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Land-Use Studies are designed to assess the contamination plumes in urban areas, existing wells concentrations and distribution of water-quality that were installed to define the downgradient extent constituents in recently recharged ambient ground of a contamination plume were not considered for water (generally less than 10 years old) associated sampling Furthermore, samples were collected from with the most significant settings of land use and only four wells that were drilled to define the quality hydrogeologic conditions in each Study Unit Two to of ambient ground water upgradient of point-source four Land-Use Studies typically are completed in each spills This well-selection process essentially excludes Study Unit The priority of potential Land-Use Studies from the study those parts of the aquifer where known is based on a combination of Study-Unit and National point-source plumes are well defined, therefore, priorities Factors considered in assigning priorities depending on the number of defined plumes within the include importance of the land-use setting to quality of study area, the water-quality data from the Urban ground water withdrawn and used in the Study Unit, Land-Use Studies may be skewed toward nonde- regional significance of the hydrogeologic setting, tectable or Iower concentrations of contaminants For contamination potential of the targeted land use, and example, if all the point-source contamination in an geographic correspondence to subunits concurrently urban area were defined by observation wells and sampled as part of the Study-Unit Survey Wells were sufficiently spread out so that the plumes did not selected for a Land-Use Study are randomly overlap, then none of the wells selected for an Urban distributed throughout the occurrences of the land-use Land-Use Study in this area would be affected by, or setting of interest within the Study Unit using a gnd would characterize, point-source spills The water- based random sampling approach (Scott, 1990) quality data from this hypothetical Urban Land-Usefp Generally,a minimum of 20 wells are sampled in each Study would charactenze only contaminants from land-use setting Some studies used existing wells, nonpomt sources In the Denver Urban Land-Use others drilled new wells, whereas still others used a Study, four wells were sampled that were drilled to combination of new and existing wells Because of the define the quality of the ambient ground water before difficulty in finding suitable wells within Atlanta, it is affected by leakage from a regulated unit as . Georgia, some springs were sampled, and the results defined by the USEPA (1992b) Samples from all four are included with data from the wells in this report of these wells had detectable concentrations of MTBE The wells sampled for the Land-Use Studies have a at 0 5, 9 6, 800, and 23,000µg/L Two of these wells short (ideally less than 3 m (meters) in length) open were upgradient of plumes that were not associated interval located near the top of unconfined aquifer with gasoline sources (Breton Bruce, U S Geological Only wells located in recharge areas underlying or Survey, written commun , 1995) although this does immediately downgradient from the land use of not mean that another undefined plume is the source interest are selected Ideally,observation wells or low- of MTBE in these wells Samples from these two capacity existing wells are selected to avoid the wells had concentrations of MTBE of 9 6 and complexities of determining contributing areas to 800 gg/L None of the other seven Urban Land-Use heavily pumped wells Many wells are installed by Studies used upgradient wells associated with point- NAWQA to meet these cntena. Well selection and source spills installation procedures are discussed in more detail by Lapham and others (in press) Sampling procedures are discussed in Koterba and others (m press) and are briefly summarized here Because there may be numerous point sources of Submersible pumps were cleaned before a sample was contamination in an urban area, a randomly located collected The normal decontamination procedure well may be affected directly by nonpoint-, or point- included an Initial flush with nonphosphate detergent, source contamination, or both MTBE may be used in a deionized VOC blank-water rinse, a methanol rinse, gasoline to meet the requirements of the 1990 Clean and a final deionized water rinse Samples were Air Act Amendments in the urban areas studied collected in an environmental chamber that protected (fig 1) MTBE also may be used in premium gasoline the samples from airborne contamination Powderless in many of the urban areas studied To avoid skewing latex gloves were wom whenever a sample was the characterization of water quality toward defined collected Samples were preserved with hydrochlonc 6 A Preliminary Assessment of the Occurrence and Possible Soun;es of UTBE In Ground Water