Table 6. Other factors affecting well casing selec-
ter, except for the FRP casings that leached Pb.
tion for monitoring wells.
Comparisons of these data with a similar study
conducted by our laboratory (Hewitt 1989) indi-
Strength Considerations
cated that FRE, PVC, PTFE, and FEP tended to
(Going from shallowest well to deepest well)
leach less than SS 304 and 316 casings, while this
PTFE (~FEP) << PVC, FRP < FRE < SS
Based on manufacturer information and information
was not always the case for FRP, especially for
in Nielsen and Schalla (1991). FEP was assumed to be
Cd and Pb.
fairly equivalent to PTFE because no strength data
It is possible that FRE and FRP casings would
were found.
leach less under dynamic conditions. Previous
Corrosion of Stainless Steel
studies by Hewitt (1989, 1992, 1994) have shown
pH < 7.0
that PVC leached Cr and Pb under static condi-
DO > 2 ppm
H2S ≥ 1 ppm
tions, but did not do so under dynamic condi-
Total Dissolved Solids > 1000 ppm
tions. In addition, several older studies (Packham
CO2 > 50 ppm
1971a,b; Gross et al. 1974; Dietz et al. 1977) have
Cl > 500 ppm
shown the leaching of metal stabilizers from PVC
Taken from Aller et al. (1989), Driscoll (1986).
pipe appears to be due to a surface phenomenon,
Softening of Plastics by Organic Solutes
i.e., concentrations of leached metal stabilizers are
Most non-fluoropolymers such as PVC, FRP, FRE, and ABS
can also be degraded by very high concentrations of some
highest initially, drop off with additional flushing
organic solvents. In order for this to happen:
with water, and then level off.
the organic solvent must be a good solvent of the poly-
With respect to sorption, none of the five mate-
mer and
rials sorbed the anions tested, As and Cr. How-
the solvent must be present in concentrations that exceed
0.1 times the chemical's aqueous solubility.
ever, all five materials sorbed the cations tested
(Cu, Cd, Pb), although the two fluoropolymers
As an example, methylene chloride is a good solvent of PVC.
It must be present at a concentration that exceeds (0.1) (~1900
were the least active. With respect to sorption of
mg/L), or ~190 mg/L.
Cd, PVC was the most active while FRE and FRP
were more sorptive of Pb and Cu (with FRE being
References: Berens (1985), Vonk (1985, 1986), Parker et al.
(1992), Parker and Ranney (1994b, 1995).
slightly more sorptive). Again, we might expect
that these effects would be mitigated in a well
Cost of Casing Materials
PVC < FRE < FRP < SS 304 < SS 316 < FEP, PTFE
where the water is replenished and sites for sorp-
Based on the price paid for the materials used in this
tion become less available with time. However,
study in 1993.
Hewitt (1994) found that PVC and PTFE well
screens continued to sorb cations under dynamic
degradation of the casing material. Table 1 sum-
conditions. A comparison of our results with a
marized most of the research studies on degrada-
similar study of sorption of metals by SS casings
tion of these polymeric materials by chemicals and
(Parker et al. 1990) also indicates that SS materials
the effects these polymeric casings have on or-
are much more sorptive than the polymeric mate-
ganic contaminants. It should be noted that while
rials we tested.
several studies (Parker et al. 1990, Gillham and
Based on our findings in this study, we would
O'Hannesin 1990, Parker and Ranney 1994a) have
tentatively rank these materials' impact on metal
shown that SS is relatively nonsorptive of a fairly
concentrations as follows:
wide range of organic solutes, Reynolds et al.
FEP, PTFE < PVC < FRE < FRP.
(1990) have shown that it can react with highly
halogenated hydrocarbons, causing their degra-
Based on our findings and those of others (Hewitt
dation. Table 6 summarizes some additional in-
1989, 1992, 1994, Parker et al. 1990), we would
formation on the strength of these materials, oth-
also rank the impact on metal concentrations of
er conditions that cause degradation of casing
four of these materials and SS as
materials, and cost.
This series of experiments clearly shows that,
FEP, PTFE < PVC < FRE < SS.
of the three new materials considered in this study
(FEP, FRE, and FRP), FRE may prove quite useful
Selecting well casings for a particular well is
in monitoring applications where either SS or PVC
highly dependent upon a number of factors in-
cannot be used, especially in deeper wells. FEP
cluding the size and depth of the well (i.e., strength
does not appear to offer any clear advantage over
considerations), the contaminants of interest, and
PTFE. When cost is also considered, PVC still re-
characteristics of the well water that might cause
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