Microbial Ecology

The dissolved organic carbon content of subsurface waters is sufficient to maintain a small but diverse population of microorganisms. Denitrifiers, sulfate-reducers, and methanogens are likely to be present in low numbers in most groundwater unless conditions strongly favoring one group exist. Consequently, when a potential energy source in the form of an organic contaminant enters the water, the group most capable of utilizing the substrate at the environmental conditions existing in the aquifer will adapt and increase in population, while the population of other indigenous microbes will remain small or possibly be eliminated.

Effects of salinity

Typical salinities in deep-well injection zones range from about 20,000 to 70,000 mg/L, which is within the optimum range (50,000 to 60,000 mg/L) for halophilic organisms.86 Many nonha-lophilic bacteria can also live within this range. For example, a test of 14 microbe genera representing widely varying groups showed that most grew in salt concentrations of up to 60,000 mg/L.95 Nitrification readily occurs at high salinities. Rubentschik96 observed the conversion of ammonia to nitrate at concentrations of 150,000 mg/L NaCl, and isolated a culture of Nitrosomonas showing optimal growth at 40,000 mg/L. However, very high concentrations may slow denitrification. Hof 95 found that it took more than three times as long for the same amount of gas to be generated from denitrification at 300,000 mg/L NaCl as at 30,000 mg/L NaCl (10 vs. 3 d).

Effects of pressure

In general, growth and reproduction of both aerobic and anaerobic bacteria occurring at near-surface conditions decrease with increasing pressures.74 However, certain barophilic (pressure-loving) bacteria have adapted to the temperature and pressure conditions in the deep-well environment. For example, aliphatic acids (acetate ions) are degraded by methanogenic bacteria in oilfield waters as long as temperatures are lower than 80°C.97 Additionally, ZoBell and Johnson74 found that certain sulfate-reducing bacteria isolated from oil-well brines located several thousand feet below the surface are metabolically more active when compressed to 400 to 600 atm (40.5 to 60.8 MPa) than at 1 atm. On the other hand, the pressures in deep-well waste injection formations may be sufficiently high to kill or otherwise severely affect the metabolic activity of microbes from surface habitats that may be indigenous to the injected wastes.98

Interactions among microbial groups

Decomposition of organic matter in anaerobic environments often depends on the interaction of metabolically different bacteria. Degradation in this situation is a multistep process in which complex organic compounds are degraded to short-chain acids by facultative bacteria and then to methane and carbon dioxide by methanogenic bacteria. In these interactions, methanogens may function as electron sinks during organic decomposition by altering electron flow in the direction of hydrogen production.99 The altered flow of interspecies hydrogen transfer that occurs during coupled growth of methanogens and nonmethanogens may result in increased substrate utilization; different proportions of reduced end products; increased growth of both organisms; and displacement of unfavorable reaction equilibria.99

Redox conditions favoring denitrification lie somewhere between those for aerobic and metha-nogenic decomposition. However, denitrification and methanogenesis are not entirely mutually exclusive. Ehrlich and colleagues100 observed evidence of both denitrifying and methanogenic bacteria in phenol-depleted zones of a creosote-contaminated aquifer and concluded that the denitrifying bacteria contributed to degradation. In this study, denitrifiers and iron reducers were the dominant anaerobes in contaminated wells. Methane production was highest in the closest wells downgradient from the contaminated site, indicatin g the development of redox zones with metha-nogenic conditions strongest where contaminant concentrations were highest, changing to stronger denitrifying conditions where contaminant concentrations were lower.

Studies by the U.S. Geological Survey at the Wilmington, NC, deep-well waste-injection facility also provide evidence of simultaneous degradation of organics by denitrifying and methanogenic organisms.101102 When the dilute waste front, containing organic acids, formaldehyde, and methanol, reached the first observation well, production of gases increased dramatically. For a period of about 6 weeks, about half the gas volume was methane and about a quarter, nitrogen. Two weeks later, nitrogen had increased to 62% and methane dropped to 33%, and after another three weeks nitrogen had increased to 68%, and methane had dropped to 12%. These relationships indicate that the methanogens were more sensitive to the increases in waste concentration as the dilute front passed the observation well and more concentrated waste reached the site. The inhibiting effects of sulfates on methane production would seem to indicate that sulfate-reduction will take place in preference to methanogenesis as long as sulfates are present.

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