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Final Phytoremediation Treatability Study
Section 5 - Evaluation and Conclusions
Nine criteria are used in evaluating remediation technologies, such as phytoremediation, for compliance with the statutory requirements. These evaluation criteria are as follows:
Effectiveness. The ability to attain media cleanup standards set by State or Federal regulations.
Protection of human health and environment. Protection of human health and environment is assessed based on the overall effectiveness of the alternative to block pathways for human exposure to the contamination.
Control the sources of releases. The ability to stop further environmental degradation by controlling or eliminating further releases that may pose a threat to human health and/or the environment is used to define the ability to control the source of release.
Short-term impacts. Risk of short-term pollution exposure, physical injury and damage to site workers, community residents, community structures, and the greater environment during implementation is considered for the remediation technology.
Long-term impacts. The ability to protect human health and the environment over the long term. Assessment of long-term effectiveness and permanence includes the durability of actions to block pollutant pathways and the minimization of monitoring requirements.
Implementability. Site conditions, project time requirements, and service availability that effect remedial design. The complexity of land use restrictions, administrative responsibilities, regulatory approval, and long-term monitoring obligations are also considered.
Reduction of toxicity, mobility, and volume. The evaluation of toxicity, mobility, and volume was based on the degree to which the metals and explosive contaminants were contained, treated, or removed from the site.
Cost. Cost estimations are based upon published cost estimation guides and engineering experience.
Compliance with waste management standards. A discussion of specific waste management activities required for the remedial technology is included.
The conclusions drawn from the benchscale study are discussed in detail in the following sections. Generally, benchscale results support the following conclusions for phytoextraction techniques for SWMU B-20 soils.
The soils at B-20 represent a fertile, clay loam, and moderately alkaline soils with moderate organic matter content. The soil would support normal plant growth and development.
An average total lead concentration in the bulk soil was measured as 833 � 182 mg/kg.
Lead distribution in the study soil was heterogeneous. Replicate analyses of soil lead concentrations ranged from 487 mg/kg to 6,215 mg/kg.
Metal concentrations in the water extracts from the soil at B-20 were below method detection limits.
Brassica juncea plants accumulated up to 4,615 mg/kg lead in the above-ground harvestable parts.
The moderately alkaline soil pH (7.7) indicates a soil system highly buffered with respect to carbonates. Because of the high levels of lead associated with the carbonate fraction, acidification treatments to decrease the soil pH below a value of 7.0 should result in a further increase in lead solubility and responsiveness to soil amendments to increase plant uptake of lead.
Phytotech�s technology has demonstrated (within laboratory settings) the potential to remove 22.49 kg of lead per acre. Assuming three crops per growing season, 66.8 to 70.85 kg of lead per acre is estimated to be removed per growing season. This also assumes that the lead removal is constant over growing seasons, which is unlikely due to the non-bioavailable lead content of the soils.
Based on the plant uptake achieved in the benchscale treatability study, phytoextraction can conceivably reduce the extractable lead concentration of B‑20 site soils at the rate of 30 to 60 mg/kg per crop.
Based on the benchscale treatability study, phytoremediation appears capable of removing lead to an average soil concentration of 133 mg/kg within a 13 to 30 year period, depending upon assumptions used.
The benchscale study included the removal of aggregates and metallic lead particles greater than 0.07 inches, prior to planting and harvesting plants.
Conclusions based on the nine evaluating criteria discussed in Section 5.1 are presented below.
The phytoremediation treatability study produced promising results for remediating SWMU B-20 lead-contaminated soils. B-20 soils have been determined to be fertile and capable of supporting Indian mustard plants, the lead hyperaccumulating plant. Although the soils tend to be slightly alkaline, the pH is within range that the soils can be treated to maximize metal extraction. Also, treatability tests indicate that the Indian mustard plant is capable of moving contaminants from the roots to the harvestable areas of the plant in site soils. Overall, the metal solubility and plant accumulation were determined to be suitable for phytoextraction.
Phytotech has extrapolated results obtained from this study to estimate lead removal rates at SWMU B-20 under a full-scale implementation remediation. It is estimated that phytoremediation has the potential to decrease soil lead concentration between 30 mg/kg to 60 mg/kg of lead per crop. Given that soils are well maintained and measures are taken in order to obtain three growing seasons within one calendar year, it is estimated that it would take over 13 to 30 years for lead contaminant levels to reach acceptable background levels. However, it is not expected that the removal efficiency attained within the benchscale tests would be directly proportional to actual field implementation of phytoremediation. Due to the estimated long term of the phytoremediation and the limited information available from the benchscale results, costs calculated for complete remediation can only be considered as estimates.
5.2.2 Protection of Human Health and Environment
The effects of accumulated metals in plant stems and leaves on local wildlife remain unclear. CSSA supports a large deer population that feeds on local vegetation and food provided by the installation. However, there is conflicting information concerning the likelihood of deer and other ungulates to eat the plants used for phytoremediation. In general, mustard plants, such as the Indian mustard plant, are considered deer resistant plants (Baccus, 1999). However, there is no information specifically for the Indian mustard plans as a food source for deer, therefore the likelihood of deer to eat the plants are highly speculative.
5.2.3 Control the Sources of Releases
Movement of metallic lead is generally not observed in soils, particularly in alkaline soils. The forms of lead which have any potential to move through soil environments is lead in an oxidized ionic form. However, even the lead in ionic compounds have limited water solubility and very poor soil mobility. Phytotech�s soil amendments may increase the mobility of lead. However, groundwater is not expected to be impacted due to the depth (<300 feet) of the aquifer.
Phytoremediation poses minimal risk to workers and surrounding environmental conditions. The process of growing hyperaccumulating plants and then disposing them following a growing season is very similar to planting and harvesting a crop. Therefore, any physical risk would most likely occur from operating equipment necessary to harvest plants. These risks may be minimized by either outsourcing maintenance of the project to trained professionals or instituting a training program for CSSA employees designated to operate the heavy equipment required by the project.
Soil amendments may be required to increase the rate of metals extraction to acceptable levels, as discussed in Section 3. Inclusion of these soil amendments will require chemical handling. Again, risks associated with chemical handling may be mitigated through worker training.
As a fairly new remediation technology, phytoremediation has rarely been implemented on a macroscale. Although the benchscale test indicates removal rates as remaining consistent throughout each growing season, it is unlikely that removal rates will stay at steady state conditions. Although long-term impacts of phytoremediation to human health may depend upon the sustainability of the metal extraction rates, the long-term effects on the surrounding environment are considered minimal.
Phytoremediation is expected to be labor intensive during the project outset and plant harvesting. Project setup requires significant material purchases, set-up labor, and possibly training. Plant harvesting, expected to be performed three times per year, requires labor for plant removal and disposal. Administrative efforts will be needed for waste disposal coordination. Administrative efforts for waste handling requires making provisions for transportation and disposal of harvested crops.
During the plant growing season, however, only cursory monitoring is necessary. Soils must be kept moist in order to foster plant growth. This maintenance is considered minimal.
Following successful remediation, soils will be placed back at B-20 and replanted with native vegetation. Soils are anticipated to achieve RRS1 concentrations, therefore long-term monitoring is not required.
5.2.7 Reduction of Toxicity, Mobility, and Volume
Phytoremediation provides an effective method of reducing lead concentration to the site risk-calculated values. Following phytoremediation, lead toxicity, mobility, and volume will be reduced at SWMU B-20. Harvested plants will be disposed of at an off-site landfill.
An estimate of the cost to remediate soils located at B-20 is provided in Appendix C. The cost is presented as an overall cost to remediate B-20. The cost to remediate SWMU B-20 soils is estimated using assumptions that include three crop harvests per year for 13 years and contamination removed from the previously excavated and sifted soils with all lead particulates >0.07 inches removed. Costs are estimated at $3.8 million per acre, or $200/cubic yard per growing season. However, further studies are required to justify assumptions made for the economic analysis. The treatability study was performed in laboratory conditions utilizing pre conditioned soils (i.e., ideal conditions). The resulting study indicated a somewhat static removal rate, which may not be viable in field test conditions. Additionally, lead at the B-20 site may be at depths greater than the phytoextraction techniques employed during the laboratory test; thus, limiting the remedial technology to surface soils only.
5.2.9 Compliance With Waste Management Standards
Phytoremediation is too new to be approved by regulatory agencies in pro forma reviews (GWRTAC, 1996). Several agencies, including the U.S. Army and the Tennessee Valley Authority, are supporting research programs to evaluate the applicability of phytoremediation to firing range soils specifically.