Superfund Research Program
Fate of Semivolatile Organic Compounds Discharged to Surface Drainage Systems from Superfund Sites
Project Leader: Phillip M. Gschwend
Grant Number: P42ES004675
Funding Period: 1995 - 2000
Project investigators characterized the discharges that carry pollutants like benzene and diphenylsulfone offsite and into the bottom of an adjacent lake, the Halls Brook Holding Area (HBHA). This was accomplished using a combination of water and salt mass balances for each sampling campaign. The result is an improved assessment of the seasonally varying salty water discharging into the bottom of the HBHA. Moreover, by using the vertical distribution of salt in the lake's water column, the values of the vertical mixing coefficient's across the lake's pycnocline have been extracted. These "diffusivities" are important for quantifying (1) the continuous delivery of pollutants from the anoxic hypolimnion to the continuously flushed epilimnion and (2) the potential for the lake's outflow to carry contaminants to the rest of the watershed.
Benzene and Methane in the Halls Brook Holding Area
Researchers determined that organic chemicals like benzene and diphenylsulfone are substantially biodegraded upon mixing from the deep hypolimnion to the oxic epilimnion. Project investigators examined the hypothesis that much of this pollutant transformation involves co-metabolism by methanotrophic bacteria. Hence, the mass balance of methane entering and exiting the lake was assessed permitting scientists to locate these methanotrophs at a narrow depth-horizon near the top of the pycnocline and characterize their activities in terms of moles of methane oxidized per volume per time.
Previous observations of the ability of methanotrophs to co-metabolize other substrates (e.g., toluene and naphthalene) were used to estimate their prospective role in biodegrading benzene in the HBHA. By comparing these predictions to the rates derived from measurements of benzene's distribution in the field, the researchers concluded that it is possible that methanotrophs are largely, if not completely, responsible for this contaminant's removal.
Hydrogen Peroxide and Iron in the Halls Brook Holding Area
Researchers investigated light-mediated phenomena for their importance to the fates of organic pollutants in the HBHA. Hydrogen peroxide was found in the surface water at 1 to 10 M, depending on the time of day and in the water just above the pycnocline at tens of micromolar levels. Additionally, the researchers identified iron entering the lake via groundwater exfiltration at about 10 mM as the ferrous form. The ferrous iron is mixed up to the pycnocline and some portion must interact with the hydrogen peroxide mixed down from above. As a result, hydroxyl radical must be generated in this "dark" water depth! Since hydroxyl radical reacts with organic compounds like benzene at near diffusion-limited rates, this very strong oxidant may play an important role in the cycling of these toxic substances.
Through mass balance modeling of ferrous iron and hydrogen peroxide, project investigators worked to estimate the concentrations of hydroxyl radical in the pycnocline water and found that the ferrous iron is mostly oxidized at the deepest depth where the water contains molecular oxygen. Also, the researchers found that the rate of oxidation appears consistent with an abiotic, heterogeneous reaction catalyzed by ferric oxyhydroxide surfaces. Moreover, by modeling the late summer and fall hydrogen peroxide profiles, investigators estimated the source strength of a deep, though unknown, input of this compound. The hydroxyl radical was estimated to occur at 0.1 fM, just above the HBHA pycnocline. Such a concentration would probably not be sufficient to contribute a major fraction of the degradation of chemicals like benzene.