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As an economical alternative to installing wells, existing monitoring, observation, or water-supply wells can be selected for Study-Unit Surveys, and in some cases for Land-Use Studies and Flowpath Studies. If the entire well network consists of water-supply wells, this can lead to bias in ground-water-quality data. Different types of water-supply wells are likely to lead to different biases (Alley, 1993). Study Units need to consider the bias introduced by the well- selection process. When selecting water-supply wells, well-construction materials, the design of the well, and the method of well installation should be determined. Certain materials, well de- sign, or installation methods can result in the well being unsuitable for either water-level mea- surements or the sampling of targeted water-quality constituents. For example, organic compounds, such as tetrahydrofuran, methylethylketone, methylisobutylketone, and cyclohex- anone can leach from the glue used to bond unthreaded polyvinylchloride casing. Bias also can result from how the well is operated; the location of the well; the depths of screened intervals and depth of well completion; well-construction materials; and the type, construction, and age of a permanently installed pump. Well construction and the length and depth of well screen(s) are critical factors in well selection. Measurement of hydraulic head is important. It is difficult to determine the hydraulic head attributable to each water-bearing unit and the source of water to a well that is screened in several units, contains multiple screens in different units, or has a long well screen. Wellbore flow, which can occur in wells having long or multiple screens, can cause mixing of waters of different quality. On the other hand, wells with screens that are short in comparison to the total thickness of an aquifer might be screened in intervals that miss major zones of interest, such as zones of high transmissivity or zones of contamination. In general, selecting several wells in proximity that differ in well-screen depth but that each have short screened intervals generally is the preferred means of obtaining depth-averaged water-quality data. Alternatively, a well that is otherwise suitable but with multiple screens can be used if the appropriate screened intervals can be isolated for sampling, for example by using packers. The well pump also can affect the chemistry of a water-quality sample. Consequently, the type of pump, and the pump and pump-riser-pipe materials are important information to consider during the selection process. For example, oil can leak from the pump casing and contaminate water coming in contact with the pump. Suction-lift pumps can induce loss of oxygen and vol- atilization of some organic compounds from a sample during withdrawal because of the drop in pressure in the sample line caused by vacuum. Jet pumps use circulation water pumped through a venturi to carry water to the surface. Both the mixing of circulation water with sample water, and the drop in pressure of the circulation water across the venturi can affect sample-water qual- ity. Because of the above, for existing wells with pumps, only those with submersible pumps are selected. Wells with water-lubricated pumps are selected in preference to wells with oil- lubricated pumps. The accessibility of a sampling point at a well is an important selection criteria, particularly when sampling for volatile organic compounds or trace elements. Many water-supply wells will have an access point for sampling; however, if the only access point is located after an on-site treatment system, a pressure tank, or a holding tank, these systems could change the chemistry of the water sample. At wells where an access point close to the well is not available, it is some- times possible to have a valve installed at the well head for sample collection. 12