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Open-File Report 2007-1430

Open-File Report 2007-1430

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Natural Attenuation Monitoring Data

The ground-water chemistry data collected by the USGS at OU 1 during June 2006 in support of long-term monitoring for natural attenuation are summarized in this section. The June 2006 data include concentrations of redox-sensitive geochemical constituents and dissolved gases at 13 wells and 9 piezometers, and concentrations of VOCs at 9 piezometers. The geochemical data include concentrations of H2, DO, organic carbon, nitrate plus nitrite, manganese, iron (II), sulfate, sulfide, methane, carbon dioxide, pH, specific conductance, ORP, and chloride. The VOC data include concentrations of a subset of the 64 measured compounds and the dissolved gases ethane and ethene. Chemical concentrations are reported as less than (<) the reporting level for samples in which the analyte was neither detected nor identified.

The ground-water chemistry data are grouped with regard to location and aquifer of the well or piezometer. “Upgradient” sites are the two upper aquifer wells (MW1-3 and MW1-20) and one intermediate aquifer well (MW1-33) upgradient of the landfill. “Northern plantation” sites are all in the upper aquifer and include six wells (1MW-1, MW1-2, MW1-15, MW1-17, MW1-18, and MW1-41) and four piezometers (P1-1, P1-3, P1-4, and P1-5) in or near the northern phytoremediation plantation; piezometer P1-2 generally is dry during June and has not been sampled. “Southern plantation” sites also are all in the upper aquifer and include four wells (MW1-4, MW1-5, MW1-6, and MW1-16) and five piezometers (P1-6, P1-7, P1-8, P1-9, and P1-10) in or near the southern phytoremediation plantation. “Intermediate aquifer” sites include six intermediate aquifer wells (1MW-4, MW1-25, MW1-28, MW1-29, MW1-38, and MW1-39) that are downgradient of the landfill; no intermediate aquifer wells are in the footprint of the former landfill.

Geochemical Data and Predominant Redox Conditions

The June 2006 data and the previous geochemical data collected by the USGS at OU 1 (Dinicola and others, 2002; Dinicola, 2003, 2004, 2006; and Dinicola and Huffman, 2004, 2006) are shown in table 2 (at back of report). The predominant redox conditions for June 2006 samples were inferred primarily by DO and dissolved H2 concentrations following guidelines described in Dinicola (2006).

For June 2006, predominant redox conditions in the upgradient wells in the upper aquifer (wells MW1-3 and MW1-20) were mildly reducing (manganese and iron reduction). These wells have varied between aerobic and sulfate reducing during the past 7 years (table 2). Concentrations of dissolved organic carbon have been consistently less than 2 mg/L. Redox conditions in the upgradient well in the intermediate aquifer (well MW1-33) have been consistently aerobic.

For June 2006, the strongly reducing conditions (sulfate reduction and methanogenesis) most favorable for reductive dechlorination of VOCs (Bradley, 2003) were inferred for 5 of 15 upper-aquifer sites (MW1-41, P1-1, P1-5, P1-6, and P1-9) sampled in the northern and southern plantations. The other upper-aquifer sites within the plantations had mildly reducing conditions (iron or manganese reductions). Methane concentrations of 0.50 to 8.5 mg/L were measured at all upper-aquifer sites within the plantations in 2006, indicating that methanogenic redox conditions are common, although not predominant, throughout the former landfill. Concentrations of dissolved organic carbon, measuring 3.2 to 26 mg/L in June 2006, have been consistently greater than the 0.4 to 1.7 mg/L range measured in upgradient wells.

Predominant redox conditions in all intermediate aquifer wells downgradient of 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 (wells MW1-25 and MW1-28), and methane concentrations were 2.4 and 0.83 mg/L, respectively. Concentrations of dissolved organic carbon in these wells (6.3 and 6.1 mg/L in 2006, respectively) have been consistently greater than those measured in the upgradient intermediate aquifer well MW1-33 (0.4 mg/L in 2006). The mildly reducing conditions are somewhat favorable for reductive dechlorination of VOCs (Bradley, 2003).

Since 2002, measured H2 concentrations in the upper aquifer generally have been lower than those previously measured, although the continued widespread detection of methane and sulfide in shallow ground water does not suggest a trend from strongly to mildly reducing predominant redox conditions. The most substantial change in H2 concentrations measured in 2006 was at piezometer P1-9 in the southern plantation where concentrations substantially increased from less than 0.1 nM (nanomolar) in 2005 to 4.4 nM in 2006. Concentrations of manganese, iron (II), sulfide, and methane also increased at piezometer P1-9 in 2006, and the inferred predominant redox condition changed from manganese reduction to sulfate reduction. Other geochemical data for piezometer P1-9, such as chloride concentrations and specific conductance, also substantially varied at piezometer P1-9, suggesting the inherent variability in ground-water chemistry beneath a landfill. Overall, no widespread changes in ground-water redox conditions were measured that should result in either more or less efficient biodegradation of chlorinated VOCs.

Volatile Organic Compounds

VOC data collected by the USGS from piezometers and selected wells at OU 1 from June 1999 to June 2006 are shown in table 3 (at back of report). Data for wells and piezometers are grouped with regard to location and aquifer of the well or piezometer. The “Total CVOCs” calculated for each sample is the sum of chlorinated VOC concentrations that had detected concentrations; concentrations less than the detection limit where considered zero. Complete analytical results for the USGS data for June 2006 and previous years are available from the USGS’s NWIS web site, or see Dinicola and others (2002), Dinicola (2003, 2004, 2006), and Dinicola and Huffman (2004, 2006).

VOC Concentrations Beneath the Phytoremediation Plantations

For the northern plantation in 2006, TCE, cis-DCE, or VC were not detected at piezometers P1-1 and P1-5. Since 1999, concentrations of most chlorinated VOCs continued to decrease at piezometers P1-3 and P1-4. The sum of concentrations of the reductive dechlorination end-products ethane and ethene were 56 and 25 µg/L at piezometers P1-3 and P1-4. The steady decrease in contaminant concentrations and the positive detections of reductive dechlorination end-products have been consistent in the upper aquifer beneath the northern plantation.

For the southern plantation in 2006, changes in chlorinated VOC concentrations at the piezometers were highly variable. At piezometer P1-9, total chlorinated VOC, as well as cis-DCE and VC in 2006, were measured at their highest concentrations to date. In contrast, total chlorinated VOC concentrations between June 2004 and June 2006 decreased from 75,000 to 1,000 µg/L at piezometer P1-9. Concentrations at piezometer P1-9 in 2006 are more consistent with previous data in that the highest contaminant concentrations at OU 1 have most often been measured at piezometer P1-9. The reasons for the 2004–05 decrease in concentrations or the 2005–06 increase in concentrations are unknown. At piezometer P1-10, the consistent temporal trend of decreasing chlorinated VOC concentrations measured since 1999 ended, and the total chlorinated VOC concentration in 2006 substantially increased. At piezometer P1-8, chlorinated VOC concentrations also increased in 2006, although they remained substantially less than the concentrations measured prior to 2003. Chlorinated VOC concentrations at piezometers P1-7 and P1-6 in 2006 generally were consistent with previous concentrations, given the relatively high year-to-year variability in contaminant concentrations in the upper aquifer beneath the southern plantation.

The sum of concentrations of the reductive dechlorination end-products ethane and ethene were 444 and 1,266 µg/L at the most contaminated piezometers P1-7 and P1-9, respectively, and were measured at 9.5 and 72 µg/L at piezometers P1-8 and P1-10, respectively. Those end-product concentrations are reliable evidence that reductive dechlorination of VOCs is ongoing.

VOC Concentrations in the Intermediate Aquifer

In 2006, the reductive dechlorination end-products ethane and ethene were the only VOC concentrations analyzed in samples from the intermediate aquifer. The sum of concentrations of ethane and ethene were 21 and 18 µg/L at wells MW1-25 and MW1-28, respectively, indicating ongoing reductive dechlorination. Farther downgradient in the intermediate aquifer beneath the Highway 308 causeway at wells MW1-38 and MW1-39, ethane and ethene were not detected.

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