Open standpipe piezometers are similar to observation wells, except that the perforated portion of the pipe and sand filter region (backfill) is sealed in a discrete zone and the riser pipe is much smaller in diameter. The smaller diameter of the pipe reduces the lag time related to changes in ground water levels. Above the sand filter, the remaining backfill should include a seal (either cement/bentonite or bentonite pellets) and surface runoff should be prevented from entering the standpipe by installing a box or monument that does not restrict the pipe from venting. Groundwater levels in standpipes may be read using an electronic dipmeter which signals when the water surface inside the pipe is encountered. The depth is measured using the graduated cable. Open standpipe piezometers have a long successful performance record and are preferable to observation wells in most applications. Open standpipe piezometers should be used only in materials of high to moderately high permeability so that lag time remains relatively short. Standpipe piezometers are less effective in low-permeable silts or clayey soils to measure fluctuations in groundwater levels.
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