Relief Well Rehabilitation and Preventative Maintenance Program at Garrison Dam, North Dakota

Routinely, the instrumentation at all the Omaha District Dam Projects is evaluated as part of the districts dam safety program. Subsequently, the Garrison Dam, West Terrace Gravel relief wells were evaluated. Relief well historic data bases such as flow measurements, relief well deposition measurements, corresponding piezomenter water levels, and experienced and projected reservoir levels are used as a means for relief well evaluation. Original specific capacities of relief wells are used to determine the extent of well capacity decline; however, no original specific capacities for these wells were available. Routine relief well flow readings and corresponding relief well piezometer water level readings indicated slight changes suggestive of a bio-fouling situation, but were not conclusive. The readings although erratic in appearance showed a slight decreasing flow trend since 1984, however the reservoir elevation also showed a decreasing trend due to the drought that had been experienced.

Visual observations of the relief well outfall pipes revealed rusty colored water and incrustation buildup, which is an indication of biofouling. Down hole video inspections conducted in 1988 were also used as a means of evaluating the condition of the relief wells, and showed that these relief wells had a large amount of bacteria growth, bacterial growth residue, and incrustation materials (bio-fouling) present in the wells. If this bio-fouling would have continued to diminish and eventually the wells would have become plugged. There are no records or evidence that these wells had ever been pumped. The lack of routine relief well pumping will allow bio-impairment to accelerate undetected until the well is fouled and a rapid reduction of flow is experienced. If high or maximum reservoir levels would have been experienced, the effectiveness of these wells would have been questionable.

These relief wells are part of a drainage system for the West Terrace Gravel which has demonstrated some unusual underseepage, and it is in the interest of dam safety to insure a reliable operational condition for these wells. To insure this, in conjunction with a Corps of Engineers, Waterways Experiment Stations (WES) research project, the wells were rehabilitated using Alford, Rogers, Cullimore, Concepts (ARCC) redevelopment techniques. ARCC is a Blended Chemical Heat Treatment (B.C.H.T.) method of disinfecting various types of wells. A preventative maintenance program was also established to continue to insure the good operational condition of the wells, since bacterial loading is a reoccurring problem. (author - includes numerous diagrams) 32 pp.