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. Advances in analytical and environmental chemistry; Increased knowledge of the adverse effects of chemicals on humans; Public awareness of ground water pollution; The advent of federal ground water quality protection legislation in the 1970s, and, Statutes relating to ground water quality enacted by the California Legislature. Since the 1970s an entire industry has developed around ground water quality monitoring and monitoring well construction. Numerous private firms are involved in providing technical services for the design and implementation of ground water quality investigations. Many firms are involved in the manufacture, distribution, and marketing of materials and equipment used in constructing and operating monitoring wells. Most monitoring wells constructed today are used to assess: .The nature and distribution of pollutants and contaminants in ground water; .The nature and distribution of naturally occurring chemical constituents; .Subsurface hydrologic conditions; and, .Hydraulic properties of strata as they relate to pollutant and contaminant movement. Some monitoring wells are designed to be multipurpose. Monitoring wells can sometimes be used as "extraction" or "injection" wells for mitigation of pollution or contamination. Although a significant number of monitoring wells constructed today are for detection and assessment of ground water quality impairment, many monitoring wells are constructed for evaluating ground water supply conditions by allowing ground water level measurement and/or aquifer testing. Still others are constructed for observing water levels associated with excavations and irrigated agriculture. . During 1989, approximately 20 percent of all well drilling in California was for monitoring wells, based on well driller's reports received by the Department of Water Resources. Monitoring wells have been constructed in nearly all California counties. The largest concentrations of water quality monitoring wells occur in metropolitan areas of the State. Large numbers of monitoring wells are installed for detection and assessment of leaks from underground storage tanks. Types of Monitoring Wells For the purpose of these standards, the term "monitoring well" is limited to wells designed to monitor subsurface water in 1he saturated zone, existing at or above atmospheric pressure (ground water); rather than water, water vapor, and/or gases contained in the unsaturated or vadose zone. Monitoring devices used for the unsaturated zone differ significantly from those used for the saturated (ground water) zone. As shown in Figure 2, three basic types of monitoring wells or "installations" are: .Individual monitoring wells; .Nested monitoring wells; and, .Clustered monitoring wells. Individual monitoring wells consist of a single casing "string" within a borehole, as illustrated in Figures 2 and 3. Individual monitoring wells are installed in unique locations apart from one another. They are the most common type of monitoring well constructed in California..-34-