Sampling Trace-Level Organics With Polymeric Tubings
Dynamic Studies
LOUISE V. PARKER AND THOMAS A. RANNEY
fuse through certain types of tubing, thereby pos-
INTRODUCTION
sibly affecting the solution chemistry of the sam-
It is important that the reported concentrations
ple as it is pumped to the surface (Holm et al.
of contaminants in samples taken from ground-
1988, Kjeldsen 1993). In this paper, we will focus
water monitoring wells accurately reflect in-situ
on the effects tubing materials can have on
values, or else decisions on whether remedial ac-
groundwater samples with respect to sorption of
tions are necessary can be erroneous. Recent stud-
organic contaminants, leaching of organic constit-
ies suggest that among the currently used sam-
uents, and desorption of sorbed organic contami-
pling methods, low-flow-rate (~100 mL/min)
nants.
purging and sampling may cause the least distur-
bance to the well during the sampling process.
Sorption of organic contaminants
Samples taken using this method have lower tur-
Several studies (Ho 1983, Barcelona et al. 1985,
bidity (Kearl et al. 1992, Puls and Powell 1992,
Reynolds and Gillham 1985, Devlin 1987, Pearsall
Puls et al. 1992, Backhus et al. 1993) and yield per-
and Eckhardt 1987, Gillham and O'Hannesin
haps the most accurate values for inorganic con-
1990, Reynolds et al. 1990) have shown that poly-
taminants among the currently used sampling
meric tubing materials can lower analyte concen-
methods (Puls and Powell 1992, Puls et al. 1992).
trations. In most cases, losses of organic solutes to
Furthermore, there is evidence indicating that
polymers were attributed to absorption within
low-flow-rate purging and sampling may also
the polymer matrix (Barcelona et al. 1985, Rey-
produce samples that reflect the in-situ values of
nolds and Gillham 1985, Gillham and O'Hannesin
at least some organic contaminants, such as poly-
1990, Parker et al. 1990, Reynolds et al. 1990). Up-
cyclic aromatic hydrocarbons (Backhus et al.
take of organic compounds by polymers is con-
1993).
sidered to proceed first by sorption by the polymer
One concern with the slow-rate pumping
surface, followed by diffusion into the polymer
methods for sampling groundwater is that the
matrix (Serota et al. 1972, Yasuda and Stannett
longer residence time in the tubing may allow
1975, Reynolds and Gillham 1985, Gillham and
possible interactions that could affect analyte
O'Hannesin 1990). Flexible tubing materials (e.g.,
concentrations. The tubing could either leach in-
silicone rubber, latex rubber, flexible PVC) have
organic or organic constituents, which may be
been shown to be highly sorptive of organic sol-
analytes of concern, or interfere with the methods
utes (Ho 1983, Barcelona et al. 1985, Devlin 1987,
of analysis and thereby give inaccurate values.
Pearsall and Eckhardt 1987). In contrast, the more
The tubing may also sorb contaminants present in
rigid polymers appear to be much less sorptive of
the sample, thereby leading to inaccurate low val-
organic solutes (Barcelona et al. 1985, Reynolds
ues. In addition, if the tubing is not dedicated to a
and Gillham 1985, Gillham and O'Hannesin
particular well, it is possible that tubing used pre-
1990). As an example, Figure 1 shows sorption of
viously to sample a well with high contaminant
ppm levels of benzene by rigid and flexible PVC
concentrations could release sorbed contami-
in a static study conducted by Gillham and
nants into the next sample, leading to inaccurate
O'Hannesin (1990). Barcelona et al. (1985) attrib-
high values. Also, gases such as oxygen can dif-
uted the reduced sorption by the more rigid poly-
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