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 This
phase of the research will occur from November 1999 to May 2000 and
is designed to develop different microcosm methods that could be used
in the laboratory to document biodegradation. When aquifers are contaminated
with chlorinated ethenes, microcosms can be used to develop a mass
balance on the contaminants and biogeochemically-important parameters.
In unconsolidated aquifers sediments, microcosms may be less useful
because there are usually fewer instances where it is impossible to
compare contaminant and by-product concentrations along the flow path.
However, in bedrock aquifers such comparisons are often difficult
to make due to a limited number of wells and the uncertainty regarding
their connectivity. Hence, microcosms could play an important role
in estimating rate constants for biodegradation, especially in the
competent bedrock. They may also be used to understand the effects
of certain parameters on biodegradation (e.g., secondary mineralization
of the rock).
Several types of microcosms have been used for unconsolidated sediments
(Tandol et al., 1994; Wiedemeier et al., 1996; Bradley and Chapelle,
1996 and 1997; Chapelle et al., 1996). Conversely, few microcosm
models have been tested for contaminated bedrock sites because rock
is more difficult to use in a microcosm. At the INEEL TAN site contaminated
with TCE, incubation tests have used pared core material or rubble,
and groundwater in serum bottles (Pyle, 1998). However, the TAN
site consists of fractured basalt flows containing sedimentary interbeds
(predominantly silts and clays) which is considerably  different
than the competent bedrock at Site 32 (well-layered, tightly-folded
quartzites and phyllites; See Section VI.B.a. and Appendix D). The
other major bedrock aquifer where microcosm studies have been conducted
is the TCE-contaminated dolomite aquifer near Niagara Falls, NY
(Yager et al., 1997). In these studies, groundwater from the site
was placed in serum bottles with a variety of chemicals to promote
different terminal electron acceptor processes. In some cases, sterile
pulverized dolomite was added to evaluate whether the rock contributed
electron donors to the system. Thus, the primary microorganisms
in the microcosms were those present in the groundwater. In the
fractures of competent bedrock, it is likely that a significant
part of the microbial community will be surface associated. Therefore,
including the fracture faces that contain these microorganisms in
the microcosm may be desirable. The microcosms developed during
Year I will use actual fracture faces from the competent bedrock.
Four microcosm models will be developed in the laboratory (i.e.,
a permeameter, a glass pipe reactor, a semi-rigid bag, and a fracture
rock chip microcosm). They will be tested using fresh core samples
in Year II. During Year II, in situ microcosms will also be developed
and evaluated. All of the microcosms will be batch or semi-batch
(fill and draw) reactors. They will be evaluated along with groundwater-based
microcosms (e.g., Yaeger et al., 1997).
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