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Table 3. Selection, design, and documentation criteria for wells used in National Water-Quality <br />Assessment Program Study-Unit Surveys, Land-Use Studies, and Flowpath Studies--Continued <br />Additional Criteria That ApUll For Land-Use Studies <br />Sampling locations are randomly distributed throughout the occurrence of the land-use setting <br />(combination of land-use and hydrogeologic setting) of interest. Sampling locations are <br />selected where land use has been stable over the past decade. <br />With the exception of reference wells, only wells located in recharge areas underlying or <br />immediately downgradient from the land use of interest are selected. <br />Install wells: Wells must be installed for urban Land-Use Studies that begin after 1995. For <br />other Land-Use Studies, existing wells might be selected, but only if the Land-Use Study <br />objective (table 1) can be met by sampling those wells. If existing wells are selected, select <br />observation, monitoring, or low-capacity water-supply wells to avoid the complexities of <br />determining contributing areas to high-discharge wells (such wells can draw water from units <br />other than the unit of interest). <br />Wells that are constructed of PVC have threaded, not glued, joints. <br />Wells have short open intervals, generally 10 feet or less in length. <br />Additional Criteria Recommended For Land-Use Studies3 <br />Well-casing material is Polyvinyl Chloride (PVC) or stainless steel. <br />Well screens are continuous-slot wire-wound screens or machine-slotted casing made of PVC <br />or stainless steel. <br />Additional Criteria That ARnly For Flowpath Studies <br />Install wells. <br />Well-casing material is Polyvinyl Chloride (PVC) or stainless steel. <br />Wells that are constructed of PVC have threaded, not glued, joints. <br />Well screens are continuous-slot wire-wound screens or machine-slotted casing made of PVC <br />or stainless steel. <br />Wells have short open intervals, generally ranging from 1 to 5 feet in length. <br />'Usually determined by field measurements of well depth and borehole logs describing the depth <br />to the top and bottom of each open interval; the depths of the hydrogeologic unit(s) at the well; and <br />checks on the integrity of well construction provided by borehole-geophysical logs, the continuous <br />pumping of sediment, and slug injection, pressure or partial vacuum tests. <br />2Uuually determined using information on well construction and installation, including the materi- <br />als used for the casing and screen, screen type, length and dimensions, the methods used to drill, com- <br />plete, and develop the well, and, if applicable, the type and operation of the pump installed in the well. <br />3It might not be possible to always meet these criteria unless wells are installed. <br />11