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Table 1. Components and attributes of the National Water-Quality Assessment Program ground-water sampling design [ mil, square miles; km2, square kilometers] Study component Feature Study-Unit Survey Land-Use Studies Flowpath Studies General To supplement existing data To examine natural and human To examine ground-water objective in providing broad overview factors that affect the quality quality along inferred of ground-water quality of shallow ground water that flowpaths and interac- within each Study Unit underlies key types of land lions of ground water use with surface water Spatial Ground-water resource domain throughout Study Unit Uppermost part of ground-water Shallow flow systems in system fir specified land-use specified settings settings Selection Aquifer system divided of areas typically into 3-5 subunits on the basis of physio- graphic and hydrogeologic features Number Minimum of 30 wells in of wells each subunit sampled General goal for spatial density is one well per 38 mi (100 km2) Well-selection Spatially distributed, with strategy random selection of suit- able water-supply or monitoring wells within each subunit Temporal Each well typically sampled sampling once during an intensive strategy period Typically, 2 -4 Land-Use Studies per Study Unit Each land-use setting represents a combination of a land-use type and a hydrogeologic subunit Minimum of 30 wells in each land-use setting Spatially distributed, with random selection of suitable water-supply wells or of sites where monitoring wells can be installed within each Land-Use Study area Each well typically sampled once during an intensive period Additional seasonal sampling at selected wells in some Study Units Typically, 1-2 Flowpath Studies per Study Unit Generally, unconsolidated shallow aquifers Upper part of flowpath generally lies within one of land-use settings examined in Land-Use Studies Typically, located in indicator basin' for surface-water sampling design Typically, 10-12 wells along flowpath and 10 wells for areal sampling Generally, wells installed by the Study Unit that are distributed at multi- ple depths along flow- path and areally in vicinity of flowpath Variable; multiple samples from most wells 'An indicator basin is a basin with homogeneous land use and physiographic conditions. Basins are chosen to be as large and representative as possible while still encompassing primarily one Environmental Setting (Gilliom and others, 1995). 4