Toxicity Factor (12) make use of essentially 

the same data bases for toxicity, and none of the options is overwhelmingly 

superior to the others. However, because there is no sc;entifically 

credible basis fo·r weighting the severity. of different health endpoints, 

as is currently done for calculating Reportable Quantities (RQs), the 

Subcommittee discourages use of the RQ method. Such weightings are subjective; 
- 12 ­

subsequently, the subjectivity may be obscured by the final numerical 


score, The strengths and weaknesses of the options are summarized here 


(see Appendix l for more detail). 


For acute toxicity, the Subcommittee prefers use of the Registry of 


Toxic Effects of Chemical Substances (RTECS) method rather than the 


Reportable Quantity (RQ) method, because RTECS facilitates route-specific 


toxicity ratings and closer attention to the raw toxicity data. 


For non-cancer chronic effects, the Subcommittee prefers use of the 


Reference Dose (RfD) method because the RQ method involves weighting the 


severity of various endpoints, and the modified ADI method has not received 


extensive review and may differ from the RfD method in rating individual 


chemicals, 


For carcinogenic potency the Subcommittee sees little difference 


between the two options presented (qt* vs. ED!O) and suggests that other 


measures, e.g., ED(l)and ED(O.!), be investigated. 


EPA addressed the question, "How Many Chemicals Should Be Evaluated?" 

·sased on information presented to the Subcommittee orally and in writing (10) 

it appears that the studies of this topic, while generally indicating 

only a few chemicals would dominate the health risk associated with a 

site, were limited in the number and type of sites evaluated and the 

sites were selected for convenience rather than representativeness. 

Because of this, and the fact that toxicological information is missing 

or scanty for many chemicals, the Subcommittee cannot recommend a fixed 

number of chemicals for OERR to examine at each site. A study to address 

this question ·is one of the Subcommittee's recommendations (see page 6). 

The Subcommittee recommends that EPA consider the risk represented 


by the numbers of chemicals known to be present, All things being equal, 


and because the toxicological effects of so many chemicals are unknown, a·· 

site with more chemicals should receive a higher priority in the Preliminary 


Assessment/Site Investigation (PA/SI) stages for further evaluation, 


Another approach is the assignment of discretionary points (see Appendix 5). 


The Subcommittee could not formulate an absolute recommendation on 

the question of whether separate toxicity ratings for a chemical should 

be developed for various exposure routes, Such a route-specific approach 

has appeal because toxicity is known to vary by exposure route for some 

chemicals. However, this toxic! ty question cannot be addressed indepe·ndently-­

it is inextricably tied to how the HRS deals with migration and exposure 

issues. If the revised HRS aggregates route-specific exposure scores, 

this total exposure score could be multiplied by a net toxicity score 

which would not need' to be route-specific, (The cruder the scheme, the 

less need for route-specific factors,) 
- 13 ­

Alternatively, a more accurate analysis would independently evaluate 


risks by route before aggregating. Route-specific toxicity factors would 


improve the accuracy of such an approach. Although such toxicity factors 


are not yet available for a large number of compounds, differences in 


toxicity between routes is potentially important. EPA may find consideration 


of route-specific toxicity on an exception basis to be a practical solution 


. especially applicable to compounds such as asbestos where a particular 

route, in this case inhalation, is well known to be of great importance. 

Whether or not route-specific toxicity is addressed in the HRS, it is of 

potential ·utility in the subsequent RI/FS stage because the more detailed 

information supports more realistic analyses of risks, 

The Subcommittee has discussed the need for two additional steps in 


the NPL process. First, some type of consistent methodology or guidelines 


is needed to select chemicals during the preliminary assessment and site 


investigation for use in the site ranking process~ Second, the option 


of using more detailed risk information, if available, could be used as 


a supplement to the HRS process (see Appendix 5). 


Air Target Distance 

The basis for this section of the review was Analysis of the Air 


Target Distance Limit in the Hazard Ranking System (9), 


The .Subcommittee is very supportive of OERR's efforts to modify 


the air pathway so that it considers the potential for releases, instead 


of relying solely on a score for observed releases, 


The air pathway in the current Hazard Ranking System evaluates 


population risks to derive a score for "air targets" using a matrix of 


population versus distance for four concentric circles with radii of 


l/4, 1/2, l, and 4 miles. 


The Subcommittee recommends that EPA consider alternatives to the ~ 

use of a single air target distance limit for all sites and derive a 


scoring system which uses site-specific population rings, The widths of 


these rings may •Tary by size 'Sod type of site to account for the decrease 


in air concentra.tion with distance. (See Appendix 2, especially the 


illustrations in Attachment B.) The Subcommittee favors such a refinement 


because the ring-weighting method relates more directly to risk at a 


site than does the current air target approach. The ring weighting 


method should be able to address risk more accurately because it considers 


both the population exposure experienced near the site and the toxicity 


factor ... 

In the ri.ng-:weighting approach, information on the chemicals present 

is used to estimate emissions factors, These emissions factors and information 

about the health effects that can be expected to occur at various levels of 

exposure can be used to develop ring-widths that relate to the exposure 

people living in those locations can expect to experience. 
- 14 ­

Although this method do~s not evaluate maximally exposed individual 

(MEI) risks~~· it does explicitly account for the higher exposures 

that ~ill occur ~!thin the ring closest to the site. These higher exposures 

would receive an appropriately weighted score. 

The Subcommittee was asked to address approaches to assessing health 

risks from airborn~ contaminants and has suggested a variety of approaches 

for EPA-s consideration. All of these approaches employ the following 

assumptions: carcinogens are non-threshold contaminants; there are thresholds 

for other toxic effects; concentrations of contaminants decrease with distance 

from the site; this decrease is more marked for particulates than for 

volatile chemicals; and acute toxicity and toxic effects from direct contact 

with particulates are the over-riding health concerns in the area closest to 

an uncontrolled waste site. 

Acute effects are addressed by Superfund-s emergency program and by the 

curr~nt HRS toxicity factor which is based on the Sax ratings (see Appendix 1). 

The Subcommittee favors a toxicity factor which includes multiple endpoints 

of toxicity including acute effects because some nearby populations 

around uncontrolled waste sites report symptoms consistent with acute 

toxicity (headaches, nausea, irritation, and respiratory effects) and 

because of the possibility of acute effects occuring in the event of a 

catastrophic release. Because of t~e weight given the inner-ring such 

acute effects could be well addressed with the ring-weighting approach 

or by a ring-weighting approach modified to account for the existence of 

a threshold concentration below which no acute effects should occur. 

However, other scientifically supportable options include: considering 

acute toxicity in a separate assessment outside the HRS (such as under 

the emergency program); assigning discretionary points to the HRS score 

for the confirmed presence of local odors or health effects reasonably 

associated with materials found at the site; a separate assessment of 

the probability of a release of materials from a catastrophic event; and 

treatment of acute effects in a direct-contact pathway. .· 

Chronic npn-cancer effects could also be addressed with the ring-weighting 

approach or by one modified to account for the existence of a threshold 

concentration below ~ich no chronic effects should occur. In the modified 

ring-weighting approach this threshold concentration can be converted to 

a distance from the site beyond which no adverse chronic effects would 

be expected. Chronic effects, particularly of particulate pollutants, 

could also be treated in a direct contact pathway. 

Carcinogens are dealt with in a reasonably precise manner by the 

ring-weighting approach, given the usual assumption that risk is directly 

related to average lifetime concentration in air. While the Subcommittee 

favors this approach, it is complex and EPA may want to ·evaluate the 

effects of different diameters to see if the use of a single diameter for 

all sites is reasonable. 
- 15 ­

The ring-weighting approach described in Appendix 2 is best suited 

to carcinogenic effects of volatile chemicals. While concentration of 

particulates decreases more sharply with distance than does the concentration 

of volatile chemicals, the Subcommittee believes that this difference is 

not large enough to require separate estimation methods for volatile and 

particulate emissions in the HRS. 

In assessing exposure, consideration of each media pathway is 

recommended, including air, water, food and direct contact. Measurement 

of exposure should also consider the route of exposure through which 

substances exert their effects, While the relationship bet~en eXposure 

level and the biologically effective dose is different for different 

pathways, not all compounds are toxicologically well characterized. As 

a result, EPA may only be able to deal with route-specific toxicity on 

an exception basis; e.g., in the case of asbestos; inhalation is more 

important than ingestion. 

Many of the Subcommittee's recommendations hinge on knowing the 

identities of the chemicals at the site, A minimum data requirement 

which mandates contaminant identification by record review or direct 

sampling would greatly improve the validity of the HRS in all pathways 

and is particularly helpful in estimating exposures to air pollutants 

and in addressing toxicity. For example, when the chemical identities 

are known, it should be possible to modify and simplify the Treatment 

Storage and Disposal Facilities (TSDF) procedures to calculate an 

approximate, order-of-magnitude emission rate for impoundments and 

landfills. 

Both the identity of chemicals at the site and emission rates can 

be confirmed using other investigative techniques, the most obvious of 

which is air concentration measurement. Because ambient monitoring is 

subject to the problem of dilution, to detect the presence and amount of 

emissions it is desirable to analyze at the source, or as close to the .. 

source as possible. There is, thus, an incentive to use methods of 

"probing" sources using such techniques as soil gas analysis. 

Large Volume Waste 

The large volume waste issues before the Subcommittee were the 

applicability of t.he HRS in scoring mining waste sites and the 

feasibility of using waste concentration data in a revised HRS. 

OERR provided The Superfund Hazard Ranking System (HRS): 

Applicability to Mining Waste Sites (13) and The Superfund Hazard Ranking 

System (HRS): Feasibility of Using Concentration Data in a Revised HRS (14) 

as the basis for the review, 
- 16 ­

There are two ways to evaluate the applicability of the HRS to 

mining sites. The Superfund Hazard Ranking System (HRS): Applicability 

to Mining Waste Sites (13) presents EPA's experience in ranking mining sites 

with the HRS system in terms of false positives and false negatives and 

then distinguishes between the sites according to potential danger (determined 

by other means). In its evaluation, the Subcommittee coupled this approach 

with an examination of the scientific issues independent of the HRS 

experience. 

In addressing large ~olume waste (LVW) issues, it is useful 

to note that clear qualitative differences exist between many LVW and 

the more numerous hazardous waste sites containing mixed synthetic 

organic/inorganic hazardous constituents in both liquid and solid form. 

LVW monofills are materials of a single type. Specific sites may have 

multiple types of wastes (e.g., mill tailings, smelting slags, etc,). 

Sites where these wastes are spatially separate are more easily characterized 

than sites where they are mixed. LVW are commonly contain low 

concentrations of hazardous trace elements (HTE) in the waste matrix 

which are ~;-eleased by ve~;-y specific geochemical proc.,ss"s and subject to 

migration constraints that are increasingly better understood. 

With r"spect to the question of the applicability of the current 

HRS to mining waste sites, the Subcommittee finds that the studies conducted 

rega~;-ding the adequacy of the HRS listing process for mining sites have 

been limited and inconclusive. The reports reviewed included the TRC (27, 28, 

29) and MITRE (24, 25) reports contained in The Superfund Hazard Ranking 

System (HRS): Applicability to Mining Waste Sites (13) as well as EPA's 

own studies. Sufficient experiential evidence has not been presented to 

show an inadequacy of the HRS in regard to mining sites. The present 

system has possibly rated mining sites no worse than other waste sites. 

Some of the findings supporting this conclusion follow. .. 

(l) 	 While the TRC (27, 28, 29) and MITRE reports (24, 25) reported 

that mining waste sites scored higher in waste quantity compared 

to non-mining sites (as expected given the large volume of 

wastes generated by the mining industry), TRC did not conclude 

that this bias caused false positives or false negatives for 

mining sites. Both sets of reports found that waste quantity 

contributed relatively little to the final HRS score. 

(2) 	 Although the TRC reports (27, 28, 29) suggested the degree of 

information available was a major factor in ~PL listing of 

mining sites, they presented no data showing that the relatively 

larger data bases for certain mining sites ~;-esulted in unfair 

scoring~ · 

(3) 	 The TRC reports (27, 28, 29) also suggested the final HRS score 

for mining sites could be predicted by population alone. Since 

the TRC reports only addressed· mining sites, they could not 

prove a bias against mining waste sites. 
- 17 ­

Nevertheless, the Subcommittee believes the present HRS is not well 

sui ted for scoring potential releases from mining site wastes because 

mobility and concentration are not included, Therefore, the present 

scoring system has a P?tential to treat mining wastes with systematic 

et"t'or. 

For the ground water pathway, the concentration of a constituent in 

a waste is of much less importance than the concentration of that constituent 

in the leachate produced at the site. Consequently, more emphasis should 

be placed on the partitioning of hazardous constituents between the 

waste and the leachate, Leachate concentration or interstitial pore 

water concentration, either known or estimated, should then be factored 

into the waste quantity term in the HRS, For wasees which are not in 

solid form, total contaminant mass quantity should continue to be used, 

Improved ways of considering the concentration, toxicity (inorganics 

have special characteristics), release (mobility in various matrixes), 

and information on the transport/transformation of chemicals would make 

the HRS more accurate. The area of contact of the site with the environmental 

compartments (air, water, humans, etc.) is also an important factor. 

The issue of mobility is especially important at mining sites because 

the large.volume of mining wastes often means large masses of the contaminant 

are available for release. However, certain conditions must exist for these 

releases to occur, The current HRS does not address specific ionic form 

of inorganic metals, and ion form affects mobility. 

Because·mobility is a more discriminating concept than persistence 

in the subsurface, the Subcommittee suggests that OERR explore means of 

incorporating important matrix characteristics in a revised HRS. Such 

characteristics include extreme acidity or alkalinity (as expressed by .• both high and low pH), any crystalline phase modifications of mining 

wastes that differ from native geologic materials, and the sorptive 

capacity of surrounding geologic materials. The latter influences 

migration tenden.cy. 

With respect to the feasibility of using concentration data in the 

revised HRS, the Subcommittee finds that including hazardous constituent 

concentration is desirable because both the severity of observed releases 

and the risk of potential releases are related to concentration. 

EPA has proposed two options for modifying the HRS to account for 

the concentration of hazardous constituents. Both approaches translate 

waste constituent concentration data into an estimate of· the total mass 

of a hazardous constituent at the site, which is then used to compute 

the waste quantity ·score for the HRS. Even the approach which uses 

"total mass" of hazardous constituent could contribute to produce false 

positives for some large volume wastes and pathways. 
- 18 ­

The Subcommittee-s preferred approach (Option 3) encourages the use 

of concentration data and provides the flexibility to substitute indirect 

estimates such as default concentrations to estimate a constituent-s 

total mass when direct measurements of concentrations are not available~ 

While the direct measurement approach (Option W2) provides the soundest 

scientific hasis for the HRS is most cases, it may not be practical, due 

to safety and cost considerations for every site at the site inspection 

stage. The Subcommittee suggests the direct use of concentration rather 

than conversion to a mass value. 

A method of determining a "representative concentration" for a site 

based on a stratified sampling strategy would be useful. In this approach, 

a complex site could be subdivided into a set of more homogeneous regions 

or strata, and a representative value for each stratum determined through 

limited sampling. Statistical techniques exist for manipulating stratified 

data, which could then be recombined with appropriate weighting into a 

final single HRS score for the site. 

Exposure 

To the extent that the HRS is intended to assess relative risk, it 

must address both toxicity and exposure sufficiently. 

Exposure occurs when there is contact between pollutants and receptors. 

It is important to address exposure when there are toxic chemicals present 

in order to determine whether there is any actual or potential risk to 

health or the environment. Where there are no toxic substances or where 

there is no exposure, there is no risk, even though there may be release 

or contamination. (However, determining the existence of a release or of 

environmental contamination is often the first step in evaluating actual 

exposure,) Where there is incomplete information on either toxicity or 

exposure, risk cannot be fully assessed. Risk can be evaluated by 

assessing four parameters: (a) the presence of chemicals, (b) their 

potential for release and mobility, (c) the probability of contact with 

humans, plants or animals and (d) their intrinsic toxicity. 

Much could be gained by adding a chemical-specific exposure score 

that could be combined with chemical-specific toxicity scores to evaluate 

which chemicals dominate the risk at a single site and how the risk 

might compare across sites that are otherwise similar, 

Given some minimal info~ation about a site and its chemical inventory, 

it may be possible to make crude estimates of partitioning among environmental 

media and, therefore, of emissions by route which could ,be used to generate 

part of a chemical~specific exposure score. The following information would 

support development of a chemical-specific exposure score: 

1. 	 An estimate of the total mass or source concentration 


of important chemicals at the site. 

- 19 ­

2. 	 A description of the site-specific factors influencing 


release, e.g.; impoundment vs. landfill. 


3. 	 Partition factors applicable to the chemical, e.g. Henry~s 

Law constant and organic carbon partition coefficient. 


Some· characterization of the potential for migration also allows 

better consideration of time as a factor in site management, 

Environmental pathways include air, water (both ground and surface 

water), soil, and plants and animals that become part of the human food 

chain. The potential for release and mobility of chemicals through 

these pathways affects risk.'estimates. Chemicals- behave 'differently in 

different environmental pathways, so all pathways need to be evaluated 

to estimate risk effectively, 

The main difficulty of dealing with exposure directly is that we 

are dealing with potential exposure as well as actual exposure, There 

are two aspects of exposure to be considered--the concentration of contaminants 

in the environment around the site snd .the number of people potentially 

exposed to these concentrations. A~tual release may be demonstrated by 

environmental sampling, Potential release may be addressed by considering 

properties of the site and characterisitics of the chemicals present, 

such as distance to ground water and volatility. Similarly, the number 

of people (receptors) present and actually exposed today can be counted, 

while the number of people who might be present at the location in the 

future can only be estimated from .studies of population dynamics. 

Measurement of exposure may be person-based or community-based. 

While estimates of individual person-based exposures can be developed 

from biological samples, from personal household samples, from self-reported 

exposures, and from self-reported symptoms, each approach has its limitations. 

Community-based exposures can be developed from ambient pathway 

measurements, from site measurements, and from modeled exposures. Numerous 

assumptions about targets and behavior are needed to impute exposure 

from ambient pathway measurements, However, even more assumptions are 

needed to impute exposure from site measurements. 

Incorporating concentration and mobility could produce a more 

comprehensive site assessment procedure. One way to do this is by using 

concentration as a weighting factor for the waste quantity. The resulting 

effective waste quantities for low volumes of high concentration waste 

might be similar to. those for large volumes of low concentration wastes. 

Similarly, the scores for toxicity/persistence and for effective waste 

quantity could be adjusted by considerations of mobility. 
- 20 ­

Algorithm 

Improving the algorithm could potentially do more to improve the 

HRS than fine-tuning individual components. 

The Subcommittee has recommended changes that will allow the HRS to 

provide a more accurate and scientifically based estimate of the relative 

risk of candidate sites, To the extent possible, the HRS scores should 

correspond to an objective analysis of relative risk at sites. However, 

this is not always feasible due to both scientific and data limitations, 

as well as the value and policy decisions implicit when considering and 

balancing human health and environmental impacts. 

A revised HRS, better designed to evaluate sites by relative risk, 

will provide an improved mechanism for determining which sites should be 

included on the NPL, and can potentially provide meaningful input to the 

subsequent prioritization of NPL sites. However, the Agency must continue 

to base this prioritization on many factors in addition to the HRS. Most 

of the changes needed to transform the current HRS into a system more 

reliably related to risk are changes in the overall algorithm and not 

changes with vast new data requirements, 

Internal consistency is important. Making rules that are consistent 

requires little up-front investment and no additional costs in the scoring 

of specific sites. These changes can be more cost-effective than some 

improvements to portions of the HRS alone, especially ones requiring 

additional data collection. 

The way'that various components of the score are combined should 

reflect how their real world counterparts interrelate. The approach for 

accomplishing this is to begin with a physically-based exposure assessment 

model for each exposure pathway, structured to properly translate expected 

nr potential releases into environmental concentrations and subsequent ·· 

exposures and effects. These models would, of necessity, be highly 

simplified for a screening assessment. Even a simplified exposure assessment 

model may not be, feasible given the time, resource and data limitations 

associated with the HRS process. However, the manner in which the pathway 

scores are estimated and combined should be consistent with the fundamental 

material balance and exposure principles of such an underlying model. This 

can be accomplished in the context of the structured value approach used 

in the current HRS system. 

When there isn't "enough" data for the HRS, it iS necessary to have 

a default value that encourages data collection (also termed missing 

value replacement), Lacking complete information on chemical inventory 

and concentration at a site may well be the rule rather than the exception, 

Therefore, a minimum data requirement will reduce the inconsistencies 

that are otherwise inevitable when different people perform the scoring. 
- 21 ­

One concern with the use of default data is that the fact of data 

limitations could indicate that the site might be different from otherwise 

comparable sites. The definition of comparable sites is crucial and may 

depend on the scoring component, 

When data are available that exceed wh.ft is needed for the HRS, 


there is the question of how to use all of it. The default or tiered 


approach OERR presented on concentration is one mechanism for using 


additional -data in the revised HRS. Another approach is the assignment 


of discretionary points. The Subcommittee discussed several scenarios 


where discretionary points could be assigned including sites where: 


a very large number· of chemicals were identified, the presence of the 


toxic materials in the surrounding population had been demonstrated, and 


toxic particulate air emissions were anticipated. For the HRS score to 


reflect relative risk, the extra points assigned must be proportional to 


the extra risk the situation presents. (See Appendix 5.) 


Recommendations to Evaluate and Improve the HRS 

The current HRS is appropriate for the original purposes OERR has 

·described because most of the factors scored are related to risk, and 

higher scores for those factors reflect higher risks, Thus, the current 

HRS is plausible, However, it is both possible and desirable to revise 

the HRS to relate more closely to the relative risk posed by uncontrolled 

waste sites. 

The HRS performance should be judged by an· empirical retrospective 

evaluation of how successfully the HRS predicts risk or on how successfully 

its components predict phenomena (such as release) which contribute to 

risk. this evaluation should be based on an in-depth technical review. 

Whatever the definition of risk, the RRS should be judged on how well it 

approximates that definition, not on how well it matches a subjective 

notion of the relative importance of the sites in a test set. .· 

The Subcommittee recognhes the possibility that sites containing 


hazardous chemicals whose toxicity is not widely known will be omitted 


from the NPL 11st.1ng, thus possibly creating false negatives. Measures 


to counter this problem include continuing to develop toxicity profiles 


on more chemicals and keeping good records so that sites with currently 


unstudied chemicals can be re-evaluated as toxicity information on these 


chemicals becomes available, 


The Subcommittee recognizes the need for some flexibility in the HRS 


to allow for the wide range of physical and chemical characteristics of 


sites, Being flexible is compatible with reducing the possibility of 


error in the HRS, EPA must reduce this possibility of error in each 


component (or "factor") of the system and, even more importantly, in the 


algorithm used to relate these components into a score that is intended 


to reflect relative risk, 

- 22 ­

To adequately evaluate the current or modified HRS, in-depth studies 

should be conducted to determine the effectiveness of the HRS to assess 

risk and to identify areas for improvement, One goal of these studies 

would be to improve the algorithm so that it can be defended as risk-related. 

Another goal is to to determine at least a crude method for estimating 

the quantity of specific chemicals present. Any HRS evaluation studies 

should be thoroughly peer reviewed before their initiation, 

Future evaluation of the HRS should consider the possibility of 


incorporating detailed numerical geochemical transport/fate models which 


are currently under development, The advantage of using such models in 


a future HRS is the explicit inclusion of quantitative representation of 


specific processes and mechanisms reponsible for the release, transport, 


transformation, and retention of hazardous compounds. In comparison to 


structured value (or "scoring") approaches, the models increase data 


requirements, but can yield less ambiguous, more detailed simulations 


appropriate for pathway calculations and, ultimately, exposure estimates. 


As m<!ntioned in the Oth"r Comments portion ·Of Appendix 3, EPA should 

develop studies both to review large-~olume waste sites ranked under the 

. model and to examine basic: parameters of the model, 

.• 
APPEND!X 1 

REPORT OF 

THE TOXICITY FACTOR WORK GROUP

• 

TO THE 


HAZARD RANKING SYSTEM SUBCOMMITTEE 


OF THE 


SCIENCE ADVISORY BOARD 

U, S. ENVIRONMENTAL PROTECTION AGENCY 


SCIENCE ADVISORY BOARD 


HAZARD RANKING SYSTEM REVIEW SUBCoMMITTEE 


TOXICITY FACTOR WORK GROUP 


ROSTER 


CHAIRMAN 


Dr. Warner North, Principal, Decision Focus Inc. Los Altos Office Center 

Suite 200, 4984 El Camino Real, Los Altos, California 94022 

MEMBERS/CONSULTANTS 

Dr. Paul Deisler, 11215 Wilding Lane, Houston, Texas 77025 

Dr. John Doull, Professor of Pharmacology and Toxicology, 

University of Kansas Medical Center, Kansas City, Kansas 66103 

Or. Joseph V. Rodricks, Environ Corporation, 1000 Potomac Street, N.W,, 

Washington, DC 20007 

Dr. Ellen Silbergeld, Chief Taxies Scientist, Toxic Chemicals Program, 

Environmental Defense Fund, 1616 P Street, N.W., Room 150, 

Washington, D.C. 20036 

Dr, Rebecca T, Zagraniski, Division of Birth Defects and Developmental 

Disabilities, Center for Environmental Health and Injury Control, 

Centers for Disease Control, Koger Center, Room 2008 (F-37), 

1600 Clifton Road, N.E., Atlanta, Georgia 30333 

EXECUTIVE SECRETARY (Until August 31, 1987) 

Mr. Eric Males, Executive Secretary, Science Advisory Board (A-101F), 

u.s, Environmental Protection Agency, 401 M Street, S.W., 

Washington, DC 20460 (Until August 31, 1987) 

EXECUTIVE SECRETARY (After August 31, 1987) 

~rs. Kathleen W, Conway, Deputy Director, Science Advisory Board, (A-101F), 

u.s, Environmental Protection Agency, 401 M St., S.W., 

~ashington, D.C. 20460 

STAFF SECRETARY 

~rs, Dorothy M. Clark, Staff Secretary, Science Advisory Board, (A-101F) 

U.S. ,Environmental Protection Agency, 401 M Street, S,W,, 


Washington, DC 20460 

TABLE OF CONTENTS 

Page Number 

GENERAL COMMENTS Al-l 

RESPONSES TO SPECIFIC TOXICITY ISSUES POSED BY OERR Al-3 


How Many Chemicals Should Be Evaluated? Al-3 


Route-specific Toxicity Scores Al-4 


Noncancer Effects of Carcinogens and Acute Toxicity Al-4 


Advantages and Disadvantages of Certain Toxicity 


Ranking Options Al-6 

Default Values Al-6 

Combining Toxicity Scores Al-6 

OTHER COMMENTS Al-6 

Flexibility and Common Sense Al-6 

General Support for a Structured Value Approach to the HRS Al-6 

Two Additional Steps in the NPL Process Al-6 

Migration Al-7 ~ 

Dose Al-7 

Future Studies Al-7 

ATTACHMENT: Discussion of Options for Revising the Hazard Ranking 

System (HRS) Toxicity Factor, May 4, 1987 pages 4Q-41 
Al-l 

GENERAL COMMENTS 

The Toxicity Factor Work Group reviewed Discussions of Options for 

Revision of the Hazard Ranking System (HRS) Toxicity Factor (12). 

The Toxicity Factor Work Group believes that toxicity is an important 

consideration in assessing sites because, if the objective of the HRS is to 

generate estimates of relative risk, the HRS must deal with toxicity and 

exposure. The toxicity factor in the current HRS makes distinctions 

among only a few of the 951 current and proposed site on the NPL. By 

pathway, fully 85% (ground water), 87% (surface water), and 97% (air) of 

these sites received the maximum toxicity/persistence score, Because 

the Agency did not present information on sites that are not on the NPL, 

the Work Group was unable to determine whether the current toxicity 

factor is adequate to distinguish between "very toxic" and "not so toxic" 

sites. ~ot only is it unlikely that the most toxic substances at different 

sites are equally toxic, it is inconceivable that they occur at equal 

concentrations even if total quantity of waste is id~ntical, Ther~fore, 

in revising the HRS, the EPA should consider approaches that would 

better discriminate between sites of differing toxicities and subsequently 

examine how well the new toxicity f~ctor actually performs, 

Some toxicity decisions are made in the Preliminary Assessment/Site 

Investigation (PA/SI) stage. For example, specific chemicals are selected 

for scoring in the HRS, while other chemicals are dropped from the process, 

The bases for these selections do not appear to be standardized at this time. 

As a result, while the toxicity data that is handled by the HRS is already 

a subset of the full information, additional (perhaps important) toxicologic 

data are not part of the quantitative process, The exposure factors are 

addressed in various ways in the HRS and most are handled quantitatively 

in the HRS. Consequently, the HRS score actually reflects a disproportionate 

amount of information on exposure and less on toxicity, thus deewphasizirig 

toxicity in the final score. Therefore, EPA should consider the development 

of some type of consistent methodology or guidelines for evaluating the 

toxicity of chemicals at a site during the PA/SI stages, 

The Toxicity Factor Work Group reviewed the following methods (12) for 

evaluating the toxicity of individual chemicals: 

o 	Chronic Toxicity (Noncancer) 


- Reference Dose Method 


- Reportable Quantity (or Composite Score) Method 


- Modified Acceptable Daily Intake (ADI) Method 


o 	Carcinogenicity 


- Cancer Potency Factor Method 


- Reportable Quantitites (RQ) or Effective Dose10 Method 


- EPA Weight-of-Evidence Method 


o 	Acute Toxicity 


- Reportable Quantities (RQ) Method 


- Registry of Toxic Effects of Chemical Substances 


(RTECS) Method 
Al-2 

The Sax chemical toxicity ratings have provided a very crude basis 

for prioritizing sites because they are largely based only on the acute 

toxicity of one constituent of the waste. The Sax system is insufficient 

because it focuses on acute adverse outcomes instead of the range of 

toxicological endpoints that need to be addressed in the HRS and because 

it does not provide citations so that toxicologic ratings can be reevaluated 

and verified f.or the purpose of the HRS. 

OERR presented three options for evaluating and scoring toxicity in the 

revised HRS (12), These are a reportable-quantities-based option (El), an 

option based on reference doses and potency factors (E2), and an option based 

on modified acceptable daily intake and 1Neight-of-the-evidence approaches. 

Each of these options takes into account several types of toxicity and uses 

quantitative measures of substance potency, 1Nhereas the Sax method is 

unidimensional and developed for protection against poisoning. The 

three options presented by OERR are much better than the Sax rating 

method, and the Work Group recommends that EPA replace the Sax method. 

Because OERR's three options make use of essentially the same data 

bases for toxicity (although the data are processed differently), the 

Work Group does not believe any of the options is overvhelmingly superior 

to the others. The options do have different strengths and weaknesses, 

however, and the Work Group has addressed these below in (page Al-5). 

Although this is really a policy decision rather than a scientific 

one, the Work Group does not encourage a system for weighting the severity 

of different health endpoints, as is currently 'done for calculating 

reportable quantities (RQs), both because such weightings are subjective, 

and because they are subsequently obscured by the final numerical score. 

Similarly, to the extent that scientifically legitimate techniques 

exist for the consideration of various toxic effects in man and in nature, 

the decision of which endpoints to address is primarily a science policy ·• 

decision rather than a purely scientific one. Nevertheless, because 

there are many endpoints relevant to human health and the environment 

for 1Nhich good methods exist; the Work Group encourages the Agency 

to address multiple endpoints as part of the HRS toxicity factor. 

The Work Group's efforts to review the toxicity issues were complicated 

by the follo1Ning factors. 

o 	 It is difficult to translate the objectives of the HRS (that 


is, a scheme that separates sites into those with risks greater 


or lesser than an arbitrary threshold, with a desire to avoid 


false negatives [false placement of high risk sites·into the 


low risk category], and to the extent possible, that ranks all 


sites by their relative risk) into operational criteria to 


design the scheme. 

Al-3 

o 	 The Agency has presented the toxicity issues in a somewhat 


theoretical manner. Few real world examples and few test cases 


such as example rankings of chemicals or sites, were presented 


to the Work Group. 


o 	 Lastly, and probably most important, the Work Group believes 

that toxicity issues are integrally linked to dose (i,e., migration 

and exposure) issues. The existing HRS system deals with exposure 

by using surrogates to address the question of whether people 

(receptors) will be exposed, and if so, how many will be exposed, 

and to what concentrations, via ground wate~, surface, and air routes. 

RESPONSES TO SPECIFIC TOXICITY ISSUES POSED BY OERR 

The full text of the specific questions can be found in the 

attachment to this report. They are repeated in abbreviated form 

here for the convenience of the reader. 

How Many Chemicals Should Be Evaluated? 

A review of past Superfund risk assessments were undertaken by OERR 

(10) to determine whether the health risk associated with a site is 

likely to be dominated by one or a few chemicals (for each pathway), or 

by a larger number of chemicals. The study, which was limited in the 

number and type of sites considered and because sites were selected for 

convenience rather than for representativeness, generally indicated that 

only a few chemicals would dominate at any given site. However, the 

Work Group did not review this study in detail and cannot recommend a 

fixed number,of chemicals for OERR to examine at each site. Instead, 

the Work Group offers the following gene(al guidance. 

EPA appropriately considers toxicity early in the site evaluation 

process. Identification of known toxic substances occurs in the PA/SI 

stages prior to the HRS evaluation. Indeed, only sites containing toxic' 

substances currently identified by the Superfund Laboratory Program are 

likely to be candidates for HRS evaluation and listing, In addition, 

the chemical rec·eiving the highest score in the toxicity-persistence 

matrix (8) is selected by field staff for scoring purposes in the HRS, 

The Work Group has concerns about the procedure now used to identify 

chemicals in the PA/SI stages and to select a single "most toxic" 

chemical for the HRS evaluation, 

Because so few chemicals have been toxicologically well characterized, 

the Agency should consider a factor fo( sheer numbers of chemicals known 

te> be present (in addition to developing scores based on a smaller number 

of chemicals of known toxicity). All things being equal, a site with 

more chemicals should receive a higher priority for further evaluation, 

most probably in the RI/FS. (See discussion of discretionary points in 

Appendix 5 of the HRS Subcommittee's report.) 
Al-4 

Should Separate Toxicity Ratings for a Chemical be Developed for 

Various Exposure Routes? 

Because this question is inextricably related to how the HRS deals 

with migration and exposure issues, the Work Group has chosen to discuss 

the advantages and disadvantages of three approaches rather than to make 

a single recommendation. 

One approach is to aggregate route-specific exposure scores into a 

net (total) exposure score for a site. This total exposure score could 

be multiplied by a net toxicity score. The toxicity score may not need 

to be route-specific because the cruder the scheme, the less the need 

for route-specific factors. 

A more accurate analysis would independently evaluate route-specific 

risks before aggregating, Because the relationship between exposure level 

and the biologically effective dose is different for different pathways, 

route-specific toxicity factors would improve the accuracy of such an 

approach, 

A third approach, which recognhes that route-specific toxicity 

factors are not yet available for a large number of compounds, is to use 

route-specific information when it is available and relevant--such as in 

the case of asbestos where the inhalation route has far greater biological 

effect than the ingestion route--and to aggregate exposure scores where 

such information is lacking or the route-specific differences are small. 

Whether or not route-specific toxicity is addressed in the HRS, it 

should be considered in the Remedial Investigation/Feasibility Study (RI/FS) 

stage because the more detailed information supports more realistic 

analysis of risks. 

How should potential carcinogens, noncancer effects and acute toxicity .. be evaluated as part of a scoring system used to determine listing on 

the NPL? 

The Work Group concludes that multiple measures of toxicity should 

be used in the process of evaluating waste sites because a variety of toxic 

effects may be experienced by the exposed human and natural populations. 

Which endpoints are of concern to EPA is primarily a science policy 

decision, However, scientific techniques exist which would permit the 

HRS to consider several endpoints which are relevant to human health and 

the environment, 

The Work Group believes the importance of acute effects in the HRS 

should be decreased relative to chronic effects because, given the likelihood 

for long-term low-level exposures around uncontrolled waste sites, chronic 

effects appear more relevant. Futhermore the Work Group understands that 

EPA has a separate emergency response program to respond quickly to those 

uncontr~lled waste sites where acutely toxic exposure levels are thought 
Al-5 

possible. As a result, the Work Group recommends replacing the Sax method 

with any of several OERR-developed approaches which include measures of 

chronic toxicity. 

Acute effects are important when they occur, and some Subcommittee 

members conclude that, since the emergency program suffices to address them, 

acute effects need not be considered in the revised HRS. However, because 

of reports of acute effects such as odors, nausea, headache, and respiratory 

irritation around unremediated and uncontrolled hazardous waste sites and 

because of the potential for catastrophic releases, or releases during 

remediation which may result in exposures sufficient to cause acute 

effects, the Work Group recommends that the revised HRS include acute 

effects among the multiple endpoints considered. 

Besides carcinogenicity, the health endpoints should include 

those used to establish reference doses (including at a minimum, fetotoxic, 

teratogenic, and neurotoxic effects); and acute toxicity (including not 

only lethality, but endpoints such as irritation, allergic sensitization, 

and neurotoxicity). The major concerns for most sites will be for chronic 

health effects (cancer and non-cancer). While acute toxicity should be 

examined, it will generally be less important than chronic toxicity in 

the site ranking process. Any of the variety of health and environmental 

endpoints considered could motivate ·a more detailed evaluation of a site. 

For acute taxies, the Work Group prefers the Registry of Toxic 

Effects of Chemical Substances (RTECS) method to the Reportable Quantity 

(RQ) method because RTECS facilitates route-specific toxicity ratings 

and closer attention to the raw toxicity data. If a more sophisticated 

use of the toxicological data is envisioned, use of the RQ evaluations 

of RTECS may be helpful to identify the preferred animal studies for 

toxicity ratings. Use of Threshold Limit Values (TLVs), which are 

contained in RTECS, may be one means of expanding the acute toxicity, 

including not only lethality but also other endpoints. , 

For non-cancer chronic effects the Work Group prefers the Reference 

Dose (RfD) method because the RQ method involves subjectively weighting 

the severity of various endpoints and the modified ADI method has not 

received extensive review and may differ from the RfD method in rating 

individual chemicals. 

For carcinogenic potency the Work Group does not perceive much 

gross difference between the two options presented (q 1 * vs. EDIO). Other 

measures (e.g., ED [I) and ED [0.1]) should be investigated because they 

refer to a more realistically meaningful risk range for human exposures and, 

where the dose-response is known to be concave upwards, they would offer an 

automatic correction to potency for this fact. In evaluating relative risk 

for carcinogens, both weight-of-evidence and potency should be considered 

in determining relative risk for carcinogens. 

The Work Group judges that the Agency's procedure in the RQ calculation 
Al-6 

for combining weight-of-evidence and potency information is reasonable for 

purposes of ranking different carcinogens. 

Are There Scientific Advantages/Disadvantages to Basing a Toxicity 

Ranking Method on a No-Adverse-Effect Level? 

The Work Group chose to address the advantages and disadvantages of 

the options presented rather than enter into an abstract discussion on 

NOAELs and alternatives. For non-cancer chronic effects the Work Group 

prefers the Reference Dose (RfD) method to the Reportable Quantities 

(RQ) method (which involves weighting the severity of various endpoints), 

and to the modified Acceptable Daily Intake (ADI) method (which has not 

received extensive review and may differ from the RfD method in rating 

individual chemicals). 

Is 1t Necessary to Attempt to Deve.lop Default Values for Rating a 

Substance's Toxicity When its Toxicity Data Base is Limited? 

The Work Group has no recommendation on this issue. Having little 

patience with "invented toxicology" the Work Group wishes to encourage 

the Agency to develop data rather than relying on default values. In 

cases where toxicological data become available after a site has been 

scored, the Work Group encourages the Agency to re-evaluate the site in 

the light·of the new findings. 

What Quantitative Procedures are Most Reasonable for Combining 

Individual Toxicity Ratings for Multiple Substances at a Site? 

The Work Group does not encourage a system for weighting the severity 

of different health endpoints, as is currently done for calculating 

Reportable Quantities (RQs). The Work Group believes that a variety of 

endpoints should be used to rank sites for further study. In the context 

of an on/off switch, any health effect may be an adequate criterion for "' 

further study of a site. 

OtllER COMMENTS 

Flexibility and Common Sense 

rhe HRS is designed to be a simple, workable, mechanistic procedure 

that generates reliable estimates of relative risk. However, considering 

the state of information available at the PA/SI stage, the complexity of 

risk, and the variability among sites, this may be an impossible goal. 

In light of these uncertainties, there are many opportunities for 

errors in both overestimating and underestimating relative risk. Thus, 

the Work Group believes thac any system must offer flexibility and the 

opportunity for common sense. 
Al-7 

General Support for a Structured Value Approach to the HRS, 

Even though, in the Work Group's view, it may not be possible to 

develop an HRS that provides reliable estimates of relative risk, the 

Work Group is supportive of a HRS concept and role which attempts to 

rank sites by relative risk. That is, the Work Group supports the use 

of a structured value analysis that includes (even if crudely) factors 

related to risk. The estimates generated by such a scheme may be simply 

one mechanism for evaluating sites. 

Two Additional Steps in the NPL Process 

The Work Group has discussed the need for two additional steps in 

the NPL process: (a) some type of consistent methodology or guidelines 

to select chemicals during the preliminary assessment and site investigation 

should be developed for use in the site evaluation process, and (b) the 

option of using more detailed risk information, if available, should be 

considered as a supplement to the HRS process., One approach, for example,· 

is to allow adjustment of point scores to take into account special 

knowledge, as discussed in Appendix 5 to the Subcommittee's report. 

Migration 

Migration of pollutants from a site to a receptor (considering both 

time and kinetics) is an important part of the exposure assessment, 

Where there is no contact between pollutants and receptors, there is no 

risk. The current HRS does not evaluate mobility at all and does not 

sufficiently evaluate migration (which the current HRS addresses in the 

ground water,' surface water·and air pathways) and, thus, can be expected 

to misclassify some sites with respect to relative risks, 

Dose .· 

Dose information is included in the HRS evaluation only through the 

very crude surrogate of total waste quantity, Concentration information 

for specific substances, if available, is not used in the HRS. The Work 

Group is concerned about dose as it relates to the types and severity of 

effect. It is an important element for predicting risk, but it is not 

apparent that any effective surrogates exist to address it in the current 

HRS. 

Future Studies 

Some members of the Work Group are concerned that si.tes containing 

hazardous chemicals.whose toxicity is not widely known will be omitted 

from the NPL listing, thus creating false negatives. Measures to counter 

this problem include continuing to develop toxicity profiles on more 

chemicals and record keeping so that sites with "unknown chemicals" can 

be re-eva.luated as toxicity information on these chemicals becomes available. 
Al-8 

The lack of toxicological information on many chemicals is hardly a 

problem unique to the Superfund program and the Work Group does not 

wish to imply that this difficulty can be solved by Superfund, or indeed 

the Agency, alone. 
SPECIFIC QUESTIONS FOR THE SCIENCE ADVISORY BOARD 

This section lists a few specific unresolved questions about 

scientific issues involved in the development of a revised toxicity 

factor for the HRS. EPA believes that these questions are significant 

and need to be addressed before a revised toxicity factor can be developed. 

(1) How many chemicals per site (or exposure medium) 

should be evaluated to serve as the basis for an 

overall toxicity factor score? If less than all 

chemicals detected, how should they be selected? 

Should selection be based on the most toxic 

substances, the most frequently detected, or those 

that appear to be present in the greatest quantities? 

(2) Should separate toxicity ratings for a chemical be 

developed for various exposure routes (i.e., should 

chemicals have three or four different ratings, 

depending on exposure route?) , or is a single 

route-independent toxicity rating adequate for HRS 

ranking purpose? [Note: It is assumed that separate 

media-specific toxicity factor scores would be 

developed for a site under any HRS revision option 

based on the chemicals evaluated for that medium, 

regardless of whether route specificity is accounted 

for in individual chemical toxicity ratings.] 

(3) Should potential carcinogens be evaluated for 

noncancer effects also, or can it be assumed that the 

carcinogenicity potential should dominate the overall 

toxicity rating? 

(4) Is it advisable to include acute toxicity evaluation 

as part of a scoring system used to determine listing 

on the National Priorities List (NPL)? If so, how 

should it be weighted? 

(5) Should more specific noncancer toxicity types, such as 

teratogenicity and mutagenicity, be broken out and 

rated separately? If so, how should they be weighted 

relative to other effect types so that single 

overall rating could be derived for each chemical? 

(6) Are there scientific advantages/disadvantages to 

basing a toxicity ranking method on a no-effect level 

NOAEL, such as in Options E2 and E3 versus an effect 

level (MED, such as in Option El)? 
- 2 ­

(7) 	 It is necessary to attempt to develop default values 

for rating a substance's toxicity when its toxicity 

data base is limited, or should site scores be 

developed based only on chemicals having "better" 

toxicity data? 

(8) 	 What quantitative procedures are most reasonable for 

combining individual toxicity ratings for multiple 

substances at a site? 

•" 
APPENDIX 2 

REPORT OF 


THE AIR TARGET DISTANCE WORK GROUP 


TO THE 


HAZARD RANKING SYSTEM REVIEW SUBCOMMITTEE 


OF THE 


SCIENCE ADVISORY BOARD 


.· 
U. S. ENVIRONMENTAL PROTECTION AGENCY 


SCIENCE ADVISORY BOARD 


HAZARD RANKING SYSTEM REVIEW SUBCOMMITTEE 


AIR TARGET DISTANCE WORK GROUP 


ROSTER 

CHAIRMAN 

Dr. Thomas Burke, Deputy Commissioner, New Jersey Department of Health 

Room 805, State Health CN360, Trenton, New Jersey 08625 

MEMBERS/CONSULTANTS 

Dr. Stephen L. Brown, Project Manager, Environ Corporation 

1000 Potomac Street, N.W., Washington, DC 20007 

Dr. Shepherd Burton, Systems Applications, 101 Lucas Valley Road 

San Rafael, California 94903 

Mr. George F. Carpenter, Michigan Department of Natural Resources, 

Environmental Response Division, Post Office Box 30028 

Lansing, Michigan 48909 

Dr. Naihua Duan, Statistician, Rand Corporation, 1700 Main Street, 

Santa Monica, California 90406 

Dr. Donald Mackay, Department of Chemical Engineering and Applied 

Chemistry and Institute for Environmental Studies, 

University of Toronto, Toronto, Canada MSS 1A4 

Dr. Mitchell Small, Department of Civil Engineering, Carnegie Mellon 

University, Schenley Park, Pittsburgh, Pennsylvania 15213 .• 

EXECUTIVE SECRETARY (Until August 31, 1987) 

Mr. Eric Males, Executive Secretary, Science Advisory Board, (A-101F), 

u.s. Environmental Protection Agency, 401 M Street, s.w., 


Washington, DC 20460 (Until August 31, 1987) 


EXECUTIVE SECRETARY (After August 31, 1987) 

Mrs. Kathleen w. Conway, Deputy Directot", Science Advisory Board, (A-101F), 

U.S. Environmental Protection Agency, 401 M St., S.W., 


Washington, D.C. 20460 


STAFF SECRETARY 

Mrs. Dorothy M. Clark, Staff Secretary, Science Advisory Board, (A-lOlF), 

U.S. Environmental Protection Agency, 401 M Street, s.w., 


Washington, OC 20460 


ATTACHMENT A 

POINTS FOR SAB CONSIDERATION 

o 	 Use of a Single Target Distance Limit at all Sites 

o 	 Applicability of Air Emissions Information fro~ 

TSDF to Superfund Sites 

o 	 Approach to assessing Health Risks 

(Is cancer most Conservative?) 

o 	 Range of Emissions Est.imates used 

.· 

• 
TABLE OF CONTENTS 


PAGE 

GENERAL COMMENTS A2-l 

COMMENTS ON THE ISSUES RAISED BY OERR A2-2 

An Alternative to the Use of a Single Target A2-2 

Distance Limit at All Sites 

Approach to Assessing Health Risks A2-4 

Applicability or Air Emissions Information from TSDFs to A2-5 

Superfund Sites and the Range of Emissions 

Estimates Calculated 

Other Factors Affecting the Potential for Release A2-8 

OTHER COMMENTS A2-8 

Relationship of the Air Pathway to the Rest of the HRS A2-8 

Determination of Actual Release A2-9 

ATTACHMENT A: Points for SAB Consideration 

ATTACHMENT B: Derivation of the Air Target Score 
• 


Al-l 

GENERAL COMMENTS 

The Air Targed Distance Work Group .reviewed OERR's Analysis of the Air 

Target Distance Limit in the Hazard Ranking System (9), 

The air pathway in the current Hazard Ranking System (HRS) uses an 

evaluation of exposed population at risk to derive a score for "air targets," 

The scheme uses a matrix of population versus distance for four concentric 

circles (with radii of 1/4, l/2, l, and 4 miles), Public comments have 

suggested that the four mile distance may be too large, EPA studies, by 

contrast, have suggested that, for sites with large emission rates of 

ca rei nogens, individual U fe time risks may remain above 10-6 for even 

greater distances, The Work Group considered this and other questions 

related to the air pathway (see Attachment A). · 

In general, there may not be significant air risks at many potential 

Superfund siees, but the HRS needs to address them when they are present, 

The Work Group is very supportive of OERR's efforts to modify the air 

pathway so that it considers potential releases, instead of relying 

solely on a score for observed releases. Because air emissions are 

often episodic or narrowly focused along a particular wind direction, 

they are difficult to observe, In -addition, there can be a declining or 

changing risk depending on the nature or integrity of containment or the 

site type·, There may have occurr!.d a significant release early in a site's 

history which has resulted in a loss of contaminants and decreasing 

emissions at the time when sites are evaluated and scored. Lastly, 

future activities at a site (such as excavation) may create the potential 

for new air ~missions. For these reasons, inclusion of a potential for 

release is an important modification of the HRS. 

Ideally, if comprehensive information was available, air emission 

models developed for other purposes could be used to estimate potential •· 

releases. These estimations, however, may require information which is not 

available to the Agency during the preliminary assessment/site investigation 

( PA/SI) stage. With information on the identities of chemicals present, an 

emission rate can be calculated for volatile chemicals or the presence 

of volatile emissions established on a crude yes/no basis. When such 

information is lacking, volatile air emissions cannot be predicted at 

all, and the subsequent consideration of air exposure routes is very weak, 

If the identities of chemicals are known, it should be possible to 

modify and simplify the procedures for modeling air emissions from hazardous 

waste transport, storage and disposal facilities (TSDFs) to calculate an 

approximate, order-of-magnitude emission rate for impoundments and landfills, 

Other investigativ~ techniques could be used to confirm emission rates, 

the most obvious of which is air concentration measurement in the illllllediate 

vicinity of the source, Because of the dificulties in measuring variable 

low-level ambient air concentrations, more direct sampling, such as soil 

gas analysis or "inverted dish pan" surface sampling, should be considered. 
A2-2 

COMMENTS ON THE ISSUES RAISED BY OERR 

Th@ full te~t of the issues raised by OERR ~an b@ found in Atta~hment A. 

Abbt"eviated versions are used here for the convenience of the read"'· 

An Alternative to the Use of a Single Target Distance Limit at All Sites. 

The Work Group recommends that EPA consider an alternative to the 

use of a single air tar:get distance limit for all sites. 

To evaluate the exposed population at risk mo•e accurately, the Work 

Group recommends that the Agency derive a scoring system that will properly 

weight the number of exposed people according to the distance from the 

site at whi~h they live. The weighting factor should be proportional to 

individual risk, as determined by concentrations that decrease with 

distance from the site. In other words, the system should use population 

rings that are based on distances from a site at which air concentrations 

decrease by a constant factor. These distances may vary by size and type 

of site. oor illustrative purposes a method of developing these rings 

is presented in Attachment B. Although this method does not evaluate 

maximally exposed individual (ME!) risks per se, it does explicitly 

a~count for the higher exposures that willoccur within the ring closest 

to the site. Therefore, the Work Group concludes that a separate or 

additional evaluation of MEI risks as part of the HRS is unnecessary. 

These rings could be incorporated into the scoring system as a 

simple refinement of the current matrix, as illustrated in table 1. 

Table 1 

Suggested Matrix for Air Pathway Population Scores .. 

Generalized Target Distance Score 

Distance from Center of Site 

Population a-ba ba-b2a

-­ b2a-b3a--­ b3a-b4a

--­ !06-10 7 10 9 8 7 

• 

105-106 

1o4-105 

9 

8 

8 

7 

7 

6 

6 

5 

103-1 o4 7 6 5 4 

J02-1o3 6 5 4 3 

10l-Joz 5 4 3 2 
• 


A2-3 

The matrix is interpreted as follows. The distance "a" is the 

radius of the site itself. (In practice, sites will not be circular and 

"a" must be taken as a typical dimension--derived, for example, by 

taking "a" as equsl to the square root of the area of the site divided 

by pi,) The dimensionless constant "b" is a multiplier of the order of 2 

to 4, determined as shown in Attachment B. For example, if the site 

was 200 yards in diameter (about 1/9 mile), and the multiplier was 3, 

the rings would be 1/9 to 1/3 mile, 1/3 to 1 mile, 1 to 3 miles, and 3 

to 9 miles, The population used is that in each annular ring (between 

"a" and "ba", etc.), determined to one significant figure. Once scores 

are assigned for each ring, the largest score is taken as adequately 

representing the site. The scores are all logarithmically related to 

population risk, so that they should be added to other scores representing 

logarithms of variables that enter risk linearly, such as the carcinogenic 

potency or the quantity of waste or emission rate. 

As shown, the scores for a large site ("a" is large) will be greater . 

than for a small site ("a" is small) for the same population distribution 

because each of the four rings will be larger and incorporate more population. 

The population in each ring will increase as a2, so if a increases by a 

factor of 3, the population will expand by a factor of 9, and the score will 

go up by one point. This behavior implies incorporation of a factor related 

to emission strength (in turn, proportional to a2) in the score. This may 

not be appropriate. Perhaps it would be better to define a standard value 

far a, taken from a typical site. 

After deriving such a ring-weighted system, the Agency may want to 

evaluate the effects of different diameters to see if the use of a single 

diameter for all sites is reasonable because a ring width that varies 

between sites, based .on their diameters, may be too complex for the HRS. 

.• 

To place the scores from the ring-weighting process on the same scale 

as ather scores in the HRS with which they might be combined, the scores 

could be increased in direct proportion by multiplying by a constant. 

For example, if one wishes to have a score of 30 in the top left corner, 

one can multiply all the Scores ·by 3 thus obtaining, for example, 3 in 

the lower right: corner and 12 in the lower left corner. But if this is 

done, an order of magnitude change in carcinogenic potency should also 

be represented by 3 points change in score, and so on. 

The above scoring system is reasonably precise for cancer for 

which individual risk, given the usual assumption that risk is directly 

related to average lifetime concentration in air, is directly related to 

average lifetime concentration in the air. For noncancer effects, 

EPA ordinarily assumes a threshold of concentration for any damage to 

occur. Thus, outside some critical distance (different for sites with 

different emission rates or different threshold toxicities), no effects 

will occur, and inside that distance one could assume conservatively 
A2-4 

that 100% incidence of the effect occurs, There seems to be no simple 

scoring schem .. to take this property into account in a population score, 

but the practical effect is that population beyond perhaps one mile (for 

example) is simply irrelevant to the air target score for noncancer 

endpoints. If, in the toxicity score, a noncancer effect is responsible 

for the greater part or the score, then the choice of points from Table 1 

should be restricted to the t~o left columns (for example). As before, 

the degree of danger for an individual in these t~o rings ~111 have 

already been captured in the toxicity score and ~hatever score is used 

as a surrogate for exposure, 

Approach to Assessing Health Risks 

As implied above, toxic! ty considerations may also be different for 

the air path~ay than for surface or ground ~aters, An exclusive evaluation 

of a contaminant from the standpoint of carcinogenicity is inadequate, 

Many compounds detected at contamination sites are not currently considered 

carcirwgens or suspected carcinogens. Acute toxicity is generally meant 

to be considered in emergency or interim response procedures prior to 

HRS scoring, In reality, sites may not pass some threshold of acute 

toxicity considered in this way snd, yet, still exhibit some subacute 

or chronic health effect. This is ·particularly true for the late-history 

site, ~here average emissions may be relatively low, but--~ith breaching 

of a fe~ drums, deterioration of a building, or removal of cover--a 

short-term, acute emission is again possible. If acute toxicity conditions 

were discovered at a site during site inspection or HRS scoring, the site 

should be automatically referred to emergency response. This seemingly 

would elimingte the need for considering acute toxicity in the HRS. !t 

does not appear that this complete separation is appropriate, ho~ever, 

given the threshold nature of emergency response or changeable site 

conditions, 

In addition, other subacute or chronic effects of noncarcinogens must 

be considered, Neurotoxicity and chemosensory effects of air pollutants, 

while occurring at differing levels of concern, are real responses to 

airborne contami-nants, and environmental toxicity is occasionally important. 

The public is also often concerned about nuisance effects, such as odor 

or nausea. 

The Work Group favors separate consideration of acute toxicity in a 

• 	 direct contact path~ay, Ho~ever, acute toxicity could instead be included 

among the multiple measures of toxicity in a revised toxicity factor, In 

addition, the exposure of the near-site population exposure to air contaminants 

could be separately assessed, or discretionary points could be added to the 

HRS score for smells or symptoms reasonably related to exposures to 

chemicals kno~ to be at the site (providing that these are confirmed in 

some suitable manner), The direct contact pathway may be especially 

useful if EPA ~!shes to eliminate or reduce the importance of scoring 

acute toxicity for the surface and ground ~ater path~ays, 
• 


A2-S 

In assessing exposure, consideration of each media path~ay is 

recommended including air, water, food, and direct contact. Measurement 

of exposure should also consider the route of exposure through which 

substances exert their effects, For example, inhalation is more important 

than ingestion in the case of asbestos. 

Applicability or Air Emissions Information from TSDFs to Superfund Sites 

and the Range of Emissions Estimates Calculated 

Because air emissions are often episodic or narro~ly focused along 

a particular wind direction, they are difficult to detect, For this 

reason, inclusion of a potential for release is an important priority 

for modification of the HRS. The procedures given in the Analysis of 

the Air Target Distance Limit in the Hazard Ranking System (6) for 

estimating air emissions from sudace impoundments and landfills at 

Superfund sites are based on model equations developed for RCRA Facilities 

(TSDFs), These procedures are appropriate for the purpose to which they · 

are applied: to determine the range of possible (long-term) emission rates 

at Superfund sites. The model equations represent the state-of-the-art 

in this relatively new area of environmental science. 

There are, however, some limitations identified below and suggestions 

for additional analysis that can result in a clearer and more defensible 

analysis. These include the need to illustrate previous validation 

studies of the models, part icula r:ly those presented in peer-reviewed 

scientific journals, the need to better illustrate the model comparisons 

at existing landfills, and better id.entification of the potential 

applicability of the models to particular sites. 

Despite the difficulties in data collection, validation studies 

with the surface impoundment and landfill models have been attempted. ·• 

Using these studies in the documentation for proposed changes to the HRS 

would provide the user with a sense of the reliability and accuracy 

of the procedures, which we s.uspect is about one-half to one order of 

magnitude. These studies have . been presented in peer-reviewed articles 

in journals such as Environmental Progress and the Journal of the Air 

Pollution Control Association. 

In addition, the comparison of predicted and measured emission 

rates (in Section 4.4.1.4 of Analysis of the Air Target Distance Limit 

in the Hazard Ranking System, July 6, 1987) would be strengthed by inclusion 

of a plot of predicted vs. observed rates, This comparison may indicate 

significant variability on a site-by-site basis, This variability is, 

in ·part, due to the simplifications·inherent in the models, but is also 

largely a result of the difficulties in identifying model parameters at 

a given site, even for the most basic of inputs, such as the identification 

of chemicals in the waste. These difficulties are pertinent to the use 

of these, or related model equations, as .part of an HRS scheme that 

considers' potential air emissions, 
A2-6 

Ideally, if comprehensive information was available, the emission 

models for TSOFs could be used. A brief review of these procedures is 

presented below in order to highlight important variables. 

Emissions from surface impoundments depend on the concentration of 

chemicals in the water. Methods to estimate such emissions apply the 

tradicional "two phase resistance" transport equations which include an 

estimate of the chemical's air-water partition coefficient or Henry's Law 

Constant. The resistances depend on factors such as wind speed, fetch, 

diffusivities, and the presence of oily films but, to a first approximation, 

it would probably be acceptable to adopt mean values of the resistances 

and apply them to all chemicals. The key items of information are chemical 

identity, chemical Henry's Law Constant, and chemical concentrations. 

In the long-term the chemical will be depleted from inactive sites unless 

there is migration from sediments to replace it. 

Air emissions from landfills depend on the resistance to transfe~ 

from the soil cover depth and porosity, coupled to the chemical's vapor 

pressure as the driving force for diffusion. The key items are chemical 

identity, chemical vapor pressure, and soil cover depth and porosity. 

Emission rates can be estimated readily for tanks and drums given a 

knowledge of the geometry and co~tents. Waste piles and other accumulations 

of waste will require separate treatment. In all cases, the key variables 

to estimate volatile emissions are chemical identity and vapor pressure. 

Information on the chemical identities is valuable because it allows 

the presence of emissions to be established and the calculation of emission 

rates for volatile chemicals. When such information is lacking, air 

emissions can not be predicted and the subsequent consideration of air 

exposure routes is impossible These may be evidence of emissions as a .• result of odors, for example. 

If the chemical identities are known, it should be possible to 

modify and simp~ify the TSDF procedures to calculate an approximate, 

order-of-magnitude emission rate for impoundments and landfills. A 

score could then be assigned based on the logarithm of the emission 

rate. The population distance matrix discussed earlier includes the 

effects of dispersion. 

• 	 Other investigative techniques could be used to confirm the emission 

rates, the moat obvious of which is air concentration measurement. 

Ambient monitoring, however, is subject to the problem of dilution, as 

the three examples on the next page show. 
A2-7 

If we consider a site 100 m by 100 m emitting 100 kg/year into a 

wind of average speed S m/s, and assume that the air is well mixed at 

the downstream sampling site to a height of 5 m, then the concentration 

at that site will be C g/m3 given by: 

100 kg/yr x 1000 g/kg • 5 m/s x 100 m x 5 m x 31.5 x 106 s/yr x c 

or C • 1.3 x 10-6 g/m3 or 1.3 ugJm3, 

This low concentration may be difficult to detect quantitatively. 

For surface impoundments, it is preferable to measure the water 

concentration and infer an emission rate. For example, in the above 

esse an emission of 100 kg/yr over 104 m2 would require a concentration 

of 0.023 g/m3, assuming an overall water mass transfer coefficient of 

0.05 m/h, 

i.e., toS g/y • 104m2 x 0.05 m/h x 8760 h/yr x 0.023 g/m3,

• 

This concentration is a factor of 18,000 greater than the air concentration. 

Likewise, if the emission is from soil and we assume an effective 

diffusivity (corrected for porosity~ of 0.01 cm2/s or 0.0036 m2/h and 

a path length of 1 m and area 104 m , then the effective diffusion 

volume exchanged is 0.0036 m2Jh'x 104m2/1m or 36m3/h. This is 

essentially the volume of soil pore vapor released per hour. If the 

emission rate was 100 kg/yr the concentrations would be 0.32 g/m3, 

i.e., 1D5 g/y • 36 m3/h x 8760 h/yr x 0.32 g/m3, 

Again, this concentration is a factor of 250,000 greater than in the 

ambient air and is much easier to measure, .· 

Based on these examples and the Work Group~s experience, to detect 

the presence and amount of emissions it is desirable to analyze at the 

sou~ce, or as close to the source as possible. Once the chemical mixes 

with the ambient· air, it suffe·rs a concentration drop by a factor on the 

order of 104 to 106, This may produce ambient concentrations below 

detection limits, 

Thus, there is thus an incentive to use methods of "probing" 

• 	 sources by using, for example, soil gas analysis or "inverted dish pan" 

surface emission samples. We believe that valuable confirmatory data 

would be obtained at quite modest cost. These investigative techniques 

could give direct. measurements of emissions rates. 
.. 


A2-8 

Site investigations of soil, ground water and surfa~e water have 

been aided by the prior existence of sound analytical protocols for 

sample collection, storage, and analysis. It appears that insufficient 

effort has been devoted to developing protocols which are appropriate 

for air sampling at waste sites, The traditional high volume sampler is 

often effective, but it is, to an extent, misapplied, More effort should 

be devoted to developing more subtle, e~onomic and useful air sampling 

procedures which are spe~ifi~ to the conditions prevailing around waste 

sites. 

Other Factors Affecting the Potential for Release 

The following additional factors should be considered in developing 

the air pathway component of the HRS: 

l. 	 The type of containment, i.e,, impoundment vs, tanks, and 

drums vs, encapsulation, should be ~onsidered, Also included 

should be an assessment of the extent to which the containment 

is likely to be disturbed as part of the remediation process 

at the site, Sites where more excavation and handling of 

wastes are required may gresent a higher potential for air 

releases. 

2. 	 Catastrophic releases may create air emissions. This category 

could include the susceptibility of the site to flooding, 

the seismic activity of the area, and the frequency of 

severe weather, e.g., extremely high winds which can lead to 

fugitive particulate emissions. 

3. 	 Reported incidents by surrounding residents, such as complaints 

concerning odors, eye irritation, or nausea, may result from a~r 

emissions. While, in the absence of air monitoring data, these 

reports may not be sufficient evidence of a "confirmed" release, 

such sites should be scored for their potential-to-release, 

OTHER COMMENTS 

Relationship of Air Pathway to the Rest of the HRS 

As part of its revision of the HRS air pathway, the Work Group • recommends that OERR assure that the scores assigned for population and 

distance are consistent with one another and with other scores with 

which they must be combined, for example, scores for to,xicity, waste 

quantity, or release potentiaL If the HRS is to represent the relative 

risk of sites, it.must be consistent with the underlying, quantitative 

relationships of the factors evaluated, Weighting the risk among pathways 

on an~ priori basis where, for example, the air pathway might be worth 

one-half the ground water pathway, is not scientifically justified. 
AZ-9 

Two aspects of risk need to be addressed. First, risk may be 

declining or changing depending on the nature or integrity of containment 

or the site type. There may have been a significant release early in a 

site's history "hich has resulted in a loss of contaminants and decreasing 

emissions in later history ("hich often coincides "ith site evaluation 

and scoring). This earlier release may have been of an acute nature 

"hich no longer is detectable or representative of the site. 

The second aspect is that the near-site population may be considered 

to be at greater risk from the air pathway than from the water pathways. 

Because of the nature of migration of air contaminants (speed of movement 

and plume entrainment) the population at highest risk may change as 

atmospheric conditions change. 

There may be a need to consider the near-site population as exposed 

to an independent direct-contact pathway rather than as a subset of the 

air pathway because acute rather. than chronic to>:icity may be more relevant 

and because of the known presence at some sites of chronic to>:icants in 

the form of particulates (such as dusts containing lead). Exposure to 

the larger respirable particulates can be a very near-site phenomenon, 

which may be de-emphasized by inclusion in the air pathway with its 

focus on greater distances. On-site workers (not cleanup staff, but 

employees at active manufacturing or landfill sites) are also at risk in 

manners different from long distance populations. E>:posures may last for 

fractions of days but be intense when they occur. Near-site populations 

are continually exposed to a variety of routes in comparison to single 

route e>:pos~re of remote populations. 

E>:posure also changes with the nature of contaminant migration. 

Near a site, the contaminants are more varied as well as at greater 

concentrations, and there are more routes of e>:posure. Volatile organic 

compounds are present and are subject to inhalation. In addition, particulates containing either metals or adsorbed organics tend to deposit 

near site boundaries and are subject to ingestion and dermal absorption 

as "ell as inhalation, Remot·e from the site, concentration and variety 

of contaminants are less, and e>:posure is more limited to the volatile 

compounds and the inhalation route. 

Determination of Actual Release 

• 

One of the important scoring factors in the current HRS is an on-off 

switch for the presence of actual release of hazardous material into the 

ambient air. The determination of actual release is usually based on a 

limited amount of ·air monitoring, The data provided by the program 

office indicate that 14.2% of the 951 NPL sites have shown observed 

release, compared to 41.6% for surface water and 73.0% for ground water. 

These statistics may be misleading and could result from differences in 

detecti?n efficiency rather than true dominance of the water routes. 
A2-!0 

The differences in detection efficiency are also thought to result in a 

reluctance to collect data for this route, a problem ~hich could be 

addressed by a minimum data requirement for the HRS. 

Air releases are often intermittent, ~ith a substantial temporal 

variation, Let "r" denote the fraction of days during ~hich a release 

occurs at a site. If r>O, the site should be classified (in the current 

HRS) as having an actual release. Strictly speaking, both the release 

fraction and the pr.,sence of actual release are relative to the time 

frame under consideration. If longitudinal air monitoring is available 

for a long time period, and measurement error is negligible, the release 

~ould be observed, In the absence of longitudinal monitoring, there can 

be a substantial probsbility that a site ~ith intermittent release ~ould 

not sho~ an observed release, If one assumes that air monitoring is 

conducted for one day, such as the day of site inspection (chosen randomly), 

For a site with release fraction,''r", (say 40%) the probability of not 

observing the release is 1-r, or 60%, Therefore, the prevalence of air. 

release for the NPL sites can be substantially higher than the value of 

!4.2%, which is based on the available data on observed air releases, 

For e~ample, if the release fraction fot all NPL sites with actual release 

is 40%, the prevalence of air releases would be about 35% (14.2%/40%), 

among which only a small fraction. 1S ident Hied by the available data as 

showing an observed release, Lt is likely that air releases can be 

substantially more prevalent than what the available data indicate, 

one possible remedy for the underestimation of actual releases is 

to increase the frequency of air monitoring, For a site with release 

fraction r· ~ 40%, if air monitoring is conducted for three instead of 

one days, chosen randomly (because of possibl"' intertemporal correlation, 

three consecutive days cannot be regarded as being random), the probability 

of~ observing the release would be 21.6% instead of 60%. However, .· 

the amount of resources required to increase the air monitoring frequency 

would be substantial, and might not be cost effective. 

As discus.sed above, alternative sampling ""'thods such as soil gas 

monitoring may be easier than ambient air monitoring, 

• 
1 (this works for n "' 2 also) 

---------------------

---------------------

-	S

x 

Pk 

( 21Tr dr) (D ----) 

rn 

~ 2iT PkD · [ ( bx) 2-n - xZ-n] 

(2-n) 

2 Tf Pk N (b2-n -1) x2-n .. (2 	- n) (b2-t)1Tx2 

2 p k N (b2-n -1) 

" (2 	- n) (b2 - 1) xn 

ATTACHMENT B 

Derivation of the Air Target Score 

suppose population N is distributed uniformly 

in a ring between distance x and bx, The 

population density is given by 

N 	 N 

~-----

Suppose further that concentration, averaged over all 

directions, declines as an inverse power of distance: 

If excess risk is proportional to concentration (as usually assumed for 

carcinogens)., the proportional! ty constant is P, · and excess risk•PC,.P(k/rn), 

then the population risk in the ring is given by 

Assuming the same population, N, the population in the next larger ring is given by 

-

The ratio of the two risks is 

• 

If we want this ratio to be 1/10, then 


10 and 
 b "-vro 

For example, if n • 2, b • 3.16. Note that if D is constant from ring 

to ring, the ratio of population risks is 

/ 11 b2-n Rbx -.,., " 

and will be greater thsn 1 if n < 2, implying that there is no "natural" 

limit for the air target distance, 
• 


Both atmospheric diffusion theory, available data, and current 

diffusion modeling practiced by EPA support a value for n of less than 

or equal to 2 for ground-level sources. At close-in distances, less 

than 10 km for e~ample, the decay in the ring-averaged concentration is 

inversely proportional to the square of the radial distan'"" from the 

source for average meteorological conditions. At distances greater than 

10 km, the decay for average meteorological conditions is slower than 

quadratic, EPA models used in performing risk assessments, such as the 

Human E~posure Model (HEM), use formulas for diffusion that have the ringaveraged concentration decaying approximately as r-1.7, for distances 

out to approximately 10 km for average meterological conditions. 

As an e~ample, one can refer to the table when a ~ 0.09 mile and 

b ~ 3,33 (n is about !.9), Figure B-1 shows the rings for these assumptions 

and Table B-1 shows the matrix of scores, 

TABLE B-1 

Air Target score for b - 3.33 

Ring Boundaries (miles) 

PoEulation 0.09 - 0.3 0.3 - !.0 !.0 - 3.3 3.3 - 11.1 

!06 - 1o7 10 9 8 7 


!05 106 9 8 7 6
 I 

t04 - !OS 8 7 6 5 


1 oJ 104 7 6 5 4 


t02 - 103 6 5 4 3 


1 ol - 'to2 5 4 3 2 


!00 - 101 4 3 2 


(The scores above the line imply population densities greater than 

30,000/miZ, which are rare.) 

rf one assumes that population is uniformly distributed at density 

0 ~ l,OOO/mi2, then the populations in the rings are about 260, 2900, 

32,000, and 390,000 people. Each ring would score 6 points; the overall 

score would, therefore, also be 6. 

The decline ia individual risk per ring is by definition one orderof-magnitude, Therefore, if one arbitrarily defines risk to equal !o-3 

for the innermost ring, then the corresponding population risk 

iS 260 x lo-3 "0.26 (less than one case in a lifetime), In ring 2 it 

is 2900 x !0-4 or 0.29, in ring 3 it is 0.32 and in ring 4 it 1S 0,39. 

Thus, total population risk continues to increase as long as the population 

density remains constant. Practically, individual risk has declined to 

10-6 in ring 4 and would be negligible (lo-7) in a fifth ring. 
F'l.GURE B-1 


· Air Target Distance Score (not to scale) 


• 


Ring 4 

Ring 3 

i 11 miles 

I 

Site Area 


(radius about 0.09 miles) 


APPENDIX 3 

REPORT OF 

THE LARGE VOLUME WASTE WORK GROUP 

TO THE 

HAZARD RANKING SYSTEM REVIEW SUBCOMMITTEE 

OF THE 

SCIENCE ADVISORY BOARD 

• 
• 


U. S. ENVIRONMENTAL PROTECTION AGENcY 


SCIENCE ADVISORY BOARD 


HAZARD 	 RANKING SYSTEM REVIEW SUBCOMMITTEE 

LARGE VOLUME WASTE WORK GROUP 

ROSTER 

CHAIRMAN 

Mr. Richard Conway, Corporate Development Fellow, Union Carbide Corporation, 

Post Office Box 8361 (770/342), South Charleston, West Virginia 25303 

XEMBERS/CONSULTANTS 

Dr. Keith Ferguson, Environmental Protection, Environment Canada, 

Kapilano 100, Park Royal, West Vancouver, British Columbia, 

CANADA V7TlA2 

Dr. C. Daniel Kealy, U. S. Bureau of Mines, Spokane Research Center, 

East 315 Montgomery Avenue, Spokane, Washington 99207 

Dr. James 0. Leckie, Department of Civil Engineering, Stanford University, 

Stanford, California 94305-4020 

~r. Bruce Napier, Battelle NW, Post Office Box 999, Richland, 

Washington 99352 

Dr, 	David Pollock, U.S. Geological Survey, National Center, Mail Stop 411, 

Reston, Virginia 22092 

EXECUTIVE SECRETARY 

~rs, Kathleen W, Conway, Deputy Director, Science Advisory Board, (A-lOIF), 

u.s. Environmental Protection Agency, 401 M Street, s.w., 


Washington, D.C. 20460 


STAFF SECRETARY 

Mrs. 	 Dorothy M. Clark, Secretary, Science Advisory Board, (A-IOlF), 

u.s. Environmental Protection Agency, 401 M Street, s.w., 


Washington, D.c. 20460 

TABLE OF CONTENTS 

BACKGROUND 

RESPONSES TO ISSUES/QUESTIONS RAISED BY OERR 

Applicability of HRS to Mining Waste Sites 

Bias Against Mining Sites Not Demonstrated 

Inadequacy of the HRS for Mining Site 

Evaluations Not Proved 

Inappropriateness of the Existing HRS for 

Mining Sites Not Demonstrated, But Has 

that Potential 

Factors to Modify the HRS for Large Volume 

Wastes 

Feasibility of Using Concentrat~on 

Value of Incorporating a Concentration. 

Factor 

Desirability of Including Hazardous 

Constituent Concentration Data in the HRS 

Options for Incorporating Concentration Data 

In the HRS 

Other Comments on Concentration 

OTHER COMMENTS 


Long-Term Considerations 


Need for Additional Studies


• 

Search for False Negatives and False Positives 

SuMMARY 

Page 	Number 

A3-l 

A3-3 

A3-3 

A3-3 

A3-4 

A3-S 


A3-S 


A3-6 


A3-6 


A3-7 

A3-8 .· 

A3-9 

A3-ll 

A3-11 

A3-12 

A3-12 

A3-l2 

ATTACHMENT A--Issues from The Superfund Hazard Ranking System (HRS): 

Applicability to Mining Waste Sites, July 22, 1987 

ATTACHMENT B--Issues from The Superfund Hazard Ranking System (HRS): 

Feasibility of Using Concentration Data in a Revised HRS, 

July 27, 1987 
.. 


A3-1 

BACKGROUND 

Mr. Henry L. Longest, Director of the Office of Emergency and Remedial 

Response (OERR), referred the large volume waste issue, "whether large 

volume wastes should be considerE!d differE!ntly from other wastes," to the 

Science Advisory Board for review in his June 25, 1987 memorandum (22). The 

issues as stated more specifically in the July 28, 1987 memorandum (20) of 

~r. Stephen A. Lingle, Director of the Hazardous Site Evaluation Division 

(OERR) are: 

"1) Applicability of the HRS in sco<ing mining waste sites; and 

2) Feasibility of using waste concent<ation. data in a <evised HRS." 

This memo<andum also states, ";,e (OERR) believe that the Subcommittee 

findings will be equally applicable to both mining and fly ash sites." 

The Work Group concentrated on mining sites and assume OERR will translate 

its findings to utility ;,aste sites. 

The primary EPA document addressing the first issue, The Superfund 

Hazard Ranking SystE!m (HRS): Applicability to Mining Waste Sites (13) was 

reviewed at the Work Group's August 20, 1987 meeting. Besides providing 

background on the HRS, it presents an overview of mining waste site 

charactedstics, summa<izes five ·previous studies on the application of 

the HRS to mining waste sites, and provides "n assessment of "potential 

dangers" at six such sites rated by the HRS. 

In the ·overvie;, of mining ;,aste site cha<,.cteristics, four major 

categories of wastes from the extraction of ores "nd minerals were 

add<essed: mine waste, mill tailings, dump and heap leach waste, "nd 

mine water. (The Work Group had the Office of Solid Wastes' Management 

of Mining Wastes (!7) and chapters 1-5 of the Draft Report to Congress: 

Wastes from the Combustion of Coal by Electric Utility Power Plants (16) 

as additional sources of information on the characte<istics of large 

volume wastes). Metals, radionuclides, asbestos, and cyanides (but not 

organics) ;,ere identified as the contaminants of conce<n at large volume 

;,aste sites. The acidity (pH) was identified as a key factor in the 

mobility of metals, and the ground water and surface water pathways were 

those usually of concern at these sites. 

• 	 Three of the five papers summarized and evaluated in the issue 

pape< were prepared by TRC Environmental Consultants, Inc. for the Ame<ican 

Mining Congress (27, 28, 29). The other two were Agency contractor reports 

developed by MITRE Co<poration (24, 25) in <esponse to issues raised by 

TRC. The TRC pape<s assert that the HRS does not adequately discriminate 

risks posed by various mining waste sites as well as it does for non-mining 

sites; the MITRE papers respond that TRC's conclusions a<e not supported 

by their analysis, or by further MITRE analysis. 
A3-2 

OERR briefed the Work Group on its comparison of scores for six 

actual mining waste sites with the results of an assessment of the 

qualitative overall risk to human health and the environment (potential 

danger) at those sites (11), Ratings for four pathways were derived: 

ground water, surface water, air and direct contact. For these six 

study sites, higher HRS scores were associated with higher potential 

danger ratings. 

In its July briefing of the Subcommittee, the Office of Policy, 

Planning, and Evaluation reported that its Site Ranking Panel (an 

in-house group of EPA personnel with varying degrees of expertised 

on Superfund issues), using a different group of sites and experts, 

apparently found virtually no correlation between the experts• ranking of 

sites and the HRS scores (18). Since the unpublished OPPE study (23) 

was not reviewed by the Subcommittee nor otherwise peer-reviewed, the 

Work Group is unable to judge what credence may be placed upon its findings. 

Taking the study at face value, it would appear that because most of the 

sites chosen were already on the NPL, this may indicate that the HRS 

discriminates poorly between listed sites, but does not necessarily mean 

the HRS does not discriminate between those that should be and should 

not be listed, 

There. are two ways to evaluate the applicability of the HRS to 

mining sites. The July 22, 1987 issue paper, The Superfund Hazard Ranking 

System (HRS): Applicability to Mining Waste Sites, uses the approach of 

"valuating the experience in ranking mining sites with the HRS system in 

terms of false positives and false negatives and differentiating according 

to th1! potential danger as determined by other means, In its evaluation, 

the Work Group coupled this approach with an examination of the scientific 

issues independent of the HRS experience. 

The second issue paper is The Superfund Hazard Ranking System (HRS): ·• 

Feasibility of Using Concentration Data in a Revised HRS, July 27, 1987, 

Besides providing general background on the HRS, the paper discusses the 

(non)use of waste concentration in the HRS, describes the use of waste 

concentration dat·a in other site ranking models, and presents three 

options for using waste concentration in a revised HRS, 

Basically, the inherent difficulties of sampling and analyzing 

hazardous wastes, the heterogeneity of waste sites, cost, and the need 

• 	 to quickly develop a NPL dissuaded the Agency from including concentration 

in the original HRS. Four .systems which address concentration in various 

ways were described: the Site Ranking System (SRS), HARM II, Site Assessment 

System, and Remedi.al Action Priority System (RAPS). OERR 'viewed the SRS 

approach, a linear s·coring system which falls just short of being a 

quantitative risk model, as having components of potential utility, but 

cautioned that any of the systems rely heavily on the professional judgement 

of field personnel. 
A.3-3 

The three options pr<>sented by the Agency for using waste concentration 

data in th<> revised HRS are: 

Wl 	 Retain the current HRS approach where waste concentration data 

are used only to identify the most toxic waste constituent. 

W2 	 Mandate the use of waste concentration at all sites to determine 

a total mass of constituent to be used in the revised HRS, 

much as the waste quantity score is currently applied. 

W3 	 Adopt a tiered approach to encourage the use of constituent 

concentration data, where available, and reliance on default 

values of concentration to estimate the total mass of a 

constituent at other sites. 

The detailed questions posed to the Work Group on large volume 

waste issue are presented in Attachments A and B. 

RESPONSES TO ISSUES/QUESTIONS RAISED BY OERR 

Applicability of HRS to Mining Waste Sites 

The full text of the questions posed by OERR can be found in 

Attachment A. An abbreviated form is used here to assist the reader. 

Bias Against Mining Sites Not Demonstrated. The TRC and Mitre 

reports showed that mining waste sites scored higher in waste quantity 

compared to non-mining sites. This is expected given the large volume 

of wastes generated by the mining industry. TRC did not show this bias 

caused false-positives or false-negatives for mining sites. Both TRC 

and Mitre found waste quantity contributed relatively little to the .· final HRS 	 score. 

TRC addressed whether the degree of site information available was 

a major factor in. NPL listing. Since some mining sites have more extensive 

data bases than do others, TRC concluded that these may be more likely 

to have higher scores and, therefore, to be placed on the NPL, compared 

with sites of equal risk but with less available information. Most 

rnajor industrial facilities also will have more extensive data bases 

than smaller sites (especially "orphan chemical dumps"). Extensive data 

• 	 bases and a regulatory history may highlight major industrial sites 

before others. Moreover, more frequent and "denser" I!IOnitoring has a 

better chance to find environmental problems and observed releases; also, 

sites with serious problems may be more thoroughly studied. In any 

event, no data were 'presented by TRC showing that the relatively larger 

data bases for mining sites resulted in unfair scoring. On the contrary, 

smaller data bases could theoretically be biased. Use of default values 

for scores where data are missing would be a better solution to the 

alleged umderscoring of sites with lesser data bases. 
A3-4 

Analysis of the relative weights of components in the Hazard Ranking 

System may suggest bias if one. kind of waste, such as mining wastes, always 

scores high in an important component irrespective of environmental hazard. 

TRC stated the final HRS score for mining sites could be predicted by 

population alone. Since the TRC studied only mining sites, it did not 

prove a bias against mining waste sites. Also, the more complete Mitre 

analysis indicated that population is the second-most important factor 

(after observed release) for scoring of all types of waste sites and, 

therefore, is a critical factor for all sites, not just mining sites. 

In two areas, the Work Group concurs with TRC. First, inorganic 

metals have certain characteristics not addressed in a system that does 

not take into account differences in mobility between specific ionic 

forms. The Work Group assumes that OJ':RR will conSider these differences 

in the revisions to the toxicity/persistence score. Second, the large 

volume of mining wastes often means large masses of the contaminant are 

available for release, but certain conditions must exist for these releases 

to occur.-. 

rn summary, while the TRC analysis ide.ntified some important ranking 

priorities of the HRS system, its findings do not present a strong argument 

indicating a bias against mining sites. 

Inadequacy of the HRS for Mining Site Evaluations Not Proven. After 

reviewing the TRC and MITRE studies as addressed in the July 22, 1987 

issue paper The Superfund Hazard Ranking System: Applicability to Mining 

Waste Sites, the Work Group finds that the studies conducted regarding the 

HRS listing process are preliminary in that they have examined only small 

subsets of the data base, and are inconclusive. Therefore, sufficient 

experiential evidence has not accumulated and bee'n presented to show an 

inadequacy of the HRS in regard to mining sites. The OPPE study (18) 

presentation at the July 16th Subcommittee meeting, "Preliminary Analysts~ 

of Alternative Models to Support Revisions of the CJ':RCLA, Hazard Ranking 

System," also· provided some data. That study involved mining sites 

and 16 nonmining sites and used a panel of in-house EPA personnel (with 

varying degrees of expertise in Superfund issues) to evaluate risk 

for comparison with the various existing ranking models. Within its 

limited study, OPPI': found no consistency between its panel~s ranking of 

risk and ratings from any of the models, including the HRS model. This 

data set for mining sites seems limited and cannot support a conclusion 

about the adequacy of the HRS scoring system• 

• 

In summary, sufficient evidence was not presented to the work Group 

to conclude that the current HRS has been adequate or inadequate for 

mining sites (or ev~n·for non-mining sites). 
• 


A3-5 

Inapptopriateness of the Existing HRS for Mining Sites Not Demonstrated, 

But Has That Potential. As stated above, the Work Group was unable to 

find sufficient evidence that the HRS has treated mining sites with 

systematic error. Further, although only a small subset of mining sites 

was available for analysis, the Work Group was unable to identify any 

sites currently listed that should not have been on the NPL. The present 

systeu has possibly rat<'!d mining sites no worse than other waste sites. 

However, the present scoring system has a potential to treat mining 

wastes (and others) with systematic error, The vast majority of listed 

mining sites and, to a slightly lesser degree non-mining sites, have 

observed releases. Since TRC reported (25, page 9) that 95% of listed 

mining sites had an observed release and sine<'! observed releases require 

l<'!ss judgement in scoring, there was a lower risk of false positives, 

The Work Group believes the present HRS is not well suited for 

scoring potential rdeases from mining waste si,tes because mobility is 

not included, as discuss<'!d below, 

Factors to Modify to Make the HRS for Large Volume Wastes, tn regard to 

improving the HRS, the following points should be considered: 

o 	Mine wastes (one of four types of mining wastes) tend to be solid 

rather than liquid, and contaminants may be less mobile. 

o 	Where mining wastes contain low concentrations of hazardous substances 

in large waste volumes only partially isolated from the environment, 

environmental conditions instead of the extent of isolation may 

control contaminant release, 

o 	Natural geochemical processes are of ten required to mobilize .. 

hazardous substances in mining wastes, such as acid generation 

by the biochemical oxidation of metal sulfides, 

o 	The concentration of hazardous substances and the potential for 

release can often be estimated for mining wastes, i.e., acid mine 

drainage (AMD) potential and leaching tests, More generally, the 

HRS doel! not adequately consider the nature of the source, release, 

transport, and transformation • 

Improved ways of considering the concentration, toxicity 

(inorganics have special characteristics), release (mobility in various 

matrices), and transport/transformation factors would make the HRS more 

accurate. 

The concentration issue is addressed in the latter part of this 

report and Appendices 1 and 2 address the toxicity issue. OERR's briefing 

of the Work Group August 19-20, 1987 in Denver indicated that it is 

considering ways to incorporate mobility factors within the HRS that may 

have cons ide rab le pr0111ise ( 26). 
AJ-6 

Revisions of the HRS.then under consideration by OERR included 

substituting a ~ability factor for the persistence factor in the ground 

water and surface water pathways. In the ground water pathway para~eters 

representative of the tendency for conta~inant ~ability and the sorptive 

capacity of the geologic ~edia would replace the persistence para~eter, 

The Work Group supports such changes because mobility is a more 

discriminating concept for both inorganics and organics than is persistence 

in the subsurface, especially with respect to inorganic co~pounds, 

The Work Group recommends that, during the development of a 

structured value representation of the mobility concept, OERR should 

explore a means of incorporating important matrix characteristics. These 

include extreme pH, any crystalline phase modifications of mining wastes 

that differ from native geologic materials, and the sorptive capacity of 

surrounding geologic materials (influencing migration tendency), 

Extreme pH is a particularly attractive characteristic because it is 

meaningful and because the pH of mining wastes can be determined relatively 

simply and accurately, Mining wastes tend to be highly buffered, either acid 

or alkaline. All fluids on a mining sites could be tested for pH and solids. 

could be tested for paste pH, particularly near the surface (30). The pH 

of pathway media and receiving waters are more difficult to determine 

accurately, However, even here, samples should be obtained and measured 

for pH in.both the field and the laboratory, 

In regard to the surface water route, OERR favors retaining 

persistence as the parameter to couple with toxicity in the waste 

characteristics portion of the HRS, but would consider four additional 

transformation processes along with the current biodegradation one. The 

Work Group concludes that inclusion of these additional factors would 

improve the existing HRS because it more closely corresponds with what 

happens in the real world. 

Mobility would be included in the air route; a gas mobility factor 

value and a particulate mobility factor value would be combined, Details 

were unavailable; but the basic approach of including mobility should be 

pursued due to its importance along with waste quantity in the exposure 

potential scenario, (See also Appendix·2 on exposures through the air.) 

Feasibility of Using Concentration Data in a Revised HRS 

• The full text of the questions posed to the Subcommittee by OERR 

can be found in Attachment B, Abbreviated versions are used below. 

Value of Incorporating a Concentration Factor. The following discussion 

assumes that the structured value model construct of HRS will be maintained 

in the short-term. 
A3-7 

Modifying the current HRS to incorporate factors which capture some 

measure of both the physical-chemical characteristics of the hazardous 

constituents and the nature of the waste matrix, as well as those site 

characteristics responsible for risk, is clearly an improvement. Modifications relating to the form and concentration of the hazardous constituents 

(HC) would add a degree of sensitivity to the HRS and possibly capture 

some measure of mobility and potential for exposure, Possible factors 

include HC concentration in the waste material, total mass of the HC, 

potential for release of the HC, mobility of the HC in the ground water, 

and concentration of the HC in the ground water. 

It appears that the most useful way to implement a modification of 

the HRS to facilitate inclusion of additional factors, as well as create 

flexibility to accommodate a range of site data quality and quantity, is 

to augment the HRS with a multi-tiered default approach for use of HC 

concentration or quantity data, Such a multi-tiered structure would 

provide alternative routes for use of data differing in quality and 

quantity, as well as provide the opportunity for inclusion of a d~cision 

point on the question of acquiring new, additional data for the HRS 

scoring process. 

Perhaps the concept of using the most appropriate tier for a specific 

situation is more important than assuming that the top tier is best, and 

only defaulting to a lower tier. ~hat is, for solid and semi-solid 

wastes at mining sites total quantity of hazardous waste makes little 

sense in the absence of mobility factors; rather, the concentration of 

the contaminants is important. For drum and other liquid wastes, the 

total quantity of hazardous material is probably important. 

Desirablity of Including Hazardous Constituent Concentration Data 

in the HRS, Because both the impact of observed releases and the risk 

of potential releases are related to source concentration, particularly .· 

for the direct contact and air pathway, including hazardous constituent 

concentration in the HRS would be desirable. 

At present, ·the HR.S h only a screening tool used by OERR, in 

combination with other science policy considerations for deciding if 

detailed (RI/FS) studies are required. SARA encourages the development 

of a more accurate and comprehensive ranking system. Addition of 

concentration data could improve the accuracy of the HRS • 

• 

EPA has proposed two options (W2 and W3) for modifying the HRS to 

account for the concentration of hazardous constituents, Both approaches 

translate waste constituent concentration data into an estimate of the 

total mass of a hazardous constituent at the site, which is then used to 

compute the waste quantity score for the HR.S. Both approaches represent 

improvements over the current HRS. However, if mobility is not addressed, 

even the approach which uses "total mass" of hazardous constituent could 

contribute to produce false positives for· some large volume wastes and 

some pathways, 
A3-8 

For the ground water pathway, the concentration of a constituent 

in a solid waste is of much less importance than the concentration 

that constituent may attain in·the leachate migrating away from a site. 

Consequently, more emphasis should be placed on the partitioning of 

hazardous constituents between the solid waste and the leachate. 

Leachate concentration (at least estimated) should then be factored 

into the HRS waste quantity term, For wastes that are not in solid 

form, total contaminant mass quantity should continue to be used. 

Options for Incorporating Concentration Data In the HRS. Option W2 

would require that waste constituent concentration data be collected at 

a site before it could be scored. In contrast, Option W3 (the Work 

Group's. preferred approach) is a tiered system that would encourage the 

use of concentration data, but would also provide the flexibility to use 

indirect estimates, or default concentrations, to estimate a constituent's 

total mass when direct measurements of concentrations are not available. 

The direct measurement approach of option W2, modified for solid wastes 

to account for partitioning between water and solid, would provide the 

soundest scientific basis for the revised HRS. In most cases, however, 

it will not be practical due to safety and cost considerations to require 

waste constituent concentration data for every site at the site inspection 

stage. Furthermore, the Work Group- recommends the direct use of 

concentration rather than conversion to a mass value for wastes in solid 

or semi-solid form. 

One of the most difficult tasks will be to decide which tier to 

use. In the case of option W3, there is a need to decide whether 

to use measured concentrations or indirect estimates of mass. It is 

likely that economic and safety considerations will determine the 

decision as to which tier will be used. If a site is relatively 

uniform and sufficient waste constituent. data can be obtained at .· reasonable cost to characterize the waste, tier one (concentration) 

could be used. On the other hand, if a site is non-uniform and costs 

of obtaining the necessary concentration data (or time and safety 

requirements) are prohibitive, it would be reasonable to use a higher 

tier. In effect~ this approach may effectively distinguish between 

inorganic (mining and utility) and organic waste sites because of the 

greater heterogeneity of organic waste sites and the large expense of 

organic analyses. Inorganic analyses are relatively inexpensive (assuming 

speciation i$ calculated, not measured); therefore, this approach would 

encourage the collection of waste constituent concentration data at • 

mining sites, where concentration data is often a critically important 

factor, as discussed above. 

In conjunction with using concentration data in the HRS, a method of 

determining a "representative concentration" for a site based on a stratified 

sampling strategy would be useful. In.this approach, a complex site 

could be subdivided into a set of more homogeneous regions or strata, 
A3-9 

and a representative value for each stratum determined through more 

limited sampling. Statistical techniques exist for manipulating stratified 

data, which could then be recombined with appropriate weighting into a 

final single HRS score for the site. 

To define a factor for converting quantity of hazardous constituents 

to quantity of hazardous waste, from a scientific point of view, the 

median concentration obtained in the September 1986 MITRE report by 

Arlene R. Wusterbarth, Hazard Ranking System Issue Analysis: Relationship 

Between Waste Quantity and Hazardous Constituent Quantity (32) should be 

used. OERR reported orally that this median concentration of all HCs 

at all sites is 450 mg/kg, but there was wide variation, This value 

relates to aggregates of constituents and only materials on certain 

lists of hazardous coru>tituents were sought. Hence, risk management 

or policy considerations might cause EPA to select another value for the 

default concentration. One alternative is to convert HC to toxicity units 

(concentration times a relative toxicity factor) rather than concentratio?• 

A specific rule for moving between levels (tiers) in option W3 

might be considered, The first level (total amount or concentration of 

hazardous constituent) should be used where data are sufficient to support 

an estimate of hazardous waste constituent quantity. This would occur 

where there are data to determine the hazardous waste quantity accurately. 

What data are available clearly show the quantity is significantly 

above the amount necessary to receive the maximum score, or significantly 

below the amount necessary for the next-to-minimum score. 

Where the quantity of hazardous constituent(s) is not known, scores 

could be calculated using the procedures of each of the first three 

levels, Then, scores for level 1 and level 2 would be compared, If the 

scores were within 25% of one another, the score for level 1 would be 

used. If the scores were greater than 25% apart, the score from level 2·· 

would be compared to that from level 3. If level 2 and level 3 scores 

were within 25% of one another, then the level 2 score would be used; if 

not, the level 3 score would be used in the HRS. The final level 4 

score would be used where data on waste quantity were virtually absent 

(especially for well fields). 

Other Comments on Concentration. The Work Group commented on five 

additional issues related to the use of concentration data in the HRS • 

• 

1. 	 Some Technical and Practical Considerations, OERR-s issue 

paper on the feasibility of using concentration (14) addresses 

only waste concentration data, The Work Group recommends 

that concentration in the receiving environment, and perhaps 

population densities, also be cons ide red as discussed in the 

Wolfinger report (31). Data on receiving water concentration 

data would aid in scoring observed releases. Fish bioassay 

data may be useful at times in ranking toxicity of releases in 

the absence of, or in addition to, chemical concentration data. 
A3-10 

Sites will have data bases of differing completeness and 

accuracy, as occurs now in site evaluation. Modified scoring 

sya tems l!IUS t use la cge data bases effectively to produce l!lore 

accurate scores while handling with equal fairness sites with 

fewer data. 

Sollie toxic contal!linants, such as metals, can be l!leasured 

in both solids and liquids at rea·sonable cost. All site 

inspections at mining sites could gather valuable data on the 

quantity of metals in the liquid and solid phases of the waste, 

2. Evaluation Techniques. The revised HRS should be tested 

for both· high strength concentration/low volume wastes 

(drums) and low strength concentration/high volume wastes 

(mine tailings), with both observed releases and potential 

releases of contaminants. 

The sensitivity of the new ranking system to concentration. 

should be tested to determine the importance of concentration 

in listing on the NFL and, thus, the need to obtain l!lore complete 

and accurate hazardous waste consitituent data. The analysis 

would also be a first step in examining possible false positives 

and false negatives. 

J, W2/W3 Modification. A non-linear scale such as that currently used 

for waste quantity may be appropriate, Such a scale would give low 

scores to small, less significant sites, while not unduly penalizing 

large sites, In a properly constructed algorithm, sites which are 

less significant in terms of risk will score lower than those sites 

which present greater risks. 

For unknown waste quantit.ies, the default scheme suggested by 

Kushner in Hazard Ranking System Issue Analysis: Sites with ' 

Unknown Waste Quantity (19) is prefered over a minimum default 

value,. 

4. QualitY of Waste Conce~tration Data. Sampling guidelines could 

be developed for each waste type (i.e. drums, tanks, tailings, 

impoundments, etc,). These could be modified by site inspection 

staff based upon site specific factors, but a minimum sampling 

scheme should be identified • 

• Perhaps a relatively large numbeJ: of samples c::ould be 

collected, but only a limited number analyzed initially. The 

initial results and calculated HRS scores could be used to 

decide on· the need for additional analyses, This approach 

would apply only to contaminants that could be preserved. 
• 


A3-ll 

More data may be required for sites with no observed release 

because they are more likely to score below the 28.5 boundary, 

and waste concentration data are critical in assessing the 

risk of potential releases. 

Composite samples should be obtained to reduce the number 

of analyses, while providing estimates of average waste 

concentration. 

5. 	 Sampling Risk. Waste sampling schemes should be developed 

based on a thorough review of existing data on hazards to 

sampling personnel. Description of the waste, anecdotal 

evidence, and data on the receiving environment should indicate 

the hazards posed to site investigators, In suspected high-risk 

sites, the sampling scheme should begin around the perimeter 

where less contamination is expected, and progress towards the 

areas of expected higher concentrations (this is also consistent 

with good sall!pling technique), Small test holes and gas sampling· 

equipment could be used on the actual site to test for hazardous 

conditions. 

Data requirements could be Nlaxed where severe onsite 

health and safety hazards exist. In any case, these sites 

should probably be addressed by the Superfund Removal Program 

for their direct contact; fire and explosion capabilities. 

OTHER COMMENTS 

Long-Terlll Considerations 

Although SARA mandates a re-evaluation of the HRS, and presumably a 

revision or alteration in a compressed time frame, it is nevertheless .. 

us.,ful to consider both the long-term and short-term requirements of the 

HRS and subsequent site evaluation process. In that context, these 

remarks focus on the issues involved in development of quantitative 

representation of .concepts involving contaminant concentration in the 

waste materials, contaminant concentration in ground water and mobility 

(a notion that combines release and transport). 

Clear qualitative differences exist between many large volume wast" 

(LVW) sites and the more numerous hazardous waste sites containing admixed 

synthetic organic/inorganic hazardous constituents in both liquid and 

solid form. LVW may be segregated into monofills of a single wast" type, 

Some sites have multiple types of wastes (e.g., mill tailings, smelting 

slag"s). Where these wastes are spatially separate, the sites may be 

r"lativ.,ly easy to characterize, Commonly, LVW sites contain low 

concentrations of hazardous trace elements (HTE) in the waste matrix, 
A3-12 

The release of these HIE is governed by very specific geochemical processes 

and is subject to migration constraints. Scientists and regulators increasingly 

understand the effects of these two factors on release. 

Long-term evolution of the HRS should include consideration of detailed 

numerical geochemical transport/fate models currently under development. 

The advantage of eventually using transport/fate models in the HRS is the 

explicit inclusion of quantitative representation of specific processes 

and mechanisms responsible for the release, transport, transformation, 

and retention of hazardous compounds. The use of these models entails 

with enlarged data requirements, but can yield less ambiguous, more 

detailed simulations appropriate for pathway calculations and, ultimately, 

exposure estimates. Considerable time and effort will be required to 

integrate these more complex transport/fate models into the overall 

framework of the HRS and to test their performance, 

Need for Additional Studies 

The studies of large volume waste presented to the Work Group contained · 

little in the way of original investigation. Looking to future improvements 

in the HRS, OERR should plan thoroughly peer reviewed studies, both to 

review large volume waste sites ranked under the HRS and to examine 

basic parameters in the model. A stud7 of the first type was suggested 

by Mr. R. Walline of EPA Region VIII which could address the actual 

reasons why ·release occurred at problem mining sites (reasons reportedly 

are different than those initially assumed). A study of the second type 

might address the characteristic(s) of mining was-te that control release 

(mobility) to ground water. 

Search for False Negatives and False Positives 

Because it is difficult to design a system that can in all cases 

make the best use of all the site-specific scientific information available,.. 

the Agency should consider the advantages of utilizing additional scientific 

judgment outside of (or adjunct to) the HRS model in making decisions on 

whether to list sites on the NPL. 

Screening models like the HRS must be simple. They do not have mucb 

resolving power and therefore, some false positives and false negatives 

are inevitable. Because of this limitation, HRS scores should not be 

overemphasized. A process should be established either to review sites 

subject to scoring or to review HRS scores in an attempt to spot false

• positives and negatives, Such a scientific review process could involve 

the use of additional models; in many cases, however, obvious false 

nega~ives or positives might be best handled by a "manual" J;eview. The 

importaut point is th~t the system be flexible enough to allow for a 

variety of approaches to be used, as needed, during the scientific review 

process. 
A3-l3 

The extent of waste characterization data to be collected at the S I 

level is part of a cost/benefit calculation. The economic impact of 

being falsely included or not included on the NPL should be considered. 

SUMMARY 

Studies of the experience of applying the HRS to mining sites have 

not proved systematic error unique to such sites. 

However, the HRS has a potential to produce abnormally high scores 

for mining sites due to the manner in which toxicity, mass, release, and 

transport/transfo~ation processes are addressed. 

.· 

• 
ATTACHMENT A 

The Superfund Hazard Ranking System (HRS): Applicability 

to Mining Waste Sites, July 22, 1987, defines the issues for SAB 

consideration as follows: 

1. 	 "Does the series of TRC analyses indicate that the HRS is 

unfairly biased against mining waste sites (as compared to 

other types of po,tential Superfund sites)? 

2. 	 "Is there sufficient evidence that the existing HRS scoring/NPt 

listing process has been inadequate with regard to mining 

sites evaluated to date (i.e., produced numerous false 

positives or false negatives)? 

3. 	 "Has it been demons era ted that the existing HRS is an inappropriate 

scoring system for determining whether mining waste sites 

should be listed on the NPL (i.e., subjected to further 

investigations and analysis)? If so, what factors should 

EPA consider modifying to make the current HRS more appropriate 

for mining sites?" 

.• 


• 

• 

ATTACHMENT B 

The Supe~fund Hazard Ranking Syst~w (HRS): Feasibility of Using 

Conc~ntratioa Data in a Revised HRS, July 27, !987 defines the issues 

for SAB consideration as follows: 

J, "Based on the information pres~nted and other data available, 

does it appear to be desir~able to include waste concentration 

data in th~ HRS? 

2. 	 "What additional t~chnical and practical considerations 

ought to be factored into the decision as to whether or 

not waste concentration data should be included in the HRS? 

), "Can the SAB suggest any systematic quantitative or s~wiquantitative methodology for deciding whether to include 

waste concentration data and for evaluating systems that 

do so in th~ HRS that would consider effectiveness of the 

system at generating risk-based scores, the costa of data 

development, and potential improvements in decision making? 

4. 	 "Which of the suggested new approachs (W2 or \o/3) appears to 

be more desireable? 

5. 	 "Can the SAB suggest any· changes in the structure of options 

\o/2 or W3 that would wake thew more desireable (e.g., changes 

in decision rules, parameter values)? 

6. 	 "What quality level of data on waste concentrations data 

quality (what types, amounts, and level of precision) 

should be required if such information is to be used in 

developing HRS scores? Is there a simple, robust way to 

specify data quality that could be easily applied by a 

wide range of field personnel at a wide variety of sites? 

7. 	 "With regard to alternative \o/3: --What general advice can 

be given regarding how to define rules for moving between 

tiers? How ·good· does waste concentration data need to 

be to allow ita use? 

--Does the approach taken to defining conversion factors 

• 	 (particularly the assumption of 200 ppm typical waste 

concentration) appear to be reasonable? What alternatives 

could be suggested? 

8. 	 "What appr·oaches can be taken to mitigating risks to field 

personnel during waste sampling? Should data requirements 

be relaxed at sites where severe on-site health and safety 

hazards exist?" 
APPENDIX 4 

COMMENTS ON EXPOSURE 


BY THE 


HAZARD RANKING SYSTEM REVIEW SUBCOMMITTEE 


OF THE 


SCIENCE ADVISORY BOARD 


• 

TABLE OF CONTENTS 

EXPOSURE 

Why is it ImportAnt to Address Actual and 

Potential Exposure 

Options for Assessing Exposure 

Relationship of Source Characteristics to Em·issions 

How Comprehensive Should the HRS Be in Addressing 

Environmental Pathways and Routes of Exposure 

EnvironmentAl Pathways 

Routes of Exposure 

MEI and Population Risks 

How Can the Revised HRS Better Account for Exposure, 

Both Actual and Potential 

• 

PAGE 

A4-l 

A4-l 

A4-2 

A4-3 

A4-3 

A4-3 

A4-3 

A4-3 

.· 
A4-1 

EXPOSURE 

Why is it Important to Address Actual and Potential Exposure 

Exposure occurs when there is contact between pollutants and receptors, 

Addressing exposure is important when there are toxic chemicals present 

because the determination of whether there is any actual or potential 

risk to health or the 'environment depends on both exposure and toxicity 

Where there are no toxic substances or where there is no exposure to 

them, there is no risk. To the extent that the RRS is intended to assess 

relative risk, it must address both toxicity and exposure sufficiently 

to determine risk, Where infor~on on either toxicity or exposure is 

incomplete, risk cannot be fully assessed, Risk can be evaluated 

by assessing four factors: (a) the presence of chemicals, (b) their 

toxicity, (c) their potential for release and 1110b1l1ty, and (d) the 

probability of human and environmental contact. 

Options for Assessing Exposure 

Ideally, a quantitative risk ranking, system would rely on data for 

all variables of the risk algorithm. Because it is not possible to 

obtain quantitative data for all the necessary variables, semi-quantitative 

approaches must be considered. Surrogates for quantitative values may 

be developed through several mea~. They include identifying the appropriate 

measure of central tendency for the known data values, modeling values 

from the few known data points for the site of, concern or for comparable 

sites, ·using indirect measures (such as total waste volume as a substitute 

for concentration), or simply applying points in the algorithm based on 

a subjective' evaluation of the available descriptive information for the 

site. An example of a tiered approach for the use of waste quantity 

information is discussed in Appendix 3. A similar strategy could be 

developed which uses measured values in preference to indirect es timat:es .· 

for a variable of exposure, When the ideal data are not available, 

however, options to estimate the parameter can be used in s tiered process 

that moves away from actual measures toward increasingly indirect measures, 

In this manner, ·the maximum value of the data is retained while the goal 

of evaluating a variable is met. 

Exposure should be assessed in a manner that is both chemical-specific 

and pathway-specific as noted in Appendix 2. The potential for release 

and mobility is related not only to the chemical of concern but also to

• the environmental media through which it must travel before exposure can 

occur. Without this specificity, any attempt to address exposure is 

likely to result in inaccurate estimates of risk. 

The most attractive option is to determine the presence of individual 

chemicals of known toxicity and then assess their potential for release, 

mobility, and contact with humans and environmental receptors. Much 

could be gained by adding a chemical-specific exposure score that could 
A4-2 

be combined with chemical-specific to~icity scores to determine which 

chemicals dominate the risk at a single site and how the risk might 

compare across sites that are otherwise similar (e.g. in population 

distribution, existence of valued environmental resources, hydrogeology, 

meteorology, and containment), While little is kno~ about the amounts 

of chemicals present and their concentrations in environmental media 

after only a site inspection, the insights to be gained from a chemicaland site-specific approach are so great that it should not be lightly 

dismissed, 

Relationship of Source Characteristics to Emissions 

The HRS has been limited by not taking into account the potential 

for mobility of specific chemicals through specific pathways. Clearly, 

detailed estimates of pollutant emissions by pathway are not feasible 

with the information and resources available for scoring a typical 

candidate site with the HRS, (Here emissions should be taken to mean 

pollutant discharges generally, not just those to the air,) Given some 

minimal information about a site and its chemical inventory, howeve~, it 

may be possible to make crude estimates of partitioning among environmental 

media and, therefore, of emissions by route which could be used to generate 

part of a chemical-specific exposure score. 

Information needed to derive-such estimates includes: 

j, 	 An estimate of the total mass or source concentration of important 

chemicals at the site. 

2. 	 A description of the site-specific factors influencing release, 

e.g.; impoundment vs, landfill. 

J, 	 Partition factors applicable to the chemical, e,g,, Henry·s law·· 

constant and organic carbon partition coefficient, 

Some characterization of the potential for migration also allows 

better consideration of time as a factor in site management, A chemical 

that quickly moves off-site through air or surface water may pose high 

risks to surrounding populations in the short-term, especially if it 

acts as a threshold toxicant, whereas an immobile (but persistent) 

chemical will remain a potential hazard for long periods, especially if 

it is a non-threshold toxicant, e.g,, a carcinogen. Estimates of the

• total mass deposited and the emission rates would permit an estimate of 

the 	site•s "lifetime" if not remediated, Such estimates might provide 

a b,etter appreciation of the urgency for remedial actions·, 

How Comprehensive Should the HRS Be in Addressing Environmental 

Pathways and Routes of Exposure? 

Both toxicity and potential for exposure depend on environmental and 

biologic processes. An evaluation of environmental pathways addresses 
A4-3 

the environmental parameters that affect risk and an assessment of routes 

of entry provides the information needed to estimate risk to hu~ans and 

and the environment. 

Environ~ental Pathways. Environmental pathways include air, water 

(both ground and surface waters), soil and inter~ediate co~part~ents 

such as plants and animals that become part of the human food chain. 

The potential for release and mobility of chemicals through these 

pathways affects risk estimates. Factors to consider include vapor 

pressure, time of transport from point of origin to populations at risk, 

chemical transformations that may occur during that ti~e, and the toxicity 

of the chemicals that are expected at the point of exposure. Chemicals 

behave differently in different environmental pathways, so all pathways 

need to be evaluated fully to estimate risk. 

Routes of Exposure. Routes of exposure (entry) occur by ingestion 

(through water, food and soil), inhalation (through air and water vapors), 

and direct contact (through water and soil). Biological processes such 

as metabolism and pharmacokinetics affect the dose delivered to tissues 

that may experience adverse effects-. Biological uptake, delivery to 

tissues and transformation of chemicals to more toxic chemicals within 

the body are aspects of risk that can not be directly assessed in the 

HRS. As .a result, where route-specific toxicity information exists, the 

toxicity and biological impact o! chemicals need to be estimated by 

route of entry and incorporated into the toxicity fa~tor of the algorithm. 

Without a syntheses of these route-specifi~ evaluations, risk ~Y be 

either incorrectly estimated or under-estimated. 

MEI and Population Risks. SARA Section 105(a)(8)(A) states that 

the HRS should take into account "the population at risk." The Maximally 

Exposed Individual (MEI) is not considered in CERCLA or SARA. While 

population risks al~ays come into. play due to the impact of collective .• 

individual risks, so!De "mini!Dal" individual risks (such as birth defects) 

associated with the ME! approach have ~jor lifelong impact. Furthermore, 

the MEI is a very widely used. measure of risk. 

The current HRS addresses MEI in considering distance to nearest 

well and population risks in considering air target distance. 

How can the revised HRS better account for exposure, both actual and potential? 

• 

Exposure requires contact between person and pollutant with risk focused 

on this point of contact. Sources; transport and fate; and the presence, 

nature,and activities of receptors are relevant to exposure. 

Measurement of exposure ~ay be person-based or community-based. 
A4-4 

The cu~~ent HRS finesses the assessment of exposu~e and instead 

uses the p~obability of release (and other factors such as persistence, 

waste quantity, distance to nearest well and population affected) as 

proxies for exposure. 

The main difficulty of accounting for direct exposure stems from 

the need to consider potential as well as actual exposure. There ate 

two aspects of exposure to be considered here--the concentration of 

contaminents in the environment around the site and the number of people 

around the site. Actual release may be demonstrated by environmental 

sampling. Potential release may be addressed by considering properties 

of the site and chemicals present such as distance to ground water and 

volatility. Similarly, the number of people (receptors) present and 

actually exposed today can be counted, while the number of people 

potentially present at the location in the future can only be estimated 

from studies of population dynamics. 

While estimates of person-based exposures can be developed from 

biological samples, from personal household samples, f~om self-reported 

exposures, and from self reported symptoms, each approach has its 

limitations. As a practical matter, biological samples are rarely 

available. The cost of personal or household samples are prohibitive 

and would allow only limited opportunities to evaluate potential ambient 

exposur.,s. Because subjectivity c·an easily enter into self-reported 

exposures, objective measurements need to be used to limit the subjective 

variability. Subjectivity also enters into reports of symptoms (whether 

self-reported or based on medical record studies) which serve as a 

proxy for exposure. Nevertheless, the reporting (by nearby residents) 

of physical symptoms consistent with acute effects of contamination from 

the site could be considered as evidence of possible, but not confirmed 

releases, This is one of the situations which could be addressed by the .• 

assignment of discretionary points to the HRS score. (see Appendix 5) 

Community-based exposures can be developed from ambient pathway 

measurements, from site measurements, or from modeled exposures. Numerous 

assumptions about targets (such as animals or humans) and their behavior 

are needed to impute exposure from ambient pathway measurements. However, 

even more assumptions are needed to impute exposure from site measurements 

(see also source characteri~ation discussion above) because the monitors 

are placed further from the potentially exposed population requiring 

• 	 additional assumptions about the time and mobility factors that operate 

between the source and target population. Even more assumptions are 

required to model exposure from similar sites because no two sites are 

exactly the same •. Modeled exposures may be based on similar sites and 

can be used to predict rare events. The latter two approaches (site 

measurements and modeling) are discussed further below. 
A4-S 


The way the current HRS addresses source characterization--including 

both the evaluation of the mobility and fate of chemicals and the consideration 

of concentration or quantity of chemical~s insufficient to model 

exposure. 

Incorporating concentration and mobility could produce a more 

comprehensive ranking scheme. One way to do this is by using 

concentration as a weighting factor for the waste quantity. The 

resulting effective waste quantities for low quantities of highconcentration waste might be similar to those for large quantities 

of low concentration wastes. Similarly, the scores for toxicity/ 

persistence and for effective waste quantity could be adjusted by 

considerations of mobility. 

From a scientific standpoint, exposure may be modeled by imputing 

an exposure for a site based on what is known of similar but more fully 

studied sites. If there are sufficient data on the actual exposures 

from sites with similar characteristics a potential exposure for the 

specific site could be imputed, but it is more difficult to define sites 

which are comparable in this context than.to define sites which are 

comparable in terms of quantity and concentration, For exposure, the 

sites have to be comparable in geology (especially in mineralogy) as 

well• 

• 


APPENDIX 5 

COMMENTS ON THE ALGORITHM 

BY THE 

HAZARD RANKING SYSTEM REVIEW SUBCOMMITTEE 

OF THE 

SCIENCE ADVISORY BOARD 

.­

• 

TABLE OF CONTENTS 


PAGE 

ALGORITHM AS-1 

Background AS-1 

Why is it Important to Pay Attention to the AS-1 

overall Alogrithw? 

Can the Revised HRS Represent Relative Risk? Or AS-2 

Can it Only Serve as an "On-Off" Switch? 

What do These Choices Mean? 

What Methods are Available to Develop a Ranked List? AS-4 

What Rules Should be Used to Develop Scores Based on AS-5 

on Risk? 

Aggregation of Pathway Scores AS-7 

How Can the HRS Make Better Use of Temporal Analysis? AS-8 

What can Be Done About the Quality of the Data Used in AS-8 

the HRS? 

What Can Be Done When There Isn't "Enough Data" For AS-9 

The HRS? 

What Should Be Done When Data Are Available That Exceed AS-10 

What Is Needed For The HRS? 

Caveat AS-11 

• 

AS-! 

ALGORITHM 

Background 

The 	 Hazard Ranking System operates through the following three steps: 

!. 	 To the extent,possible, specified information regarding the 

·properties of the site and of the chemicals deposited there 

is gathered and described quantitatively or qualitatively 

according to the needs of the HRS. 

2. 	 The information is manipulated, usually through comparison 

to categories in tables provided in the HRS manual, to 

yield a series of factor scores. The specifications of the 

conversion from raw information to factor scores may be 

called scoring rules. 

J, 	 The individual factor scores are combined within categories 

and then the category scores are further combined to yield 

route scores and eventually an overall migration score for 

the site. The specifications of how the scores are to be 

combined may be called combining rules. 

Together the scoring rules attd the combining rules form the overall 

logic and procedure, or algorithm, for the operation of the HRS. To a 

substantial extent, the methods for gathering the raw data· (step l above) 

and deciding how much effort should be expended on that step are left to 

the individuals doing the scoring. The algorithm, however, is a fixed 

procedure that undergoes OERR sponsored quality control. As such, it is 

the key to how well the HRS will perform in assessing relative risk. 

Both the relationship of the rsw data to the factor scores and the ways 

in which the factor scores are combined must be treated carefully and .· 

consistently to optimize the performance of the HRS. 

Why 	 is it important to pay attention to the overall algorithm? 

Improving the algorithm could potentially do more to improve the 

HRS than fine-tuning individual components. 

The 	algorithm should be approached by thinking about the best way 

• 	 to assess risks for·a hazardous waste site without regard to the availability 

of site-specific information and then simplifying the rules (but not the 

logic) as needed to take into account real-world constraints --that is, 

to.identify the most important variables and then develop data (or surrogates) 

and combining ruleS". 

The overall algorithm needs to meet the intended purposes of the 

HRS as stated by SARA. SARA requires EPA to modify the HRS so that, ''to 
A5-2 

the ma~imum e~tent feasible, it accu~ately assess the ~elati~e deg~ee of 

<isk to human health and the envi~onment posed by sites and facilities 

under re~iew," In Mt", Longest's June 15, 1987 memorandUlD (see Appendix 

7 for full text) he states, "OERR believes that the purpose of the HRS 

is primarily a screening tool used to determine which sites will be 

candidates fo~ Fund-financed remedial ~esponse, To the degree possible, 

the model also needs to provide a st~atification of sites based on the 

relative risks posed to human health and the envi~onment." It is not 

clear that the cur~ent system st~atifies by risk, nor that it can serve 

as a successful on-off sc~een without st~atifying by ~isk at least in 

the region of the cut-off score. 

At a minimum the HRS must discriminate sites into two risk groups--on 

or off the NPL. Ideally, the HRS would individually rank all sites 

based on accurate ascertainments of risk--resulting in as many ~elati~e 

risk ranks as there are sites to be ranked, While the binary ranking is 

minimally sufficient, it is apparent that programmatic concerns de~nd a 

ranking that is more than binary. The decision about the level of ~ankirig 

was not within the scope of the Subcommittee-s review, instead the 

Subcommittee focused on developing conclusions and recommendations for 

the scientific bases of an effective ranking system. The Subcommittee 

p~oceeded to evaluate the various components and structure of the algoritha 

to wo~k toward a scientifically valid approach to estimating risk. 

One such component relating to risk is the toxicity score, The 

additive structure of the matrix weights the lower end of the two 

components (toxicity and persistence) and thus implies that any toxic 

effect o~ p~rsistence is more important than the probability or severity 

of the effect or the degree of persistence. These baseline assumptions 

have the effect of treating toxicity and persistence in a binary manner, 

but this method of assessment may not be sufficient to meet the goals of 

the HRS and the Subcommittee did not find it to be an acceptable approach 

if the HRS outcome is expected to be more than binary. In addition--and 

as e~idenced by the very high percentages of sites receiving the maximum 

score for toxicity and persistence--the inability of the current toxicity 

module to rank substances effectively on relative risk puts a severe 

limitation on the entire HRS to discriminate between sites on the NPL 

and the~eby to prioritize appropriately on the bases of public health 

and environmental protection concerns. The scoring methodology needs to 

be re-evaluated in terms of the overall purposes of the HRS • 

• Can the revised HRS represent relative risk? Or can it only serve as an 

"on-off" switch? What do these choices mean? 

The Subcommlt.tee has suggested changes that will allow the HRS to 

provide a more accurate and scientifically based estimate of the ~elative 

risk of candidate sites. To the extent possible, the HRS scores should 
AS-3 

correspond to an objective evaluation of relative risk at sites. E:owever, 

this is not always feasible due to both scientific and data limitations, 

as well as the value and policy decisions implicit when considering and 

balancing human health and environmental impacts. 

While recognizing that the Agency must continue to base NPL listing 

decisions on many factors in addition to the HRS, the Subcommittee believes 

that a revised HRS, better designed to evaluate sites by relative risk, 

will provide an improved mechanism for determining which sites should be 

included on the NPL, and can potentially provide meaningful input to the 

subsequent assessment of NPL .sites. Most of the changes needed to revise 

the HRS are changes in the overall algorithm and not changes with vast 

new data requirements, (Pages AS-5 through AS-8 discuss desirable changes 

in the algor! thm.) 

If the HRS is to be used to set priorities based on relative degree 

of risk, many factors need to be considered systematically in setting 

priorities. 

Although it is possible to talk about an on-off switch and evaluate 

the HRS by whether it rates most sites on when they should be "on" and 

vice-versa, in fact there is a continuous range of scores with an arbitrary 

cut-off level designed simply to.get 400+ sites on the original list. 

Unless the list is in reasonably good order both above and below the 

cutoff, its fidelity in distinguishing high and lesser risk sites will 

depend markedly on the choice of the cutoff, i.e., on how many sites (or 

what percent of sites) are wanted on the list. 

There are costs associated with misclassification. If the HRS will 

be used to decide whether or not to spend $850,000 on an RI/FS, the 

economic cost of a false positive is approximately $850,000 to the fund .• 

(although less to society because even the low-risk site might need 

remediation and would benefit from the RI/FS). The social cost of a 

false positive may be reflected in decreased property values, increased 

physical stress.and community·.concern, and in extensive time and effort 

expended by public and private officials attempting to address these 

concerns. The social cost of a false negative is the difference in 

average risk between the sites that should be on and those that should 

not, less any benefit of attention, independent of Superfund. Unfortunately, 

the average risk of Superfund sites is not known, let alone the risk of

• those that were scored but which did not make the NPL. 

The difficulty remains that there has been no analysis to determine 

wnether the current HRS score is a good predictor of the risk ultimately 

generated by the formal risk assessment process conducted during the 

RI/FS phase. This evaluation needs eo be done as a guide to future 

modifications of the HRS. 
• 


A5-4 

The question remains, "tisk of what and to whom?" To create a 

risk-based RRS that evaluates sites, EPA would first need to reach an 

acceptable definition of risk which combines the health, ecological, and 

economic damages expected over the life of the facility. The Subcommittee 

favor multiple measures including a human population risk measure, The 

latter is especially important because quantitative techniques are available 

to assess it, Some subjective increases or decreases in score could be 

used to account for severity of human health effects and likelihood of 

effects on non-human biota, 

What methods are available to develop a ranked list? 

There are three common methods for developing ranked lists, 

One way is to devise a highly simplified mathematical model that 

produces a number that is approximately proportional to the true ranking 

variable--in this case, some agreed-upon definition of risk. (Actually, 

all that is required is that the number increase monotonically with 

risk,) The system referred to as "RAPS" is such a model. Although 

simplified modeling has the advantage of being related to risk, it has 

the disadvantage of being a very substantial departure from the current 

HRS. RAPS is probably too complex· to work well with the limited data 

available for most HRS candidate sites, even though it has worked well 

for DOE sites. 

Another method is to devise a series of screens that successively 

divide the list of all sites into categories, which are later ordered in 

some fashion that is approximately the same order as risk. 

The third major technique, often simply referred to as "scoring", 

is structured value analysis, of which the current HRS is an example. 

Although the other two approaches have their virtues, it is reasonable 

to concentrate on variationa .of the structured value approach because 

the current HRS uses it, Even here, two different approaches are possible. 

The curren't HRS appears to take a so-called "empirical" approach. 

That is, a superficially plausible set of rules was developed and then 

modified "empirically" to match a subjectively ranked test set of sites. 

If the subjective ranking was truly what the HRS was supposed to achieve, 

and if the test set was adequately representative of all sites to be 

scored, then this method is perfectly satisfactory• 

The other approach, a risk assessment approach, is to begin with a 

clear understanding of how to rank the list quantitatively if all the 

needed information and resources to process it were available. Then the 

risk assessment. is simplified until it is obvious how to transform it 

into a scoring system. The scoring system is further simplified until 

it can operate at a reasonable cost on the often very sparse information 
A5-5 

available. For example, the population risk at a site might be represented 

by the sum over all routes of risk by route, and each route-specific 

risk might be represented by ,the sum of all chemical-specific risks by 

route. Each chemical-specific risk might be represented by an integral 

over population at risk of the product of toxic potency and time integrated 

exposure. 

What rules should be used to develop scores based on risk? 

First of all, internal consistency is important. Making rules that 

are consistent requires little up-front investment and no additional 

costs in the scoring of specific sites. These changes may be much more 

cost-effective than additional data collection. 

The way that the various pieces/components of the score compile 

should reflect how their real world counterparts interrelate. The approach 

for accomplishing this is to begin with a physically-based exposure 

assessment model for each exposure pathway, structured properly to trans!ate 

expected or potential releases into environmental concentrations and 

subsequent exposures and effects. These models would, of necessity, be 

highly simplified for a screening assessment. Even a simplified exposure 

assessment model may not be feasib·le given the time, resource and data 

limitations associated with the HRS process. Still, the manner in which 

the pathway scores are estimated and combined should be consistent with 

the fundamental material balance and exposure principles of such an 

underlying model. The following points discuss how this can be accomplished 

in the ~ontext of the structured value approach used in the current HRS 

system. 

!n a scoring system designed to reflect relative risk, the scores 

~auld be added across routes and chemicals, with each score being a 

produ~t of scores representing population, toxic potency and exposure .. 

potential. To work properly, the scores for these three factors would 

have to be approximately proportional to the estimated values of the 

factors. For population and potency the answer would be relatively 

easy, but the exposure score would probably need to be constructed from 

scores representing various factors related to exposure, such as mass of 

chemical deposited, containment efficacy, mobility, and persistence. 

Exactly how this is done is not as important as that the rules for 

sc6ring factors are ·cons istent with the rules for combining scores, and • that both are reasonably consistent with how the factor enters the risk 

equation in a risk assessment. 

In ~ontrast, .the rules for combining scores in the current HRS do 

not always reflect, relative risk. Scores often rise as the logarithm of 

the factor value being scored. For example, each order-of-magnitude 

increase in potency may receive an additional point, and each factor ~f 

two or two-and-a-half increase, in population at risk may receive an 

' 
AS-6 

additional point. Using logarithmic scores is an appropriate way to 

account for a wide variation in magnitude of the possible values for 

factor score9; furthermore, logarithmic scores can be added to simulate 

multiplication of the underlying factors. For example, when the HRS 

assesses a site with carcinogens, the population at risk can be estimated 

by multiplying the size of the population at risk by the risk per individual 

which can be estimated as the product of the exposure of each individual 

by the potency of the carcinogen. The combining rule should add the 

logarithmic scores corresponding to the three factors. In the case 

cited, however, the logarithms are to different bases; potency is scored 

by a logarithm to the base !0, but population is scored approximately 

to the base 2.3. Because (2.3)3 is approximately equal to 10, it takes 

3 population points to be equivalent to 1 potency point, and the potency 

scores must be multiplied by 3 (or population scores divided by 3) before 

they are added. In this case, the HRS "weighting factor" of 3 is exactly 

what is called for by the existing science. It is a natural outco~e of 

the simplification process and r10t a subjective "weight". In the HRS, the 

weights do not always conform to the science, and the co~bining rules are 

not always consistent with the scoring rules or with one another. 

The use of a scoring system to place sites on the NPL is certainly 

appropriate and, as scoring systems for priority setting go, the HRS is 

already better than most. Almost all of the factors scored are plausibly 

related to risk and generally or1e point means the same in different 

parts of each scale, usually a change of sn order-of-magnitude or a 

constant factor. 

However, the current HRS is not always internally consistent in the 

number of points which relate to a change in risk. For example: 

!) 	 A single point does r10t have the same mathe~atical relationship 

to the final score in one part· of the algorithm as a single .• 

point has in another. This variation in scoring implies 

different assumptions about how the factors relate to risk. 

2) 	 Subjective weighting factors are included. For example, 

the route scores are combined by a route mean square rule 

so that when one route dominates, it alone is enough to place 

a site above the cut-off. (However, at many sites, the 

rislt through one route will usually dominate the other two. 

Whether or not such weights should be used is debatable • • However, if weighting is used, it should be explicit.) 

3) 	 The rules for combining some scoJ:"eS in the curr.ent HRS do 

not rel.at.e properly to the observed behavior of those factors. 

For example, consider the way in which the total waste 

characteristics score is de rived from its components. It 

is the sum of the component scores for quantity and toxicity/ 

persistence (toxicity and react·ivity for the sir route). 
A.S-7 

Technically speaking, a site with much harmless material could 

be eligible for the NPL if it receives the ma~imum scores for 

observed release/route characteristics containment, quantity, 

and targets .. 

This is an extreme case, However, the combination of summation and 

multiplication in the algorithm has problems beyond this rather artificial 

example. A useful interpretation of the current HRS would be that since 

both toxicity/persistence and quantity scores appear to be logarithmic, 

summing them would be correct for assessing the total waste characteristic 

score on the logarithmic scale. However, the same interpretation does not 

hold for other pares of the algorithm. For example, the component scores 

for observed release/route characteristics-containment, total waste 

quantity characteristics, and targets --are multiplied. If the same 

logarithmic interpretation is to be applied to these components, then 

all those multiplications should be changed to summations. Otherwise, 

the summation in the waste characteristics score should be changed to 

multiplication. The underlying question is whether the entire score 

should be interpreted on the logarithmic scale. 

For another important component, observed release/route characteristics. 

containment, the scale used for scoring appears to be the probability of 

release instead of its logarithm. ­

there are two important concepts here. First, the way the scores 

are combined should reflect the way those components relate to one another 

in the real world. Secondly, the final HRS score should be on some kind 

of easily understood scale (arith~etic, probability, logarithmic, 

etc.) so that the relative risks presented by various sites can be easily 

understood. "Normali~ing" scores among routes only makes sense if one 

believes that 100 points implies the same risk through each route. On 

the other hsnd, there is no easily implemented way of interpreting .. any of the scores and the equality assumption is not disprovable. 

Aggregation of Pathway Scores 

A variety of methods can be used to aggregate pathway-specific 

scores. If sufficient structural information on the pathways scores is 

available, such information should be used in choosing the aggregation 

method. For example, if the pathway scores are constructed on the 

logarithmic scale, the exponential averaging method should be used. lf

• the pathway scores are constructed on the arithmetic scale, the linear 

averaging method should be used. 

The quadratic averaging method used in the current H'RS is not 

only arbitrary but,' from a scientific viewpoint, arguably incorrect. 

OPPE reported that its unpublished Site Ranking Panel study compared 

three different aggregation rules: linear averaging, quadratic averaging, 

and cubic averaging. The results (within the limitations of the study) 

indicated that the panel's final ranking was closest to cubic averaging. 
A5-8 

In the absence of sufficient structural information on the path~ay 

scores, the choice of the aggregation method reflects the subjective 

judgment on the relative importance of single- and multiple-path~ay 

sites. An important class of aggregation rules is the k-th po~er 

averaging method in which the score for each pathway is raised to the 

power k, calculating the average across pathways, and then taking the 

k-th root, The linear averaging method is k ~ I and k~2 is the quadratic 

averaging method. Large values of k give less prominence to the path~ays 

with lower scores, making it easier for single-path~ay sites to get on 

the NPL. Small values of k give more prominence to pathways ~ith lower 

scores, making it easier for multi-pathway sites to get on the NPL. 

There are two other important aggregation rules. The worst case 

method takes the worst of the path~ay scores, The e~ponential averaging 

method e~ponentiates the pathway scores, takes the average across the 

pathways, then takes the log, The two methods should yield very similar 

results, giving little prominence to pathways with l~er scores and 

making it easier for single-pathway sites to get on the NPL. Both methods 

can be viewed as generalizations of the k-th power averaging method with 

k approaching infinity. 

How can the HRS make better use of temporal analysis? 

Analyzing how long it takes eo analyze a site through the various 

steps bet~een identification and remediation yields information which 

has implications for parts of the HRS-especially the toxicity/persistence 

score and in considering population dynamics, If, for example (and this 

currently seems to be the rule rather than the exception), the site 

remains unremediated for a long period of time, then chronic effects 

become more important than acute and the HRS should address them, 

(See Appendi~ 2 for a discussion of acute and chronic effects at 

unremediated sites.) 

By analyzing information about the source and migration (and also 

anything else that would remove wastes from the site, such as decomposition), 

it is possible to estimate the length of time a site may present a problem. 

Such an estimate has implications for remediation. 

Similarly demographic and other relevant changes which may be predicted 

over time could alter the nature and degree of risk posed by a site, 

Because such factors alter risk, the revised HRS should not ignore them • • 

What can be done about the quality of the data used in the HRS? 

A major effort·should be made to improve the overall quality of 

analytical data collected at sites. Standardized collection and laboratory 

methods currently exist for only a small fraction of substances potentially 
A5-9 

present. Expanded chemical characterization of all media, coupled with a 

strong laboratory certification program, will improve not only the HRS but 

all aspects of the Superfund process. 

Similarly, acceptable data collection procedures should be established 

(or expanded upon) for non-analytical data (such as population estimates, 

volume of ..wastes, etc.) so that the best quality data will be preferred. 

Mechanisms which could be used to encourage the collection and use of 

quality data include: a tiered data quality approach such as the one 

OERR presented to the Large Volume Waste Work Group, data collection "trees", 

and the weighting of final HRS scores to account for the quality of data 

used in generating it. 

What can be done when there isn•t "enough data" for the l!RS? 

It is necessary to have a method of default (also termed missing 

value replacement values) when data are limited or nonexistent for 

chemicals. Lacking complete information on chemical inventory and 

concentration at a site may well be the rule rather than the exception. 

Therefore, a standardized approach to the circumstance will reduce the 

inconsistencies that are inevitable ~f such decisions are left to field 

staff or if only the "better" data are somehow selected and used. 

several approaches can be used for assigning a value to a scoring 

component when data are not available. One way is to assign a zero. This 

would be· a mistake. However, because it underestimates risk. (However, 

on a logarithmic scale, assigning a zero may, in fact, overestimate 

risk, If the'risk should be very small, the logarithmic score should be 

appropriately negative--even minus infinity, if the risk is zero 

arithmetically.) 

.· From a scientific perspective, a better way is to use auxiliary 

information on the site to help impute the missing value. 

one concern with this approach is whether the fact that the data 

are missing might be an indication that the site might be 

different from otherwise comparable sites, 

The definition of comparable sites is crucial and may depend on the 

scoring component. For quan.tity and concentration, gross categories of

• sites could be used (as a minimum), Stratification by region is also 

possible, leading to a definition of comparable sites such as those of 

the same category in the same geographic region. When possible, comparable 

sites should be defined to allow the use of the greatest amount of data. 

There are two general statistical approaches for defining comparability 

in this situation. First, one can develop a metric to measure how comparable 

two sites are in tenus of the auxilliary c,harscteristics, then choose a 
A5-IO 

prespecified number of sites closest to the specific site in terms of the 

given metric. Second, one can use an empirical model to describe how the 

scoring component depends on the au~illiary characteristics, then use the 

prediction from the model to impute the missing value for the specific 

site. 

Incorporation of a large number of auxilliary characteristics is 

not a trivial task. It might not be worth the effort to go beyond the 

use of broad site categories and regions. The decision depends on whether 

other characteristics could be powerful predictors of the scoring component 

being considered. 

In some situations, limited data about the site will exist. Averaging 

those data is an easy measure of central tendency likely to represent 

the site better than any of the individual numbers. For example, if a 

few concentration measurements are available, even if variable and of 

uncertain quality, their average can be taken to show relative quantity 

in the absence of better infonnation. Alternatively, some sense of 

proportion can be gleaned from the types of wastes known to have been 

deposited at a site. For example, relatively low concentrations (compared 

with average waste materials) of all hazardous chemicals are expected in 

fly ash, but metals and products of ·incomplete combustion (dioxins and 

furans) would be more likely present than volatile organics. 

Another approach may be used ~hen some data is available. It is 

known to statisticians as the shrinkage formula and to the actuarial 

literature as the credibility method, This approach uses data both from 

the site and from sites thought to· be comparable. The Subcommittee 1s 

not in a posftion to evaluate how difficult the field staff would find 

implementation of this approach, 

What should be done when data are available that exceed what is needed 

for the HRS ? 

one approach 1s the assignment of discretionary points (which the 

Subcorumittee has sometimes called the "Bump Factor"), The Subcolllmittee 

discussed severs! scenarios where such discretionary points could be 

usefully assigned such as in the case of a site where: 

1. 	 Many chemicals were identified (only some of which were data 

available for evaluating toxicity but where the presence of 

• other implied additional risk) • 

2. 	 The presence of the toxic materials in the surrounding target 

population had been demonstrated. 

3. 	 Acute exposures to air emissions were possible from catastrophic 

release or other changes in site conditions, 
AS-11 

4. 	 One or more contaminants were known to be extremely toxic to 

indigenous natural populations. 

S. 	 Human effects had been confirmed and those effects could be 

reasonably linked to exposure to particular chemicals from 

the site. 

6. 	 Nuisance levels of chemicals in the air around the site were 

identified of and their presence confirmed (best analytically, 

but possibly by smell). 

This approach could also be used to take better account of non-human 

receptors. The idea is to assign extra points which are proportional 

to the extra risk the situation presents. 

Caveat 

If the algorithm is changed, then the 28.5 cut-off score should be 

re-evaluated. The threshold Sm, which determines whether a site is 

eligible for the NPL, needs to be adjusted when either the algorithm is 

changed or when more routes are incorporated because the same score 

will not necessarily have the same meaning. For routes already used in 

the HRS, OERR should investigate from existing data what alternative 

threshold should be used in order to yield a pre-specified proportion of 

sites on the NPL, if the algorithm is changed • 

• 


APPENDIX 6 

RECOMMENDATIONS TO EVALUATE AND IMPROVE THE HRS 

B'l THE 

HAZARD RANKING SYSTEM REVIEW SUBCOMMITTEE 

OF THE 

SCIENCE ADVISORY BOARD 

.· 

• 
• 


A6-I 

RECOMMENDATIONS TO EVALUATE AND IMPROVE THE HRS 

How Good Is the HRS? 

The HRS is appropriate for the purpose for which it is intended, 

As a scoring systems for priority setting, it is better than most, The 

factors scored (with possibly one or two exceptions) are related to risk, 

and higher scores for them reflect higher risks, Thus, the current HRS 

is plausible. The recommendations concerning the algorithm discussed in 

Appendix 5 should greatly improve it, 

HOW CAN EPA LEARN FROM EXPERIENCE TO BETTER PREPARE FOR THE NEXT REVISIONS? 

OPPE Study, In considering the HRS as a predictor for a comprehensive 

risk assessment, it is natural to clarify how well it performed in the 

past. The unpublished OPPE Site Ranking Panel study (18, 23) did not 

address this question but focused entirely on subjective evaluation, 

For parts of the HRS (route characteristics and containment), additional 

data are available, It should be possible to conduct an empirical 

evaluation using RI/FS and/or other data to evaluate the HRS score both 

for sites on the NPL and for those that did not score high enough for 

the NPL. 

Empirical Evaluation. The HRS performance should be judged by an 

empirical retrospective evaluation of how successfully the HRS predicts 

risk or by how successfully its components predict phenomena (such as 

release) which contribute to risk. This evaluation should be based on 

an in-depth technical review. Whatever the definition of risk, the HRS 

should be judged on how well it approximates that definition, not on how 

well it matches some subjective notion of the relative importance of the 

sites in a test set, 

With respect to empirical evaluations, the route characteristics 

and containment scores are estimates for the probablity of release when 

actual release is not detected. Two distinct probabilities need to be 

considered: (a) the probability that current releases are not detected, 

and (b) the probability that such releases will occur in the future. 

For the latter, the time horizon also needs to be considered. For the 

sites with sufficient RI/FS and/or data from studies by the states or 

other parti~s, it should be possible to search for the best way to 

predict release from the components of route characteristics and 

containment scores, 
A6-2 

Comparison of Data and Scores for the Same Site, Given the screening 

nature of the HRS, as a screening tool, it does not appear cost-effective 

to attempt to establish confidence intervals rigorously, However, some 

sefinition of uncertainty would be useful for interpreting the scores, 

If re-evaluations are available for some available for some sites, it 

would be useful to compare the data obtained from the separate 

inspections of the site, State data, frequently used in HRS scoring, 

might also be used for this purpose. 

What Kinds of Data Should the Agency Collect and What Kinds of Studies 

Should it Conduct to Support Future HRS Revisions? 

There are two goals for such studies. First, the Agency should 

improve the algorithm to be defended as risk-related and, second, the 

studies should develop at least a crude way to estimate the quantity 

of specific chemicals present. The planning of any evaluative studies 

should be carefully peer-reviewed before being implemented. 

As mentioned in the Other Comments portion of Appendix 3, EPA also 

should develop studies to review large volume waste sites ranked under 

the model and to examine basic parameters of the model. A study in the 

first category suggested by Mr. R. Walline of EPA Region VIII could 

address the actual reasons why release occurred at problem mining sites 

(reasons reportedly are different ~han those initially assumed) and a 

study in the second category might address the character1st1c(s) of 

mining w,aste that control release (mobility) to' ground water. 

.• 

• 


APPENDIX 7 


REQUESTS 


FROM THE 


OFFICE OF EMERGENCY AND REMEDIAL RESPONSE (SUPERFUND) 


TO THE 


SCIENCE ADVISORY BOARD 


.. 


• 

.-\7-1 

UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON, D.C. Z0460 

DEC 9 191!6 

QFIOIC5 OF 

SOLJO 'f'¥AST~ ~NO .!i:Mt:ACE:NCv ;;~£S,..nl\iS~ 

MEMORANDUM 

SUBJECT: 	 Science Advisory Board Review 


of Hazard Ranking System i/,~ 

FROM: 	 Henry L t,ongest tr, Director 


Office of Emergency and Remedi ~ s~onse 

TO: 	 Dr. Terry Yosie, Director 


Science Advisory Board 


The Office of Emergency and Remedial Response (OERR) is 

conducting a review and possible revision of the Hazard Ranking [ 

System (HRS). OERR is requestinq that the Science Advisory BoarG 

review several issues in conjunction with this effort. 

BACKGROUND 

The HRS is the principal mechanism use~ by EPA to deter~ine 

whether to place site~ on the ~ational Priorities List (NPL), 

promulgated under Section 105 of the Comprehensive Environmental 

Response, Compen<>~t ion and t,iability Act of 1980 (CERCLA). The HRS 

was promulgated on July 16, 1982 (47 FR 31219), as Appendix A of 

the National Continoency Plan (NCP). The current HRS evaluates 

the relative potential of uncontrolled hazardous substances to 

cause human health or safety problems, or ecological or environmental damaqe, hy ta~ino into account "pathways" to human or 

environmental exposure in terms of numerical scores. Those sites 

that score 28.50 or qreater on the HRS, and which are otherwise 

eligible, have been ptacerl on the NPt. A site must be on the NPL 

for it to be eligible for remedial action financed by the CERCLA 

trust fund. 

On October 17, 1986, CERCLA was amended. The Superfund • ~endment~ and Reauthorization Act of 1986 (SARA) requires E?A to 

promulgate changes to the HRS 18 months after enactment. The 

a~endments ~equire EPA to modify the HRS so that, "to the maximu~ 

extent feasible, it accu~ately assesses the relative deg~ee of 

ris~ to human health and the environment posed by sites and 

facilities subject t0 ~eview.• Specifically, the amendments 

require: 
-2­

0 An ·assessment of the human health risks associated with 

contamination or potential contamination of surface waters, 

either directly or as a result of the run-off of any ha~ardous 

subatance, pollutant or contaminant. This assessment 

should take into account the use of these waters for 

recreation and the potential migration of any ha~ardous 

substance, pollutant or contaminant through surface water 

to downstrea~ sources of drinking water · 

0 	 An evaluation of the damage to natural resources which may 

affect the human food chain and which is associated with 

any release or threatened release 

0 	 An assessment of the contamination or potential contamination 

of the ambient air which is associat·ed with a release or 

threatened release 

Section 125 of SA.RA requires EPA in its revision of the HRS, 

to specifically assess those wastes described in section 

300l(b)(3)(A)(l) of the solid waste Oisposal Act. These wastes , include fly ash waste, hattom ash waste, slag waste and flue gas[ 

emission control '"'ast~> .,enerated primarily from the combustion of 

coal or other fossil fuels. T~e amendments require EPA to consider: 

(1) 	~he quantity, to~icitv, and concentrations of 

hazardous cons t i tuen·ts which are present in such 

waste and a ca~aarison with other wastes: 

(2) 	The extent ~E, and potential for, release of such 

hazardous crJnstituents into the environment: and 

(3) 	The degree of risk to human health and the environment 

posed by such constituents. ~ 

Additionally, section 118 of the SARA states that ePA shall 

aive a high priority to facilities where the ~elease of hazardous 

substances a~ ppllutants or contaminants has resulted in the 

closing of drinking water wells, o~ has contaminated a principal 

drinking water supply. 

In the amendments to cr::RCLA, Congress has stated its intention 

that the aRS remain a screeninq tool to enable EPA to list sites 

on the NPL as expeditiously as possible, using data from the • Preliminary Assessment (PA) and Site Inspection (SI). The 

legislative history nf SARA makes cl~ar that Congress did not 

intend that the .revised HRS become a mechanism for making detailed 

risk assessments; ·~at~er, it was intended to he consistent with 

the 	limited purpose of the ~PL--sc~eening sites that might, after 

further ~tudy, wa~rant Fund-financed remedial action. 

In order to i01:J~?'I"' ~he <'lccuracy of the fiRS, the Agency is 

considering e~pandinr ~~e data collection performed befo~e a site 

is p~oposed for the ·: >:., t? o~ov ide dat.~ to i"'olement a revise•i 

fiRS. To max ill' i ze t ·, ~ use o E these 1 i•~ i te<i re.. ou~ces, EPA must 
A7-3 

-3­

target its data collection activities to those specific areas 

that would most increase the accuracy of the HRS. 

ISSUES FOR REVIEW 

Based on the public comments received during the rulemakings 

for the initial NCP (47 FR 31180, July 16, 1982), the original 

NPL (48 FR 40658, September 8, 1983), and the subsequent NPL 

updates, as well as the intent of Congress and program experience, 

OERR has begun a review and revision of the HRS. In conjunction 

with this review, OERR has analyzed the HRS and developed technical 

issue papers on selected aspects of the ranking scheme. 

Specific issues we would like the Science Advisory Board to 

review at this time include: 

• A review of the scientific factors in the 

existing HRS to recommend how these might be 

modified to enhance its effectiveness . 

• 	 An evaluation of the way the HRS evaluates waste •

•' characteristics, particularly as it relates to ~ 

mining waste sites, a critique of several reports 

on mining waste issues, and a review of a technical 

paper explorinq the potential use of concentration 

data in the HRS • 

• 	 An examination of the toxicity ranking scheme 

employed in the existing HRS and OERR's suggestions 

for modifying this scheme. 

• 	 An analysis of the distance used to determine 

the target population potentially affected by .· 

the release of hazardous substances to the air. 

OERR will outline each specific issue more fully to better 

focus the Science Advisory Board's review in a memo accompanyina 

each report. 

Review of The Scientific Factors in the HRS 

We would like the Science Advisory Board to review the 

• current HRS and those specific factors that make up each scoring 

pathway. As part of this review, we are requesting that the 

Science Advisory Board rank the existing HRS factors in order of 

th.eir impor:-tancEI in accur:-ately assessing r:-isk from a release of 

hazardous substances. We would also like the Science Advisory 

Board to evaluate the appropriateness of the weighting factors 

currently used in the HRS and recommend how these might be 

modified to enhance the effectiveness of the HRS. The intent of 

this is' to help )ER~ focus our HRS revisions effort, as well as 

our expanded data "•'\ l<'!ction activities. 
-4­

In the existing HRS, the score for a facility is based on 

the potential for harm to humans or the environment from migration of a hazardous substance from a facility by routes involving ground water, surface water or air. It is a composite of separate 

scores for each of the three routes. The score for each route 

is obtained by considering a set of factors that characterize the 

potential of the facility to cause harm. Each factor is assigned 

a numerical value (on a scale of 0 to 3, 5 or 8) according to prescribed_guidelines. This value is then multiplied by a weighting 

factor yielding the factor score. 

Waste Characteristics/Mining Waste Issues 

In scoring a site using the current HRS, EPA considers the 

quantity of hazardous waste deposited, rather than the quantity 

of hazardous constituents within these wastes. EPA also does not 

consider the quantity of hazardous constituents released into the 

ground water, surface water, or air, but only whether that release 

is significantly above background, When EP~ developed the HRS~ 

the Agency believed that ,1eterrnining the quantity of hazardous 

constituents would require a significant amount of sampling and .j analysi~ that would result in substantial delays in the ranking t 

of sites. 

In its various rulemakings on the NPL, EPA has received 

public comments on this aspect of the HRS and how it is used to 

evaluate mining waste sites, The comrnenterc. have stated that the 

HRS is biased against i'tiqh-volume, low-toxicity wastes, such as 

mining wastes, because it does not take into account quantity, 

toxicity and concentration of the hazardous constituents, and 

that EPA is unable to provide evidence that the HRS is a rational 

basis by which to ran.k mining sites for inclusion on the NPL. .• 

OERR would liKe the Rcience Advisory Board to evaluate the 

HRS in light of the~R criticisms, and to review both a technical 

paper on the HRS developed by TRC, Inc. F.or the mining industry, 

and a critique.of the TRC mininr; re:_:~ort conducted by OERR's contractors. As part of this ~eview, we would like the Science 

Advisory Board to examine the validity of the mining industry's 

concerns and to rec.ornmend ar~3s for revision of the HRS, if 

warranted, to address these concerns, keeping in mind the use 

of the RRS aa a screeninq tool for further studies • 

• 

In addition, w~ woul1 like the Sci~nce Advisory Board to 

review a technical i'"'>er developed by OERR that el(plores the 

use· of concentration :lata i'l ~valuating hazardous waste sites. 

This review shout~ focufi O'l the Eeasibil ity ?f including waste concent~~tions in the H~S, the data requirements and resource 

implications of ir"pl~·,enti~-) such a chanqe, as welt as the ~eliability of ">ii"~Pllrq ·lata to accurately det~rmine the 

quantity·of haza~1:)""' '"'~-stituents contained in wastes. 
--~7. 5 

-5­

Air Target Distance Limit 

Currently, the air pathway of the HRS relies on analytical 

data that shows level<> of a contaminant at or in the vicinity of 

a facility that significantly exceed the background levels. The 

air pathway provides a score based on the toxicity of the substance 

released via the air route, and the population within a four-mile 

radius of the site. This Population is used as an indicator of 

the population which ~ay be harmed should a hazardous substance 

be released to the air. 

OERR has analy~ed the distance used to determine the tarqet 

population potentially affected by a release of hazardous substances to the air and prepared a technical report detailing its 

findings. The report presents conclusions on the qeneral level 

of cancer risk arising from uncontrolled waste sites and examines 

the implications of the analysis for the distance factor of the 

air pathway. The study used an EPA model called Human Exposure 

Model (HEM) to make these estimates. · 

We would 1 ike the Science Advisory Board to evaluate the f 

target distance li~it used in the air pathway of the existing t 

HRS and review the technical Qaper developed on this subject. As 

part of this revi"''"'• we would like an assessment of the general 

methodology used L1 the technical paper and the implications of 

these eindings on the air pathway factor of a revised HRS. In 

addition, we would '""'lco"le any suggestions concerning general 

approaches for revising the air targets distance limit. 

Toxicity 

Many public comments have been received by EPA on the method 

used in the HRS to rank the toxicity of hazardous substances., 

The current method is based on a ratinq scheme developed by N, 

Irving sax (1975, 1979, 1984) and ~~tes the toxicity of hazardous 

substances on a scala of 0 to 3. The rating is prima~ily based 

on acute toxicity. Several technical issues have been ~aised by 

commenters that suggest th~ possible need for modification of the 

HRS is in order to ill'pr.'Jve its ability to discriminate among 

sites whose wastes have different toxicity char~cteristics. 

As a result of t~ese comments and the intent of Congress to 

• 	 ~ake the HRS as accu~~te as possible, OERR has evaluated the methon 

currently used in the HRS for ~stimatinq the relative toxicity of 

substances at hazarlous waste sites and the methods employed by 

oth~r ranking syst~<os to evaluate toxicity. Based on the review, 

OERR has sugaest~~ l~prove~ents to the HRS toxicity eactor that 

incoroo~ate ~easu~~~ •Jf acute, suh-ch~onic and chronic toxicity. 

we would like t~·~ Science Advi.sory Board to evaluate the 

toxicity· ranking ,;ch-"'"-" e!'lploy~<l in the exi-<tina HRS, to review 

OERR's technical,,.,,., 1n this subject, >~nd to sugqest imp~ovement; 

to the HRS that "'n; '"t"~ -1c::urately as~ess th~ toxicity of a 

hazardous substil'l.· ·. 
A;-6 

-6­

SCHEDULE 

The specific technical ~epo~ts Eo~ Science Adviso~y Boa~d 

~eview will be made available to you 30 days in advance of the 

Science Advisory aoa~d meeting. We would be more than happy to 

meet with the Science Advisory Boa~d to b~ief them on the existing 

RRS, if approp~iate. 

Thank you for you~ help in this p~oject. If you have any 

questions concerning OERR's ~equests, please contact Jane Metcalfe 

at 382-7393. We loo~ Eo~wa~d to wor~ing with you on the HRS 

~evisions effort. 

.f 

• 

• 

WASHINGTON, D.C. 20460 

JJN 2 5 1987 

Of~ICE 010 

SOLID WASTE AND EMeRGENCY ~!:;SPOI'.!Sf; 

MEMORANDUM 

SUBJECT: Science Adviso~y Review of thei?i 

FROM: 	 Hen~y L. Longest II, Di~ector 

Office of Eme~gency and Remedi s onse 

TO: 	 Dr. Ter~y Yosie, Director 


Science Advisory Boa~d 

In December 1986, I requested the Science Advisory Board­

(S~B) ·~ dssistance in reviewing several issues related to 

OERR's efforts to ~eview and revise the Hazard Ranking Syste~ 

(HRS) as di~ected by Congress in the Superfund Amendments and 

Reauthorization Act of 1986 (SARA). In response to this 

~equest, an Ad Hoc HRS Subcommittee was formed by the SAB. 

This Subcommittee met for the first time on May 19 & 20, 

1987. By this memorandum !'wish to express OERR's appreciation 

to the Subcommittee and SAB staff for their responsiveness to 

our request and the expressed willingness to work within the 

narrow time constraints for the revisions imposed by SARA. I 

would also like to take this opportunity to confirm several ~ey points which should facilitate the Subcommittee review. 

Purpose of the HRS 

During the initial meeting on May 19 & 20, many members 

of the Subcommittee posed questions concerning the purpose of 

the HRS. As pointed out, the purpose of the HRS is a major 

determinate of the structure and function of a model. Such a 

model is required to prioritize actual or potential hazards 

to public health and the environment posed by hazardous 

waste sit... 08M believes that the purpose of the HRS is 

primaril~~ screening tool used to determine which sites 

• 	 will be ~idates for Fund-financed remedial response. To 

the degr•• possible, the model also needs to provide a 

stratification of sites based upon the relative risks posed 

to human health and the environment. OERR does not believe 

that the HRS is-intended to assess absolute risk, since it 

is to be applied early in the remedial site evaluation process. 

OERR intends to ~evise the HRS to the maximum extent feasible, 
A7-8 

-2­

to estimate more accurately the risks associated with a site. 

tn conducting these revisions, however, OERR will balance 

the benefits of increased accuracy against the associated 

costs, avoiding revisions that would require far more extensive 

data and, in so doing, impede the ability of the HRS to 

function expeditiously. 

OERR's Objective on Seeking SAB Review 

Congress, through SARA, requested that in our review of 

the HRS, we assess several specific areas. In reviewing these 

areas, OERR is particularly concerned that our decisions 

concerning the approaches used in revisions be supported by 

sound technical and scientific analyses. Peer review of the 

technical or scientific issues under consideration will provide 

essential support for any proposed revisions. 

Specific Issues For SAB Review 

OERR requests that the Subcommittee examine three specific 

HRS revision issues: 1) the method(s) used to assess toxicity, 

2) the air target distance limit and 3) whether large volume 

wastes should be considered differently from other wastes. 

These issues are described brief1y·in the December 9, 1986 

memorandum and will be developed more fully in issue papers 

to be presented to the Subcemmittee according to the schedule 

agreed to at the May 19 & 20, 1987 Subcomittee meeting. 

As I understand, the following schedule has been agreed to by 

all parties: 

1. Toxicolog.y Panel -- June 29 & 30 

2- Meeting 2 of Subcommittee-- July 16 & 17 

3. 	 Air Panel-- July 27 & 28 (issue paper to 

be mailed July ll 

4. 	 Large Volume Waste Panel -- Aug. 19 & 20 

(issue paper available July 17) 

s. 	 Final Subcommittee . . Meeting -- Sept. 14 & 15 (tentative) . 

In my December 9, 1986 memorandum, I requested SAB review 

of the fourth issue involving the review of scientific factors 

in the au. The scope of that review was to priorithe the 

various .._ing factors in terms of their importance to 

assessinirrisk and evaluate the appropriateness of the • weightinffassigned to those factors. 

As explained by craig Wolfe during the May 19 & 20, 1987 

SAB meeting, OPPE has assembled a group of Agency officials 

to discuss the importance and weighting of various risk 

related factors and rank order 20 hazardous waste sites. I 

believe that the work of this expert panel will be most 

satisfactory in answering the questions related to the factors 

and weightings in the current HRS. consequently, I believe 

it would be duplicative to continue my request to the SAB for 

such a review and therefore, I withdraw this part of my 

request for review to the SAB. 
OERR believes that the issues p~esented to the Subcommittee 

meet the ·criteda identified in the Administe-ator's guidance 

concerning use of the SAB, and can be addressed as disc~ete 

issues. While we intend to provide updates to the Subcommittee 

0n overall proge-ess with ~espect t0 cevist<)ns •)f ~·\' '•l':.i!."e 

model, we do not believe that ceview of the ove~all model is 

eithec feasible o!." appropt:"iate foe the Subcommittee due to 

the need fo!." policy as well aN ~·:l~ntific considerations. 

OERR looks forward to continuing the open and focused 

review effort which was initiated at our first meeting. If 

you have any questions please contact Ms. Penny Hansen, Deputy 

Director of the Haza~dous Site Evaluation Division at 475-8600 . 

• 

A7-10 

UNITED STATES ENVIRONMENTAL. PROTECTION AGENCY 

WASHINGTON, D.C. 20460 

JUL 2 8 1987 

·- ·­ OF.,:;t¢£ OF 

SOt..IO W.Si'"£ ANO eM£ACi£NCY ~E:SPOfltSE 

MEMORANDUM 

SUBJECT: 	 Science Adviso~y Boa~d (SAB) Review 


of Mining Waste/Concent=ation Issi<e~ 

1; · r 	 , 

!;'ROM: 	 Stephen A. Lingle, Dil:'ectoe ~ , ·, ..._._.,U Haza~dous Site Evaluation Division ~ 

TO: 	 Teel:'y tosie, Di~ectol:' 

Science Ad visoey Boaed 


Thank you fot' the oppo~tunity to introduce the mining 

waste and concenteation issues to the full subcommittee. 

Mr. Richard Conway, chait:'l!lan of the High Volume Waste Panel 

expt'essed his desi~e to hav~ OERR further define the issues 

foe SAB consideeation. The purpose of this memorandum is to 

provide this clat'ification~ 

As :esented in the July 16, 1987 subcommittee meeting, 

the two 4Ssues ~:"elated to mining waste sites are: 

1) 	 Applicability of the HRS in scoring mining waste 

sites1 and 

2) 	 Feasibility of using waste concentration data in a , 

revised HRS. 

The fit'st issue has been eaised by the mining induStl:'y 

during publi·c comlllents on· various NPL rulemakings. The 

commentees have stated that the HRS is biased against highvolume, low concentration wastes, because it does not take' 

into account quantity, toxicity, and concent~:"ation of the 

hazardous constituents. The commentatOI:'S also stated that 

EPA is unable to p~:"ovide evidence that the HRS is a rational • basis by which to ~:"ank mining sites for inclusion on the NPL. 

. OERR would like the SAB to evaluate the HRS in light of 

these comments., and to eeview th!:'ee technical papel:'s on the 

HRS developed by TRC, Inc. fo= the mining industry, and two 

l:'epol:'ts peepaeed by OERR's contl:'actors. As part of this 

l:'eview, we would like the SAB to examine the validity of the minin~ industry's conce~:"ns with ~espect to high volume waste 
• 


A7-ll 

-2­

sites s~ch as mining sites. Although the pape~s discussed 

in the first issue pe~tained to mining waste, OERR believes 

that 	the issue is equally ~elevant to sites containing othe• 

high 	volu.a wastes such as fly ash. 

The ·s@cond issue is •elated to the fi~st as it add~esses 

the feasibility of using the concent~ation of the ha%a~dous 

constituents of wastes. In ~esponse to public comments that 

concent•ation data a~e not used in the cu~~ent HRS to dete~ine 

the quantity of haza•dous waste, and the •equi~ements of SARA 

Section 125(a), EPA developed an option that specifies th~eetie~ed methodology that allows the use of waste concent~ation 

data. This methodology is desc~ibed in the concent~ation 

issue papa~ p~esented to the SAB. 

In •evising the HRS, Section 125(a) of SARA ~equires 

EPA to add~ess quantity, toxicity and concent~ation of ha~a•dous 

constituents fo~ RCRA Section 300l(b)(3)(A)(i) special study 

wastes (fly ash, bottom ash wastes, and flue gas emission cont~ol waste gene~ated p~ima~ily f•om the combustion of coal o~ othe~ fossil fuels). Although, SARA Section lOS(g) does 

not ~equi•e ~evisions to the HRS, it does require that !PA conside~ the same factors (quantity, toxicity, and concentration) 

in the decision to p~opose new mining waste sites to the NPL 

using the current HRS until the HRS is revised. EPA believes. 

that the methodology desciibed in the third option of the 

concentration issue paper could be employed to satisfy Section 

125(a) and would also address concerns about scoring other 

high volume waste sites such as mining waste sites as well. 

Consequently, we believe that the subcommittee findings will 

be equally applicable to both mining and fly ash sites. 

I hope that this discussion p~ovidea the desired 

cla~ification of this issue. In addition, we are in the p~ocess of gathering available information on costs of waste 

sampling and analysis at large volume waste sites. This 

information.will be p~ovided to the SAB membe~s as soon as 

possible. 

OERR appreciates the time and effort the subcommittee 

members have e~pended and we look forward to our ne~t meetinq 

in Denver. If you have any additional questions, please 

contact Aqnea Ortiz of my staff, at (202) 475-9700 • 

Attachments 

cc: 	 Kathleen ·w. Conway 

SAB Subcommittee Membe~s 
APPENDIX 8 


REFERRENCES CITED 


• 
.. 


REFERENCES CITED 

!. Electric Power Research Institute. Inorganic and Organic Constituents 

in Fossil Fuel Wastes. EPRI EA-5176, Vl. August 1987. 

2. 	 Electric Power Research Institute. Leaching Studies of Utility Solid 

Wastes. EPRI EA-4215. August 1985 

3. 	 Electric Power Research Institute, Physical-Che~ical Characteristics 

of Solid wastes. EPRI EA-3236. Septe~ber 1983. 

4, Electric Power Research Institute. Round-Robin Evaluation of Regulatory 

Extraction Methods, EPRI EA-4740. Dece~ber 1986. 

5. 	 Electric Power Research Institute, Mobili~ation and Attenuation 

of Trace Ele~ents in Fly Ash. EPRI EA-4747. August 1986, 

6. 	 Electric Power Research Institute. Evaluation of tnstr~ents in 

Unsaturated Fly Ash, EPR! EA-5011, April 1987. 

7. 	 Electric Power Research Institute. SWES: Description, Status and 

Available Results. EPRI EA-532~2~-~S~R-.--A~u~g~u~s~t~1~9~8~7~.~~~~~ 

8. 	 Environmental Protection Agency. Uncontrolled Hazardous Waste Site 

Ranking System: A Users Manual (HW-10), originally published 

in the July 16, 1982. Federal Register, 1984. 

9. 	 Environmental Protection Agency, Office of Emergency and Remedial 

Response, Analysis of the Air Target Distance Li~it in the 

Hazard Ranking Syste~. Prepared by ICF Incorporated, July 1987 • 

.• 10. 	 Environmental Protection Agency, Office of Emergency and Remedial 

Response. Chemicals Contributing to Estimated Risks at Superfund 

Sites: A Limited Study. July 16, 1987, 

11. 	 Environmental Protection Agency. Office of Emergency and Remedial 

Response. Comparison of Available Data on the Concentration 

of Hazardous Constituents in Mining Waste and Non-Mining Waste, 

pages 5-10. Received August 1987 • 

• 12. 	 Environ-ental Protection Agency, Office of Emergency and Remedial 

Response, Discussion of Options for Revising the Hazard Ranking 

System (HRS) Toxicity Factor, Prepared by ICF Incorporated, 

May 4, 1987. 

!3. Environmental Protection Agency. Office of Emergency and Remedial 

Response. The Superfund Hazard Ranking System (HRS): 

Applicability to Mining Waste Sites, Prepared by ICF Incorporated. 

July 22, 1987, 
- 2 ­

14, Enviro~ental Protection Agency, Office of Emergency and Remedial 

Response, Report on the Feasibility of Using Concentration 

Data in a Revised Hazard Ranking System, Prepared by ICF 

Incorporated, July 27, 1987, 

15. 	 Environmental Protection Agency, Office of Policy Planning and 

Evaluation, Views/Approach to Hazard Ranking System Revisions, 

July 1987. 

16, Environmental Protection Agency, Office of Solid Waste. Report to 

Congress: Wastes from the Combustion of Coal by Electric 

Utility Power Plants, Chapters 1-5. June 1987. 

17, Environmental Protection Agency. Office of Solid Waste, Management 

of Mining Wastes, June 22, 1987. 

18. 	 Industrial Economics, Incorporated. Preliminary Analysis of Alternative 

Models to Support Revision to the CERCLA Hazard Ranking System, · 

Prepared for the Environmental Protection Agency's Office of Policy, 

Planning and Evaluation, Presented to the Science Advisory Board. 

July 16, 1987. 

19. 	 Kushner, L. M, Hazard Ranking System Issue Analysis: Sites with 

Unknown Waste quantity, _August 1986. 

20. 	 Lingle, S. A., Director, Hazardous Site Evaluation Division, 

Memorandum: Science Advisory Board (SAB) Review of Mining 

Waste/Concentration Issues to Terry Yosie, Director, Science 

Advisory Board, July 28, 1987. 

21. 	 Longest, H. L., Director, Office of Emergency and Remedial Response. 

Memorandum: Science Advisory Board Review of Hazard Ranking 

System to Terry Yosie, Director, Science Advisory Board, 

December 9, 1986•. 

22. 	 Longest, !{, .L., Director; Office of Emergency and Remedial Response. 

Memorandum: Science Advisory Board Review of the HRS to Terry 

Yosie, Director, Science Advisory Board, June 25, 1987. 

23. 	 Merkhofer, L, H., et al. A Site-Ranking Panel Evaluation of the 

Relative Risk Posed by Twenty Superf~nd Sites. Keystone, 

• 	 Colorado: The Keystone Center. Prepared for the u.s • 

Environmental Protection Agency's Office of Policy, Planning 

and Evaluation, Draft Received September 1987, .PP• 1-17. 
- 3 ­

24. 	 MITRE Corpo.-ation. Analysis of EPA Hazard Ranlting Systelll Scoring 

of Mining Sites on the National Priorities List. Prepared for 

U.S. EPA, 1985. 

25. 	 MITRE Corporation. Estimates of the Quantity of Hazardous 

Substances Present in Mining Waste at NPL Sites. Prepared 

for: the U.S, Environmental Protection Agency's Office of 

Policy, Planning and Evaluation. January 9, 1986, 

26. 	 Ortiz, A,, Incorporating Mobility Factors into the Hazard Ranking 

System, Office of Solid Waste and Emergency and Re=edial 

Response. August 20, 1987, 

27. 	 TRC Environmental Consultants, Analysis of Mining Sites on the 

National Priorities List, Prepared for the American Mining 

Congress, !983, 

28. 	 TRC Environmental Consultants, Analysis of Mining Sites on the 

National Priorities List: Score Changes and Newly Proposed 

Sites, Prepared for American Mining Congress, 1984a. 

29. 	 TRC Environmental Consultants. Review of Mining Sites on the 

National Priorities List: Comparison with Non-Mining Sites, 

?repared for the Ameri~an Mining Congress. 1984b, 

30. 	 Sobek, A. A., et a1. Field and Laboratory Methods Applicable to 

Overburden and Mine Soils. EPA/600-2-78-054, March 1978. 

3J. Wolf 1 nge,r, T. F. Hazard Ranking System Iasues Analysis: The trse 

of Concentration Data, EPA/68-01-7054, April 1987. 

J2, Wusterbarth, A, R. Hazard Ranking System Issue Analysis: .· Relationship Between Waste quantity and Hazardous Constituent 

Quantity. EPA/68-Ql-7054, September 1987.