Natural Attenuation Monitoring Data

The groundwater 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) east of the landfill and one intermediate aquifer well (MW1-33) south of the landfill. Northern plantation sites are all in the upper aquifer and include four wells (1MW-1, MW1-2, MW1‑17, 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 three wells (MW1-4, MW1–5, 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 four intermediate aquifer wells (MW1-25, MW1-28, 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

Geochemical data collected by the USGS from piezometers and selected wells at OU 1 from 1996 to 2008 are shown in table 2 (at back of report). Historical geochemical data for wells not sampled in 2007 or 2008 are not included in table 2, but are available in Dinicola (2006).

The predominant redox conditions for all samples were inferred following guidelines described in detail by Dinicola (2006). Redox conditions generally are considered either aerobic when DO concentrations are about 1 mg/L, or anaerobic when DO concentrations are less than 1 mg/L. Anaerobic redox conditions usually can be further specified (and are named) according to the inorganic compound acting as the predominant electron acceptor in a given part of an aquifer. Common anaerobic redox conditions in groundwater are nitrate reducing, manganese reducing, iron reducing, sulfate reducing, and carbon-dioxide reducing (methanogenic). Nitrate reduction, manganese reduction, and iron reduction commonly are together referred to as mildly reducing conditions, whereas sulfate reduction and methanogenesis commonly are referred to as strongly reducing conditions. That distinction is made because different types of biodegradation processes are favored under mildly and strongly reducing conditions. Determination of redox conditions in contaminated groundwater is not a simple task and no universally accepted procedures are available. Data from many OU-1 groundwater samples indicate multiple redox conditions near the well or piezometer. The following two examples illustrate this point.

The first example is from well MW1-41 during 2007 and 2008 where manganese, iron (II), and all methane concentrations were higher than concentrations in upgradient groundwater and were relatively high in comparison to all samples. Those data suggest either manganese- or iron-reducing or methanogenic redox conditions predominated at the time of sampling. No sulfate and very little sulfide were detected at MW1-41, indicating that the predominant redox condition has moved beyond sulfate reduction to methanogenesis. However, the dissolved H₂ concentration of 0.4 nanomolar (nM) at the well suggests iron-reducing conditions (Dinicola, 2006) at the time of sampling. The sample was assigned methanogenesis as the predominant redox condition. The second example is from well MW1‑16 during 2007 and 2008 where manganese, iron (II), sulfide, and methane concentrations were much higher than concentrations in upgradient groundwater and were relatively high in comparison to all samples. The sample was assigned sulfate-reduction as the predominant redox condition largely because sulfide is a short-lived by-product of sulfate reduction when high concentrations of reduced iron (II) are available. Elevated sulfide concentrations indicate recent and localized sulfate reduction. Dissolved H₂ could not be measured at well MW1-16 in 2007 because of poor well yield, and the dissolved H₂ concentration in 2008 was only 0.1 nM, indicating iron-reducing conditions. These two examples clearly illustrate that the assignation of predominant redox conditions has some uncertainty.

For 2007 and 2008, predominant redox conditions in the upgradient wells in the upper aquifer (wells MW1-3 and MW1-20) ranged from aerobic to mildly reducing (nitrate reduction, and manganese and iron reduction). These wells have varied between aerobic and sulfate reducing during the 10 years of monitoring (table 2). Concentrations of DOC have been consistently 2 mg/L or less. Redox conditions in the upgradient well in the intermediate aquifer (well MW1-33) have been consistently aerobic.

For the upper aquifer beneath the northern plantation in 2007 and 2008, the strongly reducing conditions (sulfate reduction and methanogenesis) most favorable for reductive dechlorination of VOCs (Bradley, 2003) were inferred for five of eight upper-aquifer wells and piezometers (MW1-17, MW1-41, P1-1, P1-3, and P1-5). The other upper-aquifer wells and piezometers within the plantation had iron-reducing or unspecified anaerobic conditions. Well MW1-17 is included with the northern plantation sites in table 2, although it is between plantations and is not downgradient from any known major contaminant sources. Methane concentrations ranged from 0.26 mg/L (MW1-2 in 2007) to 14 mg/L (P1-3 and P1-5 in 2008), indicating that methanogenic (strongly reducing) redox conditions are common, but not necessarily predominant. Concentrations of DOC, which reflect the availability of electron donor needed to sustain biodegradation processes, ranged from 6.0 mg/L (MW1-2 in 2007) to 23 mg/L (P1-3 in 2008). Concentrations of dissolved carbon dioxide, a byproduct from microbial oxidation of organic compounds, ranged from 11 mg/L (1MW-1 in 2008) to 350 mg/L (MW1-41 and P1-5 in 2007, and P1-1 in 2008). Wells and piezometers with low chlorinated VOC concentrations (MW1-41, P1-1, P1-3, and P1-5), generally had high DOC, dissolved carbon dioxide, and methane concentrations. Conversely, wells and piezometers with high chlorinated VOC concentrations (1MW-1, MW1-2, and P1-4) had low DOC, dissolved carbon dioxide, and methane concentrations. Together those data are compelling evidence that biodegradation has been and continues to be an important process for decreasing contaminant concentrations in the upper aquifer beneath the northern plantation.

For the upper aquifer beneath the southern plantation in 2007 and 2008, the strongly reducing conditions (sulfate reduction and methanogenesis) most favorable for reductive dechlorination of VOCs (Bradley, 2003) were inferred for four of eight upper-aquifer wells and piezometers (MW1-16, P1-6, P1-8, and P1-10). The other upper-aquifer wells and piezometers in the southern plantation had manganese and (or) iron reducing conditions. Methane concentrations ranged from 0.38 mg/L (P1-6 in 2007) to 7.9 mg/L (P1-8 in 2008), indicating that methanogenic (strongly reducing) redox conditions are common, but not necessarily predominant. The concentrations commonly were less than concentrations measured at northern plantation wells and piezometers. Concentrations of DOC, which reflect the availability of electron donor needed to sustain biodegradation processes, ranged from 1.4 mg/L (MW1-4 in 2007) to 18 mg/L (MW1-16 in 2007), and also were less than concentrations of DOC measured at northern plantation wells and piezometers. Concentrations of dissolved carbon dioxide ranged from less than 10 mg/L (P1-6, P1-8, and P1-9 in 2008) to 190 mg/L (MW1-16 in 2007), and again were less than concentrations of dissolved dioxide measured at northern plantation wells and piezometers. The generalized pattern of relatively high DOC, dissolved carbon dioxide, and methane concentrations at wells and piezometers with relatively low chlorinated VOC concentrations is not consistent in the upper aquifer beneath the southern plantation, although well MW1-4 is highly contaminated with chlorinated VOCs and has DOC concentrations about as low as the nearby upgradient well MW1-20.

Concentrations of H₂ measured in the upper aquifer generally have been lower than concentrations measured before 2002, and only two upper aquifer wells and piezometers (MW1-17 and P1-5) had concentrations greater than 1 nM in 2007 or 2008. However, methane and sulfide concentrations at most upper aquifer wells and piezometers beneath the landfill have been consistently elevated above levels measured upgradient, so the lower H₂ concentrations do not appear to indicate a trend from strongly to mildly reducing predominant redox conditions. Organic carbon concentrations generally decreased at most wells and piezometers for the first few years after pavement was removed in 1999 to install the phytoremediation plantations. However, that downward trend on organic carbon concentrations has not continued beyond about 2004. Overall, except for the apparent trend toward lower dissolved H₂ concentrations, no widespread changes in groundwater redox conditions were identified that should result in either more or less efficient biodegradation of chlorinated VOCs.

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) for 2007 and 2008. Methane concentrations in these wells ranged from 0.67 to 2.8 mg/L and were about twice as high in MW1-25 compared to MW1-28. Concentrations of DOC in these wells ranged from 6.4 to 7.1 mg/L, and have been consistently greater than concentrations of DOC measured in the upgradient intermediate aquifer well MW1-33 (0.5 in 2007 and 0.4 mg/L in 2008). The mildly reducing conditions are somewhat favorable for reductive dechlorination of VOCs (Bradley, 2003).

Volatile Organic Compounds

Volatile organic carbon and dissolved ethane and ethene data collected by the USGS from piezometers and selected intermediate aquifer wells at OU 1 from June 1999 to June 2008 are shown in table 3 (at back of report). The VOC data include concentrations of a subset of the 64 compounds measured using USEPA Method SW846 8260B (U.S. Environmental Protection Agency, 1996). Chemical concentrations are reported as less than the reporting level for samples in which the analyte was neither identified nor detected at concentrations equal to or greater than the reporting level. Historical VOC data for wells not sampled in 2007 or 2008 and for dates before 1999 are not included in table 3, but are in Dinicola (2006) and U.S. Navy (2008). The total CVOCs calculated for each sample is the sum of chlorinated VOC concentrations that were positively detected; concentrations reported as “less than” values were not included in the total. Complete analytical results for the USGS data for 2007–08 and previous years are available from the USGS NWIS web site http://nwis.waterdata.usgs.gov/wa/nwis/qwdata or Dinicola and others (2002), Dinicola (2003, 2004, 2006), and Dinicola and Huffman (2004, 2006, 2007). Complete analytical results for the complimentary Navy VOC data from 1995 through 2007 is available in U.S. Navy (2008).

Volatile Organic Compound Concentrations Beneath the Phytoremediation Plantations

For the northern plantation in 2007 and 2008, chlorinated VOC concentrations at most piezometers were similar to or slightly less than concentrations of chlorinated VOC measured in previous years. In 2008, chlorinated VOCs were not detected at piezometer P1-5, and the only chlorinated VOC that was positively detected at piezometer P1-1 was cis-DCE at an estimated concentration of 0.18 μg/L in P1-1 (VOCs were not analyzed at these piezometers in 2007). At piezometer P1-3, chlorinated VOC concentrations in 2007 and 2008 were at the lowest levels since monitoring began in 1999. Most VOC concentrations at piezometer P1-4 in 2007 and 2008 were similar to VOC concentrations measured in previous years except that VC concentrations increased from 280 μg/L in June 2007 to 750 μg/L in June 2008. Since 1999, total CVOC concentrations at P1-4 have been at least two times greater than CVOC concentrations measured at all other piezometers in the northern plantation, including wells 1MW-1, MW1-2, and MW1-41 that are regularly monitored by the Navy (U.S. Navy, 2008). In 2008, measurements of the sum of concentrations of the reductive dechlorination byproducts ethane and ethene were at the highest levels to date at all northern plantation piezometers except P1-1, where the compounds were not positively detected. Those byproduct concentrations provide evidence of reductive dechlorination of chlorinated VOCs.

For the southern plantation in 2007 and 2008, chlorinated VOC concentrations measured in piezometers were often extremely high and varied widely. At piezometer P1-6, the total chlorinated VOC concentration was exceptionally low in 2007 at 380 μg/L, and was then exceptionally high in 2008 at 20,000 μg/L. At piezometer P1-7 in 2008, total chlorinated VOCs, and the individual compounds TCE, cis-DCE, and VC, were at the highest concentrations to date. In contrast, total chlorinated VOC concentrations at piezometers P1-8, P1-9 and P1-10 in 2008 were relatively low compared to 1999–2007 levels. Chlorinated VOC concentrations at wells MW1-4 and MW1-16 in the southern plantation also have been highly variable over time. Since 1999, total CVOC concentrations at P1-7 or P1-9 have been the highest measured at all wells and piezometers in the northern plantation. The magnitude and persistence of dissolved-phase chlorinated VOC concentrations in these piezometers indicate non-aqueous phase chloroethenes beneath the southern plantation. The temporal variability in concentrations likely is a result of variations in precipitation and groundwater levels interacting with the non-aqueous phase liquid. In 2007 and 2008, one or both of the reductive dechlorination byproducts ethane and ethene were detected at all southern plantation wells and piezometers, with an ethene concentration as high as 850 μg/L (P1-7 in 2008). Those byproduct concentrations are evidence that reductive dechlorination of dissolved VOCs is ongoing, although non-aqueous phase liquid chloroethenes are not likely to be substantially affected by biodegradation (Dinicola, 2006).

Volatile Organic Compound Concentrations in the Intermediate Aquifer

In 2007, the reductive dechlorination byproducts ethane and ethene were the only VOC concentrations analyzed in samples from the intermediate aquifer. The sum of concentrations of ethane and ethene were 12.1 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.

In 2008, total chlorinated VOC concentrations in wells MW1-25, MW1-28, and MW1-39 were consistent with previous years. However, VC concentrations in 2008 at these wells were the highest measured to date: 510 μg/L at MW1-25, 930 μg/L at MW1-28, and 3.0 at MW1-39. Along with elevated VC concentrations, the sum of concentrations of ethane and ethene in 2008 were somewhat higher than concentrations measured to date at wells MW1-25 (24 μg/L) and MW1-28 (34 μg/L). These data suggest that either the rate of reductive dechlorination of cis-DCE to create VC may have increased, or more non-aqueous phase liquid has dissolved into groundwater.