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