Environmental Health - Toxic Substances
Hydrogen Measured in a New Test for Determining Subsurface Microbiological Activity at Contamination Sites
Scientists from the U.S. Geological Survey (USGS) and the University of Oklahoma have developed a new test for determining what microbiological processes are active in the subsurface at ground-water contamination sites. Environmental professionals and scientists can use the test for determining the effects of bioremediation systems or for assessing natural attenuation processes at cleanup sites. Few methods have been available for directly assessing the large-scale microbial activity in the field at contamination sites until now.
The new test involves injecting a solution containing a low concentration of dissolved hydrogen gas and a non-reactive (conservative) tracer into the subsurface. The resulting plume of hydrogen gas and conservative tracer is monitored through time with the injection well. Monitoring involves collecting water-quality samples as the natural flow of ground water makes it drift past the injection/monitoring well. This type of test is known as a natural gradient, single-well injection test (see first diagram). In some cases the plume is withdrawn from the aquifer by continuous pumping during the sampling phase. This type of test is called a push-pull test (see second diagram). For either type of test, the concentration of the conservative tracer is compared with the concentration of hydrogen gas in water sampled during the test, which enables scientists to estimate the rate that microbial processes consume the added hydrogen. Hydrogen is an important intermediate compound that is both produced and consumed by a wide variety of microbial processes during the biodegradation of contaminants. The rate that hydrogen is consumed can identify the types of microbial communities that are actively degrading contaminants (for example, iron-reducing, sulfate-
reducing, or methane-producing bacteria). This is useful information for assessing the performance of remediation systems—whether they are active bioremediation systems or passive systems based on natural attenuation of contaminants (monitored natural attenuation).
The hydrogen, single-well injection test was developed at two of the Toxic Substances Hydrology Program's research sites—Norman Municipal Landfill, Norman, Oklahoma site and the Cape Cod, Massachusetts site. Both of these contamination sites are used as field laboratories for developing new methods and approaches for assessing the transport and fate of contaminants in the environment.
Harris, S.H., Smith, R.L., and Suflita, J.M., 2007, In situ hydrogen consumption kinetics as an indicator of subsurface microbial activity: FEMS Microbiology Ecology, v. 60, no. 2, p. 220-228, doi:10.1111/j.1574-6941.2007.00286.x.