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2007 Ground Water Summit |
Wednesday, May 2, 2007 : 1:00 p.m.
Is Your Asr Well Clogging? No? Are You Sure?
Zone 7 Water Agency is located 30 miles east of San Francisco . Zone 7 retrofit and tested an existing well as an ASR well. Approximately 3,200 AF of water were injected during five ASR cycles over a 27-month long period. Pumping redevelopment was conducted weekly during injection.
Prior to ASR testing the specific capacity of production of the well was 100 gpm/ft. During ASR testing the well progressively lost production specific capacity to 85-95 gpm/ft. This clogging was not readily evident during the first 6 months of ASR operations. A sudden and severe clogging event near the end of injection lowered specific capacity to 55-60 gpm/ft.
An aggressive well redevelopment program was implemented resulting in a 30 percent increase in specific capacity (to 80 gpm/ft from 60 gpm/ft). This is below the pre-ASR value of 100 gpm/ft. It appears that microbiological growth was the major cause of well clogging. This clogging appears to have both near-well and far-field (aquifer formation) components. The far-field clogging component is likely a significant cause of continued low values of specific capacity.
The presence of bacteria in far-field areas may be promoted by the high transmissivity of the aquifer (175,000 gpd/ft transmissivity), high recharge rates (2,000 gpm), prolonged periods of injection, the possible presence of free ammonia in the recharge water, and relatively low chloramine disinfectant residual. It is not possible to effectively redevelop far-field areas through traditional well redevelopment techniques. It is possible that flooding the system with strongly chloraminated or chlorinated water would restore specific capacity.
Results of this ASR test have significant implications for other ASR projects and indicate many months of closely monitored ASR operations may be needed to properly assess potential clogging issues. In addition, test results indicate that even systems using “typical” ASR maintenance techniques maybe prone to clogging.
Prior to ASR testing the specific capacity of production of the well was 100 gpm/ft. During ASR testing the well progressively lost production specific capacity to 85-95 gpm/ft. This clogging was not readily evident during the first 6 months of ASR operations. A sudden and severe clogging event near the end of injection lowered specific capacity to 55-60 gpm/ft.
An aggressive well redevelopment program was implemented resulting in a 30 percent increase in specific capacity (to 80 gpm/ft from 60 gpm/ft). This is below the pre-ASR value of 100 gpm/ft. It appears that microbiological growth was the major cause of well clogging. This clogging appears to have both near-well and far-field (aquifer formation) components. The far-field clogging component is likely a significant cause of continued low values of specific capacity.
The presence of bacteria in far-field areas may be promoted by the high transmissivity of the aquifer (175,000 gpd/ft transmissivity), high recharge rates (2,000 gpm), prolonged periods of injection, the possible presence of free ammonia in the recharge water, and relatively low chloramine disinfectant residual. It is not possible to effectively redevelop far-field areas through traditional well redevelopment techniques. It is possible that flooding the system with strongly chloraminated or chlorinated water would restore specific capacity.
Results of this ASR test have significant implications for other ASR projects and indicate many months of closely monitored ASR operations may be needed to properly assess potential clogging issues. In addition, test results indicate that even systems using “typical” ASR maintenance techniques maybe prone to clogging.
Daniel E. Wendell, PG, CHG, CH2M HILL Dan Wendell has 20 years of professional experience in hydrogeology and geology and is CH2M HILL’s Global Technology Leader for Water Supply Services. Dan specializes in water resource analysis and has installed and tested numerous high-capacity ASR wells throughout California. Dan was part of the team that evaluated clogging along the seawater injection barrier that protects the coastal basins of Los Angeles. For the past 15 years Dan has worked with CH2M HILL in three of their California offices. Prior to this, Dan worked for the Goleta Water District in Southern California and was responsible for ASR testing and management of the local basin.