Ground water samples collected for analysis for organic constituents or
parameters should not be filtered in the field.
Currently, some hydrogeologists filter ground water samples in the field using a 0.45
micron filter prior to chemical analysis of certain constituents. The Agency generally does
not recommend that ground water samples that will be used to determine if there is
statistically significant evidence of ground water contamination be filtered in the field.
Nevertheless, the Agency understands that there may be circumstances where filtering ground 
water samples is appropriate. For example, some wells may produce highly turbid ground
water even though the wells have been appropriately installed and have been sampled using
procedures intended to minimize sample turbidity. The Agency believes that in these
circumstances filtering the ground water samples in the field prior to their analysis for metals
may be appropriate if filtering can be performed while still fulfilling the data quality
objectives (DQOs) for the ground water monitoring program.
There are several reasons why the Agency generally does not recommend filtering
ground water samples in the field prior to analysis for metals. One of the primary reasons is
that data generated from filtered samples provide information on only the dissolved
constituents that are present, because suspended materials are removed by the filtration
process. As discussed in greater detail below, current research in ground water sampling
protocol indicates that hazardous constituents are mobile in the subsurface in both the aqueous
(dissolved) phase and the solid phase. The research of Puls and Powell (1992), Puls and
Barcelona (1989a), Puls and Barcelona (1989b), Penrose et al. (1990), and West (1990) are
the primary sources of the discussion of field filtration that follows.
During ground water sampling, every attempt should be made to minimize changes in
the chemistry of the sample so that data representative of hazardous constituents that may be
migrating to ground water can be collected. A sample that is exposed to the atmosphere as a
result of field filtering is very likely to undergo chemical reactions (e.g., volatilization,
precipitation, chemical flocculation) that alter constituent concentrations. These reactions can
change the concentrations of organic compounds and metals if they are present in the sample.
Volatile organic compounds (VOCs), for example, are likely to partition to the atmosphere,
thereby resulting in ground water monitoring data that are not representative of constituent
concentrations. Further, precipitated and emulsion trapped constituents migrating from the
waste management unit to ground water are lost through field filtering, because they are
unable to pass through a standard 0.45 micron field filter.
Field filtration of ground water samples that will be used for metals analysis may not
provide accurate information concerning the mobility of metal contaminants. Field filtration
of ground water samples may be especially problematic in fractured or karst terranes.
Facilitated transport phenomena are more likely to occur in these types of aquifer systems that
are characterized by conduit flow, because colloidal particles can move easily through the
larger channels formed by fractures or by the dissolution of carbonates. Some metals may
November 1992
7 20






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