Technology and its Evolution

Chlorinated aliphatic hydrocarbons (CAHs) are proving to be the most widespread and abundant groundwater contaminants across the entire country. This class of compounds includes widely used solvents such as trichloroethene (TCE) and tetrachloroethene (PCE), carbon tetrachloride (CT), methylene chloride, trichloroethane (TCA) and others. In addition to their roles in many industrial processes, CAHs have historically been used for cleaning and degreasing such diverse products as aircraft engines, automobile parts, electronic components and clothing, in both the military and civilian sectors.

Enhanced reductive dechlorination (ERD) via in situ reactive zones (IRZ) is intended to facilitate and expedite the biological reductive dechlorination of CAHs. The well-documented mechanisms for PCE and TCE breakdown products are:

PCE TCE cis-DCE VC Ethene
|____-----> Trans-DCE ------>_____|

The concept of in situ reactive zones is based on the creation of a subsurface zone, where contaminants migrating in the groundwater are intercepted and permanently degraded or transformed into harmless end products (Figure 1). The ERD technology stimulates indigenous microbiological organisms through the engineered addition of electron donors, which are easily biodegradable organic carbon sources. In practice, this is implemented as an in-situ bioreactor that forms down gradient from a line of substrate injection wells placed in a line perpendicular to groundwater flow. If sufficient carbon substrate is injected, oxygen and nitrate metabolism dominates near the injection line, while sulfate reduction, methanogenesis and complete reductive dechlorination zones form farther down gradient (Figure 2).

The technology operates most effectively when groundwater is passing through the sulfate-reducing zone, while still bearing a degradable carbon load that will support methanogenesis and complete reductive dechlorination.

This “high-performance” reductive dechlorination can only be engineered when the rate of electron donor consumption exceeds the rate of electron acceptor recharge. The carbon source must be highly mobile and highly degradable, and injected at rates commensurate with the overall flux of groundwater, electron acceptors and CAHs that are moving through the treatment zone. The organic acids that form as breakdown compounds must be buffered by aquifer carbonates or by the addition of carbonates and bicarbonates to the injection mix. Certain site hydrogeologic characteristics require modification of the high-performance approach, but most can still be treated by ERD systems that are more cost-effective than alternative approaches.

The electron donors, which are soluble carbohydrates and similar substances, include both pure compounds (e.g. glucose, fructose) and complex mixtures of multiple compounds (i.e. molasses and cheese whey). The engineering application procedures and mode of action for both classes are very similar (Figures 4 and 5). We generally advocate the use of the complex food grade mixtures (i.e. molasses or high fructose corn syrup, or cheese whey) for the following reasons:

• The ability of complex mixtures to encourage growth of a more diverse microbial community.

• Rapid utilization of the injected substrate.
• Development of a diverse array of fermentation products, including hydrogen, acetate and fatty acids.

These substrates support the buildup of a broad-based community of dechlorinating bacteria.

The number of remediation sites where this technology is being implemented has grown very quickly within the last three to four years. ARCADIS has completed or is working on around 150 in-situ reductive dechlorination projects. Most of these projects are in the United States, but we have projects all over the world using this technology. We are presenting data from sites across many parts of the United States and England to counter the argument, put forth by certain segments, that “dechlorinating bacteria are not ubiquitous.”

The data presented below and the authors’ experience accumulated from the 150 sites strongly indicate that bio-stimulation — proper biogeochemical management of native microbial communities —can achieve complete reductive dechlorination if sufficient time is provided for the microbial community to shift according to the changed substrate and biogeochemical conditions. Bio-stimulation relies on coarse adjustments of the indigenous microbial habitat to induce the desired dechlorinating behaviors. In some cases, a lag phase is observed during which dechlorination, specifically the transformation of cis-DCE, proceeds slowly. But in all cases observed to date, the microbial populations emerge from a lag phase within one to six months, and rapid dechlorination to ethene is achieved. At this point in time, we believe that this lag occurs as the microbial populations adapt to increased electron donor loading of the bio-stimulation process, and as parent compound concentrations are reduced to levels that do not inhibit further degradation (high concentrations of PCE, for example, can inhibit cis-DCE degradation). This observation is substantiated by the fact that the lag period is typically longer in low-organic-carbon aquifers that have high redox levels and no daughter product development prior to starting bio-stimulation.

Below are four case studies that demonstrate the effectiveness of ERD via IRZ.

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Case history 1

An engineered anaerobic in situ reactive zone was established in a permeable, high-carbonate aquifer in the Midwestern United States that was contaminated by PCE and TCE releases prior to 1980. The organic carbon fraction (foc) in the aquifer ranged from 0.001 to 0.006 while dissolved PCE and TCE concentrations were three and five mmol/L (500 and 700 mg/L) respectively prior to treatment. At a median organic carbon fraction of 0.003, 80 percent of the PCE and 58 percent of the TCE were expected to reside in the sorbed phase prior to the formation of the reactive zone. Enhanced reductive dechlorination was induced through injections of five or ten percent molasses solution every two weeks over a two-year time period. The chlorinated alkene(s) concentration presented in (Figure 3) were observed at a groundwater monitoring well located about 100 days down gradient from the injection wells.

As reductive dechlorination proceeded, a six-fold increase in total dissolved alkenes, specifically when reductive dechlorination was complete, was observed. We believe that these increases are a beneficial natural consequence of bio-surfactant production by microbial consortia, co-solvent action of fermentation products and decrease in Koc values as chlorine atoms are removed sequentially by dechlorination. The observed concentration decreases represent the combined effects of desorption and degradation. As a result, degradation rate constants determined from field observations will be an underestimate of the “true” degradation rate.

It can be seen that when the TOC concentrations were maintained above 200 mg/L, complete reductive dechlorination was achieved within a short time frame. When the TOC concentrations dropped below 200 mg/L, cis-DCE accumulated for a short time frame and was completely degraded when the TOC levels were raised. It is important to note that vinyl chloride did not accumulate during the study period. The pre-treatment vinyl chloride concentration was 0.05 mmol/L (three mg/L, and the peak observed was only 0.2 m mol/L (12 mg/L) – occurring after 27 m mol/L (2,700 mg/L) of cis-DCE was degraded. The authors’ experience indicate that the buildup of vinyl chloride can be avoided or minimized (while achieving complete dechlorination) if the redox conditions are sufficiently reduced and maintained under methanogenic conditions.

A very important observation that needs to be noted from the breakdown products is that complete dechlorination proceeded smoothly even when there was significant production of methane within the reactive zone. Hence, we strongly believe that there is no need for the slow release “designer” organic substrate(s) to avoid competitive inhibition of dechlorination by methanogenic bacteria. We would like to advise the practitioners of this technology to use the cheapest organic substrate. We prefer molasses because it helps to drive the redox down faster due to the quick formation of sulfate-reducing conditions.

Case history 2

Performance results from another full-scale in-situ dechlorination system are presented from a site in Southeast England. This system is being implemented beneath an active manufacturing facility. Injection wells were installed below the facility prior to construction and a molasses tank was installed on the roof to facilitate gravity-fed injections (Figures 4 and 5). The plume is located in a permeable sand and gravel aquifer, approximately 20 feet below grade. The groundwater velocity is around two feet per day.

The injection system consisted of a mixing tank installed on the roof and automated controls to allow for the injection of a dilute molasses reagent on a daily basis. Electron donor feed rates were initially low due to the concerns stemming from an operating facility and was gradually increased over the period of operation. During the first two years of operation, 15,000 lbs. of organic carbon (equivalent to 21 lbs./day) was delivered into the entire plume. Later on it was increased to 57 lbs./day to induce strongly reducing conditions. The cost of organic carbon from molasses is only $0.20 per pound making the reagent cost a relatively small portion of the entire project cost.

Baseline analysis indicated that mildly reducing and anaerobic biochemical conditions were already present within this plume, substantiated by the presence of significant levels of cis-DCE. One should note that, as shown in (Figure 6), the TOC value within the plume was only increased very slowly by carefully controlling the daily gravity feed rates. The TCE concentrations declined immediately after the injections began. Increase in concentrations of the daughter products can be seen with the slightly increasing TOC levels and enhanced reducing conditions. It is important to note that the lag time required for the entire microbial community structure to build up is very minimal at this site. This is probably due to the mildly reducing biochemical condition that pre-existed within the plume prior to the engineered conditions.

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The most important observation from the performance of this IRZ is the direct correlation between the TOC levels and the extent of completion of the reductive dechlorination pathway of TCE. When the TOC levels are lower, we can see an accumulation of cis-DCE and VC and a reduction in the final end product ethene. When the TOC levels were maintained higher than 30-40 mg/L, reductive dechlorination of TCE was complete as can be seen with an absence of cis-DCE and VC buildup accompanied by an increase in ethene levels. When the injection levels of TOC were more than doubled around December 2002, we could see not only complete dechlorination but also a rapid rate of the transformations as evidenced by the quick rise in the ethene concentrations.

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Case history 3

We have chosen to present data from a site located in Northern California mainly to present the importance of maintaining a minimum TOC level to complete the reductive dechlorination pathway. Most of the contaminated groundwater plume lies beneath the paved parking lot of a retail mall. Contamination was caused by PCE released from a dry cleaner located there prior to the redevelopment of the mall.

Electron donor of choice to increase the TOC levels within the IRZ was cheese whey. It can be seen that the transformation to cis-DCE from PCE and TCE was immediate and further transformation was slowed when the TOC levels dropped (Figure 7). When the TOC levels were raised to levels above 200 mg/L complete reductive dechlorination was achieved at a rapid rate. It should also be noted that VC did not accumulate when the TOC levels were high and highly reducing conditions were achieved. Methane concentrations in the groundwater were about 1.0-2.0 mg/L and we were still able to achieve complete dechlorination.

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Case history 4

(Figure 8) presents data collected from an in-situ reactive zone established in a fractured, low permeability bedrock formation in New Jersey. Dissolved PCE concentrations in groundwater were observed at concentrations up to 90 mg/L (we are using mg/L here to make the readers easily appreciate the performance of this system under DNAPL conditions in a fractured bedrock environment). Shortly after molasses injections started, PCE concentrations drop-_ped faster and TCE and cis-DCE concentrations increased. Within a two-month period, PCE and TCE continued to decrease rapidly while cis-DCE concentrations increased to 160 mg/L, which on a molar basis is much higher than the initial chlorinated alkene concentrations. This is due to the enhanced desorption of the sorbed mass by bio-surfactants, and co-solvency of fermentation products formed within the reactive zone. Reduced Koc values for cis-DCE also keeps more mass of this compound in the dissolved phase. A very important observation to note again is that, even at these high concentrations of chlorinated ethenes, complete transformation of PCE took place in the presence of methane up to 12 mg/L. This evidence indicates that the notion of competitive inhibition to reductive dechlorination by methanogenic bacteria may only come from an academic point of view. We saw clear evidence of complete reductive dechlorination at very high concentrations of PCE with only bio-stimulation alone at this site. We are seeing some vinyl chloride and we attribute that to the very high concentrations present in the aqueous and sorbed phase and also due to the fluctuations in TOC concentrations. Complete transformation of PCE is evidenced by ethene concentrations up to 12 mg/L.

Conclusions

As described earlier, our personal experience, scientific literature and our company’s experience lead us to conclude that the bio-geo-chemical environment within the engineered IRZ is the key to designing reductive dechlorination systems. We urge the readers to be cautious and ask a lot of questions when claims are made that bioaugmentation and slow release substrates are crucial for successful reductive dechlorination.

We would like to present our observations and conclusions briefly as following:

• There should be no presumption that bioaugmentation is required to achieve enhanced reductive dechlorination at any site. The injection of bacterial cultures may shorten the lag phase, reducing the time to achieve maximum dechlorination rates, but is not required to assure the success of any project.
• Dehalorespiring bacteria are not required to achieve full dechlorination of ethenes or ethanes. There is extensive evidence from both the field and the laboratory showing complete (probably cometabolic) dechlorination under conditions which are inhospitable to Dehalococcoides ethenogenes and other species that have been proven to achieve metabolic dechlorination.
• Because it is not necessary to constrain hydrogen levels for the benefit of Dehalococcoides or any other group of organisms within the dechlorinating IRZ, it is also not necessary to limit rates of electron donor consumption in treated aquifers. In fact, this constraint is counterproductive to the objective of rapid, complete dechlorination.
• The buildup of vinyl chloride is avoidable. Vinyl chloride buildup is consistently observed at sites where electron donor consumption is unnecessarily constrained by a limited electron donor supply. The use of expensive, slow release or limited-dose donors leads to incomplete dechlorination. When high rates of electron donor consumption are achieved, vinyl chloride buildup can be avoided entirely.

These observations and conclusions were arrived upon from the data obtained at ARCADIS sites as well as published data and research from many other field applications and lab research. PE

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Remediation News

EPA Bands with 21 other federal agencies

Christie Whitman announced at the “Brownfield 2002—Investing in the Future” conference in Charlotte, N.C., that the EPA and 21 other federal agencies and departments committed to work together to redevelop Brownfields under a new Brownfields Federal Partnership Action Agenda. The action agenda is one piece of an effort by the Bush administration to address Brownfield cleanup efforts. Another example would be the Small Business Liability Relief and Brownfields Revitalization Act to help states and communities clean up and revitalize Brownfield sites. The 2003 budget request includes a proposal to more than double money earmarked for Brownfields to $200 million. Highlights of the plan are:

‡$850 million over next five years;
‡U.S. Economic Development Administration, U.S. Department of Housing and Urban Development, U.S. Department of the Interior, U.S. Department of Justice, and U.S. Department of Labor to offer funding priority to Brownfields communities through their respective grant mechanisms;
‡The National Oceanic and Atmospheric Administration’s commitment to lead an interagency “Portfields” project that will focus on the redevelopment and reuse of idled or abandoned lands in and around ports, harbors and marine transportation hubs;
‡The U.S. Army Corps of Engineers’ commitment to announce eight new pilots under its “Urban Rivers Initiative” to address restoration in and around urban rivers;
‡a new, concerted effort to share program information with interest groups, by methods such as linking web sites;
‡changing federal agency policies to facilitate Brownfields redevelopment; and
‡making funding and technical assistance to Brownfields communities a budget priority at all federal agencies.

EPA’s Brownfields assistance has leveraged more than $4.6 billion in private investment, helped create more than 20,000 jobs and has resulted in the assessment of more than 4,000 properties so far. For more information, visit www.epa.gov/brownfields/partnr.htm#fpaa.

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Drilling in Alaska

Regulatory agencies and the oil industry have worked together to reduce the environmental impacts of drilling for oil in Alaska. While many efforts have been successful, a report recently released pointed to many more items that need to be addressed.

The National Academy of Sciences reported that oil and gas production on the North Slope has brought positive and negative consequences —economic, social and environmental. New technologies are less invasive and new platforms are smaller. The report also pointed out that some migration routes had changed to avoid the noise. More people mean more refuse. The refuse brings scavengers such as bears, foxes, ravens and gulls. But, the scavengers also prey on the eggs and nestlings of local bird species. Some of these birds are on the endangered species list.

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Upcoming Events

October 27-29, 2003 Brownfields 2003 Confer-ence, Portland, Ore.
Join us in the city that is clean, green, and vibrant to see what works for Brownfield cleanup and redevelopment.

Show Contact: DynCorp Systems & Solutions LLC, Tel: (757) 252-8503