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• i► ' that no bridging of filter-pack materials occurred. Each well shall be gently surged during filter-pack installation (i.e., surging shall occur after the filter-pack is ' poured into the annulus between the casing and borehole wall, but prior to the installation of the transition and sanitary seals). 3.3 Transition and Sanitary Seals ' The remaining annular space shall be filled with a transition seal (minimum thickness of 2 feet) set on top of the filter pack, followed by a neat-cement based sanitary seal to land surface (minimum thickness of 10 feet). Bentonite or other approved material shall be used for the transition seal. For depths greater than 30 feet, and where water is present in the borehole, the material shall be installed by use of a tremie pipe or equivalent. Where bentonite exists in the borehole in ' dry form, potable water shall be added and sufficient time allowed for the bentonite to properly hydrate before placement of the sanitary seal. ' The sanitary seal shall be placed within the annular space between the top of the transition seal to land surface. Sealing material shall be selected based on required structural, handling, and sealing properties and the chemical ' environment into which it is placed. Drilling fluid or cuttings from the borehole shall not be used as the sealing material. All water used for sealing mixtures shall be potable. ' 3.4 Surface Seals Surface features shall serve to prevent physical damage; prevent the entrance of surface water, pollutants, and contaminants; and prevent unauthorized access. The top of each monitoring well shall be protected by a locking cover or ' equivalent level of protection to prevent unauthorized access. The top of the well- casing shall be sealed with a removable plug that provides a means to secure and lock the well. The sealed well-casing shall be housed in either a locking monument style well cover (above ground completion) or set below grade in a traffic rated vault box with lid (below ground completion). ' 3.4 Above Ground Completion Monitoring wells located in agricultural fields will extend at least 2 feet above land ' surface so as to be readily discernable during agricultural operations. For these above ground completions, San Joaquin County Well Standards require that a 2 ft X 2 ft concrete surface seal be constructed around the top of a monitoring well ' casing at ground surface. The surface seal shall be at least four (4) inches thick and shall slope to drain away from the well. The surface seal shall be continuous with the sanitary seal and well cover, and should be free of cracks, voids, and ' other significant defects that would allow the entry of water, pollutants, and contaminants. 1