CICEET Progress Report for the period 3/15/05 Through 9/15/05
Project Title: Development of treatment wetland technology for VOC-contaminated groundwater
Principal Investigator(s): John H. Pardue, William Moe, Fred Rainey
Project Start Date: 1 September 2002
Accomplishments
Scheduled Tasks
1. Selection and geotechnical testing of substrates. A number of peats have been tested for their suitability in a treatment system. Additional sources of peat and compost are needed for large scale implementation. For the physical-chemical testing, tests include saturated hydraulic conductivity using the falling head method, dry bulk density, wet bulk density, porosity, and the fraction of organic carbon. Although this task was completed in one of the earlier project periods, we continue to search and test new materials as they come available.
2. Sorption and biological testing of substrates. Sorption potential for a range of chlorinated VOCs will also be determined using standard batch techniques for determining partition coefficients of VOCs. Biological testing will include saturated incubations to establish the redox potential following flooding, the methane production rate, the ambient H2 concentration that develops and the organic acids present. As before, this task is being performed as new materials become available.
3. Treatability studies (effect of vegetation type). In this task, upflow mesocosm studies are being performed in the greenhouse. Specifically this task is examining the specific role of vegetation in removal efficiency. Although bench-scale studies have established an initial basis for system design, additional experiments are needed. A greenhouse mesocosm experiment is being conducted comparing system performance with Phragmites and Typha latifolia.
4. Treatability studies (effect of sulfate). In this task, treatability studies are being conducted on the effect of sulfate on dechlorination. In coastal water, saltwater instrusion can lead to elevated sulfate levels in a treatment wetland. High sulfate concentrations may create inhibitory conditions for D. ethenogenes although other halorespiring organisms may compensate. Microcosm experiments were completed during the last reporting period and challenging mesocosms with elevated sulfate concentrations was initiated during the no-cost extension period to confirm microcosm results.
5. Inoculation strategies. Successful implementation of the proposed wetland treatment technology in full-scale applications will depend on successful establishment of specific microbial populations in the treatment bed. Consequently, a reliable inoculation procedure for establishing the desired microbial populations must be developed. Experiments have been conducted using a variety of inoculation procedures that are practical methods for implementation at the full scale. These strategies were completed previously.
6. Identification of concentrations of VOCs in discharge zone (unscheduled). In late October, 2002 the group was given the opportunity to visit the location of the pilot study in the Southern Bush River and sample the adjacent river sediment porewater using 25 dialysis samplers. Analysis of metals and organics are being used to establish concentrations to be used in upflow mesocosms for design purposes. This task was completed in an earlier reporting period.
7. Refinement of design equation. Relatively simple 1-D equations have been used to design treatment wetlands. Although these may be appropriate given the uncertainty in a number of key parameters, improvements in design may be accomplished by adding dispersion to the equation and sequential dechlorination kinetics. These equations will be calibrated and validated using the mesocosm results.
8. Dissemination of results. Results will be disseminated by presentations at national conferences and in peer-reviewed publications.
Progress on Tasks
1. Selection and geotechnical testing of substrates. Two additional materials were tested during this period, two different compost samples from McGill Compost in North Carolina. These are typical mixed composts and include leaves, yard wastes, and some biosolids. Both mixtures had desirable hydraulic properties and they were biologically tested as described below. These samples represent a large category of potential wetland substrates and extend the potential of wetland construction to a potentially large pool of material.
2. Sorption and biological testing of substrates. Microcosm and sorption studies were initiated with the most promising of the compost materials from McGill. It performed very well in initial microcosm testing after inoculation with the culture containing Dehalococcoides. In fact, its performance was comparable to the Bion Soil material which was so critical in previous studies. Additional microcosm studies are being performed by the graduate student supported by the project Stephen Mbuligwe, using a wider range of chlorinated VOCs. This material appears to be a viable substitution for Bion Soil and it is produced in large quantities (70,000 yd3/year)
3. Treatability studies (effect of vegetation type). Mesocosms constructed using a mixture of Latimer peat, sand and Bion Soil and planted with Typha latifolia and Phragmites communis were utilized in this set of experiments. A simulated groundwater containing trichloroethene (TCE) and 1,1,2,2-tetrachloroethane (1,1,2,2-TeCA) has been added to the mesocosms for approximately 20 months in an upflow mode. The target influent concentrations were 1 mg/L 1,1,2,2-TeCA and 0.5 mg/L TCE during the initial phase. During February, 2004 concentrations were increased to 1.5 mg/L 1,1,2,2-TeCA and 0.75 mg/L TCE. Intensive sampling was conducted February 25-28th, 2004. During May, 2004 concentrations were increased to 2 mg/L 1,1,2,2-TeCA and 1 mg/L TCE. Intensive sampling was undertaken between May 10 and May 13. The pumping rate remained constant through each aquarium at 14.4 L/day, and as such, the hydraulic retention time in each wetland is about 3 days.
The absence of ethene and ethane, the terminal degradation products of 1,1,2,2-TeCA and TCE suggested that in the wetland mixtures, an optimized microbial population did not exist. We have observed this previously in laboratory and greenhouse and field studies over the past 5 years. Commercially-available peat and compost materials do not normally contain dehalorespiring populations that will completely dechlorinate these compounds. Rather, the presence of organisms that dechlorinate TCE to cis-1,2-DCE are commonly present but organisms capable of complete dechlorination of TCE (or 1,1,2,2-TeCA) are rarely encountered in these materials.
On May 27, 2004, we inoculated all 4 aquaria adding culture containing organisms previously observed to completely dechlorinate TCE and chloroethanes (Kassenga et al., 2004). Characteristics of the Dehalococcoides populations in this culture have been previously published (Kassenga et al., 2004). Shortly after inoculation, vinyl chloride, ethene and ethane began to be detected in both Typha and Phragmites planted mesocosms
Results for Typha-planted mesocosms from September 2004 are presented in Figure 1. Vinyl chloride, ethane and ethene were observed in samples taken from heights equal to and above 18 cm. These results imply that complete dechlorination of both 1,1,2,2-TeCa and TCE took place in the wetland, and this in turn also implies that the inoculation of the wetlands with Dehalococcoides sp. was successful.
Similar results were observed for the Phragmites mesocosms As with the Typha mesocosms, vinyl chloride, ethene and ethane were detected post-inoculation whereas before, they were non-detects. For this period, vinyl chloride was moving further up the bed than following its production. cis -1,2-DCE was not observed in appreciable quantities.
In January, 2005, data had reached steady-state for both the Typha and Phragmites mesocosms (Figure 3 and Figure 4). At this time, detected metabolites were primarily ethene and ethane. Small amounts of vinyl chloride were measured in Typha 1 and elevated ethene and ethane observed in Phragmites mesocosms. This suggests that perhaps rhizospheric microbial communities in Phragmites are better for complete dechlorination of parent VOCs. However, in both mesocosm sets, complete removal of parent and daughter chlorinated VOCs were observed over travel distances of ~10 cm.
During this period, samples were taken for microbial community structure analysis. DNA has been extracted from bulk sediments from several locations within each mesocosm and real-time PCR has been utilized to quantify the Dehalococcoides populations. Additional measurements using primers from other dehalorespiring populations will be performed during the final reporting period.
4. Treatability studies (effect of sulfate). Microcosm studies were been completed on the effect of sulfate and sulfite, two potential complicating geochemical terminal electron acceptors. A manuscript describing the experimental work was submitted during the previous reporting period and we are awaiting reviews. The finding that dechlorination can proceed under sulfate-reducing conditions is very promising for the use of treatment wetland in coastal systems. We are confirming this finding by raising sulfate levels in the feedwater to the mesocosms. Another increase in feed salinity was performed in September and we are monitoring the last results. This data will be presented in the final report.
5. Inoculation strategies. Studies have been completed on the inoculation strategies for the Bion:Latimer:sand mixture during the last reporting period. Results demonstrated that inoculation will not require specialized anaerobic procedures but can be accomplished during the normal wetland construction process.
6. Identification of concentrations of VOCs in discharge zone (unscheduled). This task was completed during an earlier reporting period and presented in several national and international conferences
7. Refinement of the design equation. Tracer studies performed during the last reporting period were used to modify the design equation to include dispersion. Dispersion measurements were identified as an important design consideration since bed depth may be underestimated if it is ignored. A range of dispersion numbers have been assessed in the mesocosms that will be useful for setting a range on design. Quantitatively the effect of dispersion on the design equation has been established and will be useful in performing the full scale design.
8. Dissemination of results. A platform presentation of research results was made at the 2nd International Conference on Remediation of Contaminated Sediments (September 30-October 4, 2003 in Venice, Italy). The presentation covered groundwater-sediment interaction at the Southern Bush river/Chesapeake Bay site and the potential for using a treatment wetland to accomplish removal of chlorinated solvents prior to groundwater discharge. The paper was well-received and spurred a number of discussions with European scientists who are studying similar problems. A presentation on temperature effects on constructed wetland biodegradation was recently made Battelle’s Third International Conference on Remediation of Contaminated Sediments (New Orleans, January 24th-27th, 2005). An invited presentation on design of treatment wetlands for chlorinated VOCs was made at the University of Massachusetts conference on Soils, Sediments, and Water on October 18th, 2005.
Difficulties
None encountered.
Anticipated Success in Meeting Project Objectives
A 1 year, no-cost extension letter was submitted to CICEET dated 8/8/2004 The extension was requested in order to run the mesocosms longer following the bacteria inoculation. This will allow for additional study prior to dissecting the mesocosms and analyzing the microbial community structure. The no-cost extension was approved.
Tasks and activities for next reporting period
Tasks for the next reporting period
Submit final report
Work plan to accomplish tasks
Write final report
Concerns or difficulties
None.