Plan for assessment of the occurrence, status, and distribution of volatile organic compounds in aquifers of the United States
The occurrence of volatile organic compounds (VOCs) in water is of national concern because of their relatively high aqueous solubility, mobility, and persistence, because many are known or suspected carcinogens, because of their widespread use, and because they have been found in drinking-water supplies. Because of this national concern, VOCs were selected for National investigation (hereafter termed "National Synthesis") by the U.S. Geological Survey's National Water-Quality Assessment (NAWQA) Program in 1994. The broad goals of this National Synthesis are to: (1) describe current water-quality conditions with respect to VOCs; (2) define trends, or lack of trends, in VOCs in surface and ground water; and (3) identify, describe, and explain causal relations among the occurrence and distribution of VOCs in surface water and ground water, and natural and human factors. The National Synthesis of VOCs in ground water has three objectives: (1) to describe their occurrence, status, and distribution; (2) to determine relations among VOCs in shallow ground water and natural and human factors; and (3) to determine, compare, and contrast the occurrence, transformation, transport, and fate of selected VOCs in the hydrologic cycle for several regionally or nationally important aquifer systems. The description of VOC occurrence, status, and distribution in ground water focuses on major aquifers of the United States. Occurrence describes the presence or absence of VOCs, their frequency of occurrence, and their ranges of concentrations. Status compares the concentrations of VOCs detected in relation to waterquality regulations or advisories, such as Maximum Contaminant Levels, Proposed Maximum Contaminant Levels, Maximum Contaminant Level Goals, and Health Advisories. Distribution describes the variability of VOCs in ground water, areally and by depth. This report describes the study design for conducting such an assessment. The assessment focuses on aquifers, or parts of aquifers, that are currently used or have the potential to be used as sources of water supplies, using data collected as part of local, State, and Federal ground-water monitoring programs since 1985. Assessment by aquifer and comparison of results among aquifers will be completed for those aquifers for which adequate spatial or depth-related data are available. Assessment of VOCs in aquifers also will be completed at regional and national scales. A set of criteria for well-network design, well construction, sample-collection methods, and methods of laboratory analysis must be met before VOC data are used for assessment. An appropriate well-network design will provide a generally unbiased, random, equal-area distribution of sampling sites throughout the aquifer, or part of the aquifer, of interest. Well-construction