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Electrokinetic Test Report for SWMU O-1
Section 5 - Conclusions and Recommendations
There are several conclusions to be drawn from the electrokinetic test data interpretations and observations. Because of the efforts to correlate VOC test data collected during this testing event, the conclusions have been separated into two categories. The first category is general conclusions, which discuss items that are related to PCE removal observations that may not be directly related to the effectiveness of the electrokinetic system at O-1. The second category is conclusions related to the chromium removal performance.
5.1.1 General Conclusions Regarding PCE Removal
The expected transport mechanism for PCE removal is electroosmosis i.e., PCE is expected to move through the soil pore volume with the available water to the cathode well. The results from the available screening data indicate that PCE levels increased in the area during the field testing of electrokinetic treatment. Transport of PCE from the surrounding area outside of the field test unit, by electroosmosis, provided the additional PCE within the treatment area.
Additionally, the technology is expected to degrade chlorinated alphatic hydrogen compounds by providing a reductive dechlorination condition during the treatment operations. The limited study budget for SWMU O-1 did not allow generating data which would support the biodegradation of chlorinated solvents by use of electrokinetic remediation. It is recommended that if additional studies for the use of electrokinetic remediation are conducted, those tests should include analysis which would prove or disprove the reductive dechlorination of chlorinated alphatic hydrocarbons.
General conclusions that are associated with PCE removal of the site are as follows:
The analysis of samples gathered from SWMU O-1 for VOC analysis by method SW-8260 during the period beginning in 1996 through 1997 was deemed rejected data by Parsons ES. This was due to invalid manual integration performed by the laboratory. The results of analysis for VOC by method SW-8260 are used as screening data for considerations of conclusions in this report.
Initial (1996) investigations (i.e., liner investigation) at SWMU O-1 resulted in significant within excavated areas.
PCE levels were detected above cleanup criteria (background) in all of the electrokinetic system 9 January 1998 soil samples (Section 4.1.2).
The lateral extent of VOC soil contamination in the main SWMU O-1 trench is not defined. VOC contamination has potentially migrated into the fractured limestone surrounding the main SWMU O-1 area, but the areal extent and potential mass of contaminants remaining in limestone fractures is unknown.
The estimated treatment area determined by the original soil gas mapping of the main area that exceed cleanup criteria is approximately 10,500 square feet, and the total volume of contaminated material in the target cleanup zone is approximately 1,600 cubic yards. This volume estimate assumes that the thickness of contamination is 4 feet bgs. The cost to remove and dispose of these soils is approximately $480,000.
5.1.2 Effectiveness of Electrokinetic System Operation
The testing activities demonstrated that due to high buffering soil capacities, in situ electrokinetic soil processing cannot be operated effectively at the SWMU O-1. Several conclusions were drawn from the electrokinetic benchscale and pilot testing that are specific to the operation of the electrokinetic system at the site. The conclusions that are specific to the effectiveness of electrokinetic soil processing at SWMU O-1 include:
The soil at the SWMU O-1 site showed a very high buffering capacity due to high limestone content. This required that a mixture of an inorganic acid, hydrochloric acid, and an organic acid, citric acid, be used as the soil conditioning solution. This solution was added to the electrode and injection wells during the process operation. Hydrochloric acid was used to enhance the process of soil acidification and citric acid was used to enhance the solubility of heavy metal contaminants in soil.
The efficiency of the electrokinetic removal of chromium, the main contaminant at the site, observed during the benchscale tests was extremely high. After 36 days of treatment, up to 99.8 percent of chromium was removed in one-third of the soil bed (near the cathode). The remaining concentration of chromium was 1 ppm, well below the target closure value of 39 ppm. The results demonstrated accumulation and transport of chromium in the direction of the anode, indicating that chromium was in the form of anions in the soil and was negatively charged. Chromium removal was 84 percent in the central region and 64 percent in the region near the anode. It is believed that with prolonged treatment, the removal below regulatory limits could be achieved throughout the whole mass of soil. The benchscale results demonstrated a clear correlation between the acidity of soil achieved during the process and chromium removal. Efficient chromium removal could be obtained when the soil was acidified to a pH of 2 to 3.
Electrokinetic remediation at CSSA O-1 site was performed in a continuous 3-month field pilot scale test. The field test results demonstrated much lower efficiency in chromium removal compared with benchscale tests. Chromium removal was entirely dependent on the efficiency of soil acidification. To enhance the acid distribution in soil, the acid conditioning solution was added to the anode and cathode wells, as well as through a series of horizontal and vertical injection wells.
Because the contaminant distribution at the site was extremely heterogeneous, both laterally and vertically, a sampling method which utilizes process "control zones" was used to recognize the trends in the contaminant transport and removal from soil. This method involved frequent sampling at predetermined locations between the wells at regular time intervals - every 20 to 22 days. Thus, it was possible to obtain temporal and spatial distribution of contaminants in soil during the process.
Using the method described above, a clear correlation between chromium removal and soil pH was verified and demonstrated to be valid for the field test. An almost linear dependence of chromium removal on soil pH was obtained. The highest chromium removal, 34 percent, was obtained near the anodes where soil was acidified down to a pH of 2 to 4. Only 13 percent chromium was removed near the cathode. Removal of chromium near the cathode indicated that transport of negatively charged chromium in that region was exclusively by dielectrophoresis and electroosmosis.
Process cost estimate showed that the energy expenditure cost was very low, only $8 per cubic yard of soil. This cost was only 1 to 2 percent of the total cost, which was entirely dominated by the cost of chemicals used in the process. The total cost for the process operation was $738 per cubic yard of soil. This includes the cost of off-site disposal of excavated soils generated from the removal of the system and the waters generated from the cathode well.
It is recommended that a potential on-site electrokinetic treatment of soil from CSSA soils could be feasible if the soil was transferred into a container and treated within the container after the removal of large limestone rocks. It is believed that the large consumption of acid was due to the rocks present in the soil and their decomposition during the acidification contributed to the high consumption of chemicals. By removing larger size soil material containing limestone, the on-site electrokinetic treatment in the container may be a more feasible and efficient approach than simple soil washing processes. This recommendation is supported by the extremely low energy cost of the electrokinetic process in which mineral and organic acids are utilized as soil conditioning solutions.
Additionally, the site investigation did not adequately address the speciation of chromium and the different interactions with citric and hydrochloric acid. It is recommended that if electrokinetic treatment of soils at CSSA is further pursued, and evaluation of the speciation of chromium and the complexation effects of citric and hydrochloric acid be included.
A potential treatment method regarding the citrate complexation with hexavalent chromium [Cr(VI)] was noted during the benchscale study. The potential ability of citric acid to reduce Cr(VI), a highly toxic form of chrome, to trivalent chromium [Cr(III)], a less toxic form of chrome is recommended for further study.