Upon immediate observation, it seems pointless to examine the waste absorption capacity of an organic compound that virtually does not biodegrade or decompose. PCBs were used in industry precisely because of its characteristic chemical stability. However, studies indicate that an ecosystem approach is potentially a valid remedial measure. The fate of chemicals like PCBs in a lake ecosystem is a variable of time when it comes to ecosystem proceses.
Despite its cessation of production in 1977, PCB loadings into lakes are continuing in some locations, in large part due to atmospheric deposition. Gas exchange (by absorption and volatilization) is the dominant atmospheric deposition process for many persistent bioaccumulative toxic pollutants. The image below shows how PCBs are still entering lake ecosystems through the atmosphere (U.S. EPA).
PCBs are virtually undetectable and quite difficult to monitor. One of the largest SuperFund sites in the U.S. is a PCB-contaminated 200 mile stretch along the Hudson River. After five and a half months of dredging a six-mile stretch of the Hudson River, only 10% of the remediation project is complete. The dredged sediments, 293,000 cubic yards from phase 1 alone, will be rail-transported to a “PCB-approved landfill” in Texas (U.S. EPA 2).
Numerous studies have indicated that dredging is often an effective remediation tool for eliminating PCB concentrations, however the method is prone to uncertain risks with the potential of exacerbating the situation. The ecosystem approach to the virtual elimination of PCBs is through natural attenuation, where a combination of various biological processes slowly disperse or degrade PCBs (Council, N.R.).
Natural attenuation is a bioremediaton approach that basically allows the already existing microbes and organisms to continue whatever degradation they can attain. Other bioremedial approaches to PCB biodegradation are biostimulation and bioaugmentaion. Biostimulation is enhancing preexisting microbes’ and organisms’ capacity to degrade contaminants with nutrients such as oxygen, food, or fertilizer. Bioaugmentation is the process of physically adding microbes and other organisms with the ability to degrade PCBs to the site of contamination. These approaches all employ natural processes to do the work of eliminating PCBs, and they have the potential of significant cost savings in comparison to dredging (Hine).
Lake Champlain Reference-
Due to its toxic effects, PCBs are of the highest priority of management action in Lake Champlain. In 2000, PCB levels were found to have exceeded federal guidelines in the Cumberland Bay area and a $35 million restoration project was initiated to eliminate the excess amounts of the contaminant. Over 140,000 tons of PCB-contaminated sludge was removed from the bottom of the bay, and now current levels are at an acceptable 15 parts per million (LCBP).
Most of the studies into the processes related to waste absorption of PCBs in lakes all focus on the time response of lake ecosystems to the contaminant. This is because PCBs don’t decompose easily, so the ecosystem approach to absorbing and eliminating PCBs is not necessarily about an array of organisms harmoniously breaking down the compound, but a combination of that and the bioaccumulation and ultimate dissipation of the PCB concentrations to negligible levels. To restate in simpler terms, the lake absorption capacity/natural attenuation approach to eliminating PCBs from lake ecosystems is mainly a process of dissipation, where the PCB concentrations become insignificant due to deep burial or effective dilution (Gobas).
There is a lack of scientific research regarding the presence and behavior of PCBs in Lake Champlain, although the Great Lakes have had extensive studies conducted. A study in Lake Champlain did discover a link between PCBs in lake sediments and its transfer to Lake Trout. A species of lake shrimp, Mysis Relicta, feed off of the lake sediments and are then consumed by the Lake Trout, making them of the primary organisms in the long chain of bioaccumulation of PCBs (McIntosh). Biological species respond to PCB concentrations at different “species-specific” rates, causing varying amounts of decline in all the different species that are contaminated. Complex scientific models have been developed to estimate the time response of the different organisms to PCB concentrations, and by combining these models scientists can develop ecosystem response models. A 1995 study concluded that, if there were to be no more additional loadings of PCBs into the lake ecosystem of Lake Ontario, then ultimately, the rates of concentration decline in water, sediments, and biota are expected to converge at a half-lifetime of approximately 5.5 years. This conclusion is based on the simplest of models and is only stated as representation of how models could be effective (Gobas).
Connecting structure to function to value…
The structure of lake ecosystems relevant to the waste absorption of PCBs involves most components of the ecosystem. The first factor to consider is that water and lake sediments have different response times when it comes to PCB concentrations. As water is free moving, and generally flowing in a certain direction, PCBs are able to dissipate and dilute more rapidly than when contained in settled sediments. This initial factor significantly implicates the ecosystem time-response to the various PCB concentrations. As PCBs respond slower in sediments than in water, organisms that feed on sediments like benthic invertebrates and Mysis Relicta (the opossum shrimp) are more likely to have slower declines in PCB concentration (McIntosh).
When PCBs enter organisms, there are numerous factors to consider when estimating the rate of degradation and decline within the organism. Certain aspects of an organism can indicate the likeliness and rate of decline in PCB concentrations: weight, lipid content, growth rate, sediment-water disequilibria, and diet composition. Organisms with high growth rates, low lipid content and small weight will have a faster time response due to what is called growth dilution. As a species reproduces rapidly, they spread the concentrations of PCBs among the population, which is ultimately the desired result, as this causes both the natural degradation of the compound, as well as further dilution (Gobas).
Figure 1 demonstrates the two different models that affect PCB concentration decline.
There is no simple solution for remediation of lakes contaminated with PCBs. Dredging is far more effective in physically removing large concentrations of PCBs from a given site. Dredging also has unintended consequences, such as habitat destruction and releasing PCBs that were buried deep in the sediments, and it is also costly (Council).
The natural attenuation approach is worth considering because it is equally as effective in the long-term and does not further disrupt the lake ecosystem. However, there is a dilemma in the natural attenuation solution, as PCBs are undoubtedly harmful to all populations of species, yet the natural attenuation approach relies on these organisms to complete the process of PCB dilution and degradation. Thus, the aim of an ecosystem approach is to promote the health and population growth of those organisms that can dissipate and degrade the PCB concentrations faster. These organisms include those that feed on sediments (Mysis Relicta and benthic invertebrates) and those with high growth rates and low lipid contents. Then the answer is time, which is certainly uncertain. Models have been created and can be applied to Lake Champlain to determine the time response of the ecosystem to the elimination of PCBs. As stated earlier, one of the simpler models predicts a half-life time of 5.5 years, but that does not indicate when the total concentrations will be eliminated (Gobas).
Environmental profile of pcbs in the great lakes. (n.d.). Retrieved from http://www.uic.edu/sph/glakes/pcb/exposure.htm
Council, N. R., & Toxicology. (2001). A Risk Management Strategy for PCB-Contaminated Sediments (1st ed.). Washington, D.C.: National Academies Press.
Gobas, F., & Z’Graggen, M. (1995). Time response of the Lake Ontario ecosystem to virtual elimination of PCBs. Environmental Science & Technology , 29 (8), 2038. Retrieved from Academic Search Premier database.
Hine, Neil A. (n.d.). Techniques for bioremediating pcb contaminated landscapes. Retrieved from http://horticulture.cfans.umn.edu/vd/h5015/99fpapers/hines.htm
LCBP (2004). Toxic substances of concern. Retrieved from http://www.lcbp.org/atlas/HTML/is_tconc.htm
McIntosh, A., & Lester, D. (1994). Accumulation of polychlorinated biphenyl congeners from Lake Champlain sediments by Mysis relicta. Environmental Toxicology & Chemistry , 13 (11), 1825. Retrieved from Academic Search Premier database.
The Great lakes wiki. (n.d.). Retrieved from http://www.greatlakeswiki.org/index.php/Lake_Champlain
U.S. EPA (n.d.). Atmospheric deposition of toxic pollutants. Retrieved from http://www.epa.gov/glindicators/air/airb.html
U.S. EPA 2 (n.d.). Hudson river pcbs. Retrieved from http://www.epa.gov/hudson/