CHAPTERV
PATIERNS OF RAINFALL {40 CFR 264.94{b){2){iv)) OSWER Directive 9481.00-6C

Precipitation is a driving factor for ground-water recharge and discharge. These two processes are basic components of the hydrogeology at a facility. To verify a claim of attenuation due to dispersion, precipitation data in support of ground- water flow and contaminant transport information must be submitted. This chapter describes the type of precipitation information that should be submitted in supportofan attenuation-based demonstration.
The permit applicant should focus the discussion of precipitation around the site's hydrologic regime. If the applicant's ACL demonstration clearly shows that ground-water discharge to surface waters is unlikely, then the discussion of precipitation events can be limited to effects on infiltration, evapotranspiration, and ground-water recharge. However, ifground-waterdischarge to surface water is an importantelementofthe ACL demonstration,then precipitation eventsshould be related to recharge and discharge ofgroundwater. Precipitation events are variable and occurwith different intensities, amounts, and durations. The geographical distribution of rainfall also varies from one area to another within a region. However, over a long period of time {years). the precipitation data for an area can be represented by events with definite amounts that occur at various frequencies. These frequencies are classified in terms of duration and yearly return periods. For example, a one-day/10-year storm event is defined as the amount of rainfall that is expected to occur during a 24-hour period once every 10 years. The amount of precipitation for a storm of specific return period and duration is used to produce an estimate ofthe amount of precipitation foragiven geographical area.
Attenuation-based demonstrationsshould contain general informationon the precipitation characteristics of a site. This includes data on rainfall and snowfall, expressed as its equivalent in rainfall. The National Oceanic and Atmospheric

5-1OSWER Directive9481.00-GC
Administration, the National Weather Service, or climate data in Ruffner (1980 and 1981) may be sources of this information if on-site data is unavailable. Regional precipitation data may be used if it was generated within 15 km of the facility. Regional data from greater than 15 km of the facility should be correlated with available on-site data. The monthly mean and range ofthis data, the specific time period from which the data was collected, and the lo<:ation of the precipitation gauge(s) in relation tothe facility should be provided. The permit applicant should discuss this information in terms of temporal effect on infiltration and seasonal ground-water recharge. These processes should be related to any effects on contaminanttransport.
Ifthe ACL demonstration does not involve surface water exposures, then the permitapplicantcan proceed tothe next chapter. However, ifthe facility is located near surface water bodies (see Chapter VI), or if surface water discharge is used as an argument in an ACL demonstration, then more detailed information on precipitation events should be submitted. The permit applicant should submit data on specific storm frequency patterns and discuss how these storms relate to flood and infiltration/discharge characteristics ofthefacility.
The predicted amount of precipitation produced over a 24-hour period by storms of return frequencies of 1, 10, 25, and 100-years should be submitted. The!- year and 10-year storm frequency information gives insight into ground-water infiltration and discharge patterns. The 25-year and 100-year storm frequency data 

### are useful in assessing dischargeduringflood conditions.
The 100-yearfloodplain should be described on a USGS topographic map. The floodplain information should be readily available to the applicant, since it is required by Section 270.14(b) permitting requirements. Federal Insurance Administration flood maps can be a useful source forthis information. Ifthe facility has any special flood prevention measures, they should also be shown on the map. These measures could include any dikes, berms, dams, and special flood retention walls. The effectofthese devices on ground-waterinfiltration and dischargeshould be discussed. Furthermore, any special site conditions that affect infiltration and discharge should be discussed. These include site topography, solar orientation of the regulated unit,and wind patterns.



### 5-2OSWER Directive 9481.00-GC

The ground-waterdischarge patterns atthe facility should also be delineated on a topographic map. All streams, ditches, culverts, and sewers that receive ground water should be clearly identified. Normal ground-water discharge patterns (1-year storm) and discharge during flood conditions (25 and 100-year storms) should be clearly marked. Snow melt pathways should be identified, if appropriate. Any discharge abatement or collection devices, such as detention basins, swales, and canals, should also be described. Evapotranspiration should also be considered in both recharge and discharge evaluations. Vegetation can use a large amount ofwater and reduce significantly the amount of infiltration reaching thewatertable, especially during the growing season.

5-3OSWER Directive 9481.00·6C

CHAPTER VI
PROXIMITY OF SURFACE WATER AND GROUND-WATER USERS (40 CFR 264.94(b)(1)(iv)and (2)(v))

This chapter and the next chapter discuss important factors necessary for assessing probable exposure pathwaysforthe ACL constituentsthrough surface and ground water. This chapter discusses the location of surface water and groundwater users in the vicinity ofthe facility. All ACL demonstrations should contain a discussion of the proximity and types of water uses near the facility. If the demonstration is attenuation-based, then more detailed information will be necessary. The uses of surface and ground water in the vicinity of the facility are discussed in ChapterVII.
A key factor involved in assessing exposure is the proximity of surface water and ground-water users to the facility. For ACL demonstrations, "proximity" is liberallydefinedto include both spatial andtemporal concepts. Lineardistance may be more appropriate forjudging potential surface water exposures, while time of travel is important for ground-water exposures. Proximity should be expressed in terms of both linear distance and time required for ground-water flow and contaminanttransport.
The level of information necessary to satisfy the proximity of users requirementdependson the basis oftheACL. lftheapplicant is attemptingtoshow thatcontaminantswill safely attenuate intoasurface water body,then data on the specific physical attributesofthesurface water bodywill be necessary. This includes information necessary to estimate the mixing potential and mechanisms of the water body. Likewise, ifthe aquifer is nonpotable, then data showing that it is not fitforconsumption must be supplied.

6-1OSWER Directive9481.00-GC

Surface Water
ACL demonstrations relying on transport mechanisms should include a discussion of the potential effects of the facility on surface waters. The initial evaluation includes assessing thefacility's proximityto surface waters and involves:
1. Identifying each surface water body in the vicinity ofthe facility, 2. Determining thedistance from thewaste management area boundaryto each surface water body, and 3. Identifying ground-waterdischarge pathwaystosurfacewaters.
Each water bodywithin five kilometers downgradient (or downstream) ofthe facility boundaryshould be identified. The owner or operator ofthe facility should supply an appropriatelyscaled map identifying each waterbody. All streams, rivers, ponds, lakes, estuaries, and marine waters should be clearly marked. All ditches, streams, sewers, and runoff pathways that serve as ground-water discharge or infiltration areas should be delineated on the map. A table specifying the name of each water body and the distance from the waste management area to the closest partofeach waterbodyshould be provided by theowneroroperatorofthe facility.
The travel time oftheACL constituentsfrom the facility tothe discharge areas should be discussed by the permit applicant. Ground water and hazardous constituents may move at various rates due to differE!nt physical and chemical properties. Therefore, discharge calculations should include estimates of both hydraulic transport and waste transport. The ground-water transport models and methodsdiscussed previously in Chapter IVshould be used to estimatethe hydraulic and hazardous constituent loading rates. Actual seepage measurements may be necessary to verify model estimates if ground-waterdischarges are estimated to be asignificant portion oftheannual hydraulic load to awater body.
A greater level ofdetail on characteristics ofsurface water bodies is needed if the applicant is attemptingtoshowthatthe contaminantswill attenuatesafely into surface water (i.e., not cause a statistically significant inc:rease in concentration). In these cases, the physical characteristics of each identified downgradient (or downstream) water body should be included in a table. Important lake and pond characteristics are:

6-2OSWER Directive9481.00·6C

1. Surface area, 2. Mean depth, 3. Volume, 4. Temperature stratification, and 5. Hydraulic residence time.
Informationon estuarine and marineareasshould include:
1. Surface area, 2. Mean depth, and 3. Tidal periodicityand amplitude.
Pertinentstream and river characteristics are:
1. Meanwidth, 2. Mean depth, 3. Flow rate, including average flow and lowest flow that would be expected to occur during a continuous 7-day period, once every 10 years (07-10), and 4. Lowest recorded flow rate.
This information is necessary in orderto estimate the discharge zones of each type of surface water in the vicinity of the facility. The temporal and spatial variability of flow rates, tidal factors, and hydraulic residence times are also essential factorsforestablishing discharge zones.
The permitapplicantshould synthesize this information to supportarguments ofnostatistically significant increase in surface waterconcentrations. The expected amount of attenuation and the zones of probable discharge areas should be discussed. The permitapplicantshould be aware that certain States have approved surface water modelsthat are used in the NPDES permitting program. Ifapproved models are available, they should be used by the applicant to determine discharge zones in surface waters. The EPA documentTechnical Support DocumentforWater Quality-Based Taxies Control (U.S. EPA 198Sa) is a good source ofthis information. In general, one quarter of the cross-sectional area of a stream may be used in



### 6-3OSWER Directive9481.00-6(
estimating the discharge zone. This area could then be applied over the width of the ground-watercontaminant plume as it enters the river to derive the discharge zone.
Ground Water
In order to assess the likelihood of exposure of current ground-water users, every ACL demonstration relying on attenuation mechanisms must discuss the proximityofground-waterusers tothefacility. This requires determining:


### I. The distance of the ground-water users' withdrawal point (i.e., well or spring) fromthefacility, and 2. The hydrologictransport time for the contaminants to reach the closest wellsorsprings.
The users of ground water within a five kilometer radius of the facility boundaryshould be identified. Both upgradientand downgradientwellsshould be included because oftheir potential to change the direction or quantity of groundwater flow. The applicant should delineate each ground-water withdrawal or injection well on a scaled site map. The distance of each well from the waste management area should be given in a table. The following uses of each well should be clearly marked:
1. Potable (municipal and residential), 2. Domestic nonpotable, 3. Industrial, 4. Agricultural, and 5. Recharge.

6-4OSWER Directive 9481.00-6C

CHAPTER VII
CURRENTAND FUTURE USES OF GROUNDWATER AND SURFACE WATER IN THE AREA (40 CFR 264.94(b)(1)(v) and (2)(vi))

Once the location of the surface water and ground-water users has been determined, the nature ofthe use should be considered. A major objective of an ACL demonstration relying on transport arguments is to show that ground-water contamination at afacility will not adversely affect any water use. The supporting arguments for the ACL can center around the fact that the ground-water contamination atthe facility is notdegrading the designated beneficial uses ofthe water resources. This requires the permit applicant to review federal, state, and local standardsorguidelinesthatgovern the uses ofboth ground and surface water to ensure that the presence of a contaminant plume is not inconsistent with any published regulations, ordinances, or guidelines. This chapter points outthe types of water uses that should be investigated, and the information that should be submitted on those water uses to support an attenuation-based ACL demonstration.
All ACL demonstrationsshould generallydiscuss current and likely future uses of water resources near their facility. However, ACL demonstrations based on attenuation arguments need to provide detailed information on these uses. Information gathered to satisfy data requirements on the proximity of water resource users (see Chapter VI) will be adequate to identify major water resources near the facility. In order to aid the permit reviewer, the water resource use information should be structured around thefollowing general categories:
1. Agricultural- irrigation and animal watering; 2. Industrial -process, cooling, and boilerwater; 3. Domestic and municipal

- potable, bathing, washing, and lawn/garden watering; 4. Environmental

- ground-water recharge or discharge, fish and wildlife propagation, uniqueareas; and



### 7-1OSWER Directive9481 .00-6C

5. Recreational- fishing, swimming, boating, and othercontact uses.
The permitapplicantshould examine pertinentaspects of both ground water and surface water uses. Both the current uses and the possible future uses of the water resources should be discussed. The specific type of ground-water use information is described in thefollowing section.
Ground-Water Uses
It should be obvious that ground-water use can be critical in the setting of ACLs at a facility. Facilities that are contaminating or have contaminated ground waters should examine local ground-water uses if their demonstration is attenuation-based.
The U.S. EPA has developed a Ground-Water Protection Strategy (U.S. EPA, 1984a) that states that ground water should be protected to its highest beneficial use. Three general classes ofgroundwaterare recognized:
Class I: Special ground waters are those that are highly vulnerable to contamination because of the hydrological characteristics of the area under which they occur and that are also characterized by eitherofthefollowing twofactors; a) Irreplaceable--no reasonable alternative source of drinking water is availabletosubstantial populations, or b) Ecologically vital--the aquifer provides the base flow for a particularlysensitive ecological system that, ifpolluted,would destroy a unique habitat.
Class II: Current and potential sources of drinking water and waters with other beneficial uses including all other ground waters that are currently used or potentially available for drinking water or other beneficial uses.
Class Ill: Ground waters not considered potential sources of drinking water and of limited beneficial use are those that are heavily saline, with

7-2OSWER Directive 9481.00-6C

total dissolved solids (TDS) levels over 10,000 mg/1, or are otherwise contaminated beyond levelsthatallow cleanup using methodsthat are reasonably employed in public water system treatment. These ground waters also must not migrate to Class I or II ground waters ordischargetosurface waterthatcould cause degradation.
Facilities that are contaminating or have the potential to contaminate Class I or Class II ground waters should incorporate human health factors (see Chapter IX) into their ACL demonstration because these resources are either potential or currentsources ofdrinkingwater. Ifthe groundwater is Class Ill,then health- based concerns may be secondary to environmental concerns in the setting ofACLs. ACLs in Class Ill groundwaterwill be assessed on acase-by-case basis.
The permitapplicantshould discuss the ground-water use in thevicinity ofthe facility in terms of these three classes or other appropriate State-approved ·classification schemes. The Agency's Office of Ground-Water Protection is developing guidance containing specific criteria fordesignation ofground wateras Class I, II, or Ill. These draftGuidelinesforclassifying ground waterwere available in December 1986. Until this guidance is finalized, we recommend only conservative use ofthe "unuseable" criterion as abasisforan ACL.
InformationfromtheState and/or local governmentas tothe beneficial use of the ground water should be included if the ground water has been classified. Otherwise, the permit applicant should have its ground-water classification data reviewed bytheState. When groundwater has been classified on a State, regional, or local level, this classification should be re-evaluated and verified using site specificdata. The State's reviewshould be included in theACL demonstration.
Surface Water Uses
Surface water uses should be discussed by the permit applicant if contaminated ground water can migrate to surface waters. The previous chapter on proximity of surface waters should aid in deciding which water bodies are of interest. If no surface water impacts are likely, then the data discussed in this section do nothaveto be submitted.



### 7-3OSWER Directive9481.00-6(

The established guidelines, criteria, and/or standards for each water body identified in Chapter VI should be examined. The permit applicant should list in a table the designated use of each water body, a citation of the local, State, or Federal regulations governing the use, and the agency responsible for implementingthe regulation. The following general use categoriesshould be used bythe permitapplicant in preparingthetable:
1. Drinking watersource (including otherdomestic uses), 2. Fish and wildlife propagation area, 3. Industrial oragricultural watersource, 4. Area ofspecial ecological concern, and 5. Recreational area.
The surfacewater use informationwill aid in determining appropriateACLs by identifying surface water exposures thatcan occur. The data gathered to fulfill the requirements ofthis section will be used to prioritize the likely exposure pathways and to determine whether human health and environment factors should be assessed in furtherdetail (see Chapters IXand X).



### 7-4OSWER Directive 9481.00-6C

CHAPTER VIII
EXISTING QUALITYOF GROUND WATER AND SURFACE WATER, AND OTHER SOURCES OF CONTAMINATION (40 CFR 264.94(b)(1)(vi) and (2)(vii))

In order for "benchmark" levels of contamination to be set, the background levels ofhazardousconstituents in the ground watershould be determined in every ACL demonstration. Ifsurface water exposure tothe ground-water contaminants is part of the ACL demonstration, the background levels of the ground-water contaminants in the surface water must also be determined. Ifthe ground water and surface water sampled for background levels appear to be contaminated, the facility owner or operator should examine the possibility of other sources of contamination in the vicinity of the facility. This chapter discusses the type of background water quality data that should be submitted in an ACL demonstration in order to adequately assess the cumulative impacts associated with any contamination emanating fromthefacility. BackgroundWaterQuality
For ACL purposes, background water quality is the quality that would be expected to be found ifthefacilitywas notleaking contaminants. Careful planning must be used in deciding where representative background water samples should be taken. UnderSection 264.97, the regulationsspecify a procedure forestablishing background levelsfor hazardous constituents for purposes ofsetting ground-water standards. Essentially, background monitoring wells must yield ground-water samples from the uppermost aquifer that represent the quality of ground water that has not been affected by leakage from a facility's regulated unit. For most sites, this is an upgradientarea that can be determined readily from the water level data. The permitapplicant is directed tothe Draft RCRA PermitWriters' Manual for Ground-Water Protection (U.S. EPA, 1983a) andthe RCRA Ground-Water Monitoring Technical Enforcement Guidance Document {U.S. EPA, 1986a) for further guidance on ground-water monitoring and station locations. Background surface water

8-1OSWER Directive9481.00-6(
quality must be assessed only in cases where surface waters receive contaminated ground-waterdischarges (see Chapters VII and X).
The permit applicant should submit a site map that identifies the location of background sampling stations and monitoring wells, and the direction of both stream flow and ground-water movement. Any flood discharge pathways and directionsshould also be shown on thesite map.
The permitapplicant may find historical ground-water monitoring studies and ambient surface water monitoring programs to be useful when assessing background water quality. The U.S. Geological Survey, U.S. EPA, State, and local environmental program offices can be good sources of historical data. The background concentrations in both ground waterand surface waterofconstituents for which ACLs are being proposed should be included in a summary table. The uppermost aquifer and any surface water bodies that receive contaminants should be listed separately. Ifadditional monitoring studies are necessary for determining background water quality, the EPA Regional Office may assist by reviewing the monitoring work plans. Regardless ofthe source ofthe background water quality data, the permit applicant should submit quality assurance and quality control information on sample collection, sample analysis, well construction, and environmental conditions. Documents from which any data were taken should be available for review iftheyare requested bythe permitwriter.
Ground-WaterContamination Sources
The permit applicant should investigate other sources of ground-water contamination ifbackground monitoring wells exhibit contamination and the ACL demonstration is based on attenuation arguments. If no contamination is found, or the POE is set at the POC, the permit applicant can omit the following discussion and proceed to the surface water discussion. The types of upgradient pollution sources and the impacts ofthe contamination on ground-water use are important and should be considered. Identifying potential pollution sources is necessary in orderto assessthe cumulative impactof pollution sources on human health and the environment. The following potential pollutionsources should be identified within afive kilometer radius ofthesite:

8-2OSWER Directive9481.00-6C

1. Other RCRA facilities and Superfund sites, 2. Solid waste management units, 3. Industrial areas, 4. Deep well injection sites, and 5. Agricultural areas. Likely sources of contamination should be delineated on a map with an appropriate scale. The distance of each source from both the facility and the upgradient monitoring wells should be discussed. Available ground-waterdata on any ofthe identified sources should be submitted and discussed. Some areas may have hazardous constituents present in the ground water because of natural processes occurring in the ground water. For example, some metals may be found atfairly high levels in certain ground waters. However, natural sources ofsynthetic organic compounds (e.g., chlorinated solvents) are not expected. If synthetic organic compounds are found in background samples, then the permit applicant should attemptto identifythesource ofcontamination.
The water-use impacts from the contamination should be discussed by the permit applicant if upgradient ground water is impaired by any source of contamination. In Chapter VII of this guidance, the current and future uses of groundwaterare discussed in more detail.
Surface WaterContamination Sources
The permit applicant should examine other sources of surface water contamination ifthe applicant's facility discharges to a surface water resource and detectable quantities of contaminants are found in the surface water. Consideration should be given to both point and nonpoint sources of contamination. Any pointsources of pollutant loading to surface waters should be identified on an appropriatelyscaled map. The pointsourcesshould include:
1. Dischargesfrom industrial facilities, 2. Dischargesfrom Publicly OwnedTreatmentWorks(POTW), and 3. Pastwaste discharges.

8-3OSWER Directive 9481 00-6C

The permit applicant should submit a table that includes the name of each pointsource and thewater body intowhich the pointsource discharges. The NPDES permitnumberofeach pointsource should also be included in thistable. Ifthey are available, the discharge rate, load allocations, permit discharge conditions, and mixing zones should be provided and discussed. The applicant should focus these discussions around the impactofthefacility's discharge on these factors. Any nonpoint sources of pollution to surface waters that may affect the ACL decision should also be discussed. The permit applicant should submit information on:
I. Urban storm run-off, 2. Agricultural run-off, 3. Ground-water infiltration,and 4. Other RCRA facilities and Superfund sites.
Actual monitoring data may be submitted along with loading model calculations, iftheyare applicable.

8-4OSWER Directive 9481.00-GC

CHAPTER IX
POTENTIAL HEALTH RISKS (40 CFR 264.94(b)(1)(vii) and (2)(viii))

A health risk assessment should be included in an ACL demonstration if human exposure tothe ground-watercontaminants is likely. The applicant need notassess possible health risks in detail ifthe probabilityofexposure can be shown to be quite low; such acase may arise ifthe pointofexposure is set atthe pointofcompliance.
The purpose of the health risk assessment is to determine allowable human exposure concentrations at a point of exposure forthe constituents for which ACLs are requested. These allowable exposure concentrations can be proposed as ACLs ortheycan be used as a basisto calculate theACLs atthe pointofcompliance.When determining potential health risks, certain assumptions are usually made when complete data on specific human effects are lacking. Both the information necessary to sufficiently support proposed allowable exposure concentrations and the areas where assumptions may be necessary are discussed in this chapter. The applicant may find the EPA document Superfund Public Health Evaluation Manual (U.S. EPA, 1986b) useful in making these determinations.
There are four components to an evaluation of health risks. An exposure assessment is thefirst component ofa health risk assessment. The exposure analysis must identify and describe the current and potential human exposure pathways. The second component, usually referred to as hazard identification, involves a qualitativeassessment ofwhetheror notachemical poses a hazard to humans. The third component, the dose-response assessment, attempts to quantitatively describe the expected human response to a given dose ofa hazardous constituent, typically relying heavily on data from long-term animal studies. The final component is the risk characterization step. For purposes ofdeveloping ACLs, this step involves the integration of the three previous steps to determine allowable human exposure levels for the ACL constituents. Depending on the results of the environmental risk analysis described in Chapter X, the allowable human exposure levels can be used astheACLs orasthe basisto calculate theACLs.

9-1OSWER Directive9481.00·6C
Exposure Assessment
The exposure assessment should followthe Agency's final guidelines (U.S.EPA, 1986c). The types of likely human exposure pathways that should be investigated include:

1. Drinking water exposure from either a ground-water or a surface water source, 2. Ingestion of contaminated food (e.g., aquatic organisms, agricultural products), and 3. Inhalation ofvolatile compounds.
The type of information needed tosatisfythe health risk requirementdepends on the exposure pathway. As an example, if the facility property boundary is located adjacent to a surface water body that is a source of drinking water and a sustained fishery, and the contaminated ground water is discharging into this surface water body, then the health risk information must be based on exposure from the consumption of contaminated ground water and aquatic organisms. In this case, the Ambient Water Quality Criteria for the protection of human health from the consumption of contaminated water and aquatic organisms could be proposed directly as the ACLs (i.e., POE= POC). If an exposure pathway is from a ground- water source of potential drinking water, the health assessment must address the consumption of contaminated drinking water. In this case, the healthbased drinkingwater levels could be proposed as theACLs.
The inhalation exposure pathwayshould be considered in cases where volatile compounds are either likely to volatilize from the contaminated ground water during use (e.g., during showering) or can be expected to penetrate subsurface structures such as basements. The permit applicant should comment on the probability of the occurrence of these two types of exposures. The applicant will have to address inhalation in the health assessment in those situations where the use of ground water or the presence of subsurface structures allows for probable exposures. It should be noted that the Agency is currently examining air releases from hazardouswaste facilities. Ifstandard inhalation assumptions are developed, they may be appropriateforACLdecisionsconcerned withthe inhalation pathway.

9-2OSWER Directive9481.00-6(

The location ofthe potential points ofexposure is discussed in Chapter I. The potential point ofexposure tothe ground-water contaminants is assumed to be at the facility's waste management boundary, the plume boundary, or the property boundaryfor mostcases in which an ACLapplication is being prepared.
Ifthe applicant is proposing thatthe POE be atthe facility property boundary and there are drinking water wells nearby, or some contaminants will be discharging into a surface water body, then the possibility of multiple exposures should be considered. For instance, ifsome members ofthe local population work: in a factory where they are exposed to substance X, and the applicant is proposing an ACL forthissubstance withthe POE atthefacility boundary,the applicantshould account for that workplace exposure in determining an allowable dose for the population. Similarly, if the potentially exposed population uses water that contains some naturally occurring compounds, the interaction of ambient constituents and the constituents for which the ACLs are proposed should be accounted for intheACL demonstration.
If any population or subgroup is or will be exposed to the ACL constituents, the cumulative effect of multiple exposures should be accounted for whenever possible. The likelihood ofsuch exposure occurring underthe Agency's ACL policy is small, but in some situations this may be a factor of concern. The National Center for Health Statistics, U.S. Departmentof Health and Human Services, may be a good source ofinformationon sensitive populationsorindividuals.
Hazard Identification
The two principal sources of data for determining hazard potential are epidemio-logical studies and experimental animal studies. The permit applicant is responsible for providing information on health effects of the hazardous constituents present in the ground waterforwhich ACLs are requested. This can be as simple as providing the Agency maximum contaminant levels (MCLs), reference doses (RfDs), or risk specific doses (RSDs) for the ACL constituents. Appendix D of this document contains a survey sheet on health effectfactors that can be used to summarize thetaxies information.



### 9-3OSWER Directive 9481.00-6C

If an ACL constituent has no MCL, RfD or RSD, then the applicant should perform a comprehensive literature search for health effects data on that contaminant. The applicant should distinguish between ground-water contaminants having threshold (toxic) and nonthreshold (carcinogenic) effects. The permit applicantshould discuss any other effects associated with the contaminants, including odor and taste, mutagenic, teratogenic, reproductive, fetotoxic, and synergistic or antagonistic effects. A reference citation and a summary should be submitted for each study that was used to determine the type of effect for each contaminant.
In the development ofACLs, the permit applicant may find that the grouping of hazardous constituents is a useful simplification. The applicant may investigate health effects data developed for entire classes ofcompounds, such as polynuclear aromatic hydrocarbons (PAHs), halomethanes, or polychlorinated biphenyls (PCBs). as well as compound-specific data. Grouping of constituents should follow a twostep process. First, the applicant should perform a qualitative assessment of the constituents' physical, chemical, and toxicological properties. Second, a quantitative analysis should be performed to determine the most hazardous compound within each group identified in thefirst step. This step will be examined in moredetail in thedose-response section ofthischapter.
The qualitative assessment should include an initial screen of the structure activity relationships (SAR) of the constituents. Constituents with similar physical and chemical structures may be initially grouped together. These groupingsshould then be further evaluated on theirtoxicological effects. The predominant adverse biological effectfor each constituent in each SAR group should be identified. The final grouping(s) should contain constituents with similar physical and chemical structures, and should result in the same toxic endpoints. This subgroup could form the basis for the exposure assumptions used in deriving the allowable concentrations forthe ground-water contaminants. All ofthis information should be presented in tabularformtofacilitate easy reference. Dose-Response Assessment
This assessment attempts to quantitatively describe the expected human response to a given dose of a hazardous constituent, typically relying heavily on



### 9-4OSWER Directive 9481.00·6C

datafrom long-term animal studies. These animal studies are usually based on exposures at high dose levels, often orders of magnitude higher than doses encountered by humans. Therefore, mathematical models are used to estimate allowabledose levels from low-dose extrapolations. The mathematical modelsyield reference doses (RfDs) for toxic compounds and potency factors (PFs) for carcinogenic compounds. The RfD refers to the amount of the chemical to which humans can be exposed on adaily basis over long periods oftime without suffering an adverse effect. A PF represents, in most cases, the largest possible linearslope at the 95% upper confidence limit of low extrapolated doses that is consistent with thedose- response data.
If Agency-compiled data on threshold (toxic) contaminants are not available, then the applicant can submit dose-response information reflecting the acute, subchronic, chronic, and "no-effect" levels for the threshold contaminants. Body surface area and weight, and absorption and excretion rates may be assumed to estimate equivalent oral doses based on data from inhalation or dermal exposure studies.
A conservative approach to risk assessment that includes groupings of compounds could reduce the amount ofdata needed to quantify potential human health effects. The quantitative step for grouping compounds consists of identifying the most potent compound in each group identified in the hazard identification step. For toxic compounds, the most potent compound would have the lowest RfD. Forcarcinogens, the compound with the lowest RSD, or highest PF, would be considered the most potent. This conservative number can then be applied to each compound within the group. It must be emphasized that this quantitativedetermination should only be performed afteraqualitativeassessment ofthe chemical, physical, and toxicological properties of each compound has been performed.
As a simplified example of the grouping process, assume that an applicant's contaminant plume contains PAHs. After analysis, the PAHs are found to be dibenzo(a,h)anthracene, benzo(j)fluoranthene, benzo(a)anthracene, benzo(b)- flouranthene, and benzo(a)pyrene. Since all ofthese aresuspected carcinogensand their chemical structures are similar, these five could be qualitatively grouped. A literature search would reveal that benzo(a)pyrene is the most potent carcinogen,

9-5OSWER Directive 9481.00-GC

and a RSD could then be obtained from EPA forthiscompound. The RSD could then be applied to the other four PAHs to calculate allowable exposure levels. To be acceptable to the Agency, this example would require much more supportive information and a numberofreferences.
Risk Characterization
Allowable exposure concentrations can be derived by using MCLs or applying appropriate exposure assumptions to established RfDs or PFs, or alternate dose levelsderived from the literature ifestablished dose levels are notavailable.
When deriving an ACL for those ground waters that are current or potential sources of drinking water, MCLs set under the Safe Drinking Water Act should be examined for use in setting the allowable concentration level at the point of exposure (POE). In those cases where a MCL does exist for the particular contaminant, and the potential exposure route is human drinking water, the MCL should normally be used as the allowable POE concentration. If a MCL does not existfora hazardous constituent, then either RfDs or RSDs should be used to set the allowableexposure levels.
There are othercircumstanceswhere itmay be necessaryto applyan allowable exposure level other than the MCL at the POE. The circumstances depend on the site-specificfactors involved and includethefollowingscenarios:

• Where there are multiple contaminants in the ground water at a site, individual contaminant levels may have to be set belowthe MCLin order toachieve an overall carcinogenic risk level inthe range of 10-4and 10-7.

• Where drinkingwater is butone ofthe possible exposure routes.
In the first case, the permit applicant should examine the RSD or RfD value and consider using some form ofadditivitytoderivethe appropriate POE concentration. In the lattercase, the MCLshould be compared tothe allowable exposure levels for the other potential receptors. The level at the POE should be set at the lowest allowable exposure level in orderto protectall potential receptors.

9-6OSWER Directive9481.00-6C

The permit applicant should use generally accepted standard factors in the exposure assessment, some of which are listed in Appendix E. The allowable exposure level can be calculated for toxic compounds using the RfD and the exposure assumptions ofa 70 kg adult consuming two liters ofwater per day. The following formula should be used to calculate the allowable exposure concentration:
RfD Exposure level (mg/1) 

# = RfD x70 

### 2
A potency factor (PF) is used to estimate a risk specific dose for a hazardous constituent concentration thatcorrespondsto a particular statistical lifetime cancer risk value. For example, a contaminant concentration corresponding to a lifetime cancer risk of 10-6, assuming thata70 kg adultconsumestwo liters ofwater perday, is estimated bythefollowing formula:
RSD Exposure level (mg/1) 

# = 70 x 10-6 2XPF
In general, the Agency has made decisions to allow concentrations of carcinogens where the individual risk values have been within the range of 10-4 to 10-7. In setting ACLs, the following factors should be considered in choosing a risk level withinthe I0-4to 10-7 range:


### I. Other environmental health factors borne by the affected population, and 2. Level ofuncertainty in thedata base and models used inthe risk analysis.
As ageneral matter, a level of I0-6, the middle ofthe range, should be used as the pointofdeparturewhen proposing a risk level within the 10-4to 10-7 range fora particular facility. Justification should then be provided for using a different level ofrisk.
In orderto account for cumulative impacts ofthe hazardous constituents for which ACLs are requested, an assessment ofthe existing concentrations ofthe ACL constituents in the potentially affected ground water or surface water should be



### 9-7OSWER Directive 9481.00-6C
performed. This data is necessary to determine the total concentration ofthe ACL constituents in the affected water resource, the health effects associated with the concentrations, and the relative exposure contribution of the ACL constituents emanating from the site to the total exposure concentration. At a minimum, an additive approach based on contaminantsthat producethe same adverse effects by similar mechanisms should be used to estimate health effects from exposure to mixturesofcontaminants.
The applicant should submit an allowable health effects exposure level for each ACL constituent. The health-risk assessment should be based on conservative health based numbers. Table I lists the compounds for which RfDs and RSDs have been derived by the U.S. EPA at the time of publication of this document (their derivation is an ongoing process). The RfD summary sheets and the RSDs can be obtained by contacting the Health Assessment Section, in the Office of Solid Waste atthe U.S. EPA in Washington D.C. (Phone number: 202-382-5219). Ifthe applicant uses less conservative numbers as a basis for the health risk assessment, the applicant must submit information tojustifythe use ofthese numbers. As discussed previously, the allowable exposure levels for a group of constituents can be based on thetoxicity ofthe most potentconstituentwithin that group ifsuch a grouping is sufficientlyjustified. Ifsufficienttoxicity information on any compound has not been submitted, and the compound cannot be grouped, the concentration limit will be set atthe background level or atthe maximum concentration listed in Table 1ofSection 264.94(a) ofthe regulations.

9-8OSWER Directive9481.00-6C

TABLE 1
A. CHEMICALSWITH POTENCY FACTORS

Acrylonitrile Aldrin Aniline Arsenic Benzene Benzidene Benz[a]anthracene Benzo[a]pyrene Bis(2-chloroethyl)ether Cadmium Carbontetrachloride Chlordane Chlorinated ethanes 1,2-Dichloroethane 1,1,2,2-Tetrachloroethane Chloroform ChromiumVI DDT Dibenzo(a,h)anthracene 3,3'-Dichlorobenzidine 1,1-Dichloroethylene (Vinylidenechloride) Dichloromethane

(Methylene chloride) Dieldrin 2.4-Dinitrotoluene 1.4-Dioxane Ethyleneoxide Hexachlorobenzene Hexachlorobutadiene 3-Methylcholanthrene 4.4-Methylene-bis-2-chloroaniline N-nitrosopyrolidine Pentachloronitrobenzene Pronamide 2.4,6-Trichlorophenol Tetrachloroethylene Trichloroethylene Vinyl chloride

9-9OSWER Directive 9481 .00-6C

TABLE 1(continued)

B. CHEMICALSWITH VERIFIED REFERENCE DOSES

Acetone Aldrin Allyl alcohol Antimony Barium Beryllium Bis(2-ethylhexy1)phthalate Bromodichloromethane Bromomethane n-Butane Carbon Disulfide Carbon Tetrachloride Chlordane Chlorobenzene Chlorodibromomethane Chloroform Crotonaldehyde Cyanide (free) 2,4-D DDT D-n-butylphthalate 1,2-Dichlorobenzene Dichlorodifluoromethane 1,1-Dichloroethylene 2,4-Dichlorophenol Diethylphthalate 2,4-Dinitrophenol Disulfoton Diphenylamine

9-10 Ethylacetate Ethylbenzene Ethyl Ether Famphur Fluoride Heptachlorepoxide Hexachlorobutadiene Hexachlorocyclopentadiene Isobutyl alcohol lsophorone Mercury (inorganic) MethyleneChloride Methanol Methyl ethyl ketone Methyl isobutyl ketone Nitrobenzene Pentachlorobenzene Pentachloronitrobenzene (PCNB) Pentachlorophenol Phenol Pyridine Silver Styrene 1,2,4,5-TetrachIorobenzene Tetrachloroethylene 2,3,4,6-Tetrachlorophenol 2,4,5-Trichlorophenol