California Water Well Standards, DWR Bulletin 74 ... - County of Glenn

Transition Seal. A bentonite-based transition seal, up to 5 feet in length, is often placed in theannular space to separate filter pack and cement-based sealing materials. The transition sealcan prevent cement-based sealing materials from infiltrating the filter pack. A short intervalof fine-grain sand, usually less than 2 feet in length, is often placed between the filter pack andthe bentonite transition seal to prevent bentonite from entering the filter pack. Also, fine sandis sometimes used in place of bentonite as the transition seal material.Fine-grain forms of bentonite, such as granules and powder, are usually employed for atransition seal if a transition seal is to be placed above the water level in a well boring. Coarseforms of bentonite, such as pellets and chips, are often used where a bentonite transition sealis to be placed below the water level.Transition seals should be installed by using a tremie pipe or equivalent. However, some formsof bentonite may tend to bridge or clog in a tremie pipe.Bentonite can be placed in the well annulus in dry form or as slurry for transition seals. Watershould be added to the bentonite transition seal prior to the placement of cement-based sealingmaterials where the bentonite is dry in the borehole. Care should be exercised during theaddition of water to the borehole to prevent displacing the bentonite.Water should be added to bentonite at a ratio of about 1 gallon for every 2 pounds ofbentonite to allow for proper hydration. Water added to bentonite for hydration or to makea slurry shall be of suitable quality and free of pollutants and contaminants.Sufficient time should be allowed for bentonite transition seals to properly hydrate beforecement-based sealing materials are placed. Normally, 1/2 to 1 hour is required for hydrationto occur. Actual time of hydration is a function of site conditions.The top of the transition seal shall be sounded to ensure that no bridging occurred duringplacement.F. Placement of Annular Seal Material. All loose cuttings and other obstructions shall beremoved from the annular space before sealing materials are placed. Sealing may be accomplishedby using pressure grouting techniques, a tremie pipe, or equivalent. Sealing materialsshall be installed as soon as possible during well construction operations. Sealing materialsshall not be installed by "free-fall" from the surface unless the interval to be sealed is dry andless than 30 feet deep.Casing spacers shall be used within the interval(s) to be sealed to separate individual wellcasing strings from one another in a borehole of a nested monitoring well. The spacers shallbe placed at intervals along the casing to ensure a minimum separation of 2 inches betweenindividual casing strings. Spacershall be constructed of corrosion-resistant metal, plastic, orother non-degradable material. Wood shall not be used as spacer material.Any metallic component of a spacer used with metallic casing shall consist of the same materialas the casing. Metallic spacer components shall meet the same metallurgical specifications andstandards as the casing to reduce the potential for galvanic corrosion of the casing..The spacing of casing spacers is normally dictated by casing materials used, the orientation andstraightness of the borehole, and the method used to install the casing. Spacers shall not bemore than 12 inches in length and shall not be placed closer than 10 feet apart along a casingstring within the interval to be sealed, unless otherwise approved by the enforcing agency.E.-44-