Executive Summary

INTRODUCTION

In recognition of the need for effective and efficient methods for protecting ground water resources from future contamination, scientists and resource managers have sought to develop techniques for predicting which areas are more likely than others to become contaminated as a result of activities at or near the land surface. Once identified, those areas could then be subjected to certain use restrictions or otherwise targeted for greater attention aimed at preventing contamination of the underlying ground water resources.

The concept that some areas are more likely than others to become contaminated has led to the use of the terminology ''ground water vulnerability to contamination." This basic concept has taken on a range of definitions in the technical literature. For the purposes of this report, ground water vulnerability to contamination is defined as:

The tendency or likelihood for contaminants to reach a specified position in the ground water system after introduction at some location above the uppermost aquifer.

As considered herein, ground water vulnerability refers to contamination resulting from nonpoint sources or areally distributed point sources of pollution and does not address individual point sources of pollution nor any situation where a pollutant is purposely placed into the ground water system. This definition of ground water vulnerability is flawed, as is any



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Ground Water Vulnerability Assessment: Contamination Potential Under Conditions of Uncertainty Executive Summary INTRODUCTION In recognition of the need for effective and efficient methods for protecting ground water resources from future contamination, scientists and resource managers have sought to develop techniques for predicting which areas are more likely than others to become contaminated as a result of activities at or near the land surface. Once identified, those areas could then be subjected to certain use restrictions or otherwise targeted for greater attention aimed at preventing contamination of the underlying ground water resources. The concept that some areas are more likely than others to become contaminated has led to the use of the terminology ''ground water vulnerability to contamination." This basic concept has taken on a range of definitions in the technical literature. For the purposes of this report, ground water vulnerability to contamination is defined as: The tendency or likelihood for contaminants to reach a specified position in the ground water system after introduction at some location above the uppermost aquifer. As considered herein, ground water vulnerability refers to contamination resulting from nonpoint sources or areally distributed point sources of pollution and does not address individual point sources of pollution nor any situation where a pollutant is purposely placed into the ground water system. This definition of ground water vulnerability is flawed, as is any

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Ground Water Vulnerability Assessment: Contamination Potential Under Conditions of Uncertainty other, by a fundamental principle that is stated here as the First Law of Ground Water Vulnerability: All ground water is vulnerable. Vulnerability is not an absolute property, but a relative indication of where contamination is likely to occur; no ground water, with possible exceptions such as deep sedimentary basin brines, is invulnerable. Furthermore, it may be necessary to consider long term effects on ground water quality, perhaps over decades, in carrying out vulnerability assessments. Ground water vulnerability is an amorphous concept, not a measurable property. It is a probability (i.e., "the tendency or likelihood") of contamination occurring in the future, and thus must be inferred from surrogate information that is measurable. In this sense, a ground water vulnerability assessment is a predictive statement much like a weather forecast, but for processes that take place underground and over much longer time scales. The potential for contaminants to leach to ground water depends on many factors, including the composition of soils and geologic materials in the unsaturated zone, the depth to the water table, the recharge rate, and environmental factors influencing the potential for biodegradation. The composition of the unsaturated zone can greatly influence transformations and reactions. For example, high organic matter or clay content increases sorption and thus lessens the potential for contamination. The depth to the water table can be an important factor because short flow paths decrease the opportunity for sorption and biodegradation, thus increasing the potential for many contaminants to reach the ground water. Conversely, longer flow paths from land surface to the water table can lessen the potential for contamination for chemicals that sorb or degrade along the flow path. Recharge rates affect the extent and rate of transport of contaminants through the saturated zone. Finally, environmental factors, such as temperature and water content, can significantly influence the degradation of contaminants by microbial transformations. An array of approaches for predicting ground water vulnerability has been developed from an understanding of the factors that affect the transport of contaminants introduced at or near the land surface. These methods fall into three major classes: (1) overlay and index methods that combine specific physical characteristics that affect vulnerability, often giving a numerical score,  (2) process-based methods consisting of mathematical models that approximate the behavior of substances in the subsurface environment, and (3) statistical methods that draw associations with areas where contamination is known to have occurred. Each of these methods requires that adequate data be available on factors that affect ground water vulnerability, such as soil properties, hydraulic properties, precipitation patterns, depth to ground water, land use and land

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Ground Water Vulnerability Assessment: Contamination Potential Under Conditions of Uncertainty cover, and other characteristics of the area to be assessed. Different types and amounts of data are necessary depending on the specific assessment method used. The product of most vulnerability assessments to date has been a map depicting areas of relative vulnerability. Some researchers have chosen instead to express results as probabilities with the associated uncertainties displayed in tabular form. It is infeasible and perhaps impossible to formulate a universal technique for predicting vulnerability, one that considers all of the ways in which contamination occurs or that is appropriate for all situations. Key elements to consider in a vulnerability assessment for a particular application include the reference location, the degree of contaminant specificity, the contaminant pathways considered, and the time and spatial scales of the vulnerability assessment. The reference location is the position in the ground water system specified to be of interest. The ground water table is the reference location used in most existing techniques. However, managers may determine that another reference location is more useful for their purposes. Vulnerability assessments may or may not account for the different behavior of different contaminants in the environment. Thus, there are two general types of vulnerability assessments. The first addresses specific vulnerability, and is referenced to a specific contaminant, contaminant class, or human activity. The second addresses intrinsic vulnerability and is for vulnerability assessments that do not consider the attributes and behavior of specific contaminants. In practice, a clear distinction between intrinsic and specific vulnerability cannot always be made. Contaminants can enter aquifers by a variety of pathways. Most existing assessment techniques address only transport that occurs by simple percolation and ignore preferential flow paths such as biochannels, cracks, joints, and solution channels in the vadose zone. The omission of preferential flow paths is likely a significant limitation of vulnerability assessments in many environments. Some overlay and index methods have attempted to address contamination that might occur by wells and boreholes by mapping those features in combination with the results derived from other assessment methods. The overall utility of a vulnerability assessment is highly dependent on the scale at which it is conducted, the scale at which data are available, the scale used to display results, and the spatial resolution of mapping. The combination of these elements makes up a vulnerability assessment method. Inherent in any such combination will be scientific uncertainties associated with errors in data, errors in method, and potential misapplication of an approach to a given area. The prediction of ground water vulnerability is an imprecise exercise, as stated in the Second Law of Ground Water Vulnerability: Uncertainty is inherent in all vulnerability assessments.

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Ground Water Vulnerability Assessment: Contamination Potential Under Conditions of Uncertainty The Vulnerability Assessment Process Although a large degree of uncertainty is associated with the results of existing vulnerability assessment methods, much useful information can be gained by going through the process of assessing ground water vulnerability. Ground water vulnerability assessment is a potentially useful management concept for guiding decisions about ground water protection and thus requires the cooperative efforts of regulatory policy makers, natural resource managers, educators, and technical experts. The process of assessing vulnerability is dynamic and iterative. It requires determination of the purpose of the assessment, followed by selection of a method, identification of the type, availability, and quality of data needed, performance of the actual assessment, and, finally, use of the information gained from the assessment process to make decisions on ground water resource management. This process involves the gathering, organization, and, ideally, critical evaluation of as much information as possible relating to the potential for contamination to occur in the area being assessed. Through this process scientists and managers can develop a better understanding of ground water systems, which should help them make better decisions on how to protect ground water resources. MANAGEMENT The intended use of the vulnerability assessment process is the most obvious and important factor to consider in selecting a vulnerability assessment approach. Uses and needs for vulnerability assessments can be grouped into four broad categories. First, assessments can be used in the policy analysis and development process to identify potential for ground water contamination and the need for protection and to aid in examining the relative impacts of alternative ways to control contamination. Second, when scarce resources prevent uniform and high levels of spending, vulnerability assessments can be used in program management to guide allocation and targeting of resources to areas where the greatest levels of effort are warranted. Third, vulnerability assessments can be used in some instances to inform land use decisions such as 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 in a vulnerability assessment the objective, scientific, and accurate product they need for making these decisions. Rather they will find that its usefulness may be severely constrained

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Ground Water Vulnerability Assessment: Contamination Potential Under Conditions of Uncertainty by scientific unknowns or lack of appropriate data. Thus, it is important that policy makers and resource managers become intelligent consumers of vulnerability assessments. Important technical and institutional considerations should be taken into account in the process of developing a vulnerability assessment. Technical considerations include an evaluation of the type and form of the results or output, the appropriateness of the method for the physical characteristics of the geographic area being addressed, the adequacy of the data available or to be collected, and the analysis of uncertainty in the output and how it may affect the consequent decisions. Important institutional issues include the time frame in which the assessment is meant to apply, how the vulnerability assessment will be 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 an assessment, and the plans and activities of other agencies and institutions that may have an interest in the assessment. These factors are not mutually exclusive. APPROACHES TO VULNERABILITY ASSESSMENT The three classes of methods for assessing ground water vulnerability range in complexity from a subjective evaluation of available map data to the application of complex transport models. Each class has characteristic strengths and weaknesses that affect its suitability for particular applications. Overlay and index methods involve combining various physical attributes (e.g., geology, soils, depth to water table, well locations). In the simplest of these methods, all attributes are assigned equal weights with no judgment being made on their relative importance. Thus areas where specified attributes mutually occur (e.g., sandy soils and shallow ground water) are rated as more vulnerable. These methods were the earliest to be used in assessing ground water vulnerability and are still favored by many state and local regulatory and planning agencies. Overlay and index methods that attempt to be more quantitative assign different numerical scores and weights to the attributes in developing a range of vulnerability classes which are then displayed on a map. Specific issues that need to be considered regarding the suitability of overlay and index methods for particular applications include the relative importance of the physical attributes in influencing vulnerability, the natural variability in the attributes used, and the availability and spatial resolution of data. The factors that affect ground water vulnerability vary from place to place, as does their relative importance. Therefore it is important that the attributes included in an assessment be appropriate for the specific situation and, if they are to be weighted, that their weights reflect the particular

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Ground Water Vulnerability Assessment: Contamination Potential Under Conditions of Uncertainty physical setting. No single set of factors or weights is suitable for all situations. Most methods use a single, average annual value for each attribute at each point location, but, attributes such as depth to ground water and recharge often vary in time, both seasonally and annually. Overlay and index methods are often preferred because the data they require are generally more available. In addition, these methods are relatively simple; while they include factors important in determining ground water vulnerability, they do not attempt to fully describe the processes that lead to contamination. Approaches using process-based simulation models require analytical or numerical solutions to mathematical equations that represent coupled processes governing contaminant transport. Methods in this class range from indices based on simple transport models to analytical solutions for one-dimensional transport of contaminants through the unsaturated zone to coupled, unsaturated-saturated, multiple-phase, two- or three-dimensional models. These approaches are distinguished from others in that many of them attempt to predict contaminant transport in both space and time. While process-based models attempt to incorporate a more complete description of the physical, chemical, and biological processes affecting ground water vulnerability, they may not necessarily provide more reliable results. The data these methods require often are not available and must be estimated by indirect means. In addition, these models do not account for flow and transport processes occurring at either smaller or larger spatial scales than those for which the models were developed, and they do not account for cases where preferential flow exists. Statistical methods generally use a contaminant concentration or a probability of contamination as the dependent variable. These methods incorporate data on known areal contaminant distributions and provide characterizations of contamination potential for the specific geographic area from which the data were drawn. Statistical methods have been developed with the availability of data keenly in mind and are designed to deal with data of varying quality and types. They do not attempt to define processes or cause-effect relationships, and results are expressed as probabilities. These methods have been used in the definition and characterization of assessment areas and the assessment of vulnerability using probability models. Statistical approaches vary in complexity and generally include multiple independent variables. The primary consideration in the use of statistical methods is that the area to which they are applied must be comparable to that in which they were developed. In addition, because statistical methods rely on information about where ground water has been contaminated, it is important that adequate monitoring and chemical use information be available.

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Ground Water Vulnerability Assessment: Contamination Potential Under Conditions of Uncertainty Uncertainty in Vulnerability Assessment Uncertainty is inherent in all methods of assessing ground water vulnerability. Uncertainties arise from errors in obtaining data, due to natural spatial and temporal variability, in computerization, processing, and storage of data, and in modeling and conceptualization. Results of vulnerability assessments are usually displayed on a map of a region depicting various subareas, called polygons or cells, having different levels of vulnerability. The distinction between each level is, however, arbitrary. Further, the estimates of vulnerability are associated with a level of uncertainty. Thus, confidence intervals associated with the numerical values assigned to neighboring cells or polygons may overlap to the point that subtle distinctions perceived in the vulnerabilities of adjacent cells are not defensible. The inability to distinguish differences between adjacent cells with differing vulnerability scores increases with increasing uncertainties in methods and data. Few published vulnerability assessments account for uncertainties from either model or data errors, although an array of methods would be appropriate for this application. It is important that uncertainty analyses be included in vulnerability assessments so that users can develop an understanding of the level of knowledge about vulnerability and the hydrologic system in the area that is being studied. In addition, uncertainty analyses can help to identify which attributes require more accurate measurements in order to reduce overall uncertainty, identify attributes for which less precise information is required and thereby save in data collection efforts, and determine whether a simpler approach would suffice or if a more sophisticated approach is needed for better reliability (Heuvelink et al. 1989). Testing and Evaluation of Vulnerability Assessments Evaluation of a vulnerability assessment must address at least two questions: (1) Is the vulnerability rating assigned to a given subarea valid? and (2) Are the values assigned to neighboring subareas sufficiently different? To answer these questions, assessment results must be compared with observations in the environment. It is not possible to test regional vulnerability assessments on even a field-scale in the same way that a site-specific simulation model can be tested, nor is it possible to make definitive statements about the predictive accuracy of one method compared to another. One difficulty is that a vulnerability assessment method may yield an index value or a probability, which, unlike a concentration, cannot be measured in the field. Also, to make meaningful comparisons of predicted levels of vulnerability and observed constituent concentrations, it is necessary either to know the history

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Ground Water Vulnerability Assessment: Contamination Potential Under Conditions of Uncertainty of contaminant loading to the subsurface or to assume that contaminant loading has been spatially and temporally uniform. Neither situation is likely to exist, except perhaps in a controlled field-plot study. Further, in that the goal of assessing ground water vulnerability is to assist in the protection of ground water resources, to allow contamination to occur on a regional scale as predicted would be counterproductive. Despite these difficulties, the validity of a regional vulnerability assessment can be inferred through several lines of inquiry. Testing and evaluating vulnerability assessments may involve a hierarchal approach that evolves through several stages. The most sensible uses of vulnerability assessment techniques, in fact, may include plans to test, review, and refine the assessment over time, perhaps over many years. Ground water vulnerability predictions are made in a relative, not an absolute, sense. Assessments only distinguish some areas in a region as being more or less vulnerable than other areas. Uncertainty is pervasive in both spatial databases and computational schemes; as a result, all vulnerability assessments are inherently uncertain. It may be fairly easy to identify areas where ground water contamination is highly probable, but may not be equally easy to delineate areas where it is highly improbable. For example, it is clear that ground water in a mature karst aquifer system or a shallow sand and gravel alluvial aquifer is highly vulnerable to contamination. However, it may be much more difficult to demonstrate that ground water underlying a clay-rich unsaturated zone indeed has low vulnerability to contamination because many difficult-to-quantify factors, such as preferential flow paths, can complicate the situation. Moreover, differentiating areas that are not highly vulnerable in terms of more subtle distinctions in vulnerability is very difficult. This concept can be summarized as the Third Law of Ground Water Vulnerability: The obvious may be obscured and the subtle indistinguishable. Computing Environments for Vulnerability Assessments Regardless of method, much data on attributes and geography are required to conduct a ground water vulnerability assessment. In addition, suitable analytical tools are required to prepare, combine, study, and display the various components of the assessment. Numerous techniques have been used to perform these tasks, normally following advances in the allied fields of computer, graphic, and statistical sciences. The two computing environments used for vulnerability assessments are grid-cell based systems and geographic information systems (GIS). Grid-cell based systems were the first to evolve and are rapidly giving way to GIS, which in many ways is considered more flexible.

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Ground Water Vulnerability Assessment: Contamination Potential Under Conditions of Uncertainty The oversimplification that can occur using any method that involves the display of results in the form of a map raises important concerns. Often these concerns can be mitigated by addressing critical questions about the intended use of the assessment and uncertainties associated with the results. GIS technology provides new opportunities to describe and display graphically uncertainty associated with each assessment, and exhaust the tabular data through the construction of many different maps. DATA AND DATABASES Most research has been concerned primarily with the processes that affect vulnerability; less attention has been paid to data collection, entry and management, and the computing environment. These issues, however, are critical to the ability to conduct a successful vulnerability assessment. Federal, state, and local government agencies collect massive quantities of data each year. The Federal Geographic Data Committee has been established to provide oversight of federal agencies involved in collecting and using spatial data and their attributes, to coordinate data collection and sharing, and to establish federal standards for geographic data exchange, content, and quality. The quality and availability of geographic and attribute information at the state and local levels is highly variable. Databases are available to varying extents for parameters relating to topography, soils, hydrogeology, weather and climate, land use and land cover, and management factors. Not all of these data are readily available in digital form or at the scale needed for different types of assessments. Accelerated efforts to improve and develop spatial and attribute databases will facilitate the improvement of ground water vulnerability assessment methods. CASE STUDIES An array of methods for assessing ground water vulnerability is being used around the country. Several examples illustrate the diversity in techniques and the factors that influenced their selection. In Iowa, a qualitative overlay method is used to assess intrinsic vulnerability; a single vulnerability map was prepared showing depth to ground water and the location of well holes. On Cape Cod, Massachusetts, where the geological setting is relatively homogeneous, deterministic models for ground water flow and solute transport in the aquifer were used to identify where contamination potentially could affect well fields. In Florida, overlay and index methods were used to assess ground water vulnerability to pesticide contamination. In the San Joaquin Valley of California an approach based on detection of contamination has been used to address potential for further pesticide contamination.

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Ground Water Vulnerability Assessment: Contamination Potential Under Conditions of Uncertainty The USDA has developed a hybrid approach using an index method coupled with a simulation model to assist decision making at the national level. Finally, on Oahu, Hawaii, process-based models were used to predict pesticide transport in the vadose zone. RESEARCH RECOMMENDATIONS Critical evaluation and understanding of uncertainty is vital to the use of any means of vulnerability assessment. The following recommendations constitute a research agenda aimed at reducing uncertainty in vulnerability assessments and improving opportunities to use them effectively. Develop a better understanding of all processes that affect the transport and fate of contaminants. Establish simple, practical, and reliable methods for measuring in situ hydraulic conductivities of the soil and the unsaturated and saturated zones. Develop methods for scaling measurements that sample different volumes of porous materials to provide equivalent measures. Develop simple, practical, and reliable methods for measuring in situ degradation rates (e.g., hydrolysis, methylation, biodegradation), and develop methods for characterizing changes in degradation rate as a function of other physical parameters (e.g., depth in soil). Develop improved approaches to obtaining information on the residence time of water along flow paths and identifying recharge and discharge areas. Develop unified ways to combine soils and geologic information in vulnerability assessments. Improve the chemical databases, currently the source of much uncertainty in vulnerability assessments. Determine the circumstances in which the properties of the intermediate vadose zone are critical to vulnerability assessments and develop methods for characterizing the zone for assessments. Establish in the soil mapping standards of USDA's Soil Conservation Service an efficient soil sampling scheme for acquiring accurate soil attribute data in soil mapping unit polygons and documenting the uncertainty in these data. A need exists to better characterize the inclusions of other soil types in soil mapping units, including fractional area of included soil and distribution of inclusions. Establish reliable transfer functions for estimating in situ hydraulic properties using available soil attribute data (e.g., bulk densities, particlesize distributions, etc.). Develop ways to determine the additional uncertainty arising from the use of transfer functions in ground water vulnerability assessments.

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Ground Water Vulnerability Assessment: Contamination Potential Under Conditions of Uncertainty Develop methods for merging data obtained at different spatial and temporal scales into a common scale for vulnerability assessment. Improve analytical tools in GIS software to facilitate integration of assessment methods with spatial attribute databases and the computing environment. Establish more meaningful categories of vulnerability for assessment methods. Determine which processes are most important to incorporate into vulnerability assessments at different spatial scales. Obtain more information on the uncertainty associated with vulnerability assessments and develop ways to display this uncertainty. Methods are needed that can identify and differentiate among more sources of uncertainty. Develop methods for accounting for soil macropores and other preferential flow pathways that can affect vulnerability. These investigations should include evaluations of the uncertainty in methods and measurements as they affect the assessment. Develop method for incorporating process-based, statistical, and qualitative information into an integrated or hybrid assessment. Identify counterintuitive situations leading to greater true vulnerability than commonly perceived. For example, develop greater understanding of the circumstances in which low-permeability materials that overlay aquifers can transmit contaminants to ground water. REFERENCE Heuvelink, G.B.M., P.A. Burrough, and A. Stein. 1989. Propagation of errors in spatial modelling with GIS. Int. Jour. Geographical Information Systems 3(4):303-322.

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