2
Considerations in the Selection and Use of Vulnerability Assessments

INTRODUCTION

Numerous agencies and organizations have conducted or plan to conduct vulnerability assessments as part of their decision making, policy making, and/or planning functions. In most cases, these assessments are analytical tools that bridge the science that seeks to understand the relationship between land use activities and ground water contamination with the sociopolitical realities of making decisions and implementing programs to protect ground water quality.

Increasingly, policy makers and managers have demanded that vulnerability assessments provide objective, scientific, and accurate evidence they can use to make difficult choices and decisions. Similarly, scientists have struggled with providing simple, readily understandable products (often maps) containing the results of sophisticated models representing complex ground water processes and sometimes questionable data. An important corollary to the Second Law of Ground Water Vulnerability is that since all vulnerability assessments are uncertain, no management decisions based on them are ever clear-cut or certain. The tension between the need for and capability of vulnerability assessments to provide accurate and useful information forms the structure for this chapter.

The intended use of the vulnerability assessment process is the most obvious and important consideration in selecting an assessment approach. The uses of vulnerability assessments in policy making range from advising



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 30
Ground Water Vulnerability Assessment: Contamination Potential Under Conditions of Uncertainty 2 Considerations in the Selection and Use of Vulnerability Assessments INTRODUCTION Numerous agencies and organizations have conducted or plan to conduct vulnerability assessments as part of their decision making, policy making, and/or planning functions. In most cases, these assessments are analytical tools that bridge the science that seeks to understand the relationship between land use activities and ground water contamination with the sociopolitical realities of making decisions and implementing programs to protect ground water quality. Increasingly, policy makers and managers have demanded that vulnerability assessments provide objective, scientific, and accurate evidence they can use to make difficult choices and decisions. Similarly, scientists have struggled with providing simple, readily understandable products (often maps) containing the results of sophisticated models representing complex ground water processes and sometimes questionable data. An important corollary to the Second Law of Ground Water Vulnerability is that since all vulnerability assessments are uncertain, no management decisions based on them are ever clear-cut or certain. The tension between the need for and capability of vulnerability assessments to provide accurate and useful information forms the structure for this chapter. The intended use of the vulnerability assessment process is the most obvious and important consideration in selecting an assessment approach. The uses of vulnerability assessments in policy making range from advising

OCR for page 30
Ground Water Vulnerability Assessment: Contamination Potential Under Conditions of Uncertainty decision makers of the need for or consequences of their actions, to providing direction for allocating resources, to informing decisions about land use activities, to educating the general public about ground water contamination potential. Existing uses of and needs for vulnerability assessments can be grouped into four broad categories. First, assessments can be used in policy analysis and development to identify the potential for ground water contamination and the need for protection and to aid in examining of the relative effects of alternative ways to control contamination. Second, when scarce resources prevent uniform and costly expenditures, vulnerability assessments can be used in program management to allocate resources to areas where the greatest effort is warranted. Third, vulnerability assessments can be used in some instances to inform land use decisions such as site selection, alteration of land use activities to reflect the potential for ground water contamination, or voluntary changes in behaviors of land owners as they become more aware of the ground water impacts of their land-based activities. Finally, and perhaps most important, is the use of vulnerability assessments to improve general education and awareness of a region's hydrologic resources. Often policy makers will not find the objective, scientific, and accurate product they need for the decisions identified above. Rather, they will find that the usefulness of a vulnerability assessment may be severely constrained by scientific unknowns or lack of suitable data. Hence, policy makers and managers need to become intelligent consumers of vulnerability assessments since the selection and use of assessments are significantly affected by several technical and institutional factors. Several key factors will affect both the technical conduct of an assessment and its effectiveness in use. The more consideration given to these technical and institutional issues, the more likely are the needs for a vulnerability assessment to be matched with a useful, scientifically-based technique. EXISTING AND POTENTIAL USES FOR VULNERABILITY ASSESSMENTS Policy Analysis and Development Vulnerability assessments can be used to aid in the development and analysis of policies to respond to potential or actual ground water contamination. In this early stage of the policy making process, assessments can be used to predict, at least qualitatively, potential ground water quality outcomes of different policy scenarios. Vulnerability assessments can also be used as a tool in assessing the effectiveness of alternative responses to a problem.

OCR for page 30
Ground Water Vulnerability Assessment: Contamination Potential Under Conditions of Uncertainty In some cases, vulnerability assessments can be used to identify or predict the existence of potential for ground water contamination in a particular geographical or jurisdictional area. Therefore, these assessments often take the form of a map of existing levels of contamination or areas where contamination is known to have the potential to occur. In these cases, the assessment documents the problem needing to be resolved and provides the justification and rationale for further discussion, action, and/or policy development. Another potential use of a vulnerability assessment in the policy development stage is to analyze proposed alternative policies that seek to respond to a particular ground water quality problem. A program manager faced, say, with a problem with the use of the herbicide atrazine in a particular region might need to examine several alternative policies—from education and technical assistance to regulation of usage rates to an outright ban on use. Various analytical tools could then be used by the policy analyst to determine the impacts of each of these options on a variety of factors including atrazine use, productivity, and ground water quality. For example, the predicted effectiveness of a policy that is implemented only in more vulnerable areas (versus all areas) can be estimated through the use of vulnerability assessments. Program Management Vulnerability assessments can also be used to guide various program-level management decisions. As stated above, assessments can document the level of severity and need to resolve a ground water contamination problem. Further, an assessment can highlight the need for financial or human resources to be directed toward the control of a particular ground water contaminant or contamination problem. Vulnerability assessments also can give managers information they need to allocate resources to areas for particular purposes. These purposes could vary from providing the greatest benefit or protection with the least expenditure to preventing the worst possible contamination problem. For example, vulnerability assessments could be used to establish routine ground water monitoring programs, to establish databases, or to ensure compliance with standards or other protection requirements. More vulnerable areas would be monitored more closely than less vulnerable areas to identify incidences of contamination. Similarly, allocation of personnel to compliance programs could be based on vulnerability assessments. Assigning additional personnel to supervise land use regulations or mitigation plans in more vulnerable areas recognizes the need for closer control of activities in areas more susceptible to contamination. In each of these instances, vulnerability assessments serve as a tool to

OCR for page 30
Ground Water Vulnerability Assessment: Contamination Potential Under Conditions of Uncertainty improve the management of ground water resources. In one case, vulnerability assessments elevate the awareness of program managers to the problem of ground water contamination and provide a call for further action. In other instances, an assessment is used to direct the allocation of resources, financial or human, to particular types of problems or particular locations. Each of these uses of vulnerability assessments serves a critical management need. Land Use Decisions The vast majority of land use decisions in the United States are made by private land owners and owners operating within a framework of publicly adopted rules and regulations. Land use planning and control functions that establish and enforce these rules have typically been performed by local (municipal and county) government, although some states (Florida, Oregon, and Vermont are prominent among them) have exercised broad authority over land use. The federal role in land use management has generally been very limited, except for federally owned lands. Land use decision making thus tends to be highly decentralized, and the application of ground water vulnerability assessments to such decisions presents special technical and institutional problems. Still, ground water vulnerability assessments may inform three principal categories of land use decision making: zoning and screening functions; permit conditioning, mitigation, and monitoring; and the voluntary behaviors of individual land users and owners. Zoning and Screening Zoning, one of the primary forms of managing land use, involves the assignment of a range of acceptable and/or unacceptable uses and activities to different areas of land. Screening is the search for suitable sites for specific activities or types of facilities. Often, suitable sites are found by eliminating unsuitable sites. The goal of both zoning and screening in protecting ground water is to preemptively direct facilities relatively likely to cause ground water contamination (such as landfills, wastewater discharges, and certain industrial uses) and activities of high contamination potential (such as the application of pesticides) away from areas of high vulnerability. Zoning or screening aimed at preventing unacceptable impacts on ground water must address the most sensitive use or function of the ground water resource. For example, ground water can be used as a drinking water supply or play an important role in freshwater and marine ecosystems. Each

OCR for page 30
Ground Water Vulnerability Assessment: Contamination Potential Under Conditions of Uncertainty of these uses or functions exhibits dramatically different tolerances for contamination that must be accounted for in identifying ground water impacts. Zoning and screening functions ultimately involve decisions affecting individual sites. Regional vulnerability assessments do not provide site-specific resolution, but may appear to contain information relevant to site-level decisions. Limitations in the ability to predict the effect of land uses and activities on ground water include scale constraints, errors, and uncertainty. While it might be tempting to construct a hypothetical ground water zoning map based on a vulnerability map, such a zoning map could not of course, be more detailed or more certain than the underlying vulnerability assessment. Unfounded application of regional assessments at the site level could result in misinformed decisions with unfortunate and even tragic consequences for the land user, land regulator, and the ground water resource itself. Conditioning, Mitigating, and Compliance Monitoring In issuing permits for activities that may contaminate ground water, regulators often establish specific requirements that relate to the characteristics of the activity or site. Conditions or required mitigation may include treatment to acceptable levels of contaminant concentration, limits on discharge volumes, prohibition of discharge, and required containment to deal with accidental spills. Vulnerability assessments also can guide the establishment of sampling routines for compliance monitoring. Land uses in comparatively more vulnerable areas may, therefore, require more monitoring for compliance assurance than uses in less vulnerable areas. As before, the utility of vulnerability assessments to the manager in setting conditions or establishing mitigation will vary with the specificity of the assessment. Vulnerability assessments conceivably could be used to establish more stringent requirements in areas of high vulnerability, including more frequent or intensive monitoring to demonstrate compliance with permit conditions. Technical Assistance The availability of a vulnerability assessment can give both land users (e.g., farmers) and managers (e.g., water supply superintendents) a proper sense of caution and some information on how to avoid excessively risky actions. A vulnerability assessment prepared by the Soil Conservation Service, for example, may be linked to a set of alternative land uses and conservation practices that would minimize contamination of the water system. Farmers would select from the acceptable alternatives based on resource protection factors as well as social and economic factors. Other responses

OCR for page 30
Ground Water Vulnerability Assessment: Contamination Potential Under Conditions of Uncertainty may include voluntary restrictions of activity by the land user (such as self-imposed limits on pesticide use) to avoid contamination or the setting aside of land near wells of surface waters to prevent direct contamination. Again, the utility of the vulnerability assessment in technical assistance will depend on its specificity, level of detail, and degree of uncertainty. General Education and Awareness Although vulnerability assessments are rarely undertaken primarily to increase the awareness and knowledge of the public and decision makers on the potential for contamination of ground water resources, that may be their most valuable use. Because ground water resources are generally invisible, public knowledge of the ground water system, its use, and its susceptibility to contamination is often poor. A vulnerability assessment can improve such awareness by highlighting the surface or near-surface activities that lead to ground water contamination, by noting the movement of contaminants, and by indicating the factors leading to higher or lower levels of vulnerability. This increased knowledge often produces a greater willingness to take the necessary steps to protect against ground water contamination. In some cases, this use of vulnerability assessments is explicitly intended. As illustrated in the Iowa case study in Chapter 5, learning more about the ground water system and its susceptibility to contamination was an essential part of that state's nonregulatory approach to ground water protection. In other situations, education can be an unstated or implicit goal of an assessment that provides the knowledge or awareness necessary to implement new policies, allocate resources, or affect land use decisions. Such assessments also may help to create public support for protective measures. For example, the ''sole source aquifer" designation under the Safe Drinking Water Act was successful in creating a widely shared image in places like Cape Cod of a fragile, interconnected, and irreplaceable resource requiring conscientious management and careful protection. More specific assessments also may contribute to greater sophistication and greater commitment to action on the part of the public, as was shown in the town meeting votes to acquire large holdings of land in wellhead protection areas on Cape Cod and elsewhere in the 1980s. FACTORS AFFECTING SELECTION AND USE OF VULNERABILITY ASSESSMENTS Varying levels of accuracy, certainty, resources, and data are required to meet each of the four uses identified above. Not all approaches or data can meet these needs. Decision makers must, therefore, reconcile their

OCR for page 30
Ground Water Vulnerability Assessment: Contamination Potential Under Conditions of Uncertainty needs with the capability of the methods and data. Several technical and institutional factors affect this match between the selection and use of the vulnerability assessment and its eventual purpose. By addressing these issues, decision makers may be better able to request an assessment that meets their needs while still understanding its limitations and weaknesses. Further, those performing the assessments may be able to provide language that clarifies the proper use of the results. Technical Considerations Many technical issues affect the design and use of vulnerability assessments. They include the type and form of the results or output, the suitability of the technique for the physical characteristics of the area being assessed, the adequacy of the available data or data to be collected, and the analysis of uncertainty in the output and how it may affect the actions influenced by the assessment. These factors are not mutually exclusive. Type and Form of Output The first critical technical issue is the form of presentation of the results of the vulnerability assessment. What do users of the assessment wish to obtain from it? A variety of types and forms of output may be needed to meet various needs. For example, the USDA case (see Chapter 5) demonstrates the need for delineating regions with different vulnerabilities so that resource allocation decisions can be made. For this purpose, a map depicting numerous areas differently colored or shaded is desirable. Alternatively, a vulnerability assessment could be used to determine areas where use of a particular pesticide should be restricted or specific management practices should be implemented. This use would require comparatively greater resolution among areas than the resource-allocation example, but may lead to a much simpler product, such as a map showing two types of regions: one where a pesticide is banned and another where use is permitted. Significant explanatory text would be essential to justify this map properly, to convey its assumptions and approach, and to illustrate its use. In most cases, the product of a vulnerability assessment is a map of various areas—each area containing essentially similar vulnerability characteristics. Shading or coloring of these areas denotes different levels of vulnerability or characteristics affecting vulnerability. Such maps are very easily understood graphical means of conveying information for decision making. Maps, however, require aggregation of characteristics into defined areas. This aggregation can either mask important distinctions or emphasize minor differences. While a map is the output of choice in most existing

OCR for page 30
Ground Water Vulnerability Assessment: Contamination Potential Under Conditions of Uncertainty vulnerability assessments, much care should go into preparing it, since the map, not the actual data, method, or process involved in preparing it, will be regarded as the true output of the assessment. The map should be supported by a technical supplement describing the underlying assumptions of the assessment, the rationale for the selection of the method of analysis, an assessment of the adequacy of the data for the selected method, the uncertainties associated with the input data and analysis, and the sensitivity of the results to key assumptions and available data. This information makes possible an independent evaluation of the technical adequacy of the vulnerability assessment. Size and Characteristics of Assessment Area The analytical technique selected for a specific vulnerability assessment should be technically defensible with respect to the physical dimensions and characteristics of the area covered by the assessment. The area may be the entire nation, a subregion, a state, a single county or multicounty region, an entire watershed or part of a watershed, or a field. The characteristics may be quite uniform or highly variable within the region to be assessed. As a rule of thumb, the mathematical complexity of the vulnerability assessment technique is inversely related to the size of the area being assessed. For smaller regions, on the level of a watershed or field, the vulnerability assessment technique may include numerical models of the physical processes of vadose zone hydrologic flow and chemical transport and fate. Depending on the data available, the characteristics to be included in the assessment, and the technical skill of the people doing the assessment, these models could be very complex. At present, when the area being assessed is larger than a watershed, such as in a state, regional, or national assessment, these detailed physical models typically would be replaced by techniques that aggregate information across watershed boundaries. Availability of Data A major consideration in selecting an assessment technique is the type and amount of data required by the analysis and the level of certainty desired. For physical process models, the requirements are very specific: measurements of hydraulic and soil properties, such as conductivities, permeabilities, and bulk densities, should be available from one or more locations within the area being modeled. Without site- or area-specific data, the credibility of the modeling is diminished. Because these data are associated with physical measurements, specific requirements for the methods

OCR for page 30
Ground Water Vulnerability Assessment: Contamination Potential Under Conditions of Uncertainty of collection can be stipulated to ensure comparability across different data collection activities. The statistical and index techniques also require data, including flow-system characteristics, vadose zone transport times, population densities, and land use information, among others. Because these types of data are less tied to specific physical measurements, and because the geographic regions will vary from assessment to assessment, it is difficult to set standards for data collection and aggregation for these methods. For any type of analysis, the quality of the data will affect the level of uncertainty associated with the assessment results. Thus, the desired level of certainty will also affect the selection of a technique. Uncertainty of Results and Impact on Use Any vulnerability assessment will be subject to uncertainty for many reasons. They include lack of available data, measurement errors in available data, incomplete understanding of the relevant environmental processes, uncertainty resulting from prediction of events, errors in aggregating information, and errors inherent in using statistical measures of association. Some of these errors or uncertainties can be measured, but others cannot. Uncertainties in data that describe field properties that can be directly measured are more easily quantified than the types of uncertainty that result from the method used. All forms of uncertainty are critical in the design and use of an assessment. Consideration should be given to the effects of uncertainty on how decisions will be made, what decisions are made, and how the results of the assessment are presented. In all cases, this discussion of uncertainty and errors should help elucidate which decisions are possible, the benefits of making correct decisions, and the consequences of making incorrect choices. Simple methods may be appropriate for areas where more comprehensive data needed for detailed evaluations are not available. However, when data are available and more detailed methods can be performed, these results usually would be preferred to those from simpler methods. The eventual use of an assessment should, more or less, reflect the technical limitations of the method. Therefore, the purpose of the vulnerability assessment, the level of uncertainty in the results that can be tolerated, and the limitations of the techniques and results will directly affect the design and use of the assessment. These and similar technical issues should be adequately considered by both the users of the assessment and the technical staff who prepare it.

OCR for page 30
Ground Water Vulnerability Assessment: Contamination Potential Under Conditions of Uncertainty Institutional Considerations In addition to purely technical issues, several institutional factors are important to the design and eventual use of a vulnerability assessment. These issues include the time frame in which the assessment is meant to apply, how the proposed vulnerability assessment is coordinated with other planning programs and needs, the cost of the assessment and the value of the information to be gained, the availability of personnel and physical resources to perform the assessment, and the plans and activities of other agencies and institutions that may have an interest in the assessment. Again, these factors are not mutually exclusive, and they overlap with the technical issues discussed above. Time Frame of a Vulnerability Assessment Since a vulnerability assessment is done to predict the possibility of contamination of ground water or the potential worsening of existing conditions, the period to be covered by the proposed vulnerability assessment must be chosen carefully. As noted in Chapter 1, sufficiently persistent and mobile contaminants may eventually be observed in ground water at significant levels even though the travel times may be long. Thus, it may be necessary to consider effects on ground water quality over longer time spans and greater distances than is commonly done in vulnerability assessments. Cost and Commitment of Personnel and Physical Resources Any vulnerability assessment, no matter how simple, will incur programmatic costs, including the commitment of personnel and physical resources. Key questions to be considered include: How much funding is available for a vulnerability assessment? Are staff with the needed expertise available? If new staff must be hired, what are the long-term personnel and financial commitments? Are the physical resources, such as ground water sampling or testing equipment, or computer facilities for data management and analysis readily available? Each of these questions considers a different feature of the human and financial costs of performing a vulnerability assessment. In general, the more resources available for the analysis, the more detailed and sophisticated it can be. In many cases, however, the intended use of the assessment may not require a detailed or sophisticated approach. Therefore, the resources invested in the analysis should be consistent with the value of the results.

OCR for page 30
Ground Water Vulnerability Assessment: Contamination Potential Under Conditions of Uncertainty Coordination with Other Planning Programs and Needs Another important institutional issue in the design and use of a vulnerability assessment is its role in other ground water planning and protection efforts. Is the assessment an integral part of a well defined strategy for ground water protection, or is it designed to respond to a particular, rather specific, ground water question? If the vulnerability assessment is to be a one-time activity—for example, a survey of the status of a region's ground water—the approach would likely be to use existing data with a readily available assessment method to produce results in relatively short order. On the other hand, a vulnerability assessment can be part of a long-term program to develop and monitor ground water protection strategies and intervention measures. In this case, the assessment would benefit from potentially significant investments in data collection, computer systems to manage the data, and personnel to produce an assessment tailored to the specific needs of the program. These longer-term assessments also may need to be flexible enough to adapt to programmatic changes and to incorporate advances in computer and data management technologies. Coordination with Other Agencies and Institutions A final consideration in planning and conducting a vulnerability assessment is the role of other organizations, agencies, or institutions in the performance or use of the assessment. Often, other agencies or institutions may need or have an interest in specific information to be produced by the assessment. In some cases, another group may be conducting or may have completed a similar vulnerability assessment. More commonly, however, several agencies may be undertaking assessments for somewhat different purposes. Coordination among these organizations and agencies may greatly decrease the cost of the assessment and increase its value and credibility. SUMMARY Vulnerability assessments can meet a variety of needs for ground water managers, land use regulators, resource conservationists, and the general public. Increasing awareness, informing land use decision making, allocating resources, and evaluating alternative policies are just a few of the uses noted in this chapter. Regardless of use, however, tension exists between managers' desire to obtain clear, incontrovertible vulnerability information and the ability of assessments to meet that need. At present, lack of knowledge, data, staff, and time prevent accurate assessments of ground water vulnerability. To reduce this tension and make progress in developing and using vulnerability

OCR for page 30
Ground Water Vulnerability Assessment: Contamination Potential Under Conditions of Uncertainty assessments, action is required from scientists and managers alike. Managers and policy makers should recognize the scientific limitations and expect less from current vulnerability assessments. To reduce uncertainty in assessments, managers will have to provide adequate support for the development and evaluation of improved approaches and data collection. Similarly, scientists should recognize manager's needs for assessments and focus their energies on responding to the uncertainties, high costs, and high levels of expertise needed. In addition, scientists should seek better methods for communicating underlying uncertainties to managers who may not have extensive technical training. Better communication among scientists and managers should surely improve the development of useful and valid vulnerability assessments and the soundness of policies informed by them.