OSWER Directive 9481.00-GC

CHAPTER II

PHYSICALAND CHEMICAL CHARACTERISTICS OFTHE WASTE CONSTITUENTS (40 CFR 264.94(b)(1)(i) and (2)(i))

The firststep in any ACL demonstration isto identifythe chemicals ofconcern. These "hazardous constituents" are chemicals listed in Appendix VIII of Part 261 of the regulations that have been detected in the ground water and may reasonably be expected to be related to the hazardouswaste facility. The applicant must also account for the degradation products of all ACL constituents, especially if those products have significanttoxicological properties. Once the hazardous constituents are identified, an ACL demonstration based on attenuation arguments must determine the physical and chemical characteristics ofthe constituents in order to effectively determine their transport through the environment and their ultimate fate. This chapter discusses the data that is needed to adequately characterize the physical and chemical propertiesofthe hazardousconstituents.

The permit applicant should already know which hazardous constituents are present in the ground water at the facility by the time an ACL demonstration is being considered. The §270.14(c) permitting requirements specify that the permit applicant must determine the extent of ground-water contamination when a significant increase in aground-watercontaminantoccurs atthe compliance point. Additional ground-watersample collection and analysis is usually not necessary for ACL purposes.

The hazardous constituents of concern during the permitting process can be any ofthe 375 contaminants listed in 40 CFR Part 261, Appendix VIII. The Agency does not require sampling for Appendix VIII substances that are unstable in ground waterorforwhich no EPA-approved analytical method exists (U.S. EPA, 1984b). The permit applicant should be aware that the Agency has proposed changing Appendix VIII monitoring requirements for ground water (July 24, 1986; 51 FR 26632). When this proposal isfinalized (expected in June 1987), theAgency will use this list along with any site-specific additions to the list made by the Regional Administrator as the basis for detection and compliance monitoring programs.

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Currently, an Appendix VIII analysis is required whenever any leakage from a facility's unit is detected by §§264.98 and 270.14(c)(4) monitoring. Assuring the absence of particular hazardous constituents emanating from a regulated unit is difficultto dosimply by record-keeping. Wastes otherthan those that are currently received might have been placed in the unit in pre-record-keeping times. In addition, the potential exists for unpredicted reactions between the constituents and theformation ofdegradation products.

The fulfillment ofthe §270.14(c) permitting requirements should result in the spatial characterization ofeach hazardousconstituentfound atthesite. The permit applicant should submit, as part of the ACL demonstration, the data gathered to satisfy these requirements and present the information in terms of threedimensional representations ofconstituentconcentrations. The three- dimensional representation of ground-water contamination may not necessitate threedimensional modeling ofthe contaminant plume. Atwo-dimensional model in the vertical and longitudinal planes may be sufficient in many cases, if the site hydrogeology is fairly homogeneous, and if sufficient monitoring data exists to describe the plume. See Chapter IVforfurtherdiscussion ofappropriate models.

The permit applicant should also submit, as part of the ACL demonstration, information on the chemical and physical characteristics of the wastes in the regulated unitthatwas gathered pursuant to §264.13. This data will give the ACL reviewera better understanding ofwhat may be expected toshow up in theground water. Additional waste constituent analyses may not be needed for the ACL demonstration ifthe applicant has fulfilled the

**requirem~!nts** of§264.13.

Several physical and chemical characteristics of hazardous constituents are critical to the modeling of contaminant transport in ground water. Permit applicants should submit data on the following characteristics of the constituents forwhich ACLs based on fate and transportare requested: density, solubility, vapor pressure, viscosity, valence state, and octanol-water partitioning coefficient. For example, consider a facility that is leaking a hazardous constituent at a concentration level near or above the constituent's solubility level. In this case, there is agood possibilitythatatwo-phase plume could result. One phase would be thedissolved constituent plume in the groundwater, and theotherwould consist of relatively pure hazardous constituent. This latter phase c:ould either be floating on

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thewatertable orsinking to an aquitard, depending on its density. The two phases would probably move atdifferent ratesduetoviscosity and densitydifferences.

Even when only one phase is present, the transport model results are dependent on the physical and chemical characteristics of the constituents; attenuation parameters for transport models depend on specific characteristics of the hazardousconstituents. The permitapplicantshould submit in tabularform the density, solubility, vapor pressure, viscosity, and octanol-water partitioning coefficient values ofthe hazardous constituents. The ability for one constituent to mobilize other constituents should also be investigated. Appendix C contains an example ofasummary sheetthatcan be used to listthe importantpropertiesofthe ACL constituents.

An ACL demonstration that is based on attenuation should be supported by data on fate-related characteristics of the ACL constituents. If a permit applicant argues that the presence of an ACL constituent at the point of compliance is not likely to cause exposure because it is not persistent in the ground water, then special fate and stability related characteristics of the constituent should be discussed in theACLdemonstration.

The stability of waste constituents in the subsurface environment can be affected by chemical, biological, and physical processes. Important chemically mediated subsurface processes may involve oxidation, reduction, and hydrolysis. Important biologically mediated processes include biodegradation and biotransformation reactions. The subsurface physically mediated processes can involve ion exchange, precipitation, and complexation reactions. If the ACL demonstration is based on any of these processes, then the results of site-specific tests should be submitted. Most of the degradation processes depend on the properties of contaminants as well as environmental factors such as microbial populations, solid surfaces, and dissolved constituents present in the ground water. Because the relevant environmental factors are unevenly distributed in nature, degradation and reaction rates are notconstant in ground-waterenvironmentsand must be assessed on asite-specific basis. Therefore, the use ofgeneral information gathered from the literature will be of limited value when assessing the stability of waste constituents.

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Degradation properties of the ACL constituents are also important in determining the effects of the constituents. Since deqradation products can be
more hazardous than the parent compound, all known and likely degradation products should be discussed when describing the characteristics of the waste constituents.
Grouping ground-water contaminants according to stability characteristics may be possible. Ifsite-specifictests supportthe grouping of constituents, then the fate and mobility of each constituent within a group can be based on the stability characteristics ofthe most mobile and most persistent compounds m the group. This would result in the fate and mobility coefficients for each constituent being set at thecoefficientvalues forthe most mobile and moststable compounds in the group. Before grouping the constituents, the applicant should investigate possible degradation products associated with the constituents of concern. The ability of
one contaminantto facilitate or hinderthe movement ofanother contaminant (cosolvent effects) should also be accounted forwhen possible. Although it is difficult todecidewhich groupingsofconstituentsare appropriate, grouping can reduce the amount of predictive modeling necessary for quantifying environmental concentrationsand exposure pathways (see U.S. EPA 1986c).

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CHAPTER Ill

HYDROGEOLOGIC CHARACTERISTICS (40 CFR 264.94(b)(1)(ii)and (2)(ii))

A general description of the hydrogeological characteristics of the facility is needed in all ACL demonstrations. A more detailed assessment of ground-water movement nearafacility is essential to ademonstration based on attenuation. The main route of exposure to ground-water contaminants usually involves the movementofthe hazardous constituentsthrough the soil tothe ground water and
on to an existing or potential point of use. This chapter describes the information needed to adequately determine the hydrogeologic properties required for characterizing ground-watermovementatasite.
Duringthe general RCRA permitting process, the permit applicant is required under §270.14(c) to identify the uppermost aquifer. The uppermost aquifer is defined under §260.10 as the geologic formation nearest the natural ground surface that is an aquifer as well as lower aquifers that are hydraulically interconnected with this aquiferwithin thefacility's property boundary. Saturated
zones above the uppermost aquifer are also of interest as contaminant migration pathways.Therefore, for an ACL demonstration based on attenuation, information
on the geologicand hydrologicpropertiesofeach ofthe individualstrata beneath a facility, thatare likelyto influence ground-watercontaminant migration, should be submitted. This data is needed to adequately characterize ground-watertransport mechanisms. Much ofthe data should already be available to the permit applicant ifthe Section 270.14(c) requirements have been fulfilled.

The importantgeologicattributesofafacility include:

1. Soil and rock characteristics, 2. Geologicstructure, and 3. Geomorphologyandtopography.



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When describing the soil characteristics of a facility, the permit applicant should use the Unified Soil Classification System or the U.S. Department of Agriculture's soil classification system. Each soil type beneath the site and within the areal extent of the ground-water contaminant plume should be investigated. The permitapplicantshould submitdata describing thethickness, areal extent, and hydraulic properties of each soil type. The soil information should be submitted in both tabular and graphic form. The areal extent ofsoil types should be presented
on a map with a scale no greater than one inch:200 feet. The applicant should submitcopies ofdrilling and boring logs from monitoring and waterwellsthat have been installed.

Ifthe applicant uses soil or other matrix attenuation mechanisms tojustify the ACL, the additional data and calculations used to define the attenuative properties should be submitted. The following attenuation mechanisms may be relevantto an ACLdemonstration:

1. Dispersion, including hydrodynamicdispersion; 2. Retardation, including all sorptive properties; and 3. Degradation, including mechanisms of biodegradation, oxidation, reduction, and hydrolysis.

The permit applicant should submit data describing the organic and mineral content,the cation and anion exchange capacity, and the grain size ofeach soil type in the area ofcontamination. Aquifer matrix characteristics that affectthe stability ofthe ACL constituents (see Chapter II) should also be described ifthey are used to support attenuation claims. The results of tests to substantiate any attenuation claims should be submitted by the applicant. Likewise, sampling and laboratory procedures used to determine the attenuation propertiE!S should be presented and results tabulated. Brady (1974) and Freeze and Cherry (1979) provide in-depth discussions of these specific soil characteristics, and the permit applicant and reviewershould consultthese references forassistance.

The permitting authorityand the applicant should keep in mind the outcome of the different types of attenuation. Although matrix effects (binding) and dispersion act to lower the concentration of the contaminants at the point of exposure, they do notdirectly reduce theirtotal mass. Constituents that attach to

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the matrix may become mobilized at a later time. However, degradation mechanisms act to eliminate the contaminants themselves. This form of attenuation is permanent and, assuming the products are harmless, can be a final solutiontoeliminatingthecontamination.

If the ACL demonstration is attenuation-based, the permit applicant should submitaset of maps that adequately depictthe subsurface stratigraphy. The nearsurface stratigraphic units in the zone ofsaturation thataffect or are likelyto affect ground-water contaminant migration should be described. The areal and vertical extent of the hydrogeologic units can be presented in several ways. For complex settings, the most desirable presentation is a series of structural contour maps for thetop orbottomofeach unit. Vertical sections and isopach maps can also be used since they are generally more graphic and are useful as supplements to the structural contour maps. Because the construction ofany ofthese diagrams involves interpolation and extrapolation ofdata,the diagrams should show the location of control points and the corresponding value at each control point. The site maps should includethedepth,thickness, and areal extentofeach stratigraphic unit. The maps should also adequately depict all stratigraphic zones and lenses within the near-surface zone of saturation. The site-specific stratigraphic maps should be detailed and have ascale no greaterthan one inch:200 feet. The applicant should also submit regional stratigraphy maps in order to show unique regional characteristics and their relationshipstothe site and tojustify claims concerning the ultimate fate of a contaminant plume. A table that summarizes the subsurface geologicdatashould be submitted.

Each of the stratigraphic units located in the zone of saturation should be characterized for the hydrologic parameters of hydraulic conductivity (vertical and horizontal), specific yield (unconfined aquifer) or specific storage (confined aquifer), and effective porosity. Hydraulic conductivity and porosity of aquifer material can be determined by using laboratory or field methods. It is recommended thatall tests thatare conducted to definethe hydraulic propertiesof each stratigraphic unit be performed in the field. Laboratory tests may be used to substantiate field test results but should not be the sole basis for determining aquifercharacteristics. Only in special caseswillthesubmittal oflaboratory analyses be considered adequate for describing aquifer characteristics. Literature value estimates forthese parameterswill rarely be acceptable.

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Each ofthe hydrologic parameters can vary from point to point, even within the same aquifer. Therefore, the areal variations of the parameters within the stratigraphic units should be characterized. The amount of data necessary to characterize a stratigraphic unitwill increase with the increasing heterogeneity of the unit. As an example, an aquifer of extensive homogeneous beach sand will require less investigation than a glacial unit consistinq of lenticular deposits of outwash sand and gravel interbedded with clayey till. However, in order to save time and effort, the applicant may choose to makE! simplified "worst case" assumptions ofhydraulicconductivity and porosity.

There are many field methods for measuring hydraulic conductivity and porosity. Hydraulic conductivity is most effectively determined from the analysis of pump test data. For units having low hydrologic conductivity, single-well tests are generally used (i.e., aslug test). For unitshaving high hydraulicconductivity, multiwell pumping tests are necessary. The pump test methods are normallydesigned to evaluate the transmissivity and storativity (storage wefficient) of the aquifer. Hydraulic conductivity is determined by dividing transmissivity by the aquifer thickness. More information on determining aquifer characteristics can be found in Freeze and Cherry (1979), Kruseman and De Ridder (1979), U.S. EPA (1983a), and Walton (1970).

Differentlaboratory methodscan be used tosubstantiate field data. Hydraulic conductivity may be determined on a core sample of the aquifer by using either a constant-head orafalling-head permeameter. A description ofthe method can be found in Todd (1980) and Bouwer (1978). In the laboratory, effective porosity can be determined as the ratio of the volume of water yielded by gravity flow to the volume ofsoil or rock material.

If an aquitard separates two distinct ground-watN zones, then the physical and hydrauliccharacteristics ofthataquitard must be provided in sufficientdetailto illustrate the degree of interconnection between the two aquifers. This requirement can be fulfilled by providing the results of an aquifer pump test designed to show the effect ofthe pumping ofthe deeper aquifer on the shallow aquifer. The shallow aquifer will exhibit significant drawdown during the pump test ifthetwo aquifersare interconnected.

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A summary of the hydraulic properties of each stratigraphic zone within the
zone ofsaturation should be submitted by the permit applicant. This data should be provided in a table that includes the aquifer name, stratigraphic zone, vertical conductivity, horizontal conductivity, specific yield, transmissivity, and storage coefficient.



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CHAPTER IV

GROUND-WATER FLOW DIRECTION AND QUANTITY (40 CFR 264.94(b)(1)(iii) and (2)(iii))

The amountor quantityofground wateratasite and the direction in which it flows are two essential components of an analysis of the fate and transport of hazardous constituents in the ground water. The ultimate fate of contaminated ground water is a principal topic of every attenuation-based ACL demonstration since a contaminant plume may discharge into and mix with other ground or surface waters. This chapter describes methods that can be used to determine ground-water flow direction and quantity at a site. The EPA publication GroundWater Monitoring Technical Enforcement Guidance Document (U.S. EPA 1986a) may also be useful in making these determinations. However, ifafacility is located over a relatively complex hydrogeologic system (e.g., fractured rock or karst aquifers), setting the POE at the POC may be the only acceptable method for establishing theACL.

The primary processes that control the migration of contaminants in subsurface environments include:



### I. Advection (movementofthegroundwater), 2. Hydrodynamic dispersion (mixing of ground water having different levelsofcontamination), and 3. Chemical reactions.

For ACL purposes, advection is defined as the migration of hazardous constituents by actual motion or flow and is generally assumed to be caused by natural ground- waterflow. Consideration of advection alone presents the worstcase calculations in terms of peak arrival times and concentration strengths. Furthermore, qualitative and quantitative evaluation ofadvection in terms offlow pathways is possible.

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The Section 270.14(c) permit requirements specify the submittal of groundwater flow information. This data should be adequate for on-site determinations of flow; however, additional data may be required if off-site determinations of ground- water flow are needed for the ACL demonstration. The permit applicant should evaluate ground-water flow in terms of the flow regime present at the facility. Flow from the facility to the water table will generally occur in the unsaturated zone, although it may go via surface water. Where subsurface heterogeneitiesare notsignificant, itis reasonableto assume thatflowthroughthe unsaturated zone will be predominantly downward. This assumption is justified because gravity isthe primaryforce acting on afluid in the unsaturated zone.

Once in the saturated zone, dissolved constituents will move with the ground water. Evaluation of advective transport in the saturated zone for miscible constituents can generally be based on the observed ground-water flow field and hydraulic properties (hydraulic conductivity gradient and effective porosity). The observed flow field can be determined by acombination ofareal water level maps and vertical sections showing water levels.

Calculations of ground-water quantity will require the use of the subsurface hydrogeologic parameters described in Chapter Ill. Ground-water quantity can be estimated by calculating the porosity of the aquifer. The use of Darcy's Jaw for determining ground-water flow quantity is acceptable and can be found in any standard ground-watertextbook (e.g., Freeze and Cherry (1979)). Darcy's law can be used to calculate specific discharge orvolume rate offlowthrough a cross-sectional
area perpendiculartotheflowdirection in relatively homogenoussystems.

The determination of ground-water flow rates and directions is simple in concept. Ifthe distribution of hydraulic head and the hydrologic properties at the site are known, then aflow netorwater level contour map in conjunction with the
use ofDarcy's lawcan be used todetermineflow rates and directions.

The permit applicant should be aware of a number of factors that can make accurate determination ofground-waterflowdifficult.These include:

I. Lowhorizontal or relativelyflatgradients, 2. High vertical or relativelysteep gradients,

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3. Temporal variations in water levels, 4. Heterogeneousproperties, and 5. Anisotropic properties.

In areas of low horizontal gradients, small errors in water level measurements
orsmall transient changes in water levels can make determination offlow direction and rates difficult. High vertical gradients often exist in surficial units. In recharge areas, head decreases with depth; whereas in discharge areas, it increases with depth. Often, a shallow water table aquifer may overlie an aquifer of higher permeability, resulting in vertical head gradients. A very common mistake is made when water level contour maps are constructed using wells or piezometers at different depths. In such a case, calculated horizontal flow directions may be inaccurate.

Water levels can vary temporally because of short-term stresses, tidal effects, atmospheric pressure variations, seasonal effects, and long-term trends. In determining flow direction, the annually averaged water levels are of primary interest. Consideration of short-term effects is more important at sites with low hydraulicgradients. In evaluating any water level data,the uncertainty introduced by neglecting short-term effects must be estimated. Seasonal variations in recharge
can result in significant water level variations in unconfined aquifers. Artificial recharge and certain types of ground-water pumpage often lead to seasonal changes in water levels. These changes may occur under both confined and unconfinedconditions.

The degree of heterogeneity in aquifers may range from fairly moderate to extreme. The potentiometricsurfaces orwater levels in heterogeneousaquifers are not smooth regular surfaces. At the contact between two geologic materials, the hydraulic gradientwill be discontinuous. For some aquifers, such as fractured rock and karst aquifers, the heterogeneity is much more complex.

Another property of an aquifer is its anisotropy. Hydraulic conductivity is a property that is dependent on direction and therefore has three principal components. Ifthe principal components are equal, then the aquifer is isotropic. If not, the aquifer is anisotropic. For anisotropic aquifers, flow lines are not perpendicular to equipotential lines or water levels. Many aquifers display a

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horizontal-vertical anisotropy. However, if the two horizontal components of hydraulic conductivity a're equal, then, from an areal perspective, flow lines will be perpendicular to lines of equal potential. Aquifers that may demonstrate horizontal anisotropy include aquifers in fluvial sandstone, fractured rocks, or steeply inclined strata. Ground-water flow direction is difficultto determine from water level data in these types ofanisotrophicaquifers.

The factors that make the determination of flow rates and directions unreliable can often be overcome by an expanded effort in water level monitoring. For seasonal variations in water levels, a higher frequency monitoring schedule is necessary. For low horizontal gradients, the effects ofshort-term changes in water levels can be analyzed by installation of continuous rec<>rders in selected wells. In aquifers having significant vertical gradients, piezometers completed at various depths may be required in orderto provide athree-dimensional description ofthe flow field. For heterogeneous and anisotropic aquifers, more water level monitoring wells and morefield tests for hydraulic propertiesare required.

The hydrogeologic portion of the ACL demonstration must include an adequate description of both horizontal and vertical ground-water flow components. This is required because itwill help determine where the hazardous constituents may migrate. The horizontal ground-water flow description should include a flow net based on ground-water elevation measurements taken from monitoring wells or piezometers screened at the same elevation in the same saturated zone. The monitoringsystem must be designed to provide reliable results ofthe ground-water flow direction in the zone of saturation. There may be sites thatwill require the applicantto monitor for hazardous constituents at more than
one ground-waterelevation. When this situation occurs, the permit applicant must be especially careful to ensure thatthe monitoring plan is designed correctly.

Information obtained from analyses of the hydrogeological properties and flow direction will allow the calculation of ground-water flow velocity. Well identifier codes, well depths, screened intervals, ground-water elevations, and sampling data should be presented in tabular form. The flow net data should be graphically portrayed on a site map that includes ground-water elevations, isopleths, and flowvectors. As discussed before,the ground-waterflowvelocity can be determined by a simple modification of Darcy's equation if the aquifer is

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relatively isotropic. All calculations and assumptions should be included in the discussion offlow rates.

Vertical ground-watergradients and flow should also be described. Facilities should have several nested piezometers for vertical gradient determinations. Vertical flow gradient will aid in determining discharge and recharge zones, aquitard characteristics, and whether the monitoring wells are located and screened atthe appropriate depths. The data that should be submitted in tabular form for each well nest includes well identification code, well depth, screened interval, ground-water elevation, and sampling date. All calculations and assumptionsshould be described in detaiL

Facilities that are located in environmental settings that exhibit temporal variation in ground-waterflowdirection should definethe extenttowhichtheflow change occurs. The main causes ofground-waterflowvariation are:

1. Seasonalityofrecharge ordischarge, 2. Ground-waterwithdrawals, 3. Undergroundinjection, 4. Surfacewaterelevation changes, and 5. Artificially induced recharge by basin flooding.

In cases ofseasonal ground-waterflowvariation, the permitapplicantshould provide information describing those temporal changes in ground-water flow direction using records compiled overaperiod ofno lessthan oneyear.

The rate ofwithdrawal ofground water is an importantfactorthat influences ground-water and contaminant movement and exposure to contaminated water. In attenuation-based demonstrations, the rate of ground-waterwithdrawal in the vicinity (5 km radius) of the facility should be summarized in tabular form and should includewell location, depth,type ofuse, and withdrawal rates. The zone of impactcreated by any majorwell orwell field withdrawal should be identified on a site map. The map should include drawdown isolines out to the 30 centimeter drawdown leveL Modeling of drawdown curves should use low recharge assumptionssuch as droughtconditions.

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Models
Although not required for an ACL demonstration, mathematical simulation models of ground-water flow and contaminanttranspClrt can be extremely useful toolsforthe applicant. Models are more appropriate for relatively simple geologic environments where conditions do not vary widely; in complex geologic areas, modeling may be less useful.

The permitapplicant is responsible forensuring thatthe models used simulate
as precisely as possible the characteristics of the site cllld the contaminants and minimize the estimates and assumptions required. All models used in the demonstration should:

1. Be compatiblewiththequalityand typeofinputdataavailable, 2. Have been demonstrated to be applicable to the environmental conditionsatthesite, 3. Have been subjected to an independent quality assurance audit or to a level of professional peer review equivalent to that for publication in a scientificortechnicaljournal, 4. Be internally consistent in the use of boundary and initial conditions, timesteps, assumptions, and code modifications, 5. Have fullydocumentedsupportavailabletotheAgency, and 6. Be calibrated and verified forthesite before being applied in apredictive mode.

As discussed in Chapter II, atwo-dimensional model ofaground-watersystem may be used; however, all simplifying assumptions must be justified. Whenever possible, input parameters and assumptions should be conservative in nature; worst-case scenarios may save much effort. If models are used, the ACL demonstration should includetables ofthe assumptions,the input parameters, and any calibration data.

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