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Circular 1224--Assessing Ground-Water Vulnerability to Contamination: Providing Scientifically Defensible Information for Decision Makers


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.


The Safe Drinking Water Act as amended in 1996 began a new era of prevention of drinking-water contamination and emphasized the importance of source-water management. The U.S. Environmental Protection Agency (USEPA) source-water assessment program, which has arisen from the Act, requires States to "assess water systems' susceptibility to contamination" (U.S. Environmental Protection Agency, 1997). Viewed as a first step in a 'multiple barrier' approach to protect drinking water, source-water assessments should consider the potential for contamination of ground-water resources and often incorporate existing wellhead protection programs. The need for ground-water vulnerability assessments is found in many other Federal, State, and local water-management programs, including the identification and location of sustainable sources of drinking water, ground-water disinfection, State pesticide management plans, underground injection of waste, and confined animal feeding operations. A National Research Council (1993) publication summarizes the broad array of definitions and approaches that are used by government as well as private and academic organizations in assessing the vulnerability of ground water to contamination. Based on specific objectives and resources available, assessments can be scoped to include individual wells or entire aquifer systems and target one contaminant, a group of similar contaminants, or contamination in general. The differences in the effectiveness of individual assessments will be linked to the degree to which the important physical/chemical processes for each particular situation have been identified and accounted for, uncertainty is addressed, and the original science and management objectives are appropriately met.

Purpose of this report

This report provides an overview of some of the common approaches used to scientifically determine the important factors controlling the vulnerability of ground-water resources to contamination. In addition, the report discusses the strengths and weaknesses of the various approaches as sources of scientifically defensible information for the water-resource management decision-making process. Descriptions of scientifically defensible methods are supported by example studies that have been conducted by the U. S. Geological Survey (USGS) often in cooperation with local, state, and regional water-resources agencies.

Two hypothetical examples of ground-water vulnerability assessments are used to define differences between management and science objectives and to highlight the process towards scientifically defensible water-resource management decision making. In this way, the report recognizes the broad array of appropriate methodologies that maintain objectivity, are scientifically defensible, and meet science objectives. These examples provide the background to show how successful ground-water vulnerability assessments blend scientifically defensible analyses with additional interpretations by water-resource decision makers in order to meet management and policy objectives.

The intended audience for this report includes experienced land- and water-resource managers, regulators, and policy makers (hereafter collectively referred to as water-resource decision makers) who must use vulnerability assessments in their work, as well as managers of scientific agencies (including the USGS) who are responsible for the design and promotion of investigations to scientifically determine the important factors controlling the vulnerability of ground-water systems to contamination. This report also may be useful as a primer to understand the science required by water-resource decision makers to assess ground-water vulnerability. For more detailed information on the scientific aspects of ground-water vulnerability assessments, examples of various approaches, and guidance, the reader is referred to the National Research Council (1993), Alley (1993), Barbash and Resek (1996), Nolan (1998), the U.S. Environmental Protection Agency (1997), the American Society for Testing and Materials (2000), or any of the case studies presented or cited in this report.

What is scientifically defensible?

The scientific method can be defined as principles and procedures for the systematic and objective pursuit of knowledge involving the recognition and formulation of a problem, the collection of data through observation and experiment, and the formulation and testing of hypotheses. The scientific method therefore requires the objective use of facts without distortion by subjective feelings or prejudices. A scientifically defensible ground-water vulnerability assessment is one that follows the scientific method and includes adequate documentation of data, observations, and method of investigation to allow for independently reproducible results.

Next: Understanding the hydrologic system and the associated behavoir of contaminants: A necessary step in scientific assessments of ground-water vulnerablity