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4 The California Site Mitigation Decision Tree Process: Solving the "How Clean Should Clean Be?" Dilemma DAVID J. LEU AND PAUL W. HADLEY One of the greatest environmental issues facing our nation dur- ing this decade is expressed by the cliche "How Clean Should Clean Be?" This cliche refers to the complex problems associated with the mitigation of soils and waters contaminated by chemicals that are produced and used by our modern society. Different federal and state agencies, together with other research and consulting groups, have developed various approaches to this issue. One real- istic approach to answering the question "How Clean Should Clean Be?" has been developed by the California Department of Health Services (DHS). This process is contained in a technical guidance document entitled The California Site Mitigation Decision Tree Manual (DHS, 1986~. At. . . . . . This decision tree manual (also referred to as the decision tree process) was created to fulfill four basic functions. First, it establishes a realistic approach to answering the question of "How Clean Should Clean Be?" Second, it identifies the key The authors of this paper would like to recognize those individuals who made the development of The California Sit c Mitigation Decuon lFcc Marshal possible. The coauthors of the manual include Michael Kiado; William Quan; Stanford Lau; James Polisini, Ph.D., California Department of Fish and Game; Stephen Reynolds; Richard Sedman, Ph.D.; Judith Tracy; and Caryn Woodhouse. At the same time the authors of this paper especially wish to acknowledge the contribution of Susan Solarz, also a coauthor of the decision tree manual, to the arsenic-contaminated site case study. 67

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68 HAZARDOUS WASTE SITE MANAGEMENT decision points needed to set cleanup criteria. Third, it establishes a technical basis for each major decision. Last, it standardizes the decisionmaking process so that it can be applied consistently to all sites. Fundamental to the decision tree process is a series of dis- tinctive aspects. One such aspect is that the process specifies a multimedia approach to site characterization activities and to establishing cleanup criteria. Specifically, the decision tree pro- cess requires one to address analytically the significance of the air, water, soil, and biotic exposure pathways for each site. It also identifies the specific parameters for which such data must be collected. This type of approach promotes a well-focused site characterization effort and minimizes the need for costly revisita- tions to collect data. Another unique aspect of the decision tree process is that it identifies preferred data gathering, handling, and analytical techniques that should be used to ensure high-quality environmental data. A critical aspect of the decision tree process is that it quickly sets statewide, health-based criteria called applied action levels (AAI`s). AAI,s are specific to substances, media, and biologic re- ceptors. They define exposure levels at which no observed adverse effect would be found. The decision tree process also allows one to set different cleanup levels for a particular site that reflect the different degrees of effectiveness of various remedial action combinations. Thus, the project manager is in a position to select the final cleanup solution that best suits the conditions of a particular site. The purpose of this paper is to discuss briefly the basic con- cepts affiliated with the decision tree process. The paper will con- clude with two case studies that illustrate how this process works quickly to reach a cleanup level that has a strong technical and scientific basis. Because this paper is an overview of the decision tree process, the reader is referred to the California site mitigation manual noted earlier (DHS, 1986) for a detailed presentation of the complete approach. COMPONENTS OF THE DECISION T1lEE PROCESS The decision tree process consists of five basic components: (1) preliminary site appraisal, (2) site assessment, (3) risk ap- praisal (4) environmental fate and risk determination, and (5)

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DECISION TREE PROCESS 69 development of site mitigation strategies and selection of remedial action. Each component is made up of several steps, procedures, and decision points. To minimize the time needed to finish a cleanup, the components are designed to be highly interactive and the last four components run concurrently. Preliniinary Site Appraisal The purpose of this component is to quickly assess a site's potential for environmental and/or public health damage. Sites that are potentially contarn~nated with hazardous substances are qualitatively assessed using conventional procedures developed for the U.S. Environmental Protection Agency. Based on the charac- teristics of the wastes that are present and the features of the site itself, the site may be determined to be sufficiently hazardous to be placed on either the National Priority List (for the federal Su- perfund) and/or dealt with through the state Superfund program. This scoring process, which is referenced in the decision tree, is based on the Mitre model approach developed for EPA and used throughout the nation. The advantage of this approach is that it quickly establishes a priority list of sites based on qualitative data obtained from each site. Newly enacted statutes within the state of California also as- sist DHS in establishing its priorities for state-managed cleanups. These statutes create three categories of sites. Each category reflects the degree of willingness and active involvement by the responsible party in addressing the problems that exist. The cate- gories range from proactive participation by the responsible party (thus requiring minimal oversight by the state) to total recalci- trance and strong state participation. (For more details on these priority categories, the reader is referred to the California Health and Safety Code, Division 20, Chapter 6.8, Article 5, Section 25356.) Site Assessment After a site has been identified, a detailed quantitative assess- ment is then conducted by activating the site assessment compo- nent. The function of this component is threefold. First, it defines

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70 HAZARD O US WA S TE SI TE MANA GEMEINT the thought process and procedures used to adequately character- ize a site. Second, it defines the parameters for which data must be collected. Finally, it identifies the preferred data collection, handling, and analytical techniques needed to ensure high-quality environmental information. This is accomplished through the use of a series of decision branches and data checklists. Through the use of these tools, the project manager is able quickly to identify the pathways of concern, the chemical contaminants of concern, and the biologic receptors of concern. The assessment also provides the project manager with site data needed in other components to determine the short-term and long-term health threat of the site. It should be noted that all of the branches presented in this component need not be used on all sites. In fact, the branching process has been designed to address certain core questions first, a method that allows one to close down a particular branch of analysis before it is pursued very far. For example, if a site only has relatively small amounts of surficial contamination, one may be able to justify not opening up the ground water pathway branch and thus save tremendous time and the costs associated with fully characterizing that medium. Furthermore, this process allows one to document the basis for a particular decision. Thus, if later questioned either through public scrutiny or in the courts, one wouIc] have a documented, technical basis for not pursuing that particular branch. Representations of transport pathways are referred to as mod- ules and are developed from data collected during this component. Each module may consist of observations, deductions, calcula- tions, numerical models, and professional judgments that allow the project manager to make scientifically and technically de- fensible statements and conclusions regarding the behavior and transport of chemical contaminants at the site. The focus of site characterization and the development of environmental modules is ascertaining what the concentrations of toxic chemicals will be at the points of exposure to biologic receptors of concern. Risk Appraisal Risk appraisal, the next component, begins while the site as- sessment process is still going on. Here the purpose is to assess quickly whether any immediate corrective action should be consid- ered to mitigate the short-term risk to the public. This assessment

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DECISION TREE PROCESS , ~Prevailing Wind Direction I AAL waver -a. ~ ~ ~ Expose I ~ ~ ~ ,,,, Point ~ 71 \ Direct AAL son >~Contact ~ BY At_ \t,_!1!~!1~!~!;~l ~ ~ ~ ~ A FIGURE 4-1 Illustration of the applied action level (AAL) concept and point of application. is done using three simple risk appraisal tests. By using these tests the project manager quickly compares the amount of contaminants reaching a biologic receptor to the statewide health-based criterion known as the AAL. As previously mentioned, the AAL is a substance-medium- biologic receptor-specific value. It defines the maximum exposure value in which no observable adverse effect would be detected. It is viewed as a statewide health-based criterion in that it does not matter where in the state the biologic receptor is located; if he is exposed above this level, he is at risk. AAL values are derived using conventional toxicologic principles and are published by DHS. Figure 4-1 illustrates the AAL concept and how it is applied at the location of the biologic receptor instead of at the site of initial contamination. The project manager can quickly assess whether or not a biologic receptor is currently at risk through the use of three simple tests contained in the decision tree process. The three tests taken together make up the risk appraisal mechanism. The first test evaluates whether a biologic receptor receives an excessive exposure to any toxic substance through contact with

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72 HAZARDOUS WASTE SITE MANAGEMENT each contaminated medium (e.g., air, water, soil, biota). The test compares the level of exposure for a substance In the medium (Cmedium) with a safe exposure level delineated by the AAI, crite- rion. Test 1 is written as follows: if Cme~ium/AALme~ium > 1, then a biologic receptor of concern is considered to be at risk to an adverse impact, the test fails, and a risk management process should be initiated. The second test determines whether a biologic receptor re- ceives an excessive exposure to any toxic substances through con- tact with all substantially contaminated media. The exposures by various media are assumed to be cumulative. Excessive exposure is determined by the cumulation of expo- sure in various media normalized to the AAL standard developed for that medium. Test 2 is written as follows: n med ium= 1 Cmedium/AALmedium > 1, then a biologic receptor of concern is considered to be at risk to an adverse impact, the test fails, and a risk management process should be initiated. The third test in the risk appraisal process determines whether a biologic receptor may receive excess exposure to an aggregate of substances that produces toxic manifestations. This test assumes additivity of such exposures across all media. The test can be modified to account for different types of interactions between toxic substances if shown to exist. Test 3 is written as follows: z ~ ~ Cmedium, sub > 1 then sub=i medium=! AALmedium, sub ~ a biologic receptor of concern is considered to be at risk to an adverse ~rnpact, the test fails, and a risk management process should be initiated. It should be noted that additional criteria may be used in lieu of AAL values. For example, if worker exposure and risk appraisal were to be assessed, it might be appropriate to use worker safety standards providing they are health based in their derivation.

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DECISION TREE PROCESS Environmental Fate and Risk Determination 73 As with the previous component, the environmental fate and risk determination component begins soon after the initiation of the site assessment component. Whereas the risk appraisal com- ponent evaluates whether a biologic receptor is currently at risk, this subsequent component assesses how the contaminants will be- have through time and then evaluates if the receptor will be at risk in the future. The environmental fate and risk determination component establishes methods and procedures to assess the en- vironmental fate of chemicals and their potential to move across media. Conservative projections are then made as to what the concentrations of a substance will be in the future at the exposure point for a biologic receptor. The process contained in this component allows one to make two critical determinations. First, it allows the project manager to establish the maximum contaminant concentration in each medium that will not pose a health risk (i.e., a health-based cleanup criterion). Second, the process allows one to project the relative efficiencies of different remedial actions and determine whether they will meet the health-based cleanup criterion just established. Because these two actions are the strength of the decision tree process, two case studies are presented later in this chapter to demonstrate each action. The first case illustrates how the decision tree process quickly establishes the cleanup criteria. The second case demonstrates how the decision tree process al- lows the project manager to evaluate the effectiveness of different remedial actions. It should be noted that the risk determination process used to establish the cleanup criteria is composed of the three simple tests that make up the risk appraisal mechanism. The difference is that now the concentration values used in each test are those derived through the environmental fate assessment. A dynamic aspect of the risk determination process is that it allows the transformation of various concentrations of contam- inants at a particular location into a single risk value. As shown by Figure 4-2, such a transformation greatly simplifies the evalua- tion of risk and males it easier for the project manager to convey this concept to the public. The risk values that are plotted out in Figure 4-2 are defined as risk index scores (RIS). Case study 2 graphically illustrates how risk index scores can be used.

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74 Concentration, mg/l A N = 0.02 C N = 0.01 E N = ND X= 1.2 X=0.80 X=0.21 B N = 0.001 D N = 0.005 X = 0.45 X = 0.60 HAZARDOUS WASTE SITE A~4NAGEMENT Naphthalene= ~ / Rlsk Index Scores ARIS = 3 0 CRIS = 1.84 ERIS = 0.34 BRIS = 0.78 DRIS = 1.25 FIGURE 4-2 A comparison between contours of ground water contamina- tion concentrations and risk index scores. The AAL values for naphthalene and xylene are 0.018 mg/1 and 0.62 mg/1, respectively. Development of a Mitigation Strategy and the Selection of Remedial Action If it is determined, either through the risk appraisal process or the risk determination process, that a biologic receptor of concern is or will be at risk, mitigation of that risk should be investigated. The development, evaluation, and selection of such remedial ac- tions are presented as elements of the last component of the deci- sion tree process. Discussing these activities in the latter portion of this section, however, does not mean that these activities begin late in the decisionmaking process. Rather, they begin during site assessment and run concurrently with the remaining components. The selection of the remedial action for a project is based on the specific site characteristics (Component 2), the existing toxic concentrations at the location of the biologic receptor (Compo- nent 3), and the ability of the contaminants to move across and within media to reach biologic receptors in the future (Component 4~. Thus, by initiating the screening process concurrent with site assessment activity, the impractical remedial actions are quickly discarded. Detailed analyses of feasible alternatives can be con- ducted along with the rest of the investigations to yield a timely solution. _ . . _ _.

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DECISION TREE PROCESS 75 Alternative site mitigation measures are identified and eval- uated in the feasibility study component of the development of a remedial action plan. A decision process for the development and evaluation of appropriate alternative remedial actions for a given site is contained in the EPA Guidance on Feasibility Studies Under CERCLA (U.S. EPA, 1985~. The discussion presented here has been adapted from the discussion presented in that more detailed document. The process for the development and evaluation of ap- propriate alternative remedial actions for a given site is shown in Figure 4-3. An example of how this component can be used to define and evaluate the various alternatives Is contained in the second case study. The reader is also referred to The California Site Mitigation Decision Tree Manual (1986) for a more detailed description of this component. APPI~G To DECISION T=E PROCESS: TWO CASE STUDIES Two case studies are presented below. The first study illus- trates how the decision tree process is used to set cleanup criteria quickly. The second study demonstrates how various remeclial actions are evaluated so that the best option is selected. Case Study 1: An Arsenic-Contam~nated Site In this first example, the preliminary site appraisal identified the site as a pesticide-formulating plant that had been in operation for more than 40 years. The facility covered over 10 acres and was located adjacent to a saltwater marsh. Samples showed that extremely high levels of arsenic compounds (up to 10,000 parts per million [ppm] total arsenic) were contained in soils underlying former waste disposal impoundments and storage areas, as well as along former loading and handling areas. Elevated levels of arsenic (up to 100 mg/~) were also observed in samples of the shallow ground water underlying the site. Although the site was located in an industrial zone, a residential neighborhood was less than one-half mile away. Site assessment activities were undertaken for a better def- inition of the characteristics of the site and neighboring areas. First, the shallow (~12 feet) ground water was determined to be

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76 HAZARDOUS WASTE SITE MANAGEMENT ~ . Characterize Problem and Identify General Response Actions rem Develop Alternative Specific Technalacilen ~ _ _ ! Technical creeping of Specific Technolonles I~ Formulate Broad Alternative Remedial A~tl~nQ Environ rental, Pubilc Health, and Institutlonal Screening Cost Screening Identity Surviving Alternative Remedial Actions Phase 1: Phase 11: Project Scoping Identification of Specific Technologies Phase 111: Screening of Alternatives Technical Institutlonal Cost An:llVals Analysis Ana veils Statutory Environmental Impact Analysis Phase IV: Detailed Analyses of Surviving Alternatives Summ;rizatlon of Alternatives . Final Feasibillty Report I FIGURE 4-3 Feasibility study process. Phase V: Summarization Phase Vl: Preparation of Feasibility Report

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DECISION TREE PROCESS 77 nearly stagnant and highly saline (about 25,000 ppm total dis- solved solids). This aquifer was shown to reside above a drinking water aquifer found at a depth of approximately 200 feet. Do- mestic wells were so located that they used this deeper aquifer, but they were hydraulically upgradient and located a considerable distance from the site. The drinking water aquifer was separated from the contaminated, shallower aquifer by approximately 100 feet of low-permeability deposits. The surface and near surface (0-12 feet) soils consisted primar- ily of silty sands. There were large areas of arsenic contamination as a result of surface transport of the contaminant by seasonal flooding and manufacturing activities. The soil concentration val- ues ranged as high as 10,000 ppm total arsenic for a few "hot spots" but were more typically confined to the I,00~ to 5,00~ppm range. In addition to soil and water data, meteorological information and marsh flora/fauna data were collected. The California De- partment of Fish and Game analyzed tissue samples from aquatic species living in the marsh and conducted a vegetation assessment. While site assessment activities were under way, a risk am praisal was conducted to assess any existing health threats. It was determined that by limiting access to the site the public would be adequately protected. To preclude any surface contamination reaching the marsh and endangering aquatic species, a berm was constructed along the marsh boundary. This barrier eliminated seasonal flooding and surface water runoff into the marsh. To set a health-based cleanup criterion for the site, the envi- ronmental fate and risk determination component was activated. To project what the future concentrations of arsenic compounds would be at the location of the biologic receptors, two conservative scenarios were created. For the first scenario the future site condi- tions were defined as an undistributed site with all buildings and structures removed; no soil cap or vegetative cover were present, and dry soil conditions existed. The biologic receptor of concern was identified as the general public, and the predominant exposure pathway (medium) was the air. It was assumed that residential development had encroached up to the site boundary. The pri- mary health concern for this first scenario was based on long-term chronic exposure to arsenic compounds. In the second scenario the site conditions were once again defined as all buildings and structures removed, no soil cap or vegetative cover present, and extant dry soil conditions. In this

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DECISION TREE PROCESS r source ~ I Area ~ -1, 87 -1 ~ 'I ,~ ~ ~ ~ _ . - it. . - din - ~- 5- ~1 ~ 1 Agricultural Wed rem b~ . _~ ll City Well ~ . ~ . ~ l l .-, _ no_ RIS> 1 Private Ne11 River ~ 1 FIGURE 4-8 Environmental fate and risk determination: potential future conditions if the city well is closed. ground water. The private well is located in the path of the con- taminated ground water plume and typically would have such a small capture zone as to preclude at-the-welIhead dilution. The continuing operation of the agricultural and municipal wells to harvest contaminated ground water and thereby work to protect the private well cannot be assumed without formal commitments from the farmer and water purveyor. Therefore, the level of chIo- roform at the private well would be expected to exceed the AAL in the future; test 1 of the risk appraisal mechanism fails; and a risk management process should be considered to protect those biologic receptors demonstrated to be at risk in the future. In addition to the existing downgradient biologic receptors, human beings who in the future may wish to use the ground water resource downgradient of the site would be considered at risk. As illustrated in Figure 4~9, this is equivalent to evaluating the site by identifying the biologic receptor of concern as a human being exploiting the ground water just downgradient of the contam~na- tion source. Thus, a second biologic receptor has been identified as being at risk, although this second receptor currently does not exploit the ground} water and, in reality, may not have been born yet.

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88 Area ~ _ ~ HAZARDOUS WASTE SITE MANAGEMENT . I >~U~_ ~- r, 4~ Private ~ 7 WeI' River FIGURE 4-9 Environmental fate and risk determination: future beneficial uses of ground water. As shown also in Figure 4-9, the flux of contaminated ground water that could eventually enter the river in this problem has been determined to be small with respect to the flow of the river. This condition would provide sufficient dilution and result in non- detectable levels of chloroform in the bulk flow of the river. Based on this analysis the aquatic species identified as the biologic recep- tors of concern would not be considered at significant risk in the future, and a risk management process would not be warranted to protect them. Development of a Mitigation Strategy and Selection of Remedial Action At this point in the case study the problems to be solved through remedial action have been identified and defined by in- vestigation and analysis. Specifically, the potential risks of future adverse impacts on biologic receptors of concern have been eval- uated and defined through the risk appraisal mechanism. Those risks deterrn~ned to be significant have been identified as media specific, receptor specific, chemical specific, and site specific. The mitigation strategy to be used must address the defined problems.

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DECISION TREE PROCESS 89 In this case, the mitigation strategy must preclude adverse health effects associated with the exposure of humans to chloroform in ground water. In the fifth component of the decision tree process, the project manager has the opportunity to evaluate various remedial alterna- tives. The effect of each alternative in reducing the risks associated with remedial actions is evaluated through the decision tree pro- cess, again employing the environmental fate modules and the risk appraisal mechanism. Both technical and nontechnical considera- tions are evaluated by the site manager before proposing plausible remedial alternatives. In this example, four remedial alternatives are evaluated. Alternative 1- No Action. The no-action remedial alternative would not alleviate or reduce the risk posed to downgradient water users, nor would it protect future human biologic receptors wishing to use the ground water resource as a drinking water supply. Although humans would be at risk here, the nonhuman bi- ologic receptors of concern, the fish in the nearby river, are not considered to be at significant risk. For this case the no-action al- ternative would be acceptable with respect to the aquatic species. Alternative 2 Aquifer Remediation. A second alternative, aquifer remediation, would intend to restore all contaminated ground water to a condition in which the AAL for chloroform is not exceeded anywhere (Figure 4-10~. At this particular site, such an alternative protects all biologic receptors of concern but has an associated cost that is extremely high. Alternative ~ Alternate Water Supply. This third remedial alternative (Figure 4-11) would protect the biologic receptors of concern that have been identified as being at risk, but it would limit the availability of the ground water resource. As shown in Figure 4-11, the alternate source of water would be the existing municipal supply well. Potential problems in implementing this alternative might arise from a reluctance on the part of the water purveyor either to operate this well in a regime that provides the necessary dilution at the welThead or to operate such a well at all. At this point it might be appropriate for the risk manager, the water purveyor, and the public to consider the risks associated with other sources

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go Source Area _ RIS ~ 1 )1 FIGURE 4-10 Remedial alternative: aquifer remediation. Capture _ Radlus ~ Source Area 1 r~ W! ~ l ~ ! Cultural ~ el ~ I d: RIS<1 _ ~- _, - FIGURE 4-11 Remedial alternative: alternate water supply. HAZARDOUS WASTE SITE MANAGEMENT ~ Hi_ ~ 9 : . ~ ~3 ~ ', River River l

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DECISION TREE PROCESS Capture Radlus - =m Source Area d' , Ha, A' 91 .~-~ _ ~ _ . _ ~ _ alit ~ r ~ _~ ~ -~ 1 C12 1 1 ! 1 . RIS > ~ '1 RIS ~ 1 River 1 FIGURE 4-12 Risk index scores for surface water supply and ground water supply. of water, such as chlorinated surface water, and compare the risk index scores associated with both sources of water. Figure 4-12 illustrates the relative risks associated with the water supply alternatives of concern. As can be seen in this figure, exposure to by-products of chlorination, including chloroform, would often be expected to place human biologic receptors at greater risk than they would be from the delivery of untreated ground water. Alternative 4 Plume Monitoring and Maintenance. A fourth alternative that might be subtitled the "don't go near the water" alternative is shown in Figure 4-13. As the figure illustrates, re- stricting the use of portions of the ground water system would preclude the exposure of humans to ground water containing chIo- roform above the AAI.. Controlling the pumping of the municipal well would protect downgradient ground water users. This alter- native also protects those biologic receptors identified as being at risk and, like other remedial alternatives, has associated costs and problems in implementation. The four remedial alternatives considered in this example are compared in summary form in Table 4-1. Only one alternative, the no-action alternative, fails to protect the biologic receptors of

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92 Capture _ Radius Source Area U HAZARDO US WASTE SITE MANAGEMENT r 'A .1 > _ `~.~ = Subcultural Shells Em- ~ ~- - 7~ ~_~ d_ mar City W 811 L, ] l RIS c 1 ll RIS < 1 - ^ w~ Private We11 . 4 ~ River . , _ _ FIGURE 4-13 Remedial alternative: plume monitoring and maintenance. concern, as discussed above. The aquifer remediation alternative is acceptable under all categories of evaluation but has a cost that is far in excess of the other alternatives. The availability of financial resources to remediate all sites to the standard implied in Alterna- tive 2 is a serious consideration for project managers. Alternatives 3 and 4 rely on administrative and resource management practices rather than the traditional soil removal/ground water treatment program; yet, if rigorously enacted, they would also meet the criterion of protecting the biologic receptors of concern. It should be noted that the traditional evaluation of "How clean is clean?" for soil contamination is applicable to only one of the four alternatives considered here. It should also be noted that such an evaluation is technically defensible only following a site assessment. As illustrated in Figure 4-14, such an evalu- ation would rely on the characterization of the soils system as represented by the unsaturated zone module. The construction of such a representation requires the input of several disciplines, as indicated in Figure 4-14. In fact, the multidisciplinary team approach to evaluating hazardous waste sites is an explicit recom- mendation made throughout the decision tree manual, but it is perhaps most important when evaluating the subsurface behavior of contaminants.

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DECISION TREE PROCESS TABLE 4-1 Remedial Alternative Analysis 93 Remedial Public Alternative Cost Technical Health Aquatic Species Concerns Public Input No action None Unacceptable Unacceptable Aquifer restoration with source control t500X Acceptable Acceptable Alternate $50X Acceptable Acceptable Acceptable water supply Plume $20X Acceptable Acceptable Acceptable monitoring maintenance Acceptable Unacceptable Acceptable Acceptable Water agency reluctant Water agency reluctant of *meteorology Hydrology COMA Inflltratlon . i~ ~ ' Jl Rate Kd a' Containment Flux ~ | Hi; Chemistry > Son Science .3~ FIGURE 4-14 Unsaturated zone module. An) '''~' Geology -

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94 HAZARDOUS WASTE SITE MANAGEMENT In summary, the project manager who must make the final recommendations regarding this case has been provided with an analysis of the various remedial alternatives considered plausible to implement. The technical basis for each alternative has been constructed through the decision tree process, and the strengths, weaknesses, and costs associated with each alternative have been compared. At this point, it becomes the state decisionmaker's job to select the alternative that is considered the "best" for this par- ticular site. He or she must balance concerns over implementability and public acceptance with the very real-worId constraint of cost. The role of the decision tree process is to provide that decision- maker with the strongest possible technical basis for making such a decision, in part with the goal of making the decision defensible in the event of a challenge in a public or legal forum. CONCLUSION The California Site Mitigation Decision Tree Manual has been created as a technical guidance document to assist project man- agers in making decisions that have a strong analytical basis and technical merit. The process specified in the document was de- signed to be flexible in application. The decision-branching format allows one to quickly identify the pathways of exposure that must be characterized for each site. Simple sites generally require sim- ple approaches; complex sites require more detailed multipathway analyses. To facilitate a scientifically based decision process the decision tree incorporates a series of unique aspects. First, it requires a mul- timedia approach to site characterization and the establishment of cleanup criteria. Second, it identifies the specific parameters for which data must be collected. Third, it identifies the preferred data gathering, handling, and analytical techniques that should be used. Fourth, it establishes statewide, health-based criteria called applied action levels that are specific to particular substances, me- dia, and biologic receptors. They define an exposure level in which no observed adverse effect would be found. Fifth, the decision tree process also allows one to set different cleanup levels for a particular site, a capability that reflects the different degrees of effectiveness of various remedial action combinations. Using the process the project manager is in a position to select the final

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DECISION TREE PROCESS 95 cleanup solution that best suits the condition of the particular site. Finally, it should be noted that DHS views the decision tree manual as a dynamic document; as new field techniques and an- alytical procedures are developed, the document will be updated accordingly. The intent is to have a process that yields decisions with the strongest technical basis. REFE1lENCES California Department of Health Services, Toxic Substances Control Division. 1986. The California Site Mitigation Decision Tree Manual. Sacramento, California. Cooper, W. C., J. Murchio, and W. Popendorf. 1979. Chryotile asbestos in a California recreation area. Science 206: 685-688. Cowherd, C. M., G. E. Muleski, P. J. Englehart, and D. A. Gillette. 1984. Rapid Assessment of Exposure to Particulate Emissions From Surface Contamination Sites. Kansas City, Mo.: Midwest Research Institute. U.S. EPA. 1985. Guidance On Feasibility Studies Under CERCLA. Prepared for Hazard Waste Engineering Research Laboratory, Cincinnati, Ohio, and Office of Emergency and Remedial Response and Office of Waste Programs Enforcement, Washington, D.C. PROVOCATEUR'S COMMENTS Joan Berkowitz The California decision tree process, which is outlined in the report that David was kind enough to send to me, is really a "how- to" manual for conducting a remedial investigation/feasibility study (RI/FS). The document presents a series of flowcharts on what data to obtain and a text on how to obtain them. The mate- rial is basically an amplification of the requirements of the national contingency plan. If the directions in the manual were followed, believe that both the R! and the FS would be of high quality and that they would be linked together. This linkage has not al- ways been achieved with RI/FS studies in the past, as Hirschhorn (1987) points out. Although the California decision tree manual provides excel- lent guidance on fact finding, the manual does not provide the last word on how those facts should be used to come to a decision on remedial action. It cannot be emphasized too strongly that facts

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96 HAZARDOUS WASTE SITE MANAGEMENT are fundamental. Without a good factual base, reasonable and defensible conclusions cannot be drawn. The decisionmaking guidelines in the California mode! center around AALs (applied action levels). These action levels are set at the point at which contaminants in air, surface water, ground water, and soils impinge on target organisms. The decision it- self is based on a comparison between the concentrations (either measured or estimated through a model) at the points of expo- sure and theoretically derived, health-based AALs. Specifically, the concentration of a chemical in a medium (Cc,m) is compared to an AAL for the same chemical in the same medium. If the ratio, Cc,m/(AAL)c,m, is greater than one, there is a potential risk. Conceptually, this is very nice. However, uncertainties in the measured or modeled concentrations, as well as in the AALs, are both reflected in even greater uncertainties in the ratio. A recent book by Wood et al. (1984), for example, shows that measured concentrations in ground water can vary by an order of magnitude in a given location over relatively short periods. This means that there will be large error bounds on the numerator (environmental concentrations). There will also be large error bounds on the de- nominator (AAL) because of uncertainty in the data that go into calculating the AAL. The uncertainties are still greater in the sum of the ratios of Cc,m/(AAL)c,m over all chemicals and all media used to reflect overall risks. Therefore, the final answer, taking into account error bounds of the input data, might range from something below one to something above one. The case example that David gave highlights an additional problem with the AALs. The contaminant selected in the example was chloroform; the AAL was set at 4.3 ppb on the basis of potential carcinogenic effects. Yet the drinking water standard for total trihalomethanes (primarily chloroform) is 100 ppb. Based on conventional dose-response extrapolations, 100 ppb happens to correspond to an increased cancer risk of about 10-4. Admittedly, the drinking water standards are technology based and not health based. In fact, however, the drinking water standards trade off the uncertain risk of cancer as a result of the presence of chloroform against the certain risk of pathogenic diseases if the water were not chlorinated. Chloroform is a byproduct of chlorine disinfection. A dual standard 4.3 ppb for cleanup and 100 ppb for drinking water- may be appropriate. Nonetheless, there is clearly some subjective judgment involved in setting the AALs.

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DECISION TREE PROCESS 97 Finally, the California decision tree process and this entire workshop are based on the premise that priority attention must be paid to protecting human health and the environment from hazardous waste sites. After the RI/FS has been completed and a decision has been made to spend, let us say, $20 million on a site, the question is never asked, "If $20 million were made available to this particular community to protect and enhance public health and the environment, what would it be spent on to achieve the maximum overall benefits?" Over the next 5 years, more than $20 billion is likely to be spent in the United States for inactive waste site cleanup; the question is never asked, "If that same $20 billion were to be put into a program to improve public welfare in the United States would it all be put into waste sites?" In an even broader context, the question is never asked, "If $20 billion were to be invested in a global public health program, would it be spent on cleaning up hazardous waste sites in the United States?" ~ am not suggesting that these questions be addressed here; we have a full agenda focused on issues of major national interest. ~ am suggesting that current national priorities may not be directly proportional to current health and environmental risks in the United States, much less worldwide. REFERENCES Hirschhorn, J. S. 1987. Superfund: A Scientifically Sound Strategy Needed. Ground Water Journal, Jan.-Feb.: pp. 3-11. Wood, E. S., R. A. Ferrara, W. G. Gray, and G. F. Pinder. 1984. Ground Water Contamination from Hazardous Waste. Englewood Cliffs, N.J.: Prentice-Hall.