Throughout the United States increasing demands for safe drinking water and requirements to maintain healthy ecosystems are leading policy makers to ask complex social and scientific questions about how to assess and manage our water resources. This challenge becomes particularly difficult as policy and management objectives require scientific assessments of the potential for ground-water resources to become contaminated from anthropogenic, as well as natural sources of contamination. Assessments of the vulnerability of ground water to contamination range in scope and complexity from simple, qualitative, and relatively inexpensive approaches to rigorous, quantitative, and costly assessments. Tradeoffs must be carefully considered among the competing influences of the cost of an assessment, the scientific defensibility, and the amount of acceptable uncertainty in meeting the objectives of the water-resource decision maker.
Purpose of this report
UNDERSTANDING THE HYDROLOGIC SYSTEM AND THE ASSOCIATED BEHAVIOR OF CONTAMINANTS: A NECESSARY STEP IN SCIENTIFIC ASSESSMENTS OF GROUND-WATER VULNERABILITY
The Ground-Water-Flow System
a. Description of a ground-water-flow system
b. Factors that control ground-water movement
c. Advective transport of contaminants through the ground-water-flow system
d. Unsaturated zone
The Geochemical System
a. Sources of contamination
b. Chemical properties
c. Diffusion and dispersion
OVERVIEW OF METHODOLOGY
Subjective rating methods
a. Index methods
b. Subjective hybrid methods
Statistical and process-based methods
a. Statistical methods
b. Process-based methods
SCIENTIFICALLY DEFENSIBLE GROUND-WATER-VULNERABILITY ASSESSMENTS
Examples of considerations in designing a scientifically defensible assessment
Hypothetical Example 1. Regional-scale mapping of ground-water resource vulnerability to a targeted contaminant.
Sources of uncertainty and limitations in the index method used for hypothetical example
Minimizing uncertainty by using sound statistical approaches to meet science objectives of hypothetical example
Hypothetical Example 2. Local-scale process-based determination of the vulnerability of a public ground-water supply to a targeted contaminant.
Sources of uncertainty and limitations in the fixed radius method used for hypothetical example 2
Minimizing uncertainty by using sound process-based approaches to meet science objectives of hypothetical example 2
Summary of scientifically defensible approaches
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