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Studies of Contaminated Ground Water Yield New Insights into Degradation of Diesel Fuel
In 1999, the North Dakota Department of Health (NDDH) was facing a big problem—a very large pool (estimated at 1.5Ð3 million gal) of oil was floating on top of the ground water beneath the downtown area in Mandan, ND. The brand new building housing the sheriff's office, for example, became uninhabitable on its lower floor owing to odor and health problems caused by the chemicals after a rainy season raised the water table. The downtown contains only an 8- to 10-square-block business area, but a large railroad facility is adjacent to it and the rest of the town (see map below). Spills of diesel fuel in the railroad yard have been common for about 50 years. The railroad acknowledged responsibility for spillage within the railroad yard, but it was unclear whether the railroad was responsible for the contamination under the downtown. Early assessments of the ground-water hydrology and chemical analyses contracted by the railroad led the company to doubt that the contamination under the downtown was all spilled diesel fuel. In 2000, the USGS, as an independent and unbiased external agency, was asked by the NDDH to do a complete characterization of the contamination and the hydrologic setting. USGS scientists taking part in this study were geochemists Keith Kvenvolden (Menlo Park, CA) and Jon Kolak (Reston, VA), chemists Fran Hostettler (Menlo Park, CA) and Colleen Rostad (Denver, CO), and hydrologists Geoff Delin (Mounds View, MN) and Larry Putnam (Rapid City, SD).
The hydrologists showed that the ground-water flow, previously assumed to be to the southeast and away from the downtown and the contaminated ground water, could vary seasonally. Recharge from precipitation seeping into the ground could change and reverse the flow direction back toward the downtown. Thus, the hydrogeologic part of the study suggested a reasonable explanation for the position of the oil under downtown Mandan. The geochemical analysis was highly complex, owing to the length of time (10Ð50 years) the oil had been in the ground and the extent of degradation of the hydrocarbons over that time. The floating oil was a complex hydrocarbon soup! After analyzing all the chemical constituents, however, the USGS scientists concluded that all the downtown oil is diesel fuel from a single source (railroad diesel produced at a local refinery), at diverse stages of biodegradation. No evidence was found of any industrial solvents or chemicals, or other hydrocarbon fuels, except for traces of gasoline at one outlying site. As the USGS scientists continued to examine their results, they found even more compelling evidence in support of their conclusions. Comparison of the degradation patterns of the hydrocarbons with those from a well-studied 1979 oil spill in Bemidji, MN, showed that the patterns were the same. Furthermore, these degradation patterns were unusual, and some of them were previously unrecognized in the scientific literature. Most oil spills are degraded by weathering and air oxidation; however, both the Mandan and Bemidji spills had been in the ground for so long that their environment had become anoxic. An anoxic environment changes the nature of the microorganisms that degrade oils, as well as the pattern of chemical degradation. The little bugs slowly eat away at the molecules, not starting with the small molecules, as is the case in air oxidation, but starting at the ends of the larger molecules, especially the paraffins and the alkylcyclohexanes. This pattern of degradation at Mandan resulted in residual oil that produced a chemical fingerprint that looked surprisingly like a mixture of slightly lower-range refinery fuels. In other words, the new degradation pattern that the USGS scientists were documenting mimics what could be interpreted as a mixture of other fuels, and so care must be taken in the interpretation. This discovery, in addition to allowing the USGS group to sort out the chemical soup at Mandan, is of interest to the scientific community because it is one of the first documented environmental occurrences of anoxic biodegradation of specific types of hydrocarbons that are the main components of fuels. It also will be significant in future forensic evaluations of fuel spills. In 2001, the USGS scientists were honored by the USGS' North Dakota District Office for their teamwork and for the timeliness and high quality of their report, entitled "Hydrologic Setting and Geochemical Characterization of Free-Phase Hydrocarbons in the Alluvial Aquifer at Mandan, North Dakota, November 2000" (see article in November 2001 Sound Waves). The paper that Fran presented at June's AMOP meeting, entitled "Alkylcyclohexanes in Environmental Geochemistry" and coauthored by Keith Kvenvolden, was an outgrowth of that award-winning research.
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in this issue:
Parasites as Indicators of Coastal-Ecosystem Health Climate-Change Effects Lecture usSEABED - Seabed Characteristics von Huene Receives Prestigious Award |
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