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Table 3. Selection, design, and documentation criteria for wells used in National Water-Quality Assessment Program Study-Unit Surveys, Land-Use Studies, and Flowpath Studies [Drains, springs, or seeps will not be used to substitute for wells, but may be sampled as a separate category] The well is suitably located in relation to the desired spatial and depth design. For example, wells are screened only within the unit defined for study. - The hydrogeologic unit (units) represented by the water level being measured is (are) known.l - The hydrogeologic unit (units) contributing water to the well is (are) known.l Monitoring or observation wells that were installed to detect a known or suspected contami- nant are to be avoided. Wells located near roads and highways are avoided because of the common use of herbicides and road-salt applications along roadsides. Where this is not possible, knowledge about road- side applications of chemicals, especially of herbicides, is documented. Wells with filter packs extending over a long interval of the annulus of the well as compared to the screened or open interval are to be avoided (the long packed interval can lead to uncer- tainty as to the source of water to the well). The top of the screen is located at least several feet below the lowest anticipated position of the water table (to reduce the chances of the well being dry during some periods of the year and to avoid problems with interpreting data from partially saturated open intervals). The integrity of well construction has been verified using verification checks where practical, such as depth-to-bottom measurements. The well construction and pumping equipment in the well are known to be of a type that are not likely to affect the water-quality constituents of concern? Possible biases caused by pumping rate have been considered. Selected or installed wells can be pumped at a rate that is adequate for sampling: typically, on the order of at least 1 gallon per minute. The sampling point should be located before any water treatment, pressure tanks, or holding tanks. 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. Wells with permanently installed suction-lift or gas-contact pumps are not selected.