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basins and aquifer systems in the Nation, and incorporate about 60 to 70 percent of the Nation's water use and population served by public water supply. Investigation in each Study Unit will be conducted on a rotational rather than a continuous basis. One-third of the Study Units will be studied intensively at a given time. For each Study Unit, a 3- to 4-year intensive period of data collection and analysis will be alternated with a 6- to 7-year period of low-intensity as- sessment activities. The first intensive period of study for 20 of the 60 Study Units began in 1991, and another 20 began, in 1994. During the first cycle of intensive study, Study-Unit Investigations will consist of four components: Retrospective Analysis; Occurrence and Distribution Assessment; Trend and Change Assessment; and Case Studies (Gilliom and others, 1995). The Retrospective Analysis forms the basis for evaluating what is known of water-quality conditions in a Study Unit and what water-quality issues need further investigation. The Occurrence and Distribution Assess- ment builds on findings of the Retrospective Analysis to complete a broad assessment of cur- rent water-quality conditions and to identify important questions about sources, transport, fate, and effects. The Trend and Change Assessment will identify long-term trends and changes in water-quality conditions in each Study Unit. Case Studies will develop an improved under- standing -of questions about sources, transport, fate, and effects. These four components are interrelated. It is anticipated that results from one component commonly will lead to changes in approaches used in the other three components. The interaction among the four components centers on the Occurrence and Distribution Assessment; therefore, the primary focus of inves- tigations by all Study Units during the first cycle of intensive study is the Occurrence and Dis- tribution Assessment. The Occurrence and Distribution Assessment will characterize the broad-scale geograph- ic and seasonal distributions of water-quality conditions,in relation to major contaminant sources and background conditions (Gilliom and others, 1995). For ground water, the focus of this assessment will be on water-quality conditions of major water-supply or potential water- supply aquifers in each Study Unit. Emphasis is on the chemical quality of ground water as- sociated with current human activities. The chemical quality of older (in excess of several decades) ground water also is studied depending on factors such as environmental and hydro- geologic settings, water use, and water-quality issues in a Study Unit. Currently (1995), the national emphasis in the Occurrence and Distribution Assessment of ground water is on characterizing the occurrence and distribution of nutrients and selected pesticides and volatile organic compounds. In addition, some Study Units are investigating local concerns, such as trace elements and radionuclides. The primary focus of the Occurrence and Distribution Assessment for ground water is on spatial characterization of ground-water quality at several areal scales within each Study Unit. Consideration of temporal changes in the water quality of the resource are incorporated in sam- pling design by the Study Unit/Program. This characterization is achieved through three pri- mary components (table 1): (1) Study-Unit Survey; (2) Land-Use Studies; and (3) Flowpath Studies. The Study-Unit Survey will be used, in conjunction with an analysis of available data, to broadly characterize ground-water quality across a Study Unit, and Land-Use and Flowpath Studies will build understanding of causal relations and processes (Gilliom and others, 1995). 3