Polycyclic Aromatic Hydrocarbons and Redox Parameters in a Creosote-Contaminated Aquifer
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A groundwater monitoring study was conducted as part of a comprehensive program to remediate a former wood-preserving site that was contaminated with creosote. Twenty-five multi-level samplers (MLSs) were installed on-site and groundwater samples were collected and tested regularly between March 1998 and July 2000. Nearly one-thousand hybrid poplar trees were planted on-site in 1997 to help contain the groundwater plume and enhance phytoremediation. Ten polycyclic aromatic hydrocarbons (PAHs) were monitored along with several terminal electron acceptors (TEAs) and their reduced end products. The focus of the study was to determine the extent of natural biodegradation in the subsurface and assess the role of the poplar trees in site remediation. Since monitoring began, considerable progress has been made remediating the site and the contaminant plume has been shrinking consistently. PAH levels in the groundwater and soil have been reduced and individual MLSs show consistently decreasing contamination. At this point in the study it cannot be conclusively determined what impact the poplar trees are having on the progressing site remediation. However, there is a wealth of evidence indicating that natural biodegradation is playing a major role in site cleanup. Monitoring of TEAs indicates suggests that there are aerobic zones in the site aquifer, but that reduced conditions exist as well. Dissolved oxygen (DO) was found in many MLS ports, but other ports were devoid of both DO and nitrate and contained large quantities of aqueous Fe(II). Oxygen, nitrate and Fe(III) are being reduced on-site and data suggests that they are being used in the biological oxidation of PAHs. Although laboratory studies document the oxidation of PAHs under sulfate-reducing conditions, high aqueous sulfate values were recorded throughout the site, regardless of the level of contamination. Several possible mechanisms are proposed to explain the coexistence of high sulfate and PAHs in the site aquifer. The system may be redox-buffered by excess solid Fe(III) and Mn(III, IV) oxides. Also, dissimilatory sulfate-reducers are strict anaerobes and oxygen-rich rainwater may be toxic to them. The presence of a layer of coal below land surface creates pyrite oxidation conditions similar to those encountered in conjunction with acid mine drainage. The MLSs most affected by the coal layer have less PAHs and DO, lower pH, and higher sulfate and Fe(II) levels than other wells. The oxidation-reduction status of each MLS, based on oxygen, nitrate and Fe(II) measurements, appears to be closely related to the level of PAH contamination, suggesting that PAHs are the primary substrate being biologically oxidized in the site aquifer. These findings tend to support the general belief that the major limitation to natural biodegradation in subsurface environments is the delivery of adequate supplies of suitable TEAs to contaminated zones.
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