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1 <br /> Ground Water Sampling <br />' carefully collected over long periods of time (i e , greater than two years) to <br /> • determine optimal sampling frequency and to delineate seasonal trends in ground- <br /> water monitoring results In Barcelona et al.'s study, groundwater was collected <br /> biweekly for 18 months and analyzed for 26 water quality and geochemical <br /> constituents The researchers determined that for the study site, ground-water <br />' sampling performed four to six times per year would result in an estimated <br /> information loss below 20% and would minimize redundancy The researchers <br /> concluded that by using careful sampling and analytical procedures, sampling and <br />' analytical errors can be controlled to approximately ±20% of the annual mean <br /> inorganic chemical constituent concentration in ground water <br /> 2.3 Ground-Water Sampling Equipment Selection and Use <br /> 1 <br /> There are three broad categories of ground-water sampling devices grab samplers(e g , <br /> bailers and syringe devices),positive displacement pumps (e g , gear drive pump, bladder <br />' pump, helical rotor pump, piston pump, centrifugal pump), and suction lift pumps (e g , <br /> peristaltic pump ), (Pohlmann and Hess, 1988 and Nielsen, 1991) Gas contact pumps <br /> are also available (Pohlmann and Hess, 1988) Table 1 is a useful guide for selecting <br /> 1 devices for sampling Cal/EPA prefers that all sampling equipment be dedicated to a <br /> particular well To encourage innovation, Cal/EPA may allow the use of other devices <br /> that are not specifically mentioned above if the RP demonstrates that the device will yield <br />' representative ground-water samples <br /> The following requirements should apply to the selection of sampling equipment <br /> lie O Sampling equipment should be chosen based on the analytes of interest and the <br /> charactenstics and depth of the saturated zone from which the sample is <br /> withdrawn For example, the choice of sampling equipment should reflect <br />' consideration of the potential for LNAPLs and DNAPLs <br /> U Sampling equipment should be constructed of inert material Sample collection <br />' equipment should not alter analyte concentrations, cause loss of analytes via <br /> sorption, or cause gain of analytes via desorption, degradation, or corrosion <br />' O Sampling equipment should cause minimal sample agitation and should be selected <br /> to reduce/eliminate sample contact with the atmosphere during sample transfer <br /> Sampling equipment should not allow volatilization or aeration of samples to the <br /> extent that analyte concentrations are altered <br />' The following sections discuss each category and type of available sampling device, <br /> including their appropriateness for use and their relative advantages and disadvantages <br /> 2.3.1 Grab Samplers <br /> There are two types of grab samplers available bailers and syringe devices <br /> Bailers <br />' Bailers are among the simplest ground-water sampling devices A bailer is simply <br /> • a rigid tube that fills with water when lowered into the well, when raised back out <br />' 9 <br />