Ground Water Vulnerability Assessment Predicting Relative Contamination Potential Under Conditions of Uncertainty (1993) / Chapter Skim
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1 Introduction
1 Introduction
Pages 13-29
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From page 13... ...
In addition, ground water is the principal source of surface water during low flow periods. About 30 percent of river and stream flow comes from ground water, where it contributes to important ecological habitat as well as surface drinking water supplies (USEPA 1991a)
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From page 14... ...
The cost of replacing contaminated sources with bottled water or other alternatives is high relative to that of existing ground water resources (Abdalla 1990~. PROTECTING GROUND WATER FROM FUTURE CONTAMINATION The seriousness and intractability of the problem of contaminated ground water has led resource managers to pursue a policy of prevention.
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From page 15... ...
Ground Water Vulnerability to Contamination As illustrated in Box 1.3 the concept of ground water vulnerability to contamination has different meanings for different people. In its broadest context, ground water vulnerability refers to whether or not an underlying aquifer will become contaminated as a result of activities at the land surface.
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From page 16... ...
for specific pesticides of concern." An SMP is supposed to describe a state's approach to ground water protection for a specific pesticide based on local differences in ground water use, value, vulnerability, and sensitivity. Thus, as a part of their ground water protection responsibilities, states are strongly encouraged to conduct vulnerability assessments.
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From page 17... ...
General Accounting Office (1 991) Hydrogeologic Vulnerability - "a function of geologic factors such as soil texture and depth to groundwater." Total Vulnerability - "a function of these hydrogeologic factors, as well as the pesticide use factors that influence the site's susceptibility." Total Risk - "This last approach is even broader, for it incorporates the size of the population at risk from potential pesticide contamination that is, the number of people who obtain their drinking water from ground water in the area." Pettyjohn et al.
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From page 18... ...
While in many places these sources and pathways may be the dominant cause of contamination, the concept of ground water vulnerability addresses only contaminants introduced by humans above the water table at or near the land surface. Other potential contamination must be addressed on a case-by-case basis using other means.
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From page 19... ...
In this sense, a vulnerability assessment is a predictive statement much like a weather forecast, but for processes that take place underground and on much longer time scales. An array of methods for predicting ground water vulnerability has been developed.
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Key elements to consider in a vulnerability assessment for a particular application include the reference location (e.g., the water table or a specified position within the ground water system) , the degree of contaminant specificity, the contaminant pathways considered, and the time and spatial scales of the assessment.
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From page 21... ...
Most measures of ground water vulnerability to contamination assume simple percolation from the land surface and ignore preferential flow paths, such as biochannels (root holes and worm holes) and cracks, joints, and solution channels in the vadose zone.
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From page 22... ...
It is difficult to incorporate these types of contaminant pathways into quantitative measures of ground water vulnerability, yet they may be the primary control on the vulnerability of deeper aquifers to contamination. One approach is to overlay information on the potential for cross-contamination of deeper aquifers (e.g., sink holes or agricultural drainage areas)
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From page 23... ...
Box 1.4 Scale, Size, and Maps Vulnerability assessments are performed over areas ranging from the national level, through state, county and hydrologic unit levels, to the field level, where a field is as small as 2 or 3 acres. The degree of resolution required for a vulnerability assessment will depend on the purpose for which it is intended.
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From page 24... ...
24 GROUND WATER VULNERABILITY ASSESSMENT The usage of the cartographer's term "scale" can be confusing. Maps that provide high resolution, such as 1:12,000 and 1 :24,000, are considered "large-scale." Maps having a lower level of resolution, such as 1 :250,000 and 1 :2,000,000, are termed "small-scale." AS a point of reference, Figures 1.2a, b, and c show a 3-acre field plot depicted on a 1 :100,000, 1 :24,000, and 1 :12,000 maps, respectively.
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INTROD UCTION 25 -- I; .11 it 0 1,000 R FIGURE 1.2b See Figure 1.2a for description.
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From page 26... ...
They include: determining the purpose of the assessment; selecting an assessment method, dealing with issues of uncertainty and evaluation; identifying the needs, availability, and quality of data; and eventually using the completed assessment in managing ground water resources. As the flowchart shows, the approach used to assess ground water vulnerability is central to the process, but is also directly affected by inputs or considerations entailed by the purpose, data availability, and management use of the assessment.
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From page 27... ...
Once an assessment is complete, various management actions may be taken to protect ground water quality or minimize contamination. Management actions could range from altering land use practices, targeting resource allocations, or disseminating vulnerability information through an educational program to collecting additional data on factors relating to vulnerability or ground water quality.
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From page 28... ...
In Iowa, the vulnerability assessments were conducted to acquire improved information that would help develop ground water protection strategies based on voluntary action, but not to support regulatory activities. In contrast, vulnerability assessments are being used in Florida and on Cape Cod to delineate land areas that will be targeted for differential management in order to protect the quality of ground water resources.
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From page 29... ...
1993. A Review of Methods for Assessing Aquifer Sensitivity and Ground Water Vulnerability to Pesticide Contamination.
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