4. Full scale field testing of bioaugmentation and niche adjustment
A major CME initiative on bioaugmentation, the Schoolcraft Project, is in the final stages of a successful demonstration of carbon tetrachloride remediation. The ongoing evaluation of bioaugmentation with Pseudomonas stutzeri strain KC has demonstrated sustained high levels of CT removal (>95%) for over 400 days. A system of closely spaced wells was used to deliver base, microbes and nutrients using 15 injection/ extraction wells oriented normal to the aquifer flow. An initial tracer test and pH adjustment of the subsurface test zone (50 ft vertical by 50 ft horizontal, 30 ft below ground surface) were followed by two cycles of inoculation with Pseudomonas stutzeri strain KC. The entire grid was inoculated at 0.9 ft3 of inoculum culture (107 cfu /ml) per linear foot of well screen, and one half the grid was re-inoculated 79 days later with a greater volume per foot of screen (3.8 ft3 inoculum per linear foot of screen, 107cfu/ml).
The second inoculation was performed due to concerns the initial inoculum may have been too flocculent or of insufficient volume to effectively colonize the test zone. While the second inoculation was non-flocculent (addition of trace metals reduced flocculation), both sections of the test grid have shown nearly identical CT degrading performance. Chloroform production was observed in the twice-inoculated section of the test grid after 6 months of operation. This chloroform production was observed at one set of peizometers, the Monitoring Well 11 (MW11) cluster at depths 75 and 65 feet. The chloroform produced (up to 20 ppb) in the vicinity of this well did not migrated downstream, and was apparently degraded between MW 11 and MW 16 locatedone meter further downgradient. This chloroform production was associated with complete nitrate removal, SO4 concentration decreases from 35 to 10 ppm, H2S production (up to 1.7 ppm) and changes in groundwater microbial flora. The perturbations to the native microbial flora at MW 11 correlated with the operational cycles of the experiment. Community diversity as measured by the Shannon Index decreased from 2.07 to 0.2 during alkaline pH adjustment of the test zone from prior to inoculation. Following inoculation and subsequent feeding events, community diversity increased to 1.61 and Pseudomonas stutzeri strain KC could be detected. This overall pattern of community changes was observed at most downgradient monitoring wells. At MW 11 and MW 16, Desulfobacterium species were identified among the high G+C population members detected 60 days after inoculation. Ion chromatographic analysis indicated that acetate was being delivered in excess of the available nitrate, and this appears to have stimulated sulfate reducers and led to chloroform production. The concentration of acetate in the weekly feedings was decreased from 100ppm to 60 ppm. The change in operations resulted in a drop in the levels of chloroform to below 2 ppb over a period of 7 months. As of September, 1999, the CT concentrations on the treatment zone range from 0.1 to 2 ppb, and chloroform concentrations remain at or below the 1-2 ppb found at this site prior to perturbation (see figure).
The Schoolcraft CT evaluation is completing the last six months of a 4 year Michigan Department of Environmental Quality supported field trial. The plume, however is but one of the nine contaminant plumes located in the vicinity of Schoolcraft, MI. Two of the contaminant plumes, Plume G (PCE/TCE) and Plume F (hexavalent chromium and arsenic) are co-mingled and present a mixed waste treatment challenge. Working with the MDEQ a two year study of remediation options for these plumes will be conducted. Intrinsic remediation, biostimulation of reducing conditions and dechlorinators, bioaugmentation with halorespiring microbes and biological generation of reducing conditions for chromium fixation will be evaluated in a series of laboratory and pilot field tests.
Return to Bioremediation Thrust Group