### OSWER DIRECTIVE 9481.00-SC EPA/530-SW-87-017



# ALTERNATE CONCENTRATION LIMIT GUI"DANCE PART I

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# POLICY AND INFORMATION

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# REQUIREMENTS INTEHiiM FINAL

OFFICE OF SOLIDWASTE WASTE MANAGEMENT DIVISION U.S. ENVIRONMENTAL PROTECTION AGENCY 401 M STREET, S.W. WASHINGTON, D.C. 20460 

## JULY 1987

•OSWER Directive9481.00-GC

DISCLAIMER
This document is intended to assist Regional and State personnel in exercising thediscretion conferred by regulation in evaluating applicationsforACLs submitted pursuantto 40 CFR 264.94. Conformance with this guidance is expected to result in ACL applicationsthatmeetthe regulatorystandard ofprotecting human health and the environment. However, EPA will not in all cases limit its review to demonstrationsthat comportwith the guidance set forth herein. This document is nota regulation (i.e., itdoes notestablish astandard ofconductwhich hasthe force oflaw) 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.

iiiOSWER 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 Salee ofthe 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 developmentofthisdocument.

ivOSWER Directive 9481.00-6C

CONTENTS Page EXECUTIVE SUMMARY ES-1 I. INTRODUCTION 1-1 II. PHYSICALAND CHEMICAL CHARACTERISTICS OFTHE WASTE 2-1 CONSTITUENTS Ill. HYDROGEOLOGICALCHARACTERISTICS 3-1 IV. GROUND-WATER FLOW DIRECTION AND QUANTITY 4-1 

## v. PATIERNS OF RAINFALL S-1 VI. PROXIMITYOF SURFACE WATER AND GROUND-WATER USERS 6-1 VII. CURRENTAND FUTURE USES OF GROUND WATER AND SURFACE 7-1 WATER IN THE AREA VIII. EXISTING QUALITYOF 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 POTENTIALADVERSE EFFECTS 11-1 XII. SUMMARYAND CONCLUSIONS 12-1 XIII. REFERENCES 13-1

vOSWER Directive 9481.00-6C

CONTENTS APPENDICES A. Outline of Informationthatmay be Cross ReferencE!d A-1 fromthe PermitApplication B. Summary ofTables, Figures, and Information Required B- 1 toSupportan ACL Demonstration 

### C. Summary Sheet of Hazardous Constituent PropertiE!S C-1 D. Summary Sheeton Health Effects Factors D-1 E. Standard Factors used in Exposure Assessments E-1

viOSWER 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 utilizedesign 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, foreach hazardousconstituententeringthegroundwaterfrom a regulated unit, a concentration limitthat cannot be exceeded. The concentration limits are the "triggers" thatdeterminewhen corrective action is required. There are three possible concentration limits that can be used to establish the GWPS: 

### I. Background levelsofthe hazardousconstituents, 2. Maximum concentrations listed in Table I of Section 264.94(a) of the regulations, or 3. Alternateconcentration limits (ACLs). The first two levels are established in the facility permit as the GWPS, unless thefacility owneroroperatorappliesfor 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 preventthe use ofan ACL altogether ifthe State has an authorized program for 40 CFR Part 264, Subpart F. This document provides guidance to RCRA facility permit applicants and writersconcerningthe establishment ofACLs. To obtain an ACL, a permitapplicant 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 ES-1OSWER Directive 9481.00-GC environmentatthe 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) ofthe 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 thetype, 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 underthe authority of the Safe Drinking Water Act, or Agency-reviewed reference doses or risk-specific levels directly, orafterasimple manipulation, as ACLs. For use as ACLs, these levels would have to be approved by EPA forthespecific site. This chapter also points out that data from the RCRA Part Bpermit application may be cross- referenced in the ACLdemonstration. Experience gained over the last several years hc:1s allowed the Agency to developa better understanding ofground-watercontamination problems. This has led EPA todevelop general policy guidelinesforthe use ofACLs at RCRA hazardous waste disposal facilities. These guidelines are designed to ensure that ACLs will be protectiveofhuman health and theenvironment. Three basic policy guidelines have been identified to assist the permit writer and applicant in implementingtheACL process for useable groundwater: 1. Ground-water contaminant plumes should not increase in size or concentration above allowable health orenvironmental exposure levels, 2. Increased facility property holdingsshould notbe used to allowagreater ACL, and ES-2OSWER Directive9481.00-6C

3. ACLs should not be established so as to contaminate off-site ground waterabove allowable health orenvironmental exposure levels. These guidelines are further described in the first chapter. Also described are theways in which ACLs should be determinedforfivetypesofsituations. Chapter I also discusses how the ACL process fits into the rest of the RCRA ground-waterprotection program. The applicant iscautionedthatan ACL based on attenuation mechanisms may not be acceptable to the permitting authority at the end ofthe post-closure care period. Atthe conclusion of post-closure care, a more stringent ground-waterstandard, based only on an allowable exposure level, may be needed because thefacilityownerhas nofurther RCRA obligationsfor managing ormonitoring 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 Fregulations. 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 submitdata on any factors relating to the stability and mobility ofthe waste constituents in the ground water ifthe 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 forwhich an ACL is requested.

ES-3OSWER Directive9481.00-6C Chapter Ill discusses the data needed to describe the hydrogeologic properties ofthe site. General descriptions ofthe 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 importantgeologicattributesofasite include: I. Soil and rock characteristics, 2. Geologicstructure, and 3. Geomorphologyandtopography. In ACL demonstrations where soil and other matrix attenuation mechanisms are used tojustifythat 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 informationthat is used to assess contaminanttransport. The general RCRA permit requirements specifythe submittal ofground-waterflowdata. This data should be adequate for on-site determinations of ground-water quantity and flow direction; however, additional data may be required ifoff-sitedetE!rminations ofquantityand flow are needed for the ACL demonstration. Ground-water quantity can be estimated from hydrologic parameterssuch as specific**yi1~ld** for unconfined aquifers and specific stotage for confined aquifers. The use of Darcy's law for determining ground-waterflowquantity is acceptableforhomogeneousand isotropicaquifers. The hydrogeologicportion ofall ACL demonstrationsshould include ageneral 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 measure

### ES-4OSWER Directive9481.00-6C
mentstaken from monitoring wells or piezometers screened atsimilar 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 extenttowhichtheflowchange occurs. ChapterVdiscussesthetypes ofprecipitation datathat should be submitted in an ACL demonstration. ACL demonstrations based on contaminant attenuation need to include adiscussion of precipitation, which should be focused on the site's hydrologic regime. Ifthe applicant'sACL 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 surfacewaterhydrology. 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 oftheACL. Only ageneral discussion ofsurface 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 ofa surface water body are necessary if the applicant is attempting to show that the contaminants will safely and quickly attenuate intoasurface water body. In ordertoassessthe likelihood ofexposure of ground-water users, ACL demonstrations should discuss the proximity of groundwateruserstothefacility. 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 needsto examine onlythe pertinentaspects ofground-waterand 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 atthe point ofcompliance. Surface water uses should be discussed bytheACL applicant ifcontaminated groundwatercan migratetosurface waters.
ES-5OSWER Directive 9481.00-6C Chapter VIII is concerned withthe existing quality ofgroundwaterand 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 qualitythatwould be expected to be found ifthe facility's regulated unit(s) was not leaking contaminants. Background monitoring wells must yield ground-watersamples 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 ofcontamination ifthe 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 ofthe 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 ofthe hazardous constituents present in the ground waterforwhich 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 constituentswhich should be useful in preparing ACLdemonstrations. ChapterXdiscusses datathatshould be submitted on the potential impactsto 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 ES-6OSWER 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 permitapplicantshould discuss specificdata thatsupport 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 environmentaltoxicityfactorsshould be examined. The permit applicant should discuss the presence of any endangered or threatened species in terrestrial orsurface water environments near the facility. If any endangered or threatened species are in the area, then the permit applicant should discuss the potential impacts ofthe contaminated ground water, including critical habitatimpacts, on thespecies. Physical structures can also be adversely affected by hazardous constituents in thegroundwater_ The determination ofpotential impactsto 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 applicantshould discuss the process by which each ACL constituent will degrade, either from a ground-water perspective, surface water perspective, oracombination ofboth, 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 ofthecriteria discussed in this guidancedocument is not required in every ACL demonstration_ Each ACL demonstration will reflect site-specific environmental 

### ES-7OSWER 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 demonstratethatthe GWPS, which may contain ACLs, has not been exceeded for a period ofthree consecutiveyears. 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 dependentuponthetechnical strength ofthedemonstration.

ES-8OSWER Directive9481.00-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 mannerthatwill prevent ground-watercontamination. 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 atthatfacility. This standard establishes a limiton the amountofgroundwatercontamination thatcan 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 fromthetreatment,storage, ordisposal of hazardouswastes. As such, the standard will indicate when ground-water clean-up is necessary to control contamination thathas emerged from a hazardouswaste facility. The principal elements ofthe ground-water protection standard are discussed in Section 264.92 of the regulations. For each hazardous constituent entering the ground waterfrom a regulated unit, a concentration limit must be established that will serve as a limh beyond which degradation ofground-waterqualitywill not be allowed. There are three possible concentration limitsthatcan be used to establish theground-waterprotection standard: 1. Background levels oftheconstituents, 2. Maximum concentration levels listed in Table 1 of Section 264.94(a) of the regulations, or 3. Alternate concentration limits (see Figure 1).

1-1OSWER 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

- ACLOSWER Directive9481.00-6C 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 ofthe lack ofcurrently available standards, specific concentration limits foronly afew 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 IofSection 264.94(a) ofthe regulations. If a constituent is not listed in Table 

### 1,thestandard becomes nodegradation beyond background waterquality. In such cases, the concentration limit is set at background level. However, variances from these standards are available where the permit applicant can demonstrate thatthe constituentswill not pose asubstantial 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) ofthe regulations. This section ofthe 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 prohibitthe release ofany pollutants intothe ground water. If the State has an authorized program for 40 CFR Part 264, Subpart F and does prohibitsuch releases, ACLs may notbe allowed. This document serves to provide guidance to permit applicants seeking ACLs and permitwriters evaluating ACL demonstrations. The document describes EPA's implementation guidelines and elaborates on the application of the 19 criteria. Nine ofthe 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 ofeach ofthe criteria in the regulation, aconclusion, 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 othertypesofwaste managementfacilities. In applying this guidanceto Superfund sites or for State permits, the users must be cognizant of any differences between

1-3OSWER 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 throughthe permit process under Parts 264 and 270. Through this process, the public is afforded opportunity to participate in the establishment ofACLs. 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 thatoftheAgency's Subpart Fground-waterprotection 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 ofCompliance (POC) and the Point of Exposure (POE). The POC is defined in the Subpart FRegulations (40 CFR § 264.95) as a "vertical surface" located atthe hydraulicallydowngradient limitofthewaste managementarea that extendsdown intothe uppermostaquifer underlyingthE! regulated unit.The POC is the place in the uppermostaquiferwhere ground-watermonitoringtakes place and the ground-water protection standard is set. The ACL, if it is established in the permit,would be set atthis 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 atthe 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 appropriatetoassume thatpeoplewill be exposed through adrinkingwaterwell to thegroundwaterimmediately atthe edge ofthe plume. The groundwateratthat 1-4

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... • X OSWER Directive 9481.00-GC

Pointof 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 thedowngradient side ofthe unit where the ground-water protection standard is met. The ACL is set here. CONTAMINANT PLUME- Tho volume ofground waterthat contains the leaking pollutants. FACILITY BOUNDARY- The property boundary of the facility, REGULATED UNIT- The areawhere th• hazardous wastes are kept (landfill, surface impoundment). Figure 2. Definitions 1-5OSWER Directive 9481.00-GC point,the POE, mustthen be safe for human consumption. Likewise, ifthe groundwatercontamination 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 ofthedischarginiJ 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 tothe allowable health or environmental exposure level, with the assumption that exposure would occur at the waste management unit boundary. However, ifthe POE is removed aspecified distance from the POC, then appropriate and conservative estimates ofcontaminantc: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 toderivetheACL. 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 ACLdemonstration. ACL Policy Experience gained over the last several years has allowed the Agency to developa better understanding ofground-watercontamination problems. This has led EPA to develop general policy guidelinesforthe use ofACLs at RCRA hazardous waste disposal facilities. These guidelines are designed to establish an ACL procedurethatwill be protectiveofhuman health andtheenvironment.

1-6OSWER Directive 9481.00-GC Three basic policy guidelines have been identified to assist the permit writer and applicant in implementingthe ACL process for useable groundwater: 1. Ground-water contaminant plumes should not increase in size or concentration above allowable health orenvironmental exposure levels; 2. Increased facility property holdingsshould not be used to allowagreater ACL; and 3. ACLs should not be established so as to contaminate off-site ground waterabove allowable health orenvironmental 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 ofcontaminants releasing to groundwaterand tothe volumetricextentof the plume itself. The implementation ofsource control measures may be necessary to preventthe release ofcontaminants above their permissible level, the ACL. This may require that a regulated unit meet the appropriate Part 264 minimum technology requirements ifthe 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 ofthe plume not move. The leading edge ofthe plume is identified by groundwaterthatdoes notexceed 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 limitingthe growth ofgroundwater contamination, the degradation of the ground-water resource is restricted and the uncertainty of eventually cleaning up the contamination is reduced. This 1-7OSWER 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, ifthe contaminantconcentrationswithin 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 determiningthe extentofan on-site plume, the permitting authorityshould 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 Bapplication, in determiningan allowableACL Corrective Action The Agency is in the process ofdeveloping a corrective action program for all solid waste management units (SWMUs) at RCRA permitted facilities. This program is mandated by Section 3004(u) ofthe 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 atthese othersolid waste management units. In developing this SWMU corrective action program, EPA will also assess the need to promulgatesome additional regulationsforground-watercorrective action at regulated units. Closure Ifa 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 policyasthey applyto ground-watermonitoring. It is likelythat ifacontaminantis left in useable groundwaterabove 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 1-8OSWER Directive 9481.00-GC situation, an owner or operator may discontinue post-closure care and groundwater monitoring onlyaftersuccessfully demonstratingthatall thegroundwaterat a site is safe for all potential receptors. A more stringent ground-water standard, based only on allowable exposure levels for unitsabove useable ground water, may be needed because thefacilityownerhas nofurther RCRA obligationsfor managing and monitoring the facility at the end of post-closure care. If, at any time during the post-closure care orthe 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 thatthe points of exposure be assumed as discussed below. These POEs were chosen because the Agency believes that they are realistic and conservative estimates ofwhere environmental or human receptors would likely be exposed tothecontaminants. The Agency believes thatthis method is conservative enough to be protective of human health and the environment in situations that would be encountered duringthesetting ofACLs. These POEswerealso deemed necessary because ofboth the persistent nature of many toxic chemicals in the environment and the need to preventthefurther migration ofthese compounds. Case 1: For regulated units located above useable ground waterthat have not detected ground-watercontamination atthetime of permit issuance, the potential POE will be assumed to be directly atthe POC (Figure 3). That is, for units atwhich no ground-water contaminant plume exists, the potential point of exposure is assumed to be atthewaste management unitboundary. Therefore, no attenuation can be presumed for contaminants that leach from a unit in the future. Fate and transportargumentscannot be used tosupportACLdemonstrationsifnoground

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Figure 3. Case 1: No Contamination at Original Permit Issuance 1-10OSWER 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 unitsthathave notdetected contamination will be heldtothis 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 ofthe existing plume (Figure 4). If the permitting authority concludes it is protective of human health and the environment, then the pointofexposure maybeset atthe 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 atthe POE should be performed to verify the attenuation mechanisms thatwere accounted for. Case 3: For units located over useable ground water, if the leading edge of the plume extends offthefacility property,the pointofexposure will be assumed to be nofartherfromthe 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 ofexposure on the facility property. At no time may the designated POE be beyondtheoriginalfacility 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 intothatsurface water bodythat result in astatistically significant increase inthe concentration ofthe contaminants inthesurfacewaterbody.

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



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Figure 5. Case 3: Contamination that Extends Off·Site 1-13OSWER Directive 9481.00-6C



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Figure 6. Case 4: Contamination Discharging into a SurfaceWater Body 1-14OSWER Directive9481.00-GC Case 5: Forthose unitsover a nonpotable aquifer, the location ofthe POE will be established on a case-by-case basis (Figure 7). Such non-potable aquifers will usually be highly saline, containing morethan 10,000 ppmtotal dissolved solids. In such situations, protection ofenvironmental receptors may be the overriding factor. In any case, theACL 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 ofthe Part BPermitApplication. AppendixA contains a listing ofthetypes 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. Ifthe unit already has caused ground-water contamination and the owneroroperatorwishes to take into account mechanisms ofattenuation in derivingtheACL (Cases 2-5), more informationwill be required. The simplest and quickest method forderiving the ACL in all situations will be to establish the POE at the POC. Even for those sites with gross contamination, settingthe POE atthe 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 oftime and money by not having to gather and organizethe additional information required forderiving an ACL thataccounts forattenuation mechanisms.

1-15OSWER Directive 9481.00-SC**~--** 

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Figure 7. Case 5: Contamination in a Non-Potable Aquifer 1-16OSWER Directive 9481.00-GC No matterwherethe POE is established, an allowable health orenvironmental 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 meantto 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 1987to helptheowneroroperatorprepare an acceptable ACLdemonstration. The appropriate allowable health or environmental exposure level will be dependent on the most vulnerable receptor near the facility. The most vulnerable receptor isthat receptor, human orenvironmental,that has the lowesttoleranceto 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, attimesthe mostvulnerable receptorwill be an environmental receptor. Agency-reviewed dose levels for humans are available to quickly determine allowable concentration levels for many hazardous constituents. Chapter IX ofthis document discusses the method for determining an allowable exposure level for humans. Essentially, the applicantcan use Maximum Contaminant Limits (MCLs), or if MCLs are not available, obtain the appropriate allowable dose level (Reference Dose (RfD) forsystemictoxicant, 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 forhumans. Ifenvironmental receptors exist in the vicinity ofthe facility, the vulnerability ofthese 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 nearthefacility. The lowest level should be chosen as the concentration level atthe POE. This method is necessary because the concentration of the contaminants at

1-17OSWER Directive9481.00-6( the point of exposure must be protective of the potential receptor that is most vulnerabletothatcontaminant. 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 paragraphsguidethe applicanttothe relevantchapters ofthis documentthat describe the types of information that should be submitted. This discussion is organized around the five case examples described previously in thischapter. Since some sites will notfit neatly intothisscheme, 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 applicantdesires a direct health or environmental exposure level withoutaccounting for mechanisms ofattenuation. This method ofsetting the ACL is the standard for new units orfor old units with no contamination atthe 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]. Ifthe cclnstituents for which ACLs are requested donothave Agency-reviewed allowabledclse orexposure levels,then additional information on human and/or environmental effects will be necessary. The type of information will be dependenton 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 ofthe horizontal and vertical extent of the uppermost aquifer [Chapter Ill]; a description ofthe 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 permitapplication.

1-18OSWER 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 ofthe 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 chemicalspecificdegradation orattenuation rates and/orprocesses [Chapter II]; adescription 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-waterdischarge pathways [ChapterV]; location ofeach nearby (5 km) surface water body and an estimate oftravel timeforground waterfrom the facilitytothewater 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 ofthe degradation ofthe 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 theACL 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 IXand X].

1-19OSWER Directive9481.00-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 ofstorm 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 ofthese 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 [ChapterVII]_ 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 nonpotable, and an assurance from the State that the ground water will notbe used, is required.

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