Laboratory experiments have clearly demonstrated that the transport of polycyclic aromatic hydrocarbons (e.g., phenanthrene, anthracene, pyrene) through porous media is enhanced by their association with natural organic matter. Natural organic matter boosts the "apparent" solubility of the PAHs, so the concentration of "mobile" PAHs increase. However, the enhanced transport of PAHs has never been observed in field studies. With this in mind, we went to the Bemidji, Minnesota, oil spill site that has been studied by U.S. Geological Survey researchers for over 15 years as a Toxic Substances Hydrology Program site to look for natural organic matter-enhanced transport of phenanthrene.
In collaboration with Dr. George Aiken of the U.S. Geological Survey in Boulder and Dr. Deb Backhus of Indiana University's School of Public and Environmental Affairs, we sampled groundwater from five wells up-gradient, down-gradient, and adjacent to the oil "blob" sitting on the water table during July 1995. The samples revealed abundant organic matter and dissolved iron. While we are still investigating the association of phenanthrene to this organic matter, which is mostly derived from the degradation of aliphatic and aromatic oil components, we decided to explore the effect of high ferrous iron concentrations on the association of phenanthrene with organic matter. Master's student Christine Hawley examined the change in phenanthrene binding to organic matter ranging from long-chain carboxylic acids to Suwannee River humic acid.in the presence of up to 50 mg/L of ferrous iron in an anoxic environment. Her results are reported in her Master's thesis (Hawley, 1996). With the addition of a few experiments planned for this year, we will prepare a manuscript on the effect of iron on organic matter binding of phenanthrene for publication.
During the same Bemidji sampling trip, we explored the effect of pumping rate on groundwater sampling for colloid- and organic matter-associated PAHs. Hawley (1996) reported the results of tests in which we sampled groundwater from a single well (adjacent to the oil "blob") at four different pumping rates from about 100 mL/min to 2 L/min. These results are still being analyzed for preparation of manuscript.
As part of the same project, Prof. Gary Amy, the PI of the grant funding this research, and his graduate students, Fung-Hwa Chi and Chris Lopez, are exploring PAH transport in laboratory columns to improve our ability to model PAH transport in the presence of organic matter.
Competitive Binding of Polycyclic Aromatic Hydrocarbons (PAHs) by Aqueous Organic Matter, Mineral-Bound Organic Matter, and Mineral Surfaces: Effects on Groundwater Transport of PAHs
- Sponsor: National Science Foundation, Engineering Directorate, Bioengineering and Environmental Systems Program
- Duration: June 1994 to May 1997
- Funding: $295,000
- PIs: Gary Amy (University of Colorado) and Joe Ryan
- Graduate RAs: Christine Hawley
Hawley C.M., 1996. A field and laboratory study of the mechanisms of facilitated transport of hydrophobic organic compounds. M.S. Thesis, University of Colorado, Boulder, 119 pp.