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In principle, toxic components of industrial wastewater should be neutralized before release into environment. However, once untreated discharges make their way underground — as has happened at Ramat Hovav and other manufacturing centers throughout the world — environmentalists, geologists, and hydrologists must seek approaches to ameliorate the accumulating subsurface pollution.
One approach, which has already come on line at Ramat Hovav, was born of the early geologic studies and rock-fracture mapping carried out by hydrologist Prof. Eilon Adar at the BIDR and the late hydrogeologist Prof. Ronit Nativ at the Hebrew University. Their investigations indicated that the water table at the higher levels of Ramat Hovav was just three meters (~10 ft) underground. It was also recognized that 80% of the pollution was concentrated at the top of the shallow aquifer. By identifying a tract of land with densely packed fractures, the researchers suggested digging long trenches along the preferential underground flow paths that would cut across the many permeable fractures. Since the trench depth hit below the water table, polluted water would exit from the many fractures along both sides of the trench and fill up the groundwater draining moat. The Ramat Hovav Local Industrial Council could then pump out and treat the water in evaporation ponds. The trenches also enabled skimming off of floating nonaqueous organic substances. Use of this trench-drainage approach has already produced a dramatic decrease in subsurface pollution.
This procedure, known as pump-and-treat, is widely used to remove dissolved pollutants from aquifers that are easy to pump. However, dealing with fracture-based aquifers, such as that at Ramat Hovav, requires special approaches. When a water table is 10’s of meters underground, the trench approach is extremely costly and impractical. In such deep, fracture-based aquifers the aquifer remediation must include wide-drill, slanted boreholes that end up piercing one or just a few permeable fractures holding ground water. But these voids hold extremely small quantities of water and any attempt to pump out the crack’s contents would yield little water for treatment. In addition, it would take an inordinately long time for additional water and pollutants to seep out of the chalk matrix and refill the space available to the borehole.
Drs. Shai Arnon and Zeev Ronen , microbiology experts in the Ramat Hovav research group, have examined the possibility of using microbes to decontaminate subsurface industrial pollutants in the Ramat Hovav fractured chalk aquitard. The use of bacteria to combat pollutants is well known: they are the treatment of choice to break down organic (such as petroleum) spills on land due to burst pipes or cracked tanks. Because pump-and-treat is not appropriate for a deep chalk aquitard, the microbiologists sought to examine whether growth of appropriate pollutant-degrading bacteria in the subsurface might aid in reducing the groundwater contamination at Ramat Hovav.
In turning to pollution-degrading bacteria, the researchers had to deal with two problems. They knew that most contaminant degradation by microbes is carried out on petroleum and their refined products, which are relatively easy to break down. The complex chemicals reaching the Ramat Hovav aquifer are not so easily biodegradable. In addition, there was little known about the mechanisms of biodegradation within fractured rock formations or whether they could be made to work in the laboratory or in the field.
To find an appropriate bacterium to do the job, the researchers assumed that soil or chalk fractures containing a particular toxic contaminant might also have organisms that had adapted to its presence. The bacteria would break down the pollutant and utilize the released energy for their metabolism. For this reason, the investigators chose the toxic compound 2,4,6-tribromophenol (TBP), which is abundant in the Ramat Hovav groundwater, as a model compound for studying biodegradation within a chalk fracture. In order to isolate a bacterium that does the job, the team carried out a laboratory study using a fractured chalk core removed from the Ramat Hovav site. By continuously injecting dissolved 2,4,6-tribromophenol into the fracture for 120 days, the researchers registered TBP breakdown (as manifested by the appearance of free bromide ions in the experimental liquid). To verify that microorganisms present in the crack were responsible for this effect, samples removed from the fracture were worked up by standard bacteriologic techniques. The team found and identified several bacterial strains — one of which, Achromobacter xylosoxidans, was shown in test tube experiments to actively degrade TBP. Further detailed laboratory studies of bacterial breakdown in the fractured core, using enhanced provision of nutrients and oxygen showed that 90% of the TBP was removed within a very short distance (10 cm) of the contaminant entry point. This indicated that bacteria operating in a fracture could be used to remove organic contaminants provided that nutrient-delivery pathways to the polluted sites could be secured.
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