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DELIVERY, DELIVERY, DELIVERY -
SLOW RELEASE VS. INJECTION VS. GROUNDWATER RECIRCULATION

Sami A. Fam, Ph.D., P.E. L.S.P., Donald F. Kidd, P.E., Innovative Engineering Solutions, Inc.

Electron donors and bioaugmentation cultures, as required, must be delivered to the area that requires enhancement and the "enhanced" subsurface conditions must be maintained for a period of time so as to degrade the soluble fraction, in addition to the contaminant fraction that will desorb from the soil over time. Desorption of contaminants from soil is enhanced by bioremediation. Solid phase (SP), slow release electron donor systems are easy to use and do not require active remediation. Electron donor compounds are slowly released and in theory can offer soluble (released) electron donor that can subsequently travel with the groundwater and be distributed downgradient. Groundwater flow however is generally laminar, predominantly horizontal and the released/mobile electron donor is highly degradable.  As such, soluble electron donors released from solid phase media can only be adequately distributed in the subsurface under high-density point installation and relatively high groundwater flow velocity.

As an example, under conditions of low electron donor demand, 300 foot per year (ft/yr) groundwater flow velocity and one foot cross-gradient spacing between the solid phase injection wells, an area of dissolved contamination 100 feet downgradient can likely be remediated before the donor is depleted. An often applied alternative to SP donor is placement of donor compounds directly into wells in a batch mode. Injection of electron donor directly into wells creates a localized groundwater mound to disperse the additives and displace contaminated groundwater with amended water. Injection volume, however, must be substantial and injection wells again need to be placed fairly close together to affect adequate coverage. Soluble electron donor injection into wells is an improvement over SP donors in that a larger annulus of electron donors is created to begin the migration "journey" with groundwater flow. Proper implementation of well injection and solid phase injection systems requires calculation/evaluation of the electron donor's half-life and the natural groundwater flow's ability to deliver the additives to where they need to go. The application of these injection systems also requires an accounting of the electron donor demand based on the mass of contaminant, including an excess amount over the calculated minimum requirement.

The most sophisticated method to distribute amendments is to recirculate groundwater to distribute the additives. This method of operation can overcome groundwater flow linearity constraints, but still must contend with the short half-lives of soluble electron donors. An additional issue that electron donor delivery must contend with is pH drops that occur if the electron donor breaks down to volatile fatty acids. Ethenogens, the organisms responsible for breakdown of cis-dichloroeethene to vinyl chloride and ethane, are not active at a pH below 6.3. This issue is especially problematic in slow groundwater movement conditions, where the electron donor remains in close proximity to the injection point and the released acids exceed the groundwater/soil buffering capacity. In order to fully remediate a large plume, the site must be broken up into �segments� where the added donor will persist at a sufficient strength to affect remediation. In this paper, several case histories will be presented along with a discussion of the required modeling and analysis of alternative electron donor methods from over 30 bioremediation full-scale projects.

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