OSWER DIRECTIVE 9481.00-SC 

EPA/530-SW-87-017 

ALTERNATE CONCENTRATION LIMIT 

GUI"DANCE 

PART I 

A~Ct POLICY AND INFORMATION --------- --~ ~ -

REQUIREMENTS 

INTEHiiM FINAL 

OFFICE OF SOLID WASTE 

WASTE MANAGEMENT DIVISION 

U.S. ENVIRONMENTAL PROTECTION AGENCY 

401 M STREET, S.W. 

WASHINGTON, D.C. 20460 

JULY 1987 

• 
OSWER Directive 9481.00-GC 

DISCLAIMER 

This document is intended to assist Regional and State personnel in exercising 

the discretion conferred by regulation in evaluating applications for ACLs submitted 

pursuant to 40 CFR 264.94. Conformance with this guidance is expected to result in 

ACL applications that meet the regulatory standard of protecting human health and 

the environment. However, EPA will not in all cases limit its review to 

demonstrations that comport with the guidance set forth herein. This document is 

not a regulation (i.e., it does not establish a standard of conduct which has the force 

of law) and should not be used as such. Regional and State personnel must exercise 

their discretion in using this guidance document as well as other relevant 

information in determining whether an ACL demonstration meets the regulatory 

standard. 

iii 
OSWER Directive 9481.00-6( 

ACKNOWLEDGEMENTS 

This document was developed by the EPA's Office of Solid Waste under the 

direction of Dr. Vernon Myers, Chief of the Ground-Water Section of the Waste 

Management Division. The document was prepared by the joint efforts of 

Dr. Myers, Mr. Mark Sa lee of the Permits and State Programs Division, and Mr. Jerry 

Garman of the Waste Management Division. Special thanks are expressed to the 

numerous secretaries who survived the multitude of revisions during the 

development of this document. 

iv 
OSWER Directive 9481.00-6C 

CONTENTS 

Page 

EXECUTIVE SUMMARY ES-1 

I. INTRODUCTION 1-1 

II. PHYSICAL AND CHEMICAL CHARACTERISTICS OF THE WASTE 2-1 

CONSTITUENTS 

Ill. HYDROGEOLOGICAL CHARACTERISTICS 3-1 

IV. GROUND-WATER FLOW DIRECTION AND QUANTITY 4-1 

v. PATIERNS OF RAINFALL S-1 

VI. PROXIMITY OF SURFACE WATER AND GROUND-WATER USERS 6-1 

VII. CURRENT AND FUTURE USES OF GROUND WATER AND SURFACE 7-1 

WATER IN THE AREA 

VIII. EXISTING QUALITY OF GROUND WATER AND SURFACE WATER AND 8-1 

OTHER SOURCES OF CONTAMINATION 

IX. POTENTIAL HEALTH RISKS 9-1 

X. POTENTIAL DAMAGE TO WILDLIFE, VEGETATION, AGRICULTURE, 10-1 

AND PHYSICAL STRUCTURES 

XI. PERSISTENCE AND PERMANENCE OF POTENTIAL ADVERSE EFFECTS 11-1 

XII. SUMMARY AND CONCLUSIONS 12-1 

XIII. REFERENCES 13-1 

v 
OSWER Directive 9481.00-6C 

CONTENTS 

APPENDICES 

A. Outline of Information that may be Cross ReferencE!d A-1 

from the Permit Application 

B. Summary of Tables, Figures, and Information Required B- 1 

to Support an ACL Demonstration 

C. Summary Sheet of Hazardous Constituent PropertiE!S C-1 

D. Summary Sheet on Health Effects Factors D-1 

E. Standard Factors used in Exposure Assessments E-1 

vi 
OSWER Directive 9481.00-6C 

EXECUTIVE SUMMARY 

The hazardous waste regulations under the Resource Conservation and 

Recovery Act (RCRA) require owners and operators of hazardous waste facilities to 

utilize design features and control measures that prevent the leaking of hazardous 

waste into ground water. Further, all regulated units (i.e., all surface 

impoundments, waste piles, land treatment units, and landfills that received 

hazardous waste after July 26, 1982) are also subject to the ground-water 

monitoring and corrective action standards of 40 CFR Part 264, Subpart F. To 

establish the facility's ground- water protection standard (GWPS) under Subpart F 

(40 CFR 264.92), the Regional Administrator is required to establish in the facility 

permit, for each hazardous constituent entering the ground water from a regulated 

unit, a concentration limit that cannot be exceeded. The concentration limits are 

the "triggers" that determine when corrective action is required. 

There are three possible concentration limits that can be used to establish the 

GWPS: 

I. Background levels of the hazardous constituents, 

2. Maximum concentrations listed in Table I of Section 264.94(a) of the 

regulations, or 

3. Alternate concentration limits (ACLs). 

The first two levels are established in the facility permit as the GWPS, unless 

the facility owner or operator applies for an ACL. However, some States have nondegradation policies (which are either regulatory or statutory requirements) that 

prohibit the release of any pollutants into the ground water. These policies may 

prevent the use of an ACL altogether if the State has an authorized program for 40 

CFR Part 264, Subpart F. 

This document provides guidance to RCRA facility permit applicants and 

writers concerning the establishment of ACLs. To obtain an ACL, a permit applicant 

must demonstrate that the hazardous constituents detected in the ground water 

will not pose a substantial present or potential hazard to human health or the 

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OSWER Directive 9481.00-GC 

environment at the ACL levels. ACLs are granted throu~Jh the permit process under 

Parts 264 and 270, and are established in the context of the facility GWPS. The 19 

factors, or criteria, that are used to evaluate ACL requests are listed in Section 

264.94(b) of the regulation. Detailed information on E!ach of these criteria is not 

required in every ACL demonstration because each demonstration requires 

different types and amounts of information, depending on the site-specific 

characteristics. A separate chapter of this document i!i devoted to each of these 

criteria. The criteria are briefly discussed, along with the type, quantity, and quality 

of information that should be provided, depending on the site-specific 

characteristics. 

Chapter I is an introduction to the ACL guidance; it describes the purpose, 

intent, and organization of the document. The chapter also defines an ACL, 

describes Agency policy, and describes how ACLs fit into the RCRA permitting 

process. A procedure is presented that allows a permit applicant to employ either 

Agency- established maximum contaminant levels (MCLs) set under the authority of 

the Safe Drinking Water Act, or Agency-reviewed reference doses or risk-specific 

levels directly, or after a simple manipulation, as ACLs. For use as ACLs, these levels 

would have to be approved by EPA for the specific site. This chapter also points out 

that data from the RCRA Part B permit application may be cross- referenced in the 

ACL demonstration. 

Experience gained over the last several years hc:1s allowed the Agency to 

develop a better understanding of ground-water contamination problems. This has 

led EPA to develop general policy guidelines for the use of ACLs at RCRA hazardous 

waste disposal facilities. These guidelines are designed to ensure that ACLs will be 

protective of human health and the environment. 

Three basic policy guidelines have been identified to assist the permit writer 

and applicant in implementing the ACL process for useable ground water: 

1. Ground-water contaminant plumes should not increase in size or concentration above allowable health or environmental exposure levels, 

2. Increased facility property holdings should not be used to allow a greater 

ACL, and 

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OSWER Directive 9481.00-6C 

3. ACLs should not be established so as to contaminate off-site ground 

water above allowable health or environmental exposure levels. 

These guidelines are further described in the first chapter. Also described are 

the ways in which ACLs should be determined for five types of situations. 

Chapter I also discusses how the ACL process fits into the rest of the RCRA 

ground-water protection program. The applicant is cautioned that an ACL based on 

attenuation mechanisms may not be acceptable to the permitting authority at the 

end of the post-closure care period. At the conclusion of post-closure care, a more 

stringent ground-water standard, based only on an allowable exposure level, may 

be needed because the facility owner has no further RCRA obligations for managing 

or monitoring the facility. 

EPA is also in the process of developing a ground-water protection program 

for all solid waste management units at RCRA permitted facilities. This program is 

mandated by Section 3004(u) of the Hazardous and Solid Waste Amendments of 

1984 (HSWA). The ACL process, as described in this document, may be useful when 

assessing corrective action measures at these solid waste management units. In 

developing this corrective action program, EPA will assess the need to promulgate 

some additional Subpart F regulations. 

Chapter II discusses the data that the permit applicant must submit on the 

physical and chemical characteristics of the waste constituents. If the permit 

application sufficiently defines the extent of contamination, additional groundwater sample collection is probably not necessary for ACL purposes. The permit 

applicant should submit ground-water contamination information in terms of 

three-dimensional representations of constituent concentrations. The permit 

applicant needs to submit data on any factors relating to the stability and mobility 

of the waste constituents in the ground water if the point of exposure is different 

from the point of compliance. These factors may include density, solubility, vapor 

pressure, viscosity, and octanol-water partitioning coefficient of each constituent 

for which an ACL is requested. 

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OSWER Directive 9481.00-6C 

Chapter Ill discusses the data needed to describe the hydrogeologic properties 

of the site. General descriptions of the geologic and hydrologic conditions at the 

facility should be part of all ACL demonstrations. The geologic and hydrologic 

properties of each of the individual strata beneath a site that are likely to affect 

ground-water contaminant migration should be submitted in the ACL 

demonstration. Much of the data should already be available to the permit 

applicant if other RCRA permitting requirements have been fulfilled. The 

important geologic attributes of a site include: 

I. Soil and rock characteristics, 

2. Geologic structure, and 

3. Geomorphology and topography. 

In ACL demonstrations where soil and other matrix attenuation mechanisms 

are used to justify that exposure to ground-water contaminants will be minimal or 

prevented, data on attenuative properties must be discussed. The near-surface 

stratigraphic units located in the zone of saturation must be characterized for the 

hydrologic parameters of hydraulic conductivity (vertical and horizontal), specific 

yield (unconfined aquifer) or specific storage (confined aquifer), and effective 

porosity. 

Chapter IV discusses the ground-water quantity and flow direction 

information that is used to assess contaminant transport. The general RCRA permit 

requirements specify the submittal of ground-water flow data. This data should be 

adequate for on-site determinations of ground-water quantity and flow direction; 

however, additional data may be required if off-site detE!rminations of quantity and 

flow are needed for the ACL demonstration. Ground-water quantity can be 

estimated from hydrologic parameters such as specific yi1~ld for unconfined aquifers 

and specific stotage for confined aquifers. The use of Darcy's law for determining 

ground-water flow quantity is acceptable for homogeneous and isotropic aquifers. 

The hydrogeologic portion of all ACL demonstrations should include a general 

description of both horizontal and vertical ground-water flow components. ACL 

demonstrations based on attenuation arguments need a more in-depth discussion 

of ground-water flow. In these situations, the horizontal ground-water flow 

description should include a flow net based on ground-water elevation measureES-4 
OSWER Directive 9481.00-6C 

ments taken from monitoring wells or piezometers screened at similar elevations in 

the same saturated zone. Facilities should have several clusters of piezometers for 

vertical gradient determinations. Facilities that are located in environmental 

settings that exhibit temporal variation in ground-water flow direction should 

describe the extent to which the flow change occurs. 

Chapter V discusses the types of precipitation data that should be submitted in 

an ACL demonstration. ACL demonstrations based on contaminant attenuation 

need to include a discussion of precipitation, which should be focused on the site's 

hydrologic regime. If the applicant's ACL demonstration clearly shows that groundwater discharge to surface waters is unlikely, then the discussion of precipitation 

effects can be limited to infiltration and ground-water recharge. However, if 

ground-water discharge to surface water is an important element of the ACL 

demonstration, then precipitation events should be related to ground-water 

recharge, discharge, and surface water hydrology. 

Chapter VI discuss~s the proximity of surface water and ground-water users 

and the information that should be submitted on these users. The level of 

information necessary to satisfy the proximity of users requirement depends on the 

basis ofthe ACL. Only a general discussion of surface water and ground-water users 

is necessary if ACLs are based on exposure to the contaminants at the point of 

compliance, immediately at the edge of the waste management area. However, 

specific data on the physical characteristics of a surface water body are necessary if 

the applicant is attempting to show that the contaminants will safely and quickly 

attenuate into a surface water body. In order to assess the likelihood of exposure of 

ground-water users, ACL demonstrations should discuss the proximity of groundwater users to the facility. 

Chapter VII discusses the factors needed to determine current and future uses 

of ground water and surface water in the vicinity of the facility. The permit 

applicant needs to examine only the pertinent aspects of ground-water and surface 

water uses. Permit applicants must submit information on the types of groundwater uses in the vicinity of the facility, unless they base the ACL on allowable 

exposure levels at the point of compliance. Surface water uses should be discussed 

by the ACL applicant if contaminated ground water can migrate to surface waters. 

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OSWER Directive 9481.00-6C 

Chapter VIII is concerned with the existing quality of ground water and surface 

water and other sources of contamination. In order for "benchmark" levels of 

contamination to be set, the background levels of hazardous constituents in the 

ground water and surface water should be established. For ACL purposes, 

background water quality is the quality that would be expected to be found if the 

facility's regulated unit(s) was not leaking contaminants. Background monitoring 

wells must yield ground-water samples from the uppermost aquifer representative 

of the quality of ground water that has not been affE!cted by leakage from the 

facility's regulated unit. Background surface water quality needs to be assessed 

only in cases where surface waters are receiving contaminated ground-water 

discharges. The permit applicant should also examine the possibility of other 

sources of contamination if the upgradient waters in the vicinity of the facility are 

contaminated and ACLs are requested above allowable exposure levels. This will 

give the permit applicant information for assessing cumulative impacts associated 

with any contamination emanating from the facility. 

Chapter· IX discusses the health risk assessment. ThE! purpose of the health risk 

assessment is to determine allowable concentrations at the point of exposure for 

the constituents for which ACLs are requested. A detailed health risk assessment 

should be submitted in all ACL demonstrations, unless the point of exposure is 

established at the point of compliance and either Agency-established MCLs or 

Agency-reviewed allowable dose levels are used at this point. If human exposure 

could occur, the permit applicant is responsible for providing information on the 

health effects of the hazardous constituents present in the ground water for which 

ACLs are requested. The health risk assessment should be based on conservative 

health assumptions. The applicant should distinguish bl!tween ground-water contaminants having threshold (toxic) and non-threshold (carcinogenic) effects. The 

Agency is currently compiling toxicity information on many of the hazardous 

constituents which should be useful in preparing ACL demonstrations. 

Chapter X discusses data that should be submitted on the potential impacts to 

the environment. The initial step in assessing possible environmental impacts is to 

determine the probable exposure pathways for hazardous constituents to reach 

environmental receptors. For ACL purposes, the receptors of concern include 

wildlife and vegetation in aquatic and terrestrial environments; agricultural crops, 

products, and lands; and physical structures. The permit applicant should examine 

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OSWER Directive 9481.00-GC 

the potential impacts to the receptors discussed above if the ACLs are based on 

attenuation and exposure to hazardous constituents is likely to occur. Otherwise, 

the permit applicant should discuss specific data that support no probable exposure 

and explain why the potential environmental impact assessment is not needed. If 

there is a likely pathway for wildlife and vegetation to become exposed to 

contaminants, then environmental toxicity factors should be examined. 

The permit applicant should discuss the presence of any endangered or 

threatened species in terrestrial or surface water environments near the facility. If 

any endangered or threatened species are in the area, then the permit applicant 

should discuss the potential impacts of the contaminated ground water, including 

critical habitat impacts, on the species. 

Physical structures can also be adversely affected by hazardous constituents in 

the ground water_ The determination of potential impacts to and contamination of 

physical structures in the area around the facility requires the examination of 

exposure pathways, waste characteristics, and construction materials and 

techniques_ Physical structures of concern include buildings, buried cables and 

pipes, railroad beds, roads, parking areas, and machinery. 

Chapter XI discusses data needed to determine the persistence of the 

contaminants in the environment and the permanence of the adverse effects. If 

ACLs are based on attenuation arguments, the applicant should discuss the process 

by which each ACL constituent will degrade, either from a ground-water 

perspective, surface water perspective, or a combination of both, depending on the 

site-specific situation. Data on the permanence of the adverse effects resulting 

from exposure to the ACL constituents will be required only if the ACL 

demonstration is based on attenuation arguments. Data on permanence is needed 

to determine the long-term effects associated with exposure to the ACL 

constituents_ 

Chapter XII presents the summary and conclusions of the ACL guidance 

document_ This chapter emphasizes the independent nature of each ACL 

demonstration and presents the time frame of the ACL process. Information on 

each of the criteria discussed in this guidance document is not required in every ACL 

demonstration_ Each ACL demonstration will reflect site-specific environmental 

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OSWER Directive 9481.00-6( 

properties and waste characteristics. As part of the ground-water protection 

standard, an ACL is in effect during the compliance period. If, at the end of the 

compliance period, the owner or operator is engaged in a corrective action 

program, the compliance period is extended until the owner or operator can 

demonstrate that the GWPS, which may contain ACLs, has not been exceeded for a 

period of three consecutive years. 

Chapter XIII is a list of references cited throughout the guidance. The permit 

applicant is advised to use these documents since approval of an ACL is highly 

dependent upon the technical strength of the demonstration. 

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OSWER Directive 9481.00-GC 

CHAPTER I 

INTRODUCTION 

Hazardous waste facilities permitted under the Resource Conservation and 

Recovery Act (RCRA) regulations (40 CFR Parts 264 and 270) are required to be 

designed and operated in a manner that will prevent ground-water contamination. 

If leakage of hazardous constituents into the ground water is detected at a RCRA 

facility, the regulations require the establishment of a ground-water protection 

standard at that facility. This standard establishes a limit on the amount of groundwater contamination that can be allowed. The ground-water protection standard is 

an essential element in the Agency's strategy to ensure that public health and the 

environment are not endangered by any contamination of ground water resulting 

from the treatment, storage, or disposal of hazardous wastes. As such, the standard 

will indicate when ground-water clean-up is necessary to control contamination 

that has emerged from a hazardous waste facility. 

The principal elements of the ground-water protection standard are discussed 

in Section 264.92 of the regulations. For each hazardous constituent entering the 

ground water from a regulated unit, a concentration limit must be established that 

will serve as a limh beyond which degradation of ground-water quality will not be 

allowed. There are three possible concentration limits that can be used to establish 

the ground-water protection standard: 

1. Background levels of the constituents, 

2. Maximum concentration levels listed in Table 1 of Section 264.94(a) of 

the regulations, or 

3. Alternate concentration limits (see Figure 1). 

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OSWER Directive 9481.00-GC 

FIGURE 1 

RCRA GROUND-WATER PROTECTION PROCESS 

DEFAULT VALUE 

MCL FOR 14 CHEMICALS 

BACKGROUND 

GROUND-WATER 

MONITORING 

LEAK 

DETECTED 

SET 

PROTECTION 

STANDARD 

FURTHER 

MONITORING 

STANDARD 

EXCEEDED 

CORRECTIVE ACTION 

AND ASSOCIATED 

MONITORING 

1-2 

SITE-SPECIFIC 

- ACL 
OSWER Directive 9481.00-6C 

Section 264.94 contains the regulatory framework for these concentration . 

limits. The approach used by the regulations is to adopt widely accepted 

environmental performance standards (i.e., MCLs in Table 1), when available, as 

concentration limits. However, because of the lack of currently available standards, 

specific concentration limits for only a few specific constituents have been included 

in the regulations. 

Maximum contaminant limits are established for 14 hazardous constituents 

under the National Interim Primary Drinking Water Standards and are listed in 

Table I of Section 264.94(a) of the regulations. If a constituent is not listed in Table 

1, the standard becomes no degradation beyond background water quality. In such 

cases, the concentration limit is set at background level. However, variances from 

these standards are available where the permit applicant can demonstrate that the 

constituents will not pose a substantial present or potential hazard to human health 

or the environment. In such cases, the applicant may ask for an "alternate 

concentration limit" (ACL) under Section 264.94(b) of the regulations. This section 

of the regulations lists nineteen criteria to be applied in ACL demonstrations. The 

applicant should, however, be aware of any State or local laws regulating ground 

water. Many States prohibit the release of any pollutants into the ground water. If 

the State has an authorized program for 40 CFR Part 264, Subpart F and does 

prohibit such releases, ACLs may not be allowed. 

This document serves to provide guidance to permit applicants seeking ACLs 

and permit writers evaluating ACL demonstrations. The document describes EPA's 

implementation guidelines and elaborates on the application of the 19 criteria. 

Nine of the criteria are for ground-water contamination pathways, and ten are for 

surface water pathways. The document is divided into 13 chapters that include an 

introduction, an explanation of each of the criteria in the regulation, a conclusion, 

and references. 

This guidance may also be useful for Record of Decision preparations pursuant 

to the EPA CERCLA program (Superfund), for State permit writers, for performing 

remedial facility investigations, or for evaluating corrective action programs for 

other types of waste management facilities. In applying this guidance to Superfund 

sites or for State permits, the users must be cognizant of any differences between 

1-3 
OSWER Directive 9481 00-6C 

the requirements of their programs and the RCRA regulations and permitting 

programs. 

Alternate concentration limits are discussed in the HCRA Standards for Owners 

and Operators of Hazardous Waste Treatment, Stora9e, and Disposal Facilities 

under Subpart F: Ground-water Protection (U.S. EPA 1982a}. The permit applicant 

and the reviewer should become familiar with the ground-water protection 

regulations. ACLs are granted through the permit process under Parts 264 and 270. 

Through this process, the public is afforded opportunity to participate in the 

establishment of ACLs. 

The Agency is developing a strategy for implemt~nting corrective action at 

solid waste management units. The overall goal of this strategy is consistent with 

that ofthe Agency's Subpart F ground-water protection program: the protection of 

human health and the environment. The ACL process described in this document 

may be useful when evaluating the corrective action programs at these solid waste 

management units. 

ACL Definitions 

To establish ACLs, two points must be defined on a RCRA facility's property 

(see Figure 2): the Point of Compliance (POC) and the Point of Exposure (POE). The 

POC is defined in the Subpart F Regulations (40 CFR § 264.95) as a "vertical surface" 

located at the hydraulically downgradient limit of the waste management area that 

extends down into the uppermost aquifer underlying thE! regulated unit. The POC is 

the place in the uppermost aquifer where ground-water monitoring takes place and 

the ground-water protection standard is set. The ACL, if it is established in the 

permit, would be set at this point. 

The point of exposure (POE) is the point at which it is assumed a potential 

receptor can come in contact, either now or in the future, with the contaminated 

ground water. Therefore, the ground-water quality at the POE must be protective 

of that receptor. For example, a facility may have a ground-water contaminant 

plume restricted to a small portion of its property. In this case, it may be 

appropriate to assume that people will be exposed through a drinking water well to 

the ground water immediately at the edge of the plume. The ground water at that 

1-4 
,---

1 

I 

I 

I 

.-·-·-·-·, i . I Regulated I ® 

j Unit 

i._,_,_,_.J 

-------, 

Contaminant 

Plume 

I 

I 

I 

0 

I 

I 

I 

I 

Point of I 

l

o Compliance Point of 

L _ Lc~lity Boun~=- _ --___ __:•:_ _ J 

PLAN VIEW 

Regulated 

~Unit~ 

-------- ... • X 

OSWER Directive 9481 .00-GC 

Point of 

Compliance 

Point of 

Exposure 

CROSS·SECTION VIEW 

POINT OF EXPOSURE (POE) -Point at which potential 

exposure to contaminants is assumed. Location is site 

specific. Allowable exposure is met here. 

POINT OF COMPLIANCE (POC)- The point on the downgradient 

side of the unit where the ground-water protection 

standard is met. The ACL is set here. 

CONTAMINANT PLUME- Tho volume of ground water that 

contains the leaking pollutants. 

FACILITY BOUNDARY- The property boundary of the 

facility, 

REGULATED UNIT- The area where th• hazardous 

wastes are kept (landfill, surface impoundment). 

Figure 2. Definitions 

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OSWER Directive 9481.00-GC 

point, the POE, must then be safe for human consumption. Likewise, if the groundwater contamination is discharging to on-site surface water, the potential receptor 

may, in some cases, be an aquatic organism. In this example, the aquatic organisms 

must be protected from adverse effects of the discharginiJ contaminants. 

Understanding and identifying the spatial relationship between the POC and 

the POE is critical in the establishment of an ACL. Mechanisms that attenuate 

contaminants may be considered only over the area between the POC and the 

downgradient POE. If the. POE is established at th1~ POC, then no form of 

attenuation will be considered in setting the ACL. In sU<:h a case (POC = POE), the 

ACL would be equal to the allowable health or environmental exposure level, with 

the assumption that exposure would occur at the waste management unit 

boundary. However, if the POE is removed a specified distance from the POC, then 

appropriate and conservative estimates of contaminant c:1ttenuation may be used in 

calculating the ACL. These mechanisms of attenuation would only be considered 

over that distance between the POC and the POE. For example, if the POE is 50 

meters downgradient of the POC, then attenuation could be conservatively 

estimated from the volumetric transport of ground water in relation to the mass 

loading of the leaking constituents over that 50 meters. The attenuation factor 

could then be applied to the allowable health or environmental exposure level at 

the POE to derive the ACL. 

The following section discusses EPA's ACL policy. After presenting the basic 

philosophy and guidelines of the policy, five case examples are described. These 

examples tell the reader where the POE should be established in similar real-world 

situations. Following these examples is a guide for employing this document to 

prepare an acceptable ACL demonstration. 

ACL Policy 

Experience gained over the last several years has allowed the Agency to 

develop a better understanding of ground-water contamination problems. This has 

led EPA to develop general policy guidelines for the use of ACLs at RCRA hazardous 

waste disposal facilities. These guidelines are designed to establish an ACL 

procedure that will be protective of human health and the environment. 

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OSWER Directive 9481.00-GC 

Three basic policy guidelines have been identified to assist the permit writer 

and applicant in implementing the ACL process for useable ground water: 

1. Ground-water contaminant plumes should not increase in size or 

concentration above allowable health or environmental exposure levels; 

2. Increased facility property holdings should not be used to allow a greater 

ACL; and 

3. ACLs should not be established so as to contaminate off-site ground 

water above allowable health or environmental exposure levels. 

Useable ground water is either a current or potential drinking water resource 

or ground water that has some other beneficial use (see Chapter VII for more 

discussion). ACLs in unuseable ground water will be assessed on a case-by-case 

basis. 

Regardless of where the POE is established, there is one overriding policy. 

That is, contaminant plumes in useable ground water will not be allowed to 

increase in size above acceptable levels. This "no growth" policy applies both to 

the mass of contaminants releasing to ground water and to the volumetric extent of 

the plume itself. The implementation of source control measures may be necessary 

to prevent the release of contaminants above their permissible level, the ACL. This 

may require that a regulated unit meet the appropriate Part 264 minimum 

technology requirements if the unit has significant leakage. All facilities with RCRA 

units that have plumes in useable ground water must prevent these plumes from 

expanding out and contaminating more ground water. This will require that the 

leading edge of the plume not move. The leading edge of the plume is identified by 

ground water that does not exceed an allowable health or environmental exposure 

level. 

The policy of not allowing plumes to increase in size or concentration is 

protective of human health and the environment by eliminating continued, 

uncontrolled releases of hazardous constituents. By limiting the growth of groundwater contamination, the degradation of the ground-water resource is restricted 

and the uncertainty of eventually cleaning up the contamination is reduced. This 

1-7 
OSWER Directive 9481 00-6( 

effect applies to both the areal. extent of the contamination and the contaminant 

concentrations within the plume. If the extent of the contamination increases in 

size, the probability of ever capturing and withdrawing the contamination is 

significantly reduced. Likewise, if the contaminant concentrations within the plume 

increase, then the aquifer matrix may leach contaminants into the ground water 

over a long period of time. If this is the case, then eventual cleanup of the 

contamination may be impossible. 

In determining the extent of an on-site plume, the permitting authority should 

consider only property that the facility owned at the time of initial permit 

application. A facility may not use recently purchased property to encompass a 

plume in order to allow the use of larger attenuation factors in their ACL 

calculations. The permit writer should consider only the original property, as 

defined in the initial Part B application, in determining an allowable ACL 

Corrective Action 

The Agency is in the process of developing a corrective action program for all 

solid waste management units (SWMUs) at RCRA permitted facilities. This program 

is mandated by Section 3004(u) of the Hazardous and Sc1lid Waste Amendments of 

1984 (HSWA). The ACL process, as described in this dc1cument, may be useful in 

determining the appropriate corrective measures for ground-water contamination 

at these other solid waste management units. 

In developing this SWMU corrective action program, EPA will also assess the 

need to promulgate some additional regulations for ground-water corrective action 

at regulated units. 

Closure 

If a RCRA facility owner or operator is considering applying for an ACL, he or 

she should be aware of EPA's closure and post-closure regulations and policy as they 

apply to ground-water monitoring. It is likely that if a contaminant is left in useable 

ground water above a health or environmental exposure level, as might be the case 

with an ACL based on attenuation arguments, the post-closure care period for the 

facility may be extended beyond 30 years (40 CFR §264.117(a)(2)(ii)). In this 

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OSWER Directive 9481.00-GC 

situation, an owner or operator may discontinue post-closure care and groundwater monitoring only after successfully demonstrating that all the ground water at 

a site is safe for all potential receptors. A more stringent ground-water standard, 

based only on allowable exposure levels for units above useable ground water, may 

be needed because the facility owner has no further RCRA obligations for managing 

and monitoring the facility at the end of post-closure care. If, at any time during 

the post-closure care or the compliance period under §264.96, a substantial threat 

to human health or the environment is identified, the permit can be modified to 

include a lower ground-water protection standard and an extended post-closure 

care program. 

Examples 

To provide national consistency in calculations used to estimate the potential 

impacts of releases of hazardous constituents to ground water from regulated 

units, Agency policy is that the points of exposure be assumed as discussed below. 

These POEs were chosen because the Agency believes that they are realistic and 

conservative estimates of where environmental or human receptors would likely be 

exposed to the contaminants. 

The Agency believes that this method is conservative enough to be protective 

of human health and the environment in situations that would be encountered 

during the setting of ACLs. These POEs were also deemed necessary because of both 

the persistent nature of many toxic chemicals in the environment and the need to 

prevent the further migration of these compounds. 

Case 1: For regulated units located above useable ground water that have not 

detected ground-water contamination at the time of permit issuance, the potential 

POE will be assumed to be directly at the POC (Figure 3). That is, for units at which 

no ground-water contaminant plume exists, the potential point of exposure is 

assumed to be at the waste management unit boundary. Therefore, no attenuation 

can be presumed for contaminants that leach from a unit in the future. Fate and 

transport arguments cannot be used to support ACL demonstrations if no ground1-9 
r---

Water Table 

OSWER Directive 9481.00-6C 

------------, 

:·-·-·-, 

I . . I® I Regulatad j \ j Unit j 

l...._._,_,_i 

Point of 

Compliance 

and 

Point of 

PLAN VIEW 

' 

------------ ®- ------·-----

~ Point of 

Compliance 

and 

Point of 

Exposure 

CROSS-SECTION VIEW 

Figure 3. Case 1: No Contamination at Original Permit Issuance 

1-10 
OSWER Directive 9481.00-6C 

water contamination plume exists at the time of permit issuance. This policy will 

help to prevent contaminants from entering the ground water above allowable 

health and environmental levels. All new units seeking permits for operation and 

old units that have not detected contamination will be held to this policy. 

Case 2: For units located above useable ground water that already have 

existing contamination that is confined to the facility property, the POE will be 

assumed to be no farther from the POC than the outer edge of the existing plume 

(Figure 4). If the permitting authority concludes it is protective of human health 

and the environment, then the point of exposure maybe set at the leading edge of 

the plume. In this situation, mechanisms of contaminant attenuation (fate and 

transport) may be considered in establishing the ACL at the point of compliance. 

Monitoring for the ACL constituents at the POE should be performed to verify the 

attenuation mechanisms that were accounted for. 

Case 3: For units located over useable ground water, if the leading edge of 

the plume extends off the facility property, the point of exposure will be assumed to 

be no farther from the POC than the facility property boundary (Figure 5). Fate and 

transport arguments may then be applied to the ground-water contamination 

between the POC and the POE at the facility boundary, assuming there is no 

possible route of exposure on the facility property. At no time may the designated 

POE be beyond the original facility boundary. 

Case 4: ACLs may be based on contaminant discharge into a surface water 

body if a facility owns the property up to the surface water body (Figure 6). The 

permitting authority should allow this only if: (1) the contaminant plume has 

already reached the surface water body, (2) the contaminants do not cause a 

statistically significant increase over background in the surface water 

concentrations of those contaminants, and (3) the contaminants will not reach a 

receptor at an unsafe level before they reach the surface water body. Though it 

may be acceptable to allow some contaminants in the ground water to discharge 

into a nearby surface water body, in no case may the ACL be derived so as to allow 

releases into that surface water body that result in a statistically significant increase 

in the concentration of the contaminants in the surface water body. 

1 "11 
OSWER Directive 948l.00-6C 

,----------------l 

' I 

I 

I 

I 

I 

I 

I 

I 

Unit 

I 

'-·-·-·-·...i 

Contaminant 

Plume 

I 

' I 

' 

I 

I , 

I . Point of j I Compliance 1 

L__ J~l~y Bou:d:_ _ --_________ j 

PLAN VIEW 

Facility 

Boundary\ 

Water Table 

----------

Point of 

Exposure 

--------

Point of 

Compliance 

CROSS·SECTION VIEW 

,X 

Point of 

Exposure 

Figure 4. Case 2: Contamination Confined to F!tcility Property 

1-12 
OSWER Directive 9481.00-SC 

;-------------------l 

I . i r·-·-·-· ~ Attenuation •!'~" ' i I j Regulated 

, i Un~ 

I i '-·-·-·-·-' 

' I Point of 

i Compliance 

L _ _£_ ~acility _Boundary_ -- _ -- _ -- _ -- _ _j 

PLAN VIEW 

Water Tabla 

Point of 

Exposure 

---------- ---------------------

Point of 

Compliance 

.. 

. . 

CROSS·SECTION VIEW 

Point of 

Exposure 

Figure 5. Case 3: Contamination that Extends Off·Site 

1-13 
OSWER Directive 9481.00-6C 

,-----------------

1 

I 

I 

I 

I 

I 

I 

I 

11-o'"ot-----Attenuation ------•1 

:·-·-·-,__ _________ _ 

I i ® -... ~, j Regulated X Contaminant \ 

j Unit i \ Plume I 

L. ____ .J \ ~~ / x,; 

~i~~ - ~~' j Compliance No Statistical 

L _

1 :acility ~oundar~ --- --- --~i:ific~=--

r 

Facility 

Boundary Regulated 

~Unit~ 

PLAN VIEW 

.. 

I 

Point of 

Compliance 

No Statistical 

Significance 

CROSS-SECTION VIEW 

Figure 6. Case 4: Contamination Discharging into a Surface Water Body 

1-14 
OSWER Directive 9481.00-GC 

Case 5: For those units over a non potable aquifer, the location of the POE will 

be established on a case-by-case basis (Figure 7). Such non-potable aquifers will 

usually be highly saline, containing more than 10,000 ppm total dissolved solids. In 

such situations, protection of environmental receptors may be the overriding factor. 

In any case, the ACL must be established so as to pose no unacceptable risk to public 

health and the environment. To apply this option, the permit applicant must 

thoroughly demonstrate that the nonpotable aquifer is isolated from any potable 

aquifer. 

ACL Information Requirements 

The type and amount of information needed for an ACL demonstration 

depends on the placement of the point of exposure (POE) and the site-specific 

characteristics. An ACL demonstration may cross reference many sections of the Part 

B Permit Application. Appendix A contains a listing of the types of information that 

may be cross referenced from the permit application. For new units, units with no 

ground- water contamination (Case 1), or units for which the owner or operator 

desires an ACL to be set at the allowable health or environmental exposure level, 

relatively little additional information beyond that already supplied in the permit 

application will normally be required. If the unit already has caused ground-water 

contamination and the owner or operator wishes to take into account mechanisms 

of attenuation in deriving the ACL (Cases 2-5), more information will be required. 

The simplest and quickest method for deriving the ACL in all situations will be 

to establish the POE at the POC. Even for those sites with gross contamination, 

setting the POE at the POC may be the least expensive option because high levels of 

contamination will usually require major source control and corrective action 

measures, regardless of the ACL. By setting the POE at the POC, the owner or 

operator may save significant amounts of time and money by not having to gather 

and organize the additional information required for deriving an ACL that accounts 

for attenuation mechanisms. 

1-15 
OSWER Directive 948 1.00-SC 

~--.. ------------------- ---~ 

I ' I ! 

1 

I 

I 

I 

1 

I 

I 

f-1401------Attenuation ------

~·-·-·-·-·~ 

! Regulated 1

• ® 

I Unit . )...--..._ 

i . L.._,_,_,...J 

Contaminant 

Plume 

I ' Point of I I Compliance 1 

L _ _L_:::~~~~d:__ -------·---_j 

Water Table ---------

PLAN VIEW 

Facility 

ll<>undary ~ 

Point of 

Exposure 

Point of 

Compliance 

Point of 

Exposure 

CROSS-SECTION VIEW 

Figure 7. Case 5: Contamination in a Non-Potable Aquifer 

1-16 
OSWER Directive 9481.00-GC 

No matter where the POE is established, an allowable health or environmental 

exposure level must be determined for each constituent for which an ACL is 

requested. The following discussion outlines the method used to choose the 

appropriate health or environmental exposure level, and the types of information 

necessary to derive the attenuation factor(s). This section is meant to be used as a 

guide to the rest of this document; detailed technical discussions are reserved for 

the following chapters. Five ACL Case Studies will be available in the Summer of 

1987 to help the owner or operator prepare an acceptable ACL demonstration. 

The appropriate allowable health or environmental exposure level will be 

dependent on the most vulnerable receptor near the facility. The most vulnerable 

receptor is that receptor, human or environmental, that has the lowest tolerance to 

the hazardous constituent(s) for which the ACL is being requested. In most cases, 

the receptors will be humans exposed to the contaminated ground water via 

ingestion (i.e., drinking). However, at times the most vulnerable receptor will be an 

environmental receptor. 

Agency-reviewed dose levels for humans are available to quickly determine 

allowable concentration levels for many hazardous constituents. Chapter IX of this 

document discusses the method for determining an allowable exposure level for 

humans. Essentially, the applicant can use Maximum Contaminant Limits (MCLs), or 

if MCLs are not available, obtain the appropriate allowable dose level (Reference 

Dose (RfD) for systemic toxicant, Risk Specific Dose (RSD) for carcinogens) from EPA 

and use the appropriate formula on page 9-7 of this document to derive the 

allowable concentration level for humans. 

If environmental receptors exist in the vicinity of the facility, the vulnerability 

of these receptors must be investigated. Chapter X provides a discussion of these 

environmental receptors. In many cases, the critical environmental receptor will be 

aquatic life in the nearest downgradient surface water body (including bogs and 

wetlands); however, threatened and endangered species, terrestrial organisms, 

agricultural products, and physical structures should not be overlooked. The 

applicant should compare the allowable exposure levels for the various receptors 

near the facility. The lowest level should be chosen as the concentration level at the 

POE. This method is necessary because the concentration of the contaminants at 

1-17 
OSWER Directive 9481.00-6( 

the point of exposure must be protective of the potential receptor that is most 

vulnerable to that contaminant. 

Once the allowable exposure concentrations for the point of exposure are 

chosen, the applicant needs to collect and organize any other necessary additional 

information. The following paragraphs guide the applicant to the relevant chapters 

of this document that describe the types of information that should be submitted. 

This discussion is organized around the five case examples described previously in 

this chapter. Since some sites will not fit neatly into this scheme, some interpolation 

may be necessary. Appendix B contains a chapter-by-chapter summary of this 

information. 

Case 1 

The information requirements for this case are applicable in all situations in 

which the permit applicant desires a direct health or environmental exposure level 

without accounting for mechanisms of attenuation. This method of setting the ACL 

is the standard for new units or for old units with no contamination at the time of 

permit issuance. If the applicant is dealing with a site that has ground-water 

contamination, the application must include a descripticm of all plumes, including 

isopleth maps of all hazardous constituents in the ground water. All ACL 

demonstrations using allowable exposure levels must include the information 

necessary to select the appropriate POE level [Chapters IX and X] and information 

on the general ground-water use [Chapter VI]. If the cclnstituents for which ACLs 

are requested do not have Agency-reviewed allowable dclse or exposure levels, then 

additional information on human and/or environmental effects will be necessary. 

The type of information will be dependent on the potential receptors [Chapters IX 

and X]. A general description of the site and the types and characteristics of the 

wastes handled is also required [Chapter II]. General ground-water information is 

also necessary and includes: a description of the horizontal and vertical extent of 

the uppermost aquifer [Chapter Ill]; a description of the ground-water flow in the 

aquifer [Chapter IV]; and a listing of the background concentrations of all 

hazardous constituents in the aquifer. Most or all of this information should 

already be available in the permit application. 

1-18 
OSWER Directive 9481.00-6C 

Case 2 

If the facility has on-site ground-water contamination and the permit 

applicant desires to account for attenuation mechanisms then the ACL 

demonstration should include all oft he information required in Case 1, in addition 

to the following: a description of the chemical and physical characteristics of the 

contaminants for which an ACL is requested [Chapter II]; data on the chemicalspecific degradation or attenuation rates and/or processes [Chapter II]; a description 

of each soil type beneath the facility [Chapter Ill]; any other attenuation-related 

information, including soil properties [Chapter Ill]; description of the extent and 

hydrological properties of each local stratigraphic unit [Chapter Ill]; aquitardrelated data [Chapter Ill]; if applicable, information on the temporal variations in 

ground-water flow and any withdrawal drawdown effects [Chapter IV]; if a 

mathematical simulation model is used, verification that it meets the criteria listed 

in Chapter IV; monthly precipitation data and effects on seasonal recharge [Chapter 

V]; any local ground-water discharge pathways [Chapter V]; location of each nearby 

(5 km) surface water body and an estimate of travel time for ground water from the 

facility to the water bodies [Chapter VI]; additional information on the current and 

projected demography and ground-water use when applicable [Chapter VI]; state 

certification of the ground- water's beneficial use [Chapter VII]; if background 

ground-water is contaminated, information on the sources and associated groundwater quality data [Chapter VIII]; and an assessment of the degradation of the ACL 

constituents, including degradation products and rates, when possible [Chapter XI]. 

It is very important that this information be synthesized to present a conservative 

attenuation argument. 

Case 3 

The required information for this case, where the contamination extends offsite and the ACL is based on attenuation mechanisms, is similar to that required for 

Case 2. However, because the contamination extends off-site, a more in-depth 

study of the human health and environmental effects will probably be necessary 

[Chapters IX and X]. 

1-19 
OSWER Directive 9481.00-GC 

Case 4 

For this case, information will be necessary to demonstrate both that the 

proposed ACL is acceptable and that the contaminants allowed to attenuate into 

the surface water body will not cause any statistically significant increase in 

concentration in the surface water body. To do this, the demonstration must 

include the information required in the earlier cases (1, 2, and 3) in addition to the 

data necessary to show no statistical significance [Chapter X]. If a model is used, it 

should meet the requirements discussed at the end of Chapter IV. More specific 

data on the effects of storm events and flooding will alsc) be necessary [Chapter V]. 

Additional information on the physical characteristics and discharge zone of each 

water body [Chapter VI] may also be necessary. The uses of these surface waters is 

also important information that should be documented [Chapter VII] along with a 

discussion of and data on other sources of surfac.e water contamination 

[Chapter VII]_ 

Case 5 

The required information for this case includes thE! information specified for 

Cases 1, 2, and 3 as well as information demonstrating that any off-site 

contamination will not pose a threat to any potential receptor or degrade any 

beneficial use. Additional information pertaining to the hydraulic isolation of the 

uppermost aquifer [Chapter Ill]. the possibility of E!xposure due to off-site 

contamination [Chapters VI and VII]. the ultimate fate of the contaminants 

[Chapters II and XI]. and the uses of local ground waters [Chapter VII] is also very 

important_ Of course, background ground-water quality data demonstrating that 

the aquifer is non potable, and an assurance from the State that the ground water 

will not be used, is required. 

1-20