The following recommendations apply to the use and operation of ground water
Check valves should be designed and inspected to ensure that fouling problems
do not reduce delivery capabilities or result in aeration of samples.
Sampling equipment should never be dropped into the well, as this will cause
degassing of the water upon impact.
Contents of the sampling device should be transferred to sample containers in a
controlled manner that will minimize sample agitation and aeration.
Decontaminated sampling equipment should not be allowed to come into
contact with the ground or other contaminated surfaces prior to insertion into
Ground water samples should be collected as soon as possible after the well is
purged. Water that has remained in the well casing for more than about 2
hours has had the opportunity to exchange gases with the atmosphere and to
interact with the well casing material (USEPA, 1991b).
The rate at which a well is sampled should not exceed the rate at which the
well was purged. Ideally, the rate of sample collection should be
approximately the same as the actual ground water flow rate. Because this is
typically not possible, low sampling rates, approximately 0.1 L/min, are
suggested. Low sampling rates will help to ensure that particulates, immobile
in the subsurface under ambient conditions, are not entrained in the sample and
that volatile compounds are not stripped from the sample (Puls and Barcelona,
1989b; Barcelona, et al., 1990; Puls et al., 1991; Kearl et al., 1992; USEPA,
1991b). Pumps should be operated at rates less than 0.1 L/min when collecting
samples for volatile organics analysis.
Pump lines should be cleared at a rate of 0.1 L/min or less before collecting
samples for volatiles analysis so that the samples collected will not be from the
period of time when the pump was operating more rapidly.
Pumps should be operated in a continuous, non pulsating manner so that they
do not produce samples that are aerated in the return tube or upon discharge.
When sampling wells that contain LNAPLs, a stilling tube should be inserted
in the well as described in Section 7.2.4. Ground water samples should be
collected from the screened interval of the well below the base of the tube.