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

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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

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