Groundwater Remediation at Concord Naval Weapons Station
The benefits of In Situ Chemical Reduction was compared to Biologically Mediated Enhanced Reductive Dechlorination to treat groundwater affected by trichloroethene at Installation Restoration Site 29 at Concord Naval Weapons Station, Calif.
By Daniel P. Leigh, PG, C.Hg, M.SAME, and Neil Hey, PG
Fig. 1: Installation Restoration Site 29 at Concord Naval Weapons Station, Calif. TCE plume is shown in yellow in both photographs. IMAGES COURTESY PEROXYCHEM
Soil and groundwater at Installation Restoration Site 29 at the Concord Naval Weapons Station, Calif., has been affected by a discharge of trichloroethene (TCE). The U.S. Navy intends divest the property under the Base Realignment and Closure program as rapidly as possible.
The project was awarded to CB&I as a performance based contract with payment primarily based on achieving remedial goals within a specified timeframe. To expedite the Navy’s goals, CB&I selected an aggressive treatment approach, In Situ Chemical Reduction, to rapidly remediate chlorinated ethenes in the groundwater at the site.
EVALUATING SITE CONDITIONS
Installation Restoration Site 29 is located on the inland portion of Concord Naval Weapons Station, which is located northeast of Oakland in California's East Bay region. The affected aquifer consists of unconsolidated silt, sands and clays. Groundwater, at approximately 50-ft below ground surface, is highly aerobic (dissolved oxygen ~7-mg/L) and mildly oxidizing (oxidation reduction potential ~250-mV).
The TCE source is a building previously used to refurbish munitions. TCE was discharged, likely through drain lines east of the building. The TCE plume extends 700-ft down hydraulic gradient from the source area and up to 100-ft below ground surface. Minimal conversion of TCE to dichloroethene (DCE) and vinyl chloride (VC) had occurred and minor amounts of discharged DCE were detected combined with the TCE. The site is shown in Figure 1.
BIOTIC ONLY PILOT TEST
A Biotic Only pilot test was previously conducted in the TCE source area in 2011. The Biotic Only test used buffered emulsified oil substrate, which was augmented with VC respiring microbial consortium SDC-9 to stimulate enhanced reductive dechlorination. Sodium lactate was added to establish reducing conditions in the injection water prior to bioaugmentation and injection.
A total of 10 injections were conducted to a depth of 65-ft below ground surface at 2.5-ft vertical intervals in locations 12-ft on center to achieve a 6-ft radius of influence. The degradation of chlorinated ethenes during the Biotic Only pilot test was measured in wells S29MW10 and S29MW11 (shown in Figure 2). The Biotic Only pilot test resulted in complete degradation of the TCE and daughter products with concentration from approximately 5,000-µg/L to less than 1-µg/L in 550 days.
Fig. 2: Biotic only and DOT pilot test well and injection locations.
IN SITU CHEMICAL REDUCTION
In Situ Chemical Reduction was selected for this site to enhance the biotic process demonstrated in the Biotic Only pilot test. The biotic degradation of TCE results in the stoichiometric sequential production of toxic daughter products, including DCE VC, which subsequently reduce to non-toxic ethene and ethane. The abiotic degradation process primarily bypasses the generation of these toxic degradation product by the beta-elimination pathway, temporarily generating unstable chlorinated acetylenes, which may be converted to ethene and ethane. The hypothetical biotic and abiotic degradation pathways are shown in Figure 3.
The In Situ Chemical Reduction process synergistically incorporates both abiotic and abiotic process to more aggressively degrade chlorinated organics. This abiotic processes is applied by distributing microscale zero valent iron (ZVI) to degrade TCE. In Situ Chemical Reduction enhanced the biological degradation process by amending the ZVI with an organic substrate, Emulsified Lecithin Substrate, an organic substrate that is emulsified to generate hydrophilic micelles with enhanced distribution properties. In addition, the lecithin structure incorporates essential nutrients nitrate and phosphorous in its molecular structure to enhance biological activity.
Fig. 3: Hypothesized biotic and abiotic degradation pathways.
The injection solution consisted of groundwater and hydrant water—with the addition of sodium lactate to the injection solution creating highly reducing conditions prior to bioaugmentation and injection helping to establish the bioaugmentation culture in the aerobic aquifer. Emulsified Lecithin Substrate was distributed through the aquifer to provide a long-lasting organic substrate to treat potential residual contaminants. Bioaugmentation was conducted in both pilot tests using a VC respiring culture (SDC-9).
In Situ Chemical Reduction Pilot Test. The In Situ Chemical Reduction pilot test was conducted in the TCE source area in wells (S29MW01 and S29MW03) not affected by the Biotic Only pilot test, as shown in Figure 2. In Situ Chemical Reduction substrate was injected at three locations at a distance of 10-ft from well S29MW01 and three locations at a distance of 15-ft from well S29MW03. Distribution was conducted by Vironex using direct push technology.
Contaminant Degradation Monitoring. Baseline samples were collected 38 days prior to the beginning of the DOT. Groundwater samples then were collected at day four, 21, 35, 56, 93 and 155 following injection. Samples were analyzed for volatile organic compounds, dissolved gasses, nitrate, sulfate, arsenic, manganese, total organic carbon, and alkalinity. The data generated during the DOT were plotted on the same graphs as the Biotic Only pilot test to compare the effectiveness of the In Situ Chemical Reduction treatment to the application of Biotic Only processes (see Figure 4).
Fig. 4: Change in arsenic concentration during pilot tests.
Physical and geochemical parameters and contaminant concentrations were monitored during the Biotic Only pilot test and the DOT.
Arsenic. During the establishment of highly reducing condition necessary for reductive dechlorination, dissolved arsenic (As) concentration increases as insoluble As(V) is reduced to soluble As(III). During the Biotic Only pilot test, dissolved As concentrations increased to >50-μg/L, and maintained concentrations in excess of the primary drinking water standard (10-μg/L) for the duration of the pilot test. During the In Situ Chemical Reduction test, however, only slight increases in As were observed and concentrations never exceeded the primary drinking water standard.
Volatile Organic Compounds. As highly reducing conditions were established, rapid reductive dechlorination of the TCE was observed in both tests. Notable differences were observed in the production and degradation of chlorinated daughter products DCE and vinyl VC in the two tests. DCE concentrations increased substantially in both tests; however, DCE degradation occurred more rapidly in the In Situ Chemical Reduction test and persisted longer in the in the Biotic Only test. As DCE concentration decreased, concentrations of VC increased—though at substantially lower concentrations than the stoichiometric amount of TCE degradation would predict and approximately one order of magnitude lower than the Biotic Only approach. (The CE concentrations during both tests are presented in Figure 5.)
Complete reductive dechlorination of the chlorinated ethenes was confirmed by the near stoichiometric conversion of the chlorinated ethenes to ethene and ethane observed by Day 56 of the In Situ Chemical Reduction pilot test. The observed amount of ethene and ethane can be attributed to the complete reductive dechlorination of 1,839-μg/L of TCE, which is approaching the 2,300-μg/L concentration of TCE observed in the baseline samples from well S29MW01.
Fig. 5: Comparison of chlorinated ethene data collected during the In Situ Chemical Reduction and Biotic Only pilot test.
The total chlorinated ethenes concentration decreased at a much faster rate by In Situ Chemical Reduction than in the Biotic Only Test. This resulted in a reduction in total mass of 99.8 percent within 155 days—whereas, 500 days was required in the Biotic Only test to achieve the same amount of mass reduction. The higher rate of chlorinated ethenes degradation by the In Situ Chemical Reduction appears to be attributable to the rapid and primarily abiotic degradation of cis 1,2-DCE during the In Situ Chemical Reduction process.
Because most of the DCE was degraded by the beta-elimination pathway, minimal production of VC is attributable to minor contemporaneous biological reductive dechlorination of DCE. This minor amount of VC was subsequently degraded to ethene and ethane by biological reductive dechlorination and by hydrogenation to ethane. (The total chlorinated ethenes mass concentrations and percent mass removal are shown in Figure 6.)
Fig. 6: Comparison of ethene, ethane and total chlorinated ethene data collected during the In Situ Chemical Reduction and Biotic Only pilot tests.
In Situ Chemical Reduction represents a significant improvement to standard enhanced biological reductive dechlorination for treatment of chlorinated ethenes. The symbiotic processes that constitute the approach more rapidly achieve remedial goals. Emulsified Lecithin Substrate was confirmed to be a highly effective electron donor for biologically enhanced reductive dechlorination processes in the In Situ Chemical Reduction technology. Significantly, the primary degradation process of the cis 1,2-DCE was abiotic beta-elimination resulting from contact with the incorporated ZVI. This process was demonstrated to be much quicker than the biotic reductive dechlorination of cis 1,2-DCE and minimized the production of VC.
Emulsified Lecithin Substrate was demonstrated to be a highly efficient electron donor that rapidly established highly reducing conditions, which were buffered by the incorporated ZVI. The combination of these processes allowed the supplied bioaugmentation culture to efficiently dechlorinate the minor amount of residual VC. the remedial goals for this project were achieved within the DOT area in 155 days as compared to more than 500 days using the Biotic Only approach.
The application of this technology will allow the contractor to achieve the goals of its performance based contract in one-third the time required than if relying on the Biotic Only approach.
Ultimately, the reduced remedial time will allow for the Navy to achieve its goal of divesting the base in a timely fashion.
*The authors would like to acknowledge Scott Anderson; Valerie Harris, P.E., Amy Estey, and Eliot D. Cooper for their contributions to this article.