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' Ground Water Sampling <br /> • initial site characterization A change in ground-water flow may necessitate <br />' modification to the design of the ground-water monitoring system Water level <br /> elevations have been measured using a number of devices and methods, including <br /> the following <br />' O Steel tape coated with carpenter's chalk(wetted-tape method), <br />' a Float-type devices, <br /> o Pressure transducers, and <br />' o Electric water level sounders <br /> These devices and methods are described in more detail in Aller et al (1989), <br />' U.S.EPA (1987), and Nielsen(1991) Nielsen (1991)provides the water level <br /> measurement accuracy of each of these devices The SAP should specify the <br /> device to be used for water level measurements, as well as the procedure for <br />' measuring water levels <br /> Regardless of the method or device chosen to measure the water level elevation <br />' in a monitoring well or piezometer, the following criteria should be met when <br /> determining water level elevations <br /> • After well construction and development,water levels in piezometers and <br />'. wells should be allowed to stabilize for a minimum of 24 hours prior to <br /> measurement In low yield aquifers, recovery may take longer, and <br /> several water level measurements should be made over a period of several <br />' days to ensure that recovery has occurred <br /> • Water levels(depth to standing water)should be accurately measured with <br />' a precision of fO 01 foot Water levels should be measured from the <br /> survey datian on the top of the inner well casing The method or device <br /> used to measure water levels should be sufficiently sensitive so that a <br />' measurement to 0 01 foot can be obtained reliably In general, the <br /> wetted-tape method is the only method for water level measurement that <br /> consistently has an accuracy of 0 O1 foot(Nielsen, 199 1) <br />' • Water level measurements from boreholes, piezometers, or monitoring <br /> wells used to define the water table or a single potentiometnc surface <br /> should be collected within less than 24 hours This practice is adequate <br />' if the magnitude of change is small over that period of time In certain <br /> situations,water level measurements should be made within a shorter time <br /> interval These situations typically include <br /> t (1) tidally influenced aquifers, <br />' .(2) aquifers affected by river stage, bank storage, impoundments, <br /> and/or unlined ditches, <br /> 3 <br /> 1 <br />