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Screen lengths for monitoring wells installed by NAWQA for Land-Use and Flowpath <br />Studies typically should range from 2 to 10 ft. 'The actual length used should reflect study ob- <br />jectives and aquifer conditions. For example, a screen length of 5 ft might be too long for a well <br />used in a NAWQA Flowpath Study, if information suggests that marked vertical differences in <br />the distribution of hydraulic head or water quality occur on the order of a few feet or less. A <br />5-ft screen placed immediately below the water table, however, probably is appropriate for most <br />NAWQA Land-Use Studies. As a general rule, screen lengths of 10 ft or less generally are <br />appropriate for most NAWQA Land-Use Studies and screen lengths of 1 to 5 ft in length gener- <br />ally are appropriate for most NAWQA Flowpath Studies (table 3). <br />The diameter of monitoring wells for water quality typically range from one-half to 6 in, <br />with the 2-in diameter well being the USGS and industry norm. Ideally, the preferred well- <br />casing diameter would be suitable for running aquifer tests as well as for collecting water-quality <br />data. Typically, wells for an aquifer test consist of one large-diameter pumping well (4-in diam- <br />eter or greater) that is associated with wells that can be of smaller diameter in which drawdown <br />is measured as pumping proceeds. The larger diameter normally is required for the pumping well <br />to ensure that the well can be pumped at a rate sufficient to cause measurable drawdown in the <br />observation wells. Therefore, a well with small diameter (2 in or less) generally is not suitable <br />as the pumping well for aquifer testing. The hydraulic data acquired from aquifer tests are par- <br />ticularly important to meet objectives of the Flowpath Studies. Therefore, plans for Flowpath <br />Studies should include installation of at least one larger diameter well per well cluster along the <br />flowpath that will be suitable for aquifer testing, if (1) a larger diameter well is needed to obtain <br />hydraulic data, (2) if a suitable well does not already exist, or (3) if the hydraulic data required <br />are not available. <br />It is not always possible to select the optimum construction material and screen design. For <br />example, for a Flowpath Study of trace-element concentrations in ground water in an area inac- <br />cessible to a drill rig, a hand-driven monitoring well constructed of steel casing and drive point <br />might be the only alternative. The quality of data obtained from such a monitoring well may be <br />difficult to interpret. Where a less-than-optimum well design is used, the increased risk of data <br />bias needs to be considered and any potential bias must be explicitly identified, defined, and <br />reported. <br />Decontamination of well-installation equipment and materials <br />Decontamination of well-installation equipment prior to use reduces contamination of drill <br />holes, aquifers, pore water at the screened interval, and cross-contamination between wells. Pro- <br />cedures for decontamination of equipment, casing and screens, and other materials used for well <br />installation are provided in table 7 and in greater detail in Aller and others (1989), U.S. Environ- <br />mental Protection Agency (1987), Moberly (1985), and Richter and Collentine (1983), and in <br />W. Lapham, U.S. Geological Survey, written commun., 1995--see footnote 1. <br />Decontamination of equipment and materials and documenting decontamination proce- <br />dures and quality-control data is to be a standard operating procedure for NAWQA Study Units. <br />The frequency of decontamination depends on subsurface conditions at the drill site and objec- <br />tives of the sample collection. Decontamination of equipment between drill sites also is an <br />32