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0 0 Clayton Group Services Page 4 Standard Operating Procedures SPO -5 - Monitoring Well Installation 21. If a sieve analysis has not be conducted on the aquifer material, typically #3 sand is used as a filter pack in the annular space (space between the well casing and wall of the boring) adjacent to the screen and a screen size of 0.02 -inch is used. However, it is advisable and often required by the regulatory agency to characterize the aquifer material by performing a sieve analysis and selecting the filter pack and screen size on this basis. Less than 10-20 percent of the filter pack should enter the well. ASTM standards point out that the filter pack usually is selected to have a 30% finer (d-30) grain size that is about 4 to 10 times greater than the 30% finer (d-30) grain size of the hydrologic unit being filtered. In general, the filter pack should be emplaced from the bottom up using a tremie pipe to avoid bridging or segregation during emplacement. The slot size should retain at least 90% and preferably 99% of the filter pack. 22. Monitoring well slotted casing and/or filter pack shall not be emplaced in multiple aquifers as this provides an avenue for cross contamination between aquifers. 23. A bentonite -based transitional seal, up to 5 feet, typically is placed in the annular space above the filter pack and separates the filter pack from the overlying cement seal. This bentonite commonly is emplaced with a tremie pipe to avoid bridging or clogging. 24. A cement -based sealing material should then be emplaced from the top of the bentonite seal to the surface with the installation of a well cover. The annular surface seal should be no less than 20 feet except in shallow wells where this is not possible, if a bentonite seal as described above is in place. 25. Surface Well Box - The surface construction of the monitoring well should serve to prevent damage to the well, prevent the entrance of surface water and pollutants, and prevent unauthorized access. Surface construction commonly includes: • Cover - The surface well box should have a bolted cover to protect the well and provide a traffic cover in areas of vehicle traffic. • Locking Cap - The top of the monitoring well should be fitted with a locking cap and lock to prevent unauthorized access to the well and seal it against pollution and water influx. • Flooding - The top of the well box should be slightly above grade to prevent influx of surface drainage, except where not practical such as in vehicle areas. • Base - The well box generally should be emplaced in concrete placed around the top of the well casing and contact the annular space. The base should be at least 4 inches thick and slope away from the well casing. The base should extend at least 2 feet laterally away from the outside of the well boring and be poured before the cement of the upper annular has set. 26. Each monitoring well shall be marked identifying it as a monitoring well and have an identifier, such as a metal tag, with the well identification number. S_AEmu\ProjASPOs\SOP-5.doc