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Table 4. Advantages and disadvantages of high-capacity and low-capacity water-supply wells for <br />water-quality studies (modified from Alan Welch, U.S. Geological Survey, written commun., <br />1992) <br />HIGH-CAPACITY WATER-SUPPLY WELLS <br />Advantages: <br />- Documentation of well construction commonly is good. <br />- High-capacity wells generally are well developed and frequently purged. <br />- Long-term access may be possible, particularly for municipal wells. <br />- High-capacity wells generally provide a larger vertical mix of water in an aquifer or aquifer <br />system than lower-capacity wells, and thus can provide a more integrated measure of regional <br />ground-water quality than low-capacity wells. <br />- Much of the water produced for irrigation and municipal use is from high-capacity wells, <br />allowing a direct sample of the used resource. <br />- Long-term water-quality data may be available. <br />Disadvantages: <br />- High-capacity wells may not have flow-rate controls and a sampling point near the well <br />head. Sample collection at high flow rates can be difficult. Losses of VOCs are possible. <br />- Pumping schedules could be irregular: for example, irrigation wells generally are pumped <br />seasonally, and could lead to seasonal variations in water quality that actually are an artifact <br />of the pumping regime. <br />- The well might have a long vertical gravel pack, or open intervals might span more than one <br />aquifer or aquifer system. <br />- Wells with high pumping rates can draw water from water-bearing units other than those <br />screened, even if the well is screened solely within one unit; thus, the vertical integration of <br />water from water-bearing units might be unknown. <br />- Local hydraulics may be atypical of regional ground-water movement as a result of compac- <br />tion or enhanced downward flow. <br />- Municipal wells that produce water not meeting water-quality standards are usually aban- <br />doned, implying that the remaining population of municipal wells is biased toward acceptable <br />water quality. <br />- Downhole chlorination may affect water quality. <br />- Depth-dependent differences in water quality could be lost, given water sampled could reflect <br />a mixture of water obtained at different depths. <br />- Irrigation wells without antisiphon devices that are used for chemigation can lead to ground- <br />water contamination. <br />- Pump oil can cause local downhole contamination. <br />14