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 support of an 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, if ground-water discharge to surface water 

is an important element of the ACL demonstration, then precipitation events should 

be related to recharge and discharge of ground water. 

Precipitation events are variable and occur with 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 of the amount of precipitation 

for a given geographical area. 

Attenuation-based demonstrations should contain general information on the 

precipitation characteristics of a site. This includes data on rainfall and snowfall, 

expressed as its equivalent in rainfall. The National Oceanic and Atmospheric 

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OSWER Directive 9481.00-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 of this data, the specific time 

period from which the data was collected, and the lo<:ation of the precipitation 

gauge(s) in relation to the 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 

contaminant transport. 

If the ACL demonstration does not involve surface water exposures, then the 

permit applicant can proceed to the next chapter. However, if the 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 of the facility. 

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 discharge during flood conditions. 

The 100-year floodplain 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 for this information. If the 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 effect of these devices on ground-water infiltration and discharge should 

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. 

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OSWER Directive 9481.00-GC 

The ground-water discharge patterns at the 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 of water and reduce significantly 

the amount of infiltration reaching the water table, especially during the growing 

season. 

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OSWER 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 pathways for the ACL constituents through surface and 

ground water. This chapter discusses the location of surface water and groundwater users in the vicinity of the 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 Chapter VII. 

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 

liberally defined to include both spatial and temporal concepts. Linear distance may 

be more appropriate for judging 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 

contaminant transport. 

The level of information necessary to satisfy the proximity of users 

requirement depends on the basis of the ACL. lfthe applicant is attempting to show 

that contaminants will safely attenuate into a surface water body, then data on the 

specific physical attributes ofthe surface water body will be necessary. This includes 

information necessary to estimate the mixing potential and mechanisms of the 

water body. Likewise, if the aquifer is non potable, then data showing that it is not 

fit for consumption must be supplied. 

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OSWER Directive 9481.00-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 the facility's proximity to surface waters and involves: 

1. Identifying each surface water body in the vicinity of the facility, 

2. Determining the distance from the waste management area boundary to 

each surface water body, and 

3. Identifying ground-water discharge pathways to surface waters. 

Each water body within five kilometers downgradient (or downstream) of the 

facility boundary should be identified. The owner or operator of the facility should 

supply an appropriately scaled map identifying each water body. 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 

part of each water body should be provided by the owner or operator of the facility. 

The travel time of the ACL constituents from the facility to the 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 

methods discussed previously in Chapter IV should be used to estimate the hydraulic 

and hazardous constituent loading rates. Actual seepage measurements may be 

necessary to verify model estimates if ground-water discharges are estimated to be 

a significant portion ofthe annual hydraulic load to a water body. 

A greater level of detail on characteristics of surface water bodies is needed if 

the applicant is attempting to show that the contaminants will attenuate safely 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: 

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OSWER Directive 9481.00·6C 

1. Surface area, 

2. Mean depth, 

3. Volume, 

4. Temperature stratification, and 

5. Hydraulic residence time. 

Information on estuarine and marine areas should include: 

1. Surface area, 

2. Mean depth, and 

3. Tidal periodicity and amplitude. 

Pertinent stream and river characteristics are: 

1. Mean width, 

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 order to 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 factors for establishing discharge zones. 

The permit applicant should synthesize this information to support arguments 

of no statistically significant increase in surface water concentrations. The expected 

amount of attenuation and the zones of probable discharge areas should be 

discussed. The permit applicant should be aware that certain States have approved 

surface water models that are used in the NPDES permitting program. If approved 

models are available, they should be used by the applicant to determine discharge 

zones in surface waters. The EPA document Technical Support Document for Water 

Quality-Based Taxies Control (U.S. EPA 198Sa) is a good source of this information. 

In general, one quarter of the cross-sectional area of a stream may be used in 

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OSWER Directive 9481.00-6( 

estimating the discharge zone. This area could then be applied over the width of 

the ground-water contaminant 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 

proximity of ground-water users to the facility. This requires determining: 

I. The distance of the ground-water users' withdrawal point (i.e., well or 

spring) from the facility, and 

2. The hydrologic transport time for the contaminants to reach the closest 

wells or springs. 

The users of ground water within a five kilometer radius of the facility 

boundary should be identified. Both upgradient and downgradient wells should be 

included because of their 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. 

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OSWER Directive 9481.00-6C 

CHAPTER VII 

CURRENT AND FUTURE USES OF GROUND WATER 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 of the use should be considered. A major objective of an 

ACL demonstration relying on transport arguments is to show that ground-water 

contamination at a facility will not adversely affect any water use. The supporting 

arguments for the ACL can center around the fact that the ground-water 

contamination at the facility is not degrading the designated beneficial uses of the 

water resources. This requires the permit applicant to review federal, state, and 

local standards or guidelines that govern the uses of both 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 out the 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 demonstrations should generally discuss 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 the following general categories: 

1. Agricultural- irrigation and animal watering; 

2. Industrial -process, cooling, and boiler water; 

3. Domestic and municipal - potable, bathing, washing, and lawn/garden 

watering; 

4. Environmental - ground-water recharge or discharge, fish and wildlife 

propagation, unique areas; and 

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OSWER Directive 9481 .00-6C 

5. Recreational- fishing, swimming, boating, and other contact uses. 

The permit applicant should examine pertinent aspects 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 the following 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 of ground water are 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 

either of the following two factors; 

a) Irreplaceable--no reasonable alternative source of drinking 

water is available to substantial populations, or 

b) Ecologically vital--the aquifer provides the base flow for a 

particularly sensitive ecological system that, if polluted, 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 

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OSWER Directive 9481.00-6C 

total dissolved solids (TDS) levels over 10,000 mg/1, or are otherwise 

contaminated beyond levels that allow cleanup using methods that 

are reasonably employed in public water system treatment. These 

ground waters also must not migrate to Class I or II ground waters 

or discharge to surface water that could 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 

current sources of drinking water. If the ground water is Class Ill, then health- based 

concerns may be secondary to environmental concerns in the setting of ACLs. ACLs 

in Class Ill ground water will be assessed on a case-by-case basis. 

The permit applicant should discuss the ground-water use in the vicinity of the 

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 for designation of ground water as 

Class I, II, or Ill. These draft Guidelines for classifying ground water were available in 

December 1986. Until this guidance is finalized, we recommend only conservative 

use ofthe "unuseable" criterion as a basis for an ACL. 

Information from the State and/or local government as to the 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 by the State. When ground water has been classified on a State, regional, 

or local level, this classification should be re-evaluated and verified using site 

specific data. The State's review should be included in the ACL 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 not have to be submitted. 

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OSWER Directive 9481.00-6( 

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 

implementing the regulation. The following general use categories should be used 

by the permit applicant in preparing the table: 

1. Drinking water source (including other domestic uses), 

2. Fish and wildlife propagation area, 

3. Industrial or agricultural water source, 

4. Area of special ecological concern, and 

5. Recreational area. 

The surface water use information will aid in determining appropriate ACLs by 

identifying surface water exposures that can occur. The data gathered to fulfill the 

requirements of this section will be used to prioritize the likely exposure pathways 

and to determine whether human health and environment factors should be 

assessed in further detail (see Chapters IX and X). 

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OSWER Directive 9481.00-6C 

CHAPTER VIII 

EXISTING QUALITY OF 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 of hazardous constituents in the ground water should be determined in every 

ACL demonstration. If surface water exposure to the 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. If the 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 from the facility. 

Background Water Quality 

For ACL purposes, background water quality is the quality that would be 

expected to be found if the facility was not leaking contaminants. Careful planning 

must be used in deciding where representative background water samples should 

be taken. Under Section 264.97, the regulations specify a procedure for establishing 

background levels for hazardous constituents for purposes of setting 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 upgradient area that can be determined readily from the water level 

data. The permit applicant is directed to the Draft RCRA Permit Writers' Manual for 

Ground-Water Protection (U.S. EPA, 1983a) and the 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 

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OSWER Directive 9481.00-6( 

quality must be assessed only in cases where surface waters receive contaminated 

ground-water discharges (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 

directions should also be shown on the site map. 

The permit applicant 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 water and surface water of constituents 

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. If additional monitoring studies are necessary for determining 

background water quality, the EPA Regional Office may assist by reviewing the 

monitoring work plans. Regardless of the source of the 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 if they are requested by the permit writer. 

Ground-Water Contamination Sources 

The permit applicant should investigate other sources of ground-water 

contamination if background 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 of the contamination on ground-water use are important 

and should be considered. Identifying potential pollution sources is necessary in 

order to assess the cumulative impact of pollution sources on human health and the 

environment. The following potential pollution sources should be identified within 

a five kilometer radius of the site: 

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OSWER Directive 9481.00-6C 

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-water data on 

any of the 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 

at fairly high levels in certain ground waters. However, natural sources of synthetic 

organic compounds (e.g., chlorinated solvents) are not expected. If synthetic 

organic compounds are found in background samples, then the permit applicant 

should attempt to identify the source of contamination. 

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 

ground water are discussed in more detail. 

Surface Water Contamination Sources 

The permit applicant should examine other sources of surface water 

contamination if the 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 point sources of pollutant loading to surface waters should be 

identified on an appropriately scaled map. The point sources should include: 

1. Discharges from industrial facilities, 

2. Discharges from Publicly Owned Treatment Works (POTW), and 

3. Past waste discharges. 

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OSWER Directive 9481 00-6C 

The permit applicant should submit a table that includes the name of each 

point source and the water body into which the point source discharges. The NPDES 

permit number of each point source should also be included in this table. If they 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 impact of the facility's discharge on these factors. 

Any non point 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, if they are applicable. 

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OSWER 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 to the ground-water contaminants is likely. The applicant need not assess 

possible health risks in detail if the probability of exposure can be shown to be quite 

low; such a case may arise if the point of exposure is set at the point of compliance. 

The purpose of the health risk assessment is to determine allowable human 

exposure concentrations at a point of exposure for the constituents for which ACLs 

are requested. These allowable exposure concentrations can be proposed as ACLs 

or they can be used as a basis to calculate the ACLs at the point of compliance. 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 the first component of a 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 

qualitative assessment of whether or not a chemical poses a hazard to humans. The 

third component, the dose-response assessment, attempts to quantitatively 

describe the expected human response to a given dose of a hazardous constituent, 

typically relying heavily on data from long-term animal studies. The final 

component is the risk characterization step. For purposes of developing 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 as the ACLs or as the basis to calculate the ACLs. 

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OSWER Directive 9481.00·6C 

Exposure Assessment 

The exposure assessment should follow the 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 of volatile compounds. 

The type of information needed to satisfy the health risk requirement depends 

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 drinking water levels could be proposed as the ACLs. 

The inhalation exposure pathway should 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 hazardous waste facilities. If standard inhalation assumptions are developed, 

they may be appropriate for ACL decisions concerned with the inhalation pathway. 

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OSWER Directive 9481.00-6( 

The location of the potential points of exposure is discussed in Chapter I. The 

potential point of exposure to the ground-water contaminants is assumed to be at 

the facility's waste management boundary, the plume boundary, or the property 

boundary for most cases in which an ACL application is being prepared. 

If the applicant is proposing that the POE be at the 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, if some members of the local population work: 

in a factory where they are exposed to substance X, and the applicant is proposing 

an ACL for this substance with the POE at the facility boundary, the applicant should 

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 in the ACL 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 of such exposure occurring under the 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. Department of Health and Human Services, may be a good 

source of information on sensitive populations or individuals. 

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 water for which 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 effect factors that can be used to 

summarize the taxies information. 

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OSWER 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 applicant should 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 of ACLs, 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 of compounds, 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 the first step. This step will be examined 

in more detail in the dose-response section ofthis chapter. 

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 groupings should 

then be further evaluated on their toxicological effects. The predominant adverse 

biological effect for 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 for the ground-water contaminants. All of this information should 

be presented in tabular form to facilitate 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 

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OSWER Directive 9481.00·6C 

data from 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 

allowable dose levels from low-dose extrapolations. The mathematical models yield 

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 a daily basis over long periods of time without suffering 

an adverse effect. A PF represents, in most cases, the largest possible linear slope at 

the 95% upper confidence limit of low extrapolated doses that is consistent with 

the dose- 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 of data 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. For carcinogens, 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 

quantitative determination should only be performed after a qualitative assessment 

of the 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 

d ibenzo(a,h)anthracene, benzo(j)fluoranthene, benzo(a)anth racene, benzo(b)-

flouranthene, and benzo(a)pyrene. Since all of these are suspected carcinogens and 

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, 

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OSWER Directive 9481.00-GC 

and a RSD could then be obtained from EPA for this compound. 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 number of references. 

Risk Characterization 

Allowable exposure concentrations can be derived by using MCLs or applying 

appropriate exposure assumptions to established RfDs or PFs, or alternate dose 

levels derived from the literature if established dose levels are not available. 

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 

exist for a hazardous constituent, then either RfDs or RSDs should be used to set the 

allowable exposure levels. 

There are other circumstances where it may be necessary to apply an allowable 

exposure level other than the MCL at the POE. The circumstances depend on the 

site-specific factors involved and include the following scenarios: 

• Where there are multiple contaminants in the ground water at a site, 

individual contaminant levels may have to be set below the MCLin order 

to achieve an overall carcinogenic risk level in the range of 10-4 and 10-7. 

• Where drinking water is but one of the possible exposure routes. 

In the first case, the permit applicant should examine the RSD or RfD value and 

consider using some form of additivity to derive the appropriate POE concentration. 

In the latter case, the MCL should be compared to the allowable exposure levels for 

the other potential receptors. The level at the POE should be set at the lowest 

allowable exposure level in order to protect all potential receptors. 

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OSWER Directive 9481.00-6C 

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 of a 70 kg adult consuming two liters of water per day. The 

following formula should be used to calculate the allowable exposure 

concentration: 

RfD Exposure level (mg/1) = RfD x 70 

2 

A potency factor (PF) is used to estimate a risk specific dose for a hazardous 

constituent concentration that corresponds to a particular statistical lifetime cancer 

risk value. For example, a contaminant concentration corresponding to a lifetime 

cancer risk of 10-6, assuming that a 70 kg adult consumes two liters of water per day, 

is estimated by the following formula: 

RSD Exposure level (mg/1) = 70 x 10-6 

2 X PF 

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 within the I0-4 to 10-7 range: 

I. Other environmental health factors borne by the affected population, 

and 

2. Level of uncertainty in the data base and models used in the risk analysis. 

As a general matter, a level of I0-6, the middle of the range, should be used as 

the point of departure when proposing a risk level within the 10-4 to 10-7 range for a 

particular facility. Justification should then be provided for using a different level 

of risk. 

In order to account for cumulative impacts of the hazardous constituents for 

which ACLs are requested, an assessment of the existing concentrations of the ACL 

constituents in the potentially affected ground water or surface water should be 

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OSWER Directive 9481.00-6C 

performed. This data is necessary to determine the total concentration of the 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 contaminants that produce the same adverse effects by 

similar mechanisms should be used to estimate health effects from exposure to 

mixtures of contaminants. 

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 

at the U.S. EPA in Washington D.C. (Phone number: 202-382-5219). If the applicant 

uses less conservative numbers as a basis for the health risk assessment, the 

applicant must submit information to justify the use of these numbers. As discussed 

previously, the allowable exposure levels for a group of constituents can be based 

on the toxicity of the most potent constituent within that group if such a grouping 

is sufficiently justified. If sufficient toxicity information on any compound has not 

been submitted, and the compound cannot be grouped, the concentration limit 

will be set at the background level or at the maximum concentration listed in Table 

1 of Section 264.94(a) of the regulations. 

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OSWER Directive 9481.00-6C 

TABLE 1 

A. CHEMICALS WITH POTENCY FACTORS 

Acrylonitrile 

Aldrin 

Aniline 

Arsenic 

Benzene 

Benzidene 

Benz[a]anthracene 

Benzo[a]pyrene 

Bis(2-chloroethyl)ether 

Cadmium 

Carbon tetrachloride 

Chlordane 

Chlorinated ethanes 

1 ,2-Dichloroethane 

1,1 ,2,2-Tetrachloroethane 

Chloroform 

Chromium VI 

DDT 

Di benzo(a,h)anthracene 

3 ,3' -Dich lorobenzidine 

1, 1-Dichloroethylene 

(Vinylidene chloride) 

Dichloromethane 

(Methylene chloride) 

Dieldrin 

2.4-Di nitrotol uene 

1.4-Dioxane 

Ethylene oxide 

Hexachloro benzene 

Hexachlorobutadiene 

3-Methylcholanthrene 

4.4-Methylene-bis-2-chloroaniline 

N-nitrosopyrolidine 

Pentachloronitrobenzene 

Pronamide 

2.4,6-Trichlorophenol 

Tetrachloroethylene 

Trichloroethylene 

Vinyl chloride 

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TABLE 1 (continued) 

B. CHEMICALS WITH VERIFIED REFERENCE DOSES 

Acetone 

Aldrin 

Allyl alcohol 

Antimony 

Barium 

Beryllium 

B is(2-ethylh exy 1) 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 

Ethyl acetate 

Ethyl benzene 

Ethyl Ether 

Famphur 

Fluoride 

Heptachlor epoxide 

Hexachlorobutadiene 

Hexachlorocyclopentadiene 

Isobutyl alcohol 

lsophorone 

Mercury (inorganic) 

Methylene Chloride 

Methanol 

Methyl ethyl ketone 

Methyl isobutyl ketone 

Nitrobenzene 

Pentachlorobenzene 

Pentachloronitrobenzene 

(PCNB) 

Pentachlorophenol 

Phenol 

Pyridine 

Silver 

Styrene 

1,2,4,5-Tetrach I oro benzene 

Tetrachloroethylene 

2,3 ,4,6-Tetrach lorophenol 

2,4,5-Trich lorophenol