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Section 12. Casing. The term "casing" in its broadest sense includes all tubular materials that are permanent features of a well. Screens, collars, risers, liners, and blank casing in monitoring wells maintain the well bore and provide a passage for ground water level measurement and/or sample-collection devices. Protective casing serves to prevent accidental or intentional damage to a well. Protective casing normally consists of heavy gauge metallic pipe placed over the portion of the well casing that extends above ground surface. Conductor casing usually functions as a temporary means of shoring the walls of a well boring to allow drilling and the placement of well construction materials. If used, temporary conductor casing is usually driven into place during drilling and is withdrawn at the same time filter pack and annular seal materials are installed around the well casing. Sometimes conductor casing is left in place and is made a permanent feature of the completed well structure. Requirements for sealing permanent conductor casing in place are contained in Section 9. For the purpose of these standards, the term "casing" applies to screens, collars, risers,and blank casing,and other specialized products used to maintain the well bore. General discussion and standards for casing materials are contained in Section 12 of the Water Well Standards. Special considerations that apply to monitoring well casing are described below: A. Casing Material. 1. Chemical Compatibility. Special consideration shall be given to the selection of casing , materials for monitoring wells installed in environments that are chemically"hostile". The selected casing shall resist chemical attack and corrosion. Special consideration should be given to the selection of casing materials for wells to be used for sensitive water-quality determinations.Chemical interaction between casing materials and pollutants,contaminants,ground water,filter pack material,and geologic ' materials could bias ground-water quality determinations. 2. Used Casing. Used casing may be acceptable in certain cases, at the approval of the enforcing agency. 3. Plastic and Steel Casing. Plastic and steel well casing materials are commonly used for monitoring wells. The principal plastics used for water-quality monitoring wells are thermoplastics and fluorocarbon resins. Standards for thermoplastic well casing are in Section 12 of the Water Well Standards. The principal thermoplastic material used for water quality monitoring wells is polyvi- nyl chloride (PVC). Fluorocarbon casing materials include fluorinated ethylene propylene (FEP) and polytetrafluoroethylene (PTFE). Fluorocarbon resin casing materials are generally considered immune to chemical attack. Fluorocarbon casing materials shall meet the following specifications, including the latest,revisions thereof: a. ASTM D3296, Standard Specification for FEP-Fluorocarbon Tube. b. ASTM D3295,Standard Specifications for PTFE Tubing. -47-