SITE INFORMATION AND CORRESPONDENCE_1985-1997

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dient experiment conducted+Mackay et al. (1986). create "04oncrete." The concrete mixture was For information on the general geology and hydrogeo- formed or broken into small cubical briquet-shaped logical characteristics of the site, the reader is referred forms, approximately 2 inches by 2 inches by 2 inches to Mackay et al.(1986),Sudicky(1986),and MacFarlane in dimension. As described below, both types of ORC (1983).For purposes of this paper,it is sufficient to note formulations were tested in this experiment. that the relatively uniform sandy aquifer is unconfined; According to the manufacturer, the total available water table elevation at this site varies between ground oxygen in the ORC powder used in this work was 6.5 surface and 2 in below ground surface, and hydraulic percent (t0.3 percent) by weight; after incorporation gradient in the area varies between approximately into the concrete,the available oxygen content remain- 0.0035 and 0.0054 throughout the year in response to ing in the ORC was 5.2 percent(±0.3 percent)by weight snowmelt and precipitation. (Plant Research Laboratories, unpublished data). PRL In previous studies of the fate and transport of BTEX reports that pure ORC powder, when activated by in the Borden Aquifer,it has been shown that biodegra- hydration, has a two-phase oxygen release profile. In dation occurs most rapidly when oxygen is the electron the first phase, after a brief lag time, the ORC begins acceptor. These studies also demonstrated that BTEX releasing oxygen at a rate approximating first order biodegradation during uncontrolled migration is limited kinetics. The duration of the first phase, as described primarily by oxygen availability (Barker et al. 1987). below, depends on environmental conditions. The sec- When sufficient oxygen and nutrients are available,bio- and phase involves a slower release rate that approxi- degradation is controlled by growth of naturally occur- mates zeroth order kinetics. Placement of ORC in con- ring BTEX-degrading microorganisms (Allen 1991). trete tends to truncate the rapid first order release phase However, when excess oxygen was supplied under and results in a more constant oxygen release through- natural conditions, nitrogen availability limited the out the entire lifetime of the ORC. BTEX biodegradation rate. This rate limitation was Laboratory studies conducted by PRL indicated that removed when nitrogen,supplied as nitrate,was added the release rate was increased by certain environmental (Allen 1991). Nitrogen-limited aerobic degradation of factors, including microbial activity, lower pH, and aromatic compounds in ground water samples has also higher temperature. Therefore, it was expected that been observed by Aelion and Bradley (1991) and by actual field oxygen release rates (as determined by dis- Armstrong et al. (1991). When anaerobic, denitrifying solved oxygen measurements) would differ somewhat conditions were induced in the Borden Aquifer by from laboratory results. Laboratory oxygen release nitrate addition, benzene was recalcitrant, and toluene, experiments with samples of the ORC concrete used in ethylbenzene, and xylenes were shown to be incom- this work suggested that oxygen would be produced for pletely biodegraded (Barbaro et al. 1993). approximately 13 months in a clay matrix and for Since the BTEX-degrading capabilities of the approximately 30 months in a loam matrix.These times microbes indigenous to the Borden Aquifer have are about four to five times longer than those estimated already been shown to be limited primarily by the availa- for the pure ORC powder. bility of dissolved oxygen, the Borden site serves as the Based on the laboratory studies conducted by PRL simplest case for field testing the ORC. If the use of using the ORC concrete and ORC powder that were ORC did not lead to an increase in the available oxygen used for this study, it was expected that the oxygen and therefore a decrease in benzene and toluene concen- release rates were approximately the following (in mg trations at this site,it would seem unlikely that it could of 02 per day per gram of ORC): 0.21 for the first 25 offer promise elsewhere. days and 0.12 for the next 375 days (ORC concrete), Because of knowledge gained from previous experi- and 1.6 for the first 4 days and 0.4 for the next 150 days ments conducted at the site (e.g.. Barker et al. 1987), no (ORC powder). The behavior of the ORC at the end effort was made in this study to confirm that the disappear- of the second phase is presently unknown, but presum- ance of benzene and toluene was in fact due tobiodegrada- ably the oxygen production diminished relatively tion. Thus, throughout the following discussion, it is quickly as the last of the available oxygen was released. assumed that the transformation process that is acting on Experimental Design these compounds is biologically mediated. The experimental design was chosen to maximize the information that could be obtained from a short ORC Characteristics duration test (Figure 1). A 5-m-wide source was used ORC is currently provided by the manufacturer to inject benzene,toluene,potassium nitrate(as a nutri- Plant Research Laboratories (Corona del Mar. Cali- ent). and sodium bromide (as a ground water tracer) fornia) as a fine-grained powder. Because the powder into the saturated zone. For this initial demonstration, itself is relatively impermeable to water, an alternate nitrate was added to decrease both the cost and duration formulation was desired which would allow water to of the experiment. The target injection concentrations flow through a porous solid source. One such formu- for benzene and toluene were 4 mg/L for each com- lation was previously co-developed by Dr. S. Koenigs- pound. The target injection concentrations of the inor- berg from Plant Research Laboratories(PRL)and Pro- ganic salts were 25 mg/L nitrate and 300 mg/L bromide. fessor R. Borden (North Carolina State University). In The source was made of 16 5-foot sections of that method. ORC powder was mixed with cement to 10-inch O.D. PVC well screen jetted to approximately WINTER 1994 GWMR ■ 121