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F. Backflow Prevention. All pump discharge pipes not discharging or open to the atmosphere shall be equipped with an automatic device to prevent backflow and/or back siphonage into a well. Specific backflow prevention measures are required for drinking water supply wells, as prescribed in Title 17,Public Health,California Code of Regulations (Sections 7583-7585 and 7601-7605, effective June 25, 1987). Irrigation well systems, including those used for landscape irrigation, and other well systems that employ, or which have been modified to employ, chemical feeders or injectors shall be equipped with a backflow prevention device(s) approved by the enforcing agency." Section 12. Casing. Items 3,5,and 6 of Subsection A(page 43 of Bulletin 74-81)have been revised. The remainder of Subsection A is unchanged. Subsection B (page 45 of Bulletin 74-81) has been revised. The revisions are as follows: "A. Casing Material. 3. Plastic. Two basic types of plastic are commonly used for plastic well casing: thermoplastics and thermosets. Thermoplastics soften with the application of heat and reharden when cooled. Thermoplastics can be reformed repeatedly using heat and sometimes can unexpectedly deform. Attention should be given to the effect of heat on thermoplastic casing from the setting and curing of cement. Additional discussion on sealing material and heat generation is in Section 9, Subsection D, 'Sealing Material'. Thermoplastics used for well casing include ABS(acrylonitrile butadiene styrene),PVC (polyvinyl chloride), and SR (styrene rubber). PVC is the most frequently used thermoplastic well casing in California. Styrene rubber is seldom used. Unlike thermoplastics, thermoset plastics cannot be reformed after heating. The molecules of thermoset plastic are'set'during manufacturing by heat,chemical action, or a combination of both. The thermoset plastic most commonly used for well casing is fiberglass. Thermoplastics. Thermoplastic well casing shall meet the requirements of ASTM F480, Standard Specification for Thermoplastic Well Casing Pipe and Couplings Made in Standard Dimension Ratios (SDR), SCH 40 and SCH 80, including the latest revision thereof. (Note: A 'dimension ratio' is the ratio of pipe diameter to pipe wall thickness.) Pipe made in Schedule 40 and 80 wall thicknesses and pipe designated according to certain pressure classifications are listed in ASTM F480, as well as casing specials referencing the following ASTM specifications: (1) ABS Pine. ASTM D1527, Standard Specification for Acrylonitrile- Butadiene-Styrene (ABS) Plastic Pipe, Schedules 40 and 80. (2) PVC Pine. ASTM D1785, Standard Specification for (Poly Vmyl Chloride) (PVC) Plastic Pipe, Schedules 40, 80, and 120. (3) Pressure-Rated PVC Pine. ASTM D2241,Standard Specifications for Poly (Vinyl Chloride) (PVC) Pressure-Rated Pipe (SDR Series). -24-