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E. Transition Seal. A bentonite-based transition seal,up to 5 feet in length,is often placed in the annular space to separate filter pack and cement-based sealing materials. The transition seal can prevent cement-based sealing materials from infiltrating the filter pack. A short interval of fine-grain sand,usually less than 2 feet in length,is often placed between the filter pack and the bentonite transition seal to prevent bentonite from entering the filter pack. Also,fine sand is 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 a transition seal if a transition seal is to be placed above the water level in a well boring. Coarse forms of bentonite,such as pellets and chips, are often used where a bentonite transition seal is to be placed below the water level. Transition seals should be installed by using a tremie pipe or equivalent. However,some forms of 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. Water should be added to the bentonite transition seal prior to the placement of cement-based sealing materials where the bentonite is dry in the borehole. Care should be exercised during the addition 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 of bentonite to allow for proper hydration. Water added to bentonite for hydration or to make a slurry shall be of suitable quality and free of pollutants and contaminants. Sufficient time should be allowed for bentonite transition seals to properly hydrate before cement-based sealing materials are placed. Normally, 1/2 to 1 hour is required for hydration to 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 during placement. F. Placement of Annular Seal Material. All loose cuttings and other obstructions shall be removed from the annular space before sealing materials are placed. Sealing may be accom- plished by using pressure grouting techniques,a tremie pipe,or equivalent. Sealing materials shall be installed as soon as possible during well construction operations. Sealing materials shall not be installed by"free-fall" from the surface unless the interval to be sealed is dry and less than 30 feet deep. Casing spacers shall be used within the interval(s) to be sealed to separate individual well casing strings from one another in a borehole of a nested monitoring well. The spacers shall be placed at intervals along the casing to ensure a minimum separation of 2 inches between individual casing strings. Spacers shall be constructed of corrosion-resistant metal, plastic,or other 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 material as the casing. Metallic spacer components shall meet the same metallurgical specifications and standards 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 and straightness of the borehole, and the method used to install the casing. Spacers shall not be more than 12 inches in length and shall not be placed closer than 10 feet apart along a casing string within the interval to be sealed, unless otherwise approved by the enforcing agency. -44-