The U.S. Environmental Protection Agency recently established a new maximum contaminant level of 10 micrograms per liter for arsenic in drinking water in the United States. Ground water is the primary source of drinking water for half the population of the United States. Several national assessments have found that high arsenic concentrations (above 10 micrograms per liter) are widespread in drinking-water aquifers in the western United States, the Great Lakes region, and New England. Moderate to high concentrations were identified in ground water in parts of the central and southern United States. This chapter summarizes national trends in the use of ground water as drinking water, and national estimates of arsenic occurrence in potable ground water. The chapter also briefly describes several studies on arsenic in specific settings and water-use scenarios; these studies illustrate by example the potential power of a regional approach to understanding and managing arsenic in drinking water.
Characterizing all possible chemical mixtures in drinking water is a potentially overwhelming project, and the task of assessing each mixture's net toxicity even more daunting. We propose that analyzing occurrence information on mixtures in drinking water may help to narrow the priorities and inform the approaches taken by researchers in mixture toxicology. To illustrate the utility of environmental data for refining the mixtures problem, we use a recent compilation of national ground-water-quality data to examine proposed U.S. Environmental Protection Agency (EPA) and Agency for Toxic Substances and Disease Registry (ATSDR) models of noncancer mixture toxicity. We use data on the occurrence of binary and ternary mixtures of arsenic, cadmium, and manganese to parameterize an additive model and compute hazard index scores for each drinking-water source in the data set. We also use partially parameterized interaction models to perform a bounding analysis estimating the interaction potential of several binary and ternary mixtures for which the toxicological literature is limited. From these results, we estimate a relative value of additional toxicological information for each mixture. For example, we find that according to the U.S. EPA's interaction model, the levels of arsenic and cadmium found in U.S. drinking water are unlikely to have synergistic cardiovascular effects, but the same mixture's potential for synergistic neurological effects merits further study. Similar analysis could in future be used to prioritize toxicological studies based on their potential to reduce scientific and regulatory uncertainty. Environmental data may also provide a means to explore the implications of alternative risk models for the toxicity and interaction of complex mixtures.
The U.S. Geological Survey (USGS) National Water-Quality Assessment (NAWQA) Program is designed to assess the status of and trends in the quality of the Nation's wj ter resources, and to gain a better understanding of the natural and human factors that affect water quality. The Central Columbia Plateau is one of 60 NAWQA study units (major river basins and parts of aquifer systems) located throughout the Nation. In the Central Columbia Plateau, nitrate concentrations for 19% of the 573 wells shown below exceed the U.S. Environmental fflrotection Agency (EPA) maximum contaminant level (MCL) for drinking water. These concentrations include USGS samples from 1942-94, although 93% of the data are from 1980-94. one analysis was available for a well, this document refers to the mean concentration as the nitrate concentration for the well. Land use is the greatest influence over nitrate concentration. Nitrate concentration is related in gener and water usage. Nitrate concentrations ground water vary grej generally increased since
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