is the main user of fresh water, amounting to approximately 70% worldwide and up to 90% in developing countries. Together, irrigation and drainage (especially drainage water reuse) are a major source of salts and toxic trace elements in arid and semiarid regions.

Water, in both quantity and quality, is inextricably linked to public health. The availability of safe water is a basic human necessity and often is the first resource to become critically short in a natural disaster (such as recently encountered along the U.S. Gulf Coast in the aftermath of Hurricane Katrina). The availability and sustainability of safe water from surface and underground sources, particularly in the context of climate changes and population pressure, is a critical area of research that is beyond the scope of this report. This chapter will be limited to a description of the constituents in drinking water as potential benefits to public health (e.g., fluoride) and as potential hazards to public health (e.g., microbial contamination or dissolved toxic elements). The chapter will focus on the threshold research areas, with particular attention to the aspects of water and health that are directly influenced by earth science and the geological framework.

Drinking water contains a variety of substances that result from interactions with geological materials or from other sources such as atmospheric deposition, land application of fertilizer and wastes, mine drainage, and discharge of waste to surface water bodies. These include metals, major and trace elements, natural and anthropogenic organic substances, and microorganisms. Some of these constituents are essential nutrients; many have unknown or only suspected health benefits; and others are clearly health hazards.

In groundwater, inorganic constituents are transported primarily in a dissolved or nanoparticulate form. These constituents are able to enter the drinking water distribution system unless the water is subjected to appropriate treatment processes. Natural organic matter is also a component of natural waters, with largely unknown direct health implications. These substances have a well-established ability to form complexes with metals and potentially enhance the dissolution of minerals and mobilize sparingly soluble metal ions (Hem, 1985). In addition, natural organic matter interacts with chlorine and other drinking water disinfectants to form a dilute mixture of disinfection byproducts that may be mutagenic and/or carcinogenic (Jolley et al., 1984; NRC, 1987; Gerba et al., 2006). Anthropogenic organic materials have a wide variety of potential source points and a wider range of potential health impacts. For virtually all anthropogenic organic compounds, the geological framework—including surface topography, soils, and the vadose zone—exerts a fundamental control on the transport properties from source point to receiver.



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