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Electrokinetic Remediation - CASE STUDY 3: Naval Air Weapons Station (NAWS) Point Mugu, California

 

CASE STUDY 3: Naval Air Weapons Station (NAWS) Point Mugu, California

Site History

The NAWS located in Ventura County, California, was chosen for demonstrating electrokinetic remediation. The site was under study and the area had been used for electroplating and metal finishing operations. The area of the site was approximately half an acre including two former waste lagoons located in the center. These lagoons are unlined and were in use between 1947 and 1978 when it received wastewater from the electroplating and metal finishing plants. The plating shop discharged about 95 million gallons of plating rinse solution and some organic solvents into the lagoons. In addition to this, the lagoons were also the dumpsite for 60,000 gallons of waste photographic fixer solution and rocket fuel (USAEC. July 2000). In 1994 an emergency removal action was decided and the surface samples from the lagoons showed high levels of Cadmium and Chromium (25,100 mg/kg and 1810 mg/kg respectively) far exceeding the allowable standards (2,500 mg/kg and 10 mg/kg respectively) of California.

Remediation

Before starting the remediation process, many laboratory tests were conducted with the soil samples from the site. These included testing for contaminant and soil characteristics, grain size distribution analysis, pH, Total Organic Carbon (TOC), Toxicity Characteristic Leaching Procedure (TCLP), Cation exchange capacity and hydraulic conductivity. These tests were followed by determination of electrode placement, migration rates, electrolyte requirements, removal efficiency and power requirements. This initial laboratory study was conducted by ERDC and Lynntech, Inc. and showed that the method can be successfully implemented at the site for remediation. The major method of contaminant transport was expected to be electromigration. (USAEC. July 2000)

System Setup

The setup consists of an array of cathodes and anodes as shown in the figure 9. 

 

14. case 3.1

Figure 9: Schematic showing electrode wells (USAEC. July 2000)

 

The anode wells are 4 inch diameter with slotted PVC casings. They are wrapped in a linen fabric woven tight. The well is also packed with Kaolinite and sand. The cathodes are 3 inch diameter each with porous ceramic walls. These were designed by Lynntech (USAEC. July 2000).

 

15.case3.2

Figure 10: Electrode well Construction (USAEC. July 2000)

 

The site was subjected to 30 V and 333 A for remediation. Continuous monitoring was performed to check for soil heating. The sensors attached to each cell monitored temperature, pH, Voltage and current. Due to public and regulatory concerns mineral acids, nitric and sulphuric acids were not used in the site. Only citric acid was used for enhancement since it was available readily, environmentally friendly and was relatively inexpensive.

 

Decontamination Issues in the site

The site had sodium chloride presence and this led to production of chlorine gas along with oxygen at the anode wells. This required additional off-gas treatment in the design system. In addition to this, the metal sulphides present in the site led to the production of Hydrogen sulphide gas at the cathode wells. A scrubber system was introduced to treat the gas and it reacted with Sodium hydroxide. To increase chlorine gas removal efficiency, an air sparging system was installed.

After 22 weeks, low concentration of contaminants were discovered in some areas. But the progress review showed that some factors were retarding the remediation progress. Further inestigation was needed before implementation on a full scale basis could be considered and the field demonstration was suspended in 1998 October (USAEC. July 2000).

 

Results

After extensive evaluation, it was concluded that the bench scale tests did not accurately reflect the effects that would occur due to the remediation process. This was specifically true with respect to the competing ions having a retarding effect on the pH front development and contaminant transport. The bench tests also could not predict the efficiency or the duration accurately. The performance goals was to bring down the contaminants to California State regulatory levels but no contaminant was extracted during this process and samples were not collected after treatment. Hence the target levels were not met (USAEC. July 2000). The transport of electrokinetically mobilized contaminants within confined and unconfined areas were not assessed since the performance was poor during the demonstration. The technology had an adverse effect on the organic contaminants of the soil since they showed an increase.

 

Observation

The VOC data indicated an increase of vinyl chloride in some wells. This increase in contamination was mainly due to the dehalogenation process of PCE and TCE during electrokinetic remediation (USAEC. July 2000). The site characteristic that was primarily responsible for the poor performance of the remediation treatment was the presence of high chloride concentration in the soil. The chloride ions retard the development of pH front and in turn slow down the process of contaminant transport towards electrodes. The site conditions thus proved to be disadvantageous for electrokinetic remediation. 

 

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