Studying the Impact of Microorganisms on Organic Contaminants in Groundwater

Data documenting microorganisms and their activities at Site 24

The Site 24 Story: Environmental fate of coal tar waste in a South Glens Falls, NY shallow aquifer

The Electric Power Research Institute (EPRI) and Niagara Mohawk Power Company (NIMO) collaborated to assess how removing the contaminant source at a coal tar disposal site would affect natural attenuation of dissolved contaminants from coal tar remaining in groundwater . The South Glens Falls, NY coal tar waste-contaminated study site was used for disposal of tar from a single event that occurred in the early 1960s at a plant that manufactured combustible gases used to fuel municipal lighting. The site was ideal for study because the contaminant source and hydrogeology are relatively simple. Because the contamination resulted from a single disposal event, there was only one contamination source, making source characterization relatively straightforward compared to more complex sites. The hydrogeologic setting is an aquifer composed of coarse to fine silty sands, with a confining layer 6 to 9 m (20 to 30 ft) below ground surface preventing extensive downward migration of the contamination. The groundwater velocity is bout 12 m (40 ft) per year.

Source removal commenced in May 1991 with installation of a sheet pile enclosure driven to a depth of more than 12 m (40 ft),well below the confining layer 7.6 m (25 ft). Despite the relatively simple hydrologic setting, locating and thoroughly removing the coal tar source material were technically difficult. The enclosed area was about 1,000 m2 (0.25 acre), and approximately 7,200 m3 (9,400 /yd3) of tar-contaminated soil and overburden were removed. In October 1991, the sheet pile was removed, and the excavated area was filled with clean native soil with a grain size similar to that of the removed soil.

Groundwater concentrations of naphthalene – one of the key constituents dissolved from the coal tar were monitored annually since source removal. The concentration contours were developed from numerous monitoring wells and multilevel groundwater samplers. Over the 10 year monitoring period, much of the region downgradient from the source removal area was found to contain less than 10 µg/liter naphthalene, which is below the New York State Department of Environmental Conservation drinking water standard. Phenanthrene, and other dissolved constituents from coal tar, also began at relatively high concentrations that substantially diminished in time.

Eight types of evidence indicated the involvement of microorganisms in the attenuation of naphthalene, phenanthrene, and related pollutants:

1. In situ aerobic respiration stimulated by coal tar waste--oxygen is depleted at the center of the plume where naphthalene concentrations were highest, and an inverse relationship between oxygen and naphthalene concentrations has been found throughout the plume;
2. In situ anaerobic respiration stimulated by the coal tar waste--robust signs of anaerobic physiological processes occurring inside, but not outside, the contaminant plume include nitrate depletion, production of bicarbonate alkalinity, and production of methane.
3. Adaptation of the native microbial community to the contaminants-- rapid metabolism of naphthalene and phenanthrene in soil microcosms from inside, but not outside, the contaminant plume;
4. Development of a food chain in situ within the plume--protozoan predation of high numbers of bacteria inside, but not outside, the plume;
5. Metabolite Detection -- a unique transient intermediary metabolite (naphthalene cis-dihydrodiol) was found by GC/MS analysis inside the plume of contamination
6. Gene expression-- expressed naphthalene biodegradation genes (messenger ribonucleic acid [mRNA] for the nahAc naphthalene dioxygenase gene) within contaminated waters.
7. In situ respiration-- production of 13CO2 from stable isotopically labeled (13C) naphthalene added to site sediment.
8. Stable Isotope Probing--the 13C-labeled naphthalene was traced into sediment DNA, the heavy 13C-DNA was separated and taxonomic genes in this labeled DNA was sequenced to identify the active bacteria; those bacteria were isolated in pure culture and found to grow on naphthalene.

The data from these studies show that natural attenuation due to biodegradation was effective in controlling contamination once the source of organic contamination was removed.