Ground Water Vulnerability Assessment Predicting Relative Contamination Potential Under Conditions of Uncertainty (1993) / Chapter Skim
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Executive Summary
Executive Summary
Pages 1-12
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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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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.
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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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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.
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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.
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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.
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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.
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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.
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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.
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Develop ways to determine the additional uncertainty arising from the use of transfer functions in ground water vulnerability assessments.
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· Establish more meaningful categories of vulnerability for assessment methods. Determine which processes are most important to incorporate into vulnerability assessments at different spatial scales.
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