Disadvantages of stainless steel well casing and screen materials:

May corrode under some geochemical and microbiological conditions;

May sorb cations and anions;

May contribute metal ions (iron, chromium, nickel, manganese) to ground 

water samples;

High weight per unit length; and

Type 304 and Type 316 stainless steel are unsuitable for use when monitoring

for inorganic constituents.

Thermoplastic Materials

Thermoplastics are human made materials that are composed of different formulations

of large organic molecules. These formulations soften by heating and harden upon cooling,

and therefore, can be easily molded or extruded into a wide variety of useful shapes including

well casings, screens, fittings and accessories. The most common types of thermoplastic well

casings and screens are polyvinyl chloride (PVC) and acrylonitrile butadiene styrene (ABS).

PVC plastics are produced by combining PVC resin with various types of stabilizers,

lubricants, pigments, fillers, plasticizers and processing aids. The amounts of these additives

can be varied to produce different PVC plastics with properties tailored to specific

applications.

PVC materials are classified according to ASTM standard specification D 1785, which

covers rigid PVC compounds (ASTM, 1986). This standard categorizes rigid PVC by

numbered cells designating value ranges for certain pertinent properties and characteristics,

including: impact strength, tensile strength, rigidity (modulus of elasticity), temperature

resistance (deflection temperature), and chemical resistance. ASTM standard specification

F 480 covers thermoplastic water well casing pipe and couplings made in standard dimension

ratios. This standard specifies that PVC well casing can be made from only a limited number

of cell classification materials, predominantly PVC 12454 B, but also including PVC 12454 C

and PVC 14333 C and D (American Society for Testing and Materials, 1981).

ABS plastics are produced from three different monomers: 1) acrylonitrile, 2)

butadiene, and 3) styrene. The ratio of the components and the way that they are combined

can be varied to produce plastics with a wide range of properties. Acrylonitrile contributes

rigidity, impact strength, hardness, chemical resistance, and heat resistance; butadiene

contributes impact strength; styrene contributes rigidity, gloss, and ease of manufacturing

(National Water Well Association and Plastic Pipe Institute, 1981). The ABS used for well

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