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Ground Water Samplmg <br /> • 253 S ecia Handling Considerations p g <br /> During ground-water sampling, every attempt should be made to minimize <br /> changes in the chemistry of the samples so that data representative of subsurface <br /> hydrogeochemistry are collected Cal/EPA agrees with the following U S EPA <br /> protocols that will assist in preserving the natural chemistry of ground-water <br /> samples 1)do not routinely filter ground-water samples in the field, 2) do not <br /> transfer samples from one sample container to another, and 3) do not allow <br /> zf hcadspace in the containers of samples that will be analyzed for volatile organics <br /> Ground-water samples used to determine if there is statistically significant <br /> evidence of ground-water contamination by organic compounds should not be <br /> field-filtered Data generated from filtered samples provide information on only <br /> the dissolved constituents that are present, as suspended materials are removed by <br /> the filtration process The analytical results of these ground-water samples are <br /> used to determine if a facility is releasing hazardous constituents to ground-water <br /> As discussed in greater detail below, current research to ground-water sampling <br /> protocol indicates that hazardous constituents are mobile in the subsurface to both <br /> the aqueous (dissolved) phase and the solid phase The research of Puls and <br /> Barcelona (1989a), Puls and Barcelona (1989b), Penrose et al (1990), and West <br /> (1990)are the primary sources of the discussion of field filtration that follows <br /> During ground-water sampling, every attempt should be made to minimize <br /> changes in the chemistry of the sample so that data representative of hazardous <br /> constituents that may be migrating to ground-water can be collected A sample <br /> �• that is exposed to the atmosphere as a result of field filtering is very likely to <br /> undergo chemical reactions (e g , volatilization, precipitation, chemical <br /> flocculation)that alter constituent concentrations These reactions can change the <br /> 1 concentrations of organic compounds and metals if they are present in the sample <br /> Volatile organic compounds(VOCs) are likely to partition to the atmosphere if <br /> exposed, thereby resulting in ground-water monitoring data that are not <br /> representative of constituent concentrations Further, precipitated and emulsion <br /> trapped constituents migrating from the facility to ground-water are lost through <br /> field filtering, because they are unable to pass through a standard 0 45 micron <br /> field filter <br /> For metals analysis of ground-water samples, however, the situation is not as <br /> clear The argument against filtering is that it will not provide accurate <br /> information concerning the mobility of metal contaminants Some metals may <br /> move through fractured,karstic, and porous media not only as dissolved species, <br /> but also as precipitated phases, and/or polymeric species, some metals may be <br /> adsorbed to, or encapsulated in, organic or inorganic particles (e g , colloid-size <br /> particles),that are likely to be removed by filtration In addition, field filtration <br /> may introduce oxygen into the sample, which can oxidize dissolved ferrous iron <br /> to form a ferric hydroxide precipitate(Fe(OH3),),dus may enmesh other metals to <br /> the sample, removing them from solution The precipitate and the entrapped <br /> constituents would be removed by field filtration <br /> The argument for filtering samples (prior to analysis for inorganic constituents)is <br /> that small differences in sample turbidity can mean very large differences in <br /> 22 <br />