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amec- <br /> 2. <br /> Groundwater circulation: groundwater is recirculated through the subsurface by <br /> continuous extraction, amendment with substrate (and/or microbes), and subsequent <br /> re-injection. <br /> Achieving the target concentration of foreign microbes could be a challenge with direct <br /> pressurized-injection approaches because these approaches usually affect a small percentage <br /> of the pore volume (about 1 — 5%). Groundwater circulation methods (using a series of <br /> extraction and injection wells) appear to have a higher chance for successful distribution of <br /> injected microbes where large volumes (several pore volumes) of amended groundwater can <br /> be continuously passed through the treated area. The transport distance of injected microbes <br /> increases with faster pore velocities because the frequency of collisions between mobile <br /> microbes and stationary aquifer solids is reduced slightly (Li and Logan, 1999), therefore, <br /> sustained groundwater circulation may facilitate post-injection transport of microbes because <br /> of artificially enhanced pore velocities between injection and extraction wells. Therefore, <br /> groundwater circulation would likely be used as a delivery method to distribute microbes <br /> throughout the target treatment zone of the WWTP if bioaugmentation was believed to be an <br /> effective remedy for this specific application. <br /> 2.4.3 Injection and Extraction Well Spacing <br /> Groundwater circulation involves the extraction of groundwater from the affected aquifer <br /> (target treatment zone), and re-injection into the same portion of the aquifer, thereby <br /> establishing hydraulic control and maintaining fluid movement (groundwater and amendments) <br /> through a fixed portion of the aquifer. Because injected groundwater is captured by the <br /> extraction well, the lateral spacing of these wells needs to be such that injected groundwater is <br /> drawn back towards the extraction well, and the amendment (in this case, added microbes, <br /> nutrients and substrates) will be distributed between the groundwater circulation wells (GCWs) <br /> at the target concentration. The hydraulic conductivity of the aquifer system is also an <br /> important consideration that will impact the extraction and injection rates, which in turn affects <br /> pore volume turnover rates. Appropriate spacing of GCWs requires a balance between cost <br /> and anticipated performance (with closer GCW interwell spacing, the likelihood of success is <br /> increased, but so is the cost). The transport distance of injected microbes is the primary <br /> control on the anticipated performance and therefore on the minimum interwell spacing. <br /> There are several factors controlling the transport distance (and therefore well spacing) of <br /> injected microbes, including: <br /> • The sticking coefficient (a), which depends on the specific attributes of the microbe, <br /> the ionic strength of the water solution, and the sediment properties. Li and Logan <br /> (1999) measured average sticking coefficients for laboratory derived cultures; these <br /> ranged from 0.57 to 1.7 (it is unclear to AMEC how a>1 can be measured but this <br /> value implies that each collision between microbe and sediment grain immobilizes the <br /> AMEC Geomatrix, Inc. <br /> \\oad-fs1\doc_safe\9000s\9837.006\4000 REGULATORYTFS Assessment_Apx B_01 2711\Attachment B.2\Attach B-2.doc 132-6 <br />