Scientific Investigations Report 2006-5056
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006-5056
Geochemical and contaminant concentration data from 2001 to 2004 indicate that substantial biodegradation of chlorinated volatile organic compounds (CVOCs) in ground water beneath the landfill at Operable Unit 1 (OU-1) has continued. Contaminant concentrations in ground water decreased throughout much of the site, and biodegradation is a primary cause for the decrease. However, dissolved phase contaminant concentrations in the tens of milligrams per liter persisted in a localized area in the southern part of the southern phytoremediation plantation. A residual source of non-aqueous phase liquid chloroethenes likely is present in that area, and biodegradation was only partly effective at reducing the dissolved-phase contaminants that are generated from that source. During 2004, chloroethenes continued to discharge to the adjacent marsh at concentrations that exceeded quantitative remediation goals for the site. Chloroethene concentrations in ground water discharging to a 100-foot long reach in the southern part of the marsh were at least 300 times greater than concentrations in ground water discharging elsewhere in the marsh.
No widespread changes in ground-water redox conditions were measured since 2000 that could result in either more or less efficient biodegradation. Anaerobic redox conditions have prevailed at the site, and despite inter-annual variation, no consistent trend developed towards either more strongly or more mildly reducing conditions. Detections of sulfide, widespread detection of methane, and frequent detection of dissolved hydrogen at concentrations greater than 1 nanomolar indicated that the strongly reducing conditions most favorable for reductive dechlorination of chloroethenes are present beneath much of the landfill and in the intermediate aquifer downgradient from the site. The mildly reducing conditions detected at the remainder of the site are less favorable for biodegradation through reductive dechlorination, but are more favorable for microbial oxidation of vinyl chloride (VC) and cis-1,2-dichloroethene (cis-DCE).
The 2004 concentrations of total CVOCs and chloroethenes in the upper aquifer generally were less than concentrations measured in 1999–2000. Lower concentrations were measured throughout the northern phytoremediation plantation, in the northern part of the southern plantation, and in areas outside of the plantations. CVOC concentrations for 2004 were as high as 103,000 micrograms per liter (µg/L) in the southern part of the southern plantation where non-aqueous phase liquids likely are present. Chloroethene concentrations measured in the intermediate aquifer near the downgradient margin of the landfill decreased less consistently between 2000 and 2004. At the farthest downgradient (western) monitoring points beneath the Highway 308 causeway, TCE remained undetected in the intermediate aquifer, and cis-DCE and VC consistently were measured at concentrations between 0.3 and 2 µg/L.
Biodegradation was a substantial cause for the downward trend in contaminant concentrations beneath the northern plantation. Continued reductive dechlorination of trichloroethene (TCE), cis-DCE, and VC was indicated, and reductive dechlorination to non-chlorinated end-products was reliably indicated by ethane plus ethene concentrations as high as 60 µg/L. Microbial oxidation of VC and cis-DCE also may have occurred, but no unique diagnostic byproducts were detected to indicate that process. Chloroethenes migrating from beneath the northern plantation were further biodegraded beneath the adjacent marsh. Chloroethene concentrations at most passive-diffusion sampler sites in the northern part of the marsh decreased between 2000 and 2004, and the highest total CVOC concentration decreased from 570 to 130 µg/L. Methane concentrations as much as 290 milligrams per liter at the passive-diffusion sampler sites indicate methanogenic redox conditions in marsh ground water that are favorable for reductive chlorination, and ethane plus ethene concentrations as high as 68 µg/L were reliable evidence for reductive dechlorination to non chlorinated end-products.
In the northern part of southern plantation, increased dilution over time and biodegradation led to attenuation of CVOC concentrations. TCE and other chloroethene concentrations decreased over time, although no clear indication for reductive dechlorination of TCE exists. Continued reductive dechlorination of cis-DCE and VC were indicated by ethane plus ethene concentrations as high as 217 µg/L. The attenuation of chloroethene concentrations was partially counteracted by a localized residual (albeit waning) source of dissolved TCE to ground water.
In contrast to the rest of the site, chloroethene concentrations in the southern part of the southern plantation have remained exceptionally high despite reliable evidence for continued reductive dechlorination of all chloroethene compounds. Data indicate a localized persistent source for dissolved TCE to ground water. Beneath the marsh near the southern plantation, chloroethene concentrations increased during 2000–04 even though reductive dechlorination to non chlorinated end-products beneath the marsh was indicated by ethane plus ethene concentrations of 1,100 µg/L. Methane concentrations during 2004 in the marsh were 530 milligrams per liter, indicating methanogenic redox conditions in marsh ground water, which are highly favorable for reductive chlorination. Increased chloroethene concentrations measured in 2004 beneath the marsh did not clearly indicate an increasing trend because the marsh ground water was sampled only twice. Chloroethene concentrations were more frequently monitored in marsh surface water just downgradient from the southern plantation. Chloroethene concentrations increased immediately after pavement removal from the southern plantation during early 1999, decreased through late 2003, and began to increase again during 2004. More data are needed to determine if the 2004 concentrations at site MA-12 represent an upward trend in contaminant discharge to the marsh.
Flux calculations based on 2004 data indicate that most dissolved-phase chloroethene mass in the upper aquifer beneath the landfill is degraded before discharging to surface water in the adjacent marsh. The total chloroethene flux of 72 grams per day estimated at the landfill transect during 2004 was reduced by 87 percent due to biodegradation in the upper aquifer before discharging to surface water. Most chloroethene flux from the landfill was from the southern plantation. Although the calculated mass degradation rates were greater during 1999–2000 and the discharge to surface water rates was greater during 2004, the results are extremely sensitive to a few measured chloroethene concentrations at highly contaminated wells and passive-diffusion samplers in the southern plantation. Total chloroethene concentrations measured by the Navy in marsh surface water at site MA-12 actually were higher during 2000 compared to 2004.
Calculated flux of TCE to marsh surface water was likely underestimated for both 1999–2000 and 2004, indicating that the TCE mass degradation rates were overestimated because not all TCE flux from the landfill was represented by measured concentrations from the passive-diffusion sampler sites. In contrast, the calculated flux of VC to marsh surface water was overestimated by nearly 600 percent for 2004, indicating that the VC mass degradation rates were underestimated. There appears to be additional attenuation of VC concentrations between shallow ground water in the marsh and site MA-12. Likely mechanisms behind that attenuation include microbial degradation of VC in the upper 2 feet of organic-rich marsh sediments, and volatilization of VC from surface water before reaching site MA-12. Overall, biodegradation of chloroethenes in ground water throughout OU-1 continued through 2004 and prevents most of the mass of dissolved-phase chloroethenes in the upper aquifer beneath the landfill from discharging to surface water.
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