3.1 - Summary of Activities

The purpose of this section is to summarize the activities that were performed by Parsons ES and Lynntech, Inc. (Lynntech) during the June 1997 installation and fall 1997 testing of the electrokinetic soil processing system at SWMU O-1. The treatability study activities included a laboratory benchscale test and a field pilot scale test. A detailed description of methods and techniques used for the treatability test is presented in Appendix A.

The first activity was the siting and construction of six anode and two cathode wells for the electrokinetic test system. Soil samples were collected during system installation to characterize the pre-study conditions of the soil contamination prior to the implementation of remedial actions and to provide soil for use in the benchscale study.

The treatability study activities were initiated immediately following the installation of the electrokinetic test system. The testing activities were generally performed in accordance with the protocols detailed in Lynntech's Work Plan for SWMU O-1 (Lynntech, 1997b), provided in Appendix A with exceptions noted in the discussions of the specific testing methodology.

3.2 - Technical Approach

3.2.1   Lynntech, Inc. Electrokinetic Soil Remediation Design

Electrokinetics involves applying an electrical potential to a conducting media containing ionic contaminants. Cationic or positively charged species move toward the negatively charged electrode, or the cathode. Anions move toward the anode. Organic species can also be mobilized in a process called electroosmosis and migration. To supply a sufficiently strong field to effect movement of organics, usually requires a relatively high voltage. Low conductivity media such as those with low levels of dissolved species or low moisture, require greater voltages to mobilize ionic species. High voltages can introduce problems, such as excessive generation of heat.

Even with moderate voltages needed at SWMU O-1, electorlytice degradation of water occurs. Hydrogen gas and hydroxide ions (base) are generated at the cathode, oxygen and hydrogen ions (acid) at the anode. The gases move to the surface, but the acid and base move towards the opposite electrodes. If they are not controlled, the base precipitates many of the cationic species, and the acid neutralizes anionic species. In other words the system can produce a collision zone of neutralization between the electrodes where the ions halt movement. The Lynntech system maintained the pH control by replacing the liquid around the anode with fresh water, and at the cathode with a base-neutralizing (hydrochloric acid) and metal mobilizing (citrate) solution.

In Lynntech's electrokinetic process, an electric field is applied between the electrodes positioned in the soil; anodes are positive electrodes and cathodes are negative electrodes. Contaminants are mobilized by reducing the surrounding soils pH level to 3 or less. Lynntech uses a pulsed electric field up to 300 volts per meter (V/m), and at a pulsing rate with on/off pulse duration in the range; on equals 1 to 5 seconds and off equals 10 to 30 seconds. In theory, application of pulsed electric fields reduces both the time of the soil treatment and the energy costs. In the process both DC (electromigration, electroosmosis, electrophoresis) and AC (dielectrophoresis) effects are induced which control the transport of contaminants and reagents in soil. Lynntech's design of their non-uniform electric field test unit is further described in Lynntech's technical report provided in Appendix A.

3.2.2   Electrode Well Construction

Drilling and electrokinetic well construction activities began on July 7, 1997, and were completed by August 17, 1997. The well borings were drilled to total depths ranging from 3 to 4 feet bgs. Lynntech's process uses a proprietary design of the electrode wells which are constructed from low porosity ceramic tubing surrounded by a layer of packing material. The packing material, a clay/sand mixture with adjusted hydraulic and electrokinetic permeability, serves to further concentrate the contaminants at both electrode wells and to minimize the effluent volume. For the electrode process to occur, the electrodes must be surrounded by water. Since the O-1 site is unsaturated, the electrodes were placed in wells and maintained at all times with water. The ceramic material, as well as the packing material surrounding the wells, forms the well casing that both retains water and promotes the electrokinetic processes through the casing.

3.2.3   Soil Sample Collection and Analytical Program

Soil samples were collected during drilling of the electrode wells for chemical and geotechnical analysis from July 7 - 8, 1997. The analytical protocol for these characterization soil samples consisted of analysis for VOCs and metals to establish baseline contaminant concentrations, and to estimate the total mass of contaminants present. The analytical methods that were performed on the soil samples are summarized in Table 3.1.

Table 3.1 - Methods of Chemical Analysis for Soil and Soil Gas Samples, SWMU O-1

In an attempt to identify contaminant concentrations at varying depths, at least three samples for PCE and metals analysis were collected from every electrode well drilled in July 1997. The varying depths were identified as surface (0-1 feet bgs), mid (1-2 feet bgs), and bottom (2-4 feet bgs). Geotechnical tests that were performed on SWMU O-1 soils include soil moisture, bulk density (porosity), permeability, total organic carbon, and particle size distribution (see Table 3.2 for a summary of geotechnical test methods).

Table 3.2 - Methods of Geotechnical Analysis and Physical Parameters for Soil Samples, SWMU O-1

Quality control (QC) samples were collected during the establishment of baseline conditions and at the completion of the field testing in accordance with the approved Quality Assurance Project Plan (QAPP) (Parsons ES, 1996d). Because of laboratory problems noted during the field testing events, the data collected for PCE concentrations are considered only screening data. metal analysis, conducted by the contracted laboratory, meet data quality levels as specified by the QAPP. Only sample analyses which were collected for metal analyses during the initial (July 97) sampling event was reviewed by a Parsons ES data validator. A Parsons ES data validator reviewed all sample analyses collected during the final sampling event. Results of the validation effort are documented in an informal technical information report (ITIR) presented in Appendix B. Metal analyses data collected by Lynntech is considered screening data as well. The numbers and types of samples (including QC samples) are summarized in Table 3.3 for each sampling event.

Table 3.3 - 1997 Treatability Study Sampling Summary, SWMU O-1

Decontamination procedures followed those detailed in the "Sampling Analysis Plan for SWMU Closures " (Parsons ES, 1996d).

A composite sample of soil from drums containing soils removed from the treatment area was collected on 9 January 97, and analyzed using EPA SW-1411 toxicity characteristic leaching procedure (TCLP) for chromium, cadmium, and PCE. A grab sample of the effluent generated at the cathode well was analyzed for TCLP chromium, cadmium, and PCE. This sampling was performed to characterize the drums for applicable disposal requirements.

Upon receipt of validated IDW characterization data, proper IDW classification as specified in 30 TAC 335 subchapter R was determined. The results and disposition of the IDW are discussed in Section 4.4.

Rinse fluids were temporarily containerized during the decontamination activities. In accordance with the approved addendum work plan, these fluids were poured onto the ground within the source area where the decontamination fluids were generated at the conclusion of the sampling activities. Miscellaneous debris such as used personal protective equipment was double-bagged and placed in a general refuse dumpster at CSSA.

3.3 - Treatability Study Activities

Several types of treatability tests were performed to collect data to determine the efficacy of electrokinetic remediation of SWMU O-1 soil. These tests are described in this section in the order they were performed. The tests include:

1. A laboratory benchscale treatability test which included a determination of the optimum extractant and valence state of chromium in SWMU O-1 soils; and

2. A field pilot scale treatability test within SWMU O-1, which included a soil acid additive test to control swelling of O-1 soils.

3.3.1   Laboratory Benchscale Test

Bulk soil sample (approximately 5 gallons) was collected from the chosen SWMU O-1 area for the laboratory benchscale test. The soil was dried, sieved to less than quarter inch, grinded and homogenized to obtain uniform distribution of contaminant in soil for the experiment. The laboratory tests included:

1. Batch tests for determining optimum extractant for chromium from O-1 soils;

2. Determination of the valence state of chromium contaminant after extraction; and

3. Electrokinetic tests for removal of chromium from O-1 soils.

The batch leachability tests were performed in 100-ml beakers using 5 grams of soil and adding a predetermined volume of acid/leachant. Two types of batch tests were performed. The first involved a titration of soil with the acid additive and continuous measurements of pH in the soil/leachant slurry. This extraction procedure provided results on extraction of chromium and cadmium at different pH using a series of leaching agents. The second test involved the addition of an acid/leachant solution for a period of extended time, approximately 16 hours, and provided log-term extraction results. The soil was mixed with 50 ml of leachant solution. Several organic acids and their combinations were tested as potential additives to the soil for the electrokinetic process.

A determination of the valence state of chromium contaminant after extraction was performed as part of the benchscale tests. The determination was performed to assess concerns that the chromium (IV) ion contaminant might be generated during the electrokinetic process. Chromium (IV) ions were determined according to standard spectrophotometric method (Method 3500 CR-D, Standard Methods for the Examination of Water and Wastewater) using diphenyl carbazine as a colorimetric reagent.

The efficiency of chromium removal from O-1 soils by electrokinetic treatment was studied in a laboratory experimental system as shown in Figure 2 of Lynntech's Technical Report presented in Appendix A. The homogenized soil was placed in an 18" by 6" by 6.5" polyvinyl chloride (PVC) soil bed with electrode wells consisting of ceramic tubing made of the same material used for the construction of the electrode wells in the field. The experimental setup allowed addition of fluids and additives to the soil.

Voltage distribution in the soil was measured using voltage probes positioned between the electrode wells. The changes in the voltage between the voltage probes indicates changes in resistance of soil which can be caused by changes in moisture content of soil and chemical changes in the soil. Thus, measurements of voltage distribution in soil provided an indication of chemical processes occurring in soil during the electrokinetic treatment. Pulsed voltage was applied to the soil with on/off periods equal to 20 seconds/2 seconds. Voltage amplitude was regulated in the range of 50 to 150 volts. The current changed during the experiment and was in the range between 0.2 amps to 1.1 amps. This current range provided an optimum operation of the electrokinetic process to prevent overheating of the electrode wells.

The pH in the cathode well was controlled by addition of citric acid. The experiment was monitored by taking soil core samples at five locations between the electrode wells. Samples were taken after each week of continuous process operation. Soil core samples were analyzed for chromium, moisture content, and pH.

3.3.2   Field Pilot Scale Test

The field equipment used to perform an electrokinetic soil remediation test of SWMU O-1 soils consisted of a Sorenson power supply for generation of a pulsed electric field; stable electrodes; electrode wells; well fluid management; soil pH management; and contaminant collection. A detailed description of the field equipment used for the treatability test is included in Section II.A.2 and Figure 1 of Lynntech's technical report (Lynntech, 1997a) included in Appendix A.

The size of the treatment area was 6 ft by 7 ft and 4 ft in depth. The treatment area encompassed two zones: (1) a disturbed zone in which the soil was excavated by previous investigations; and (2) a nonexcavated zone.

The site system performance test was conducted by taking core samples in the vicinity of each electrode well as well, as along two profiles (see Figure 3.1) identified as (1) anode well 1 - middle point - cathode well 1, and (2) anode well 6 - midpoint - cathode well 2. Core samples were taken by Lynntech at the control profiles approximately every 20 to 22 days. Parsons ES split control profile samples taken during the start, midway and ending sampling events for observations of PCE removal by the electrokinetic soil processing system.

For the final sampling event, several trenches were excavated and the soil sampled at three depths at locations corresponding closely to the locations of the core samples taken during the process operation.

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