move through fractured, karstic, and porous media not only as dissolved species, but also as
precipitated phases, and/or polymeric species; some metals may be adsorbed to, or
encapsulated in, organic or inorganic particles (e.g., colloid size particles) that are likely to be
removed by filtration. In addition, field filtration may expose a sample to the atmosphere,
introducing oxygen into the sample that can oxidize dissolved ferrous iron to form a ferric
hydroxide precipitate (Fe(OH) ). The ferric hydroxide precipitate may enmesh other metals in
the sample, removing them from solution. The precipitate and the entrapped constituents
would be removed by field filtration. This phenomenon (which may be common because of
the ubiquity of dissolved iron in ground water and iron colloidal particles, such as goethite, in
the subsurface), also could result in an inaccurate measurement of metals concentrations in
ground water at the facility. The Agency's position to prohibit field filtration of ground water
samples is even more crucial in fractured or karst terranes. Colloidal transport phenomena
are more likely to occur in aquifer systems characterized by conduit flow, because colloidal
particles can move easily through the larger channels formed by fractures and the dissolution
of carbonates.
Several recent studies demonstrate that metals can migrate in ground water with
colloidal particles (via a phenomenon known as facilitated transport), and that those colloids
will not pass through a standard 0.45 micron field filter. Studies of the behavior of several
persistent chlorinated organic compounds such as DDT, PCBs, and dioxin, also have
demonstrated that the solubility of those substances is greatly increased by the presence of
surfactants. Surfactants form a microemulsion in water, trapping the organic compounds while
allowing them to stay dissolved in water and to continue moving throughout an aquifer.
These emulsion trapped organic compounds have similar contaminant fate and transport
characteristics to that of metals bound up in colloids. Field filtering ground water samples for
organic compounds or metals analyses would remove these constituents and therefore lead to
inaccurate measurements of their concentration in ground water.
The Agency is aware that many hydrogeologic field crews have routinely field filtered
ground water samples in an effort to decrease the sample turbidity. Some of this removed
fraction may represent hazardous constituents that are mobile in ground water under natural
conditions, and some of this fraction may represent immobile constituents. In many cases,
however, proper well development and maintenance procedures (e.g., development of the well
after installation to remove fine grained materials, and periodic re development of wells to
counter the effects of siltation) are sufficient to reduce sample turbidity. In addition, the
selection of an appropriate filter pack material (both composition and grain size) and screen
slot size are important components of monitoring well design that can reduce sample
turbidity. Further, lower well purging rates and sampling rates (e.g., less than 1.0
liter/minute) will minimize the amount of material flowing into the well without removing the
fraction of the sample that may contain potential hazardous constituents that are mobile in the
subsurface under natural conditions. Common sampling techniques often involve the use of
bailers that do not allow low flow rate sampling.
November 1992
7 21






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