BBC: Development of Microcosm Methods 
  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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