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+Y STUDY METHODS Unit into aquifer zones that are expected to be homogeneous in water-quality characteristics The NAWQA design for ground water focuses on compared with the Study Unit as a whole Tlus assessing the water-quality conditions of major subdivision is based mainly on identification of major aquifers in each Study Unit with emphasis on the hydrogeologic settings For each aquifer subunit, the quality of recently recharged ground water associated well-selection process and sampling strategy are with present and recent human activities (Gillicim and designed to achieve a preliminary assessment from a others, 1995) The sampimg design is based on the combination of new and existing data As a general need to examine ground-water quality at a range of planning guideline, an areal sampling density of at spatial scales (table 1) "Study-Urut Surveys"are used least one well per 100 km2 (square kilometer) is in conjunction with an analysis of available data to desired, and at least 20 wells are selected in each broadly characterize ground-water quality across the subunit. Perhaps the most significant factor that Study Unit. Water samples were collected from 412 affects the utility of Study-Unit Surveys is that Study-Unit Survey wells in 9 Study Units representing locations of existing wells are biased relative to the regionally extensive aquifers (fig 1) during 1993-94 ground-water resource Because existing drinking- "Land-Use" Studies are done on an intermediate scale water wells generally will not contain large concentra- to build an understanding of causal relations and tions of contaminants generally associated with point- processes The Land-Use Studies are directed, for the source contamination, the Study-Unit Surveys will be most part, toward the effects of human acdviues on biased against large concentrations of contaminants, shallow ambient ground-water quality There were however, the Study Unit Surveys will probably not be 211 wells and springs sampled in 8 urban Land-Use biased relative to smaller concentrations of contami- Studies and 562 wells sampled in 22 Agricultural nants generally associated with nonpoint-source Land-Use Studies contamination Different type of wells (domestic, public water supply, or irrigation) may be biased in The first step in the design of a Study-Unit Survey different ways In general, each Study-Unit Survey is the division of the ground-water resource into uses as few different types of wells as needed to obtain •aquifer subunits, generally 3 to 5 per Study Unit, adequate spatial (areally and with depth) distribution priority was given to those aquifer subunits where of ground-water samples from the aquifer system ground water is used for human consumption The Wells are selected for sampling using a gnd-based subunits serve as a first-order subdivision of the Study random sampling approach(Scott, 1990) Table 1 Components and attributes of the National Water-Qualrty Assessment ground-water sampling design Study-Unit Survey Urban Land-Use Agricultural Land-Use Studies Studies General objective To supplement existing data in To examine natural and human factors that affect the quality of providing a broad overview of shallow ambient ground water that underlie urban and agricultural the quality of"used"ground areas water within each Study Unit_ Spatial scale Ground-water resource in study Uppermost part of ground-water system in specified land-use unit Spatial density of wells is settings generally one well per 100 km2 Number of areas sampled 9 Study-Unit areas representing 8 urban areas 22 agricultural areas regionally extensive aquifers Number or kind of wells 412 total 5 drinking water wells 562 total sampled 194 monitoring wells 12 springs in Atlanta 211 total Study Methods 5