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4 2 3 2 Extraction Rates If groundwater modeling and water level data indicates that capture of the Plume can be accomplished at reduced extraction rates, then reducing the groundwater extraction rate should be considered In extraction well E-9, where galvanic corrosion has been observed, the flow rate was decreased from 800 to 300 gpm, and capture of the plume has apparently not been compromised As stated above, wells E-1 and E-5 (and perhaps others) may not be needed at all (See above paragraph on Subsurface Performance and Response ) Reducing the flow rates can directly save P0w0T costs for the well pumps, reduce pump repair costs, and perhaps allow changes in the air strippers that further save power Taking these two wells offline would save an estimated $49,000,not countin savings in well and pump maintenance In order to more accurately evaluate the extraction system, g recommend that spinner logging of the multiple-screened-interval wells (e g , E-6, E-13) be considered A spinner log allows one to identify the proportion of flow contributed from each depth interval There are no identifiable cost savings associated with this, but the data allows improvement in the computer model used to design and optimization of the system Finally, the removal of accumulated sediment should be conducted before it reaches a thickness equal to 50% of the screened interval, say at 20% of the screened Interval 4 2 3 3 Sequestration of Inorganic Precipitates SHMP isbeing fed up stream strippers at a concentration of 8 mg/L to prevent deposition of solids from the air in the piping and the injection wells Metcalf& Eddy performed the original sequestrant study, in consultation with Calgon, the SHMP supplier, to determine the concentration of SHMP that should be added to the influent The annual cost for SHMP has ranged from $175,000 to $225,000 The Sequestrant Study Report should be reviewed again to assess whether it would be possible to reduce the dosage of SHW and still control inorganic precipitates If the concentration of SHMP could be reduced to 6 mg/L, reduced by$50,000 annually Note that thecosts for SHMP would be team to review Sequestrant Study Report was not available for the RSE A 2 3 4 Alternatives to SHMP The use of carbon dioxide to control precipitates (in place of SHNT) deserves further investigation Carbon dioxide is stripped from the water in the air stripper This causes the pH of the water to rise and subsequently causes calcium carbonate to precipitate Feeding carbon dioxide replaces that which was air stripped and reduces the PH of water and can reverse or prevent precipitation of calcium carbonate, the primary inorganic precipitate A preluninary cost estimate indicates that the capital costs of installing a carbon dioxide feed system (approximately $22,OOp) are well below the annual costs for SHMp The annual costs for a carbon dioxide feeds stem was estimated to be $57,000 per year ($12,000 to lease a storage tank, $3,000 for electricity, and y $42,000 for carbon dioxide) To further refine the cost estimate, the required dosage of carbon dioxide should be verified To verify carbon dioxide dosage, a test should be performed using a sample of treated groundwater from the site The test could be perforl-ned either at the site, or in a laboratory, and would involve adding carbon dioxide while monitoring Pre-Pare eparG a rG'1111 L + r_ $pH This test should be completed, and used to ed 4Vsl ej11111a LG 1V1 a Rrbon ui0itidC fcCu jyat�ili The prelixiairiary'e�tlrnate iixuitiutea cost savings of approximately$121,000 in the first year, and $143,000 per year for each year thereafter The condition of the road serving the treatment plant may make delivery of carbon dioxide in typical tanker trucks difficult a few times during the winter/spring Adding extra storage capacity for carbon dioxide, improving the road, or maintaining the SFEM? feed system as a backup may be needed to assure adequate prevention of scale 06/07/99 Page 11 arm