Open-File Report 2006-1374
U.S. GEOLOGICAL SURVEY
Open-File Report 2006-1374
Ground-water chemistry data collected by the USGS at OU-1 during June 2005 in support of long-term monitoring for natural attenuation are summarized in this section. The June 2005 data include concentrations of redox-sensitive geochemical constituents at 12 wells and 9 piezometers, and concentrations of VOCs at 9 piezometers, 3 intermediate aquifer wells, 10 passive-diffusion sampler sites beneath the marsh stream adjacent to the southern plantation, and 2 surface-water sites in the marsh stream adjacent to the southern plantation. The geochemical data include concentrations of H2, DO, organic carbon, nitrate plus nitrite, manganese, iron (II), sulfate, sulfide, carbon dioxide, pH, specific conductance, ORP, and chloride. The VOC data include concentrations of a subset of the 64 compounds on Schedule 8260B, and the dissolved gasses ethane, ethene, and methane.
For convenience in following the discussion, the sampled wells and piezometers are grouped with regard to location and aquifer. “Upgradient” sites are the two upper aquifer wells and one intermediate aquifer well upgradient from the landfill. “Northern plantation” and “southern plantation” sites are all upper aquifer wells and piezometers in or near the respective phytoremediation plantations. “Intermediate aquifer” sites are all intermediate aquifer wells downgradient from the landfill; no intermediate aquifer wells are in the footprint of the former landfill.
The June 2005 data and the previous geochemical data collected by USGS at OU-1 (Dinicola and others, 2002; Dinicola, 2003, 2004, and 2006; and Dinicola and Huffman, 2004) are shown in table 2 (at back of report). The predominant redox conditions for June 2005 samples were inferred primarily by dissolved oxygen and dissolved H2 concentrations following guidelines described in Dinicola (2006).
For June 2005, predominant redox conditions in the upgradient wells in the upper aquifer (MW1-3 and MW1-20) were mildly reducing (manganese and iron reduction). These wells have varied between aerobic and sulfate reducing during the past 4 years (table 2). Dissolved organic carbon concentrations have been consistently less than 2 mg/L. Redox conditions in the upgradient well in the intermediate aquifer (MW1-33) have been consistently aerobic.
For June 2005, the strongly reducing conditions (sulfate reduction and methanogenesis) most favorable for reductive dechlorination of chlorinated VOCs were inferred for 6 of 16 upper-aquifer sites in the northern and southern phytoremediation plantations. Mildly reducing conditions (iron or manganese) were detected at the other upper-aquifer sites within the plantations. Methane concentrations of 1.3 to 10 mg/L were measured at 8 of 9 piezometers in 2005, indicating that methanogenic redox conditions are common, although not predominant, throughout the former landfill. Dissolved organic carbon concentrations, measuring 2.7 to 22 mg/L in June 2005, have been consistently greater than those measured in upgradient wells.
Predominant redox conditions in all intermediate aquifer wells downgradient from the landfill have been consistently anaerobic (table 2). Mildly reducing conditions (iron reduction) were inferred for the intermediate aquifer wells at the downgradient margin of the landfill (MW1-25 and MW1-28) and methane concentrations were 1 to 2.1 mg/L. Dissolved organic carbon concentrations in these wells (5.9 to 6.3 mg/L in 2005) have been consistently greater than those measured in the upgradient well MW1-33 (0.5 mg/L in 2005).
Measured H2 concentrations generally decreased at OU-1 over the past 5 years, suggesting a trend from strongly- to mildly-reducing predominant redox conditions. However, sulfide and methane concentrations were consistent during that period, indicating persistent sulfate reduction and methanogenesis. Overall, no widespread changes in ground-water redox conditions were measured that could result in either more or less efficient biodegradation of chlorinated VOCs.
VOC data collected by the USGS from piezometers, selected wells, and passive-diffusion samplers at OU-1 from June 1999 to June 2005 are shown in table 3 (at back of report). The data for wells and piezometers are grouped and presented as described for table 2, followed by data for marsh passive-diffusion samplers and surface-water grab samples. Complete analytical results for the USGS data for June 2005 and previous years are available from the USGS NWIS web site http://waterdata.usgs.gov/wa/nwis/qwdata, or in Dinicola and others (2002), Dinicola (2003, 2004, and 2006), and Dinicola and Huffman (2004).
For the northern plantation in 2005, no TCE, cis-DCE, or VC was detected at piezometers P1-1 and P1-5, and chlorinated VOC concentrations at P1-3 and P1-4 were slightly lower than previously measured. The sum of concentrations of the reductive dechlorination end-products ethane and ethene were measured at 74 and 27 µg/L at sites P1-3 and P1-4. The temporal decrease in contaminant concentrations beneath the northern plantation, and the end-products evidence for reductive dechlorination are consistent with 2000-04 results (Dinicola, 2006).
For the southern plantation in 2005, changes in chlorinated VOC concentrations at the piezometers were variable. Most notable was a substantial decrease in total chlorinated VOC concentrations at P1-9 from 75,000 to 1,000 µg/L between 2004 and 2005. The highest chlorinated VOC concentrations at OU-1 most often were measured at P1-9 in prior years. The reason for the substantial decrease is unknown. Chloride concentration at P1-9 in 2005 (12 mg/L) was about one-half that measured in 2004 (26 mg/L) suggesting some dilution, but the possible 2-fold dilution does not explain the 75-fold decrease in chlorinated VOC concentrations. At site P1-10, chlorinated VOC concentrations decreased about 25 percent, a historical low for that site. The total chlorinated VOC concentrations at P1-7, the most highly contaminated sampling location at OU-1 in 2005, was 92,000 µg/L and consistent with previous levels. Chlorinated VOC concentrations at P1-8 decreased substantially after 2002, and remained relatively low during 2005. The total chlorinated VOC concentration increased to 7,600 µg/L during 2005 at one piezometer (P1-6) from the measured historical low of 1,100 µg/L in 2004, although the 2005 concentration was less than that measured during 1999-2002. The sum of concentrations of the reductive dechlorination end-products ethane and ethene were measured at 620 and 525 µg/L at the most contaminated sites P1-6 and P1-7, respectively, and were estimated at 4 and 10 µg/L at sites P1-9 and P1-10, respectively. Those end-product concentrations are reliable evidence that reductive dechlorination of chlorinated VOCs is ongoing.
In 2005, chlorinated VOC concentrations measured in the intermediate aquifer near the downgradient margin of the landfill (MW1-25 and MW1-28) were consistent with previous data. The sum of concentrations of the reductive dechlorination end-products ethane and ethene were measured at 19 and 26 µg/L at sites MW1-25 and MW1-28, respectively. Further downgradient in the intermediate aquifer beneath the Highway 308 causeway, well MW1-39 could not be sampled due to recent road work. As in previous years, no VOCs, ethane, or ethene were detected in the adjacent well MW1-38.
In 2005 and previous years, the highest chlorinated VOC concentrations measured in the marsh passive-diffusion samplers were at a site (S-4) about mid-way along the sampled stream reach. In 2005, the total chlorinated VOC concentration was 70,000 µg/L, which exceeded the previous values of 40,000 and 25,000 µg/L measured in 2004 and 2000, respectively. It is not certain that the measured increase in concentrations is representative of site conditions, largely because the passive-diffusion samplers are not deployed in exactly the same location from year to year. However, at the two most contaminated passive-diffusion sampler sites that were sampled three times (S-4 and S-5), the total chlorinated VOC concentrations have increased each time at both sites. In 2005, the second highest chlorinated VOC concentrations were measured at site S-5B (about 75 ft upstream of site S-4), where the total chlorinated VOC concentration was 14,000 µg/ L. At sites S-4B and S-5 that are located between the highly contaminated sites S-4 and S-5B, concentrations of chlorinated VOCs were substantially less, indicating a non-uniform pattern of VOC migration towards the marsh creek. Chlorinated VOCs were positively detected at all passive-diffusion sampler sites, and the reductive dechlorination end-products ethane and ethene were detected at all sites except S-4B.
In surface water, low concentrations of cis-DCE and VC (6.5 and 2.0 µg/L, respectively) were measured at the upstream site (SW-S6). At the downstream site (MA-12), the total chlorinated VOC concentration was 830 µg/L, similar to that measured by the Navy in 2004.
U.S.
Department of the Interior | U.S. Geological
Survey
Persistent URL: http://pubs.water.usgs.gov/ofr20061374
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