The 24 MGD Andover Water Treatment Plant included four sedimentation tanks that were put on-line in 1974. The tanks which measured approximately 120 ft. long by 18 ft. wide by 16 ft. deep, were suffering from severe concrete erosion below the water line caused by an aggres- sive water chemistry. Concrete that is exposed to water with a pH of 6 or less, produces new calcium com- pounds which are very soluble and can easily erode. The presence of

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