tained. After removing it from the substrate, the
By convention most monitoring wells are in-
exterior of the barrel should be quickly wiped with
stalled in a vertical orientation and located so their
a clean disposable towel, and then the subsample
screened interval, which is typically a 3-m-long
is extruded into a tared VOA vial. Transferring a
section, intercepts the contaminant plume(s).
sample and closing the sample vial should be done
However, the low-stress groundwater sampling
rapidly (<10 seconds) to limit volatilization losses.
procedure is not only limited to wells of this con-
Samples of hard or cementitious material may
figuration; it can also be used in horizontal wells
be obtained by fragmenting a larger portion of the
or ones were the screened interval extends from
material using a clean chisel to generate
the top of the groundwater table to bedrock. When
aggregate(s) of a size that can be placed into a VOA
large screened intervals exist, this sampling pro-
vial or bottle. When sampling gravel, or a mixture
cedure can also be used to determine if there is a
of gravel and fines that cannot be obtained or
vertical or horizontal concentration profile within
transferred using coring tools, a spatula or scoop
the well. The sampling procedures presented in
can be used as a last resort. Caution should be
this report are consistent with revision 2 of "low
taken in the interpretation of the data obtained
stress (low flow) purging and sampling procedure
from materials that fit either of these two descrip-
for the collection of ground water samples from
tions. In the case of cementitious materials, losses
monitoring wells" (U.S. EPA Region I 1996a).
of VOCs by using this procedure would depend
Sample preservation and subsample preparation
on the location of the contaminant relative to the
follows the recommended procedures in Method
surface of the material being sampled. For gravel
5030B (U.S. EPA 1997b).
and loose fines, losses of VOCs are likely because
Prior to installing a pump for purging and sam-
of the nature of the sampling method and the
pling, synoptic water level and well depth mea-
noncohesive nature of the material (Hewitt et al.
surements should be performed. If the well depth
1995).
measurement is not made the day prior to the sam-
pling event, this task should be performed after
sample collection so as not to increase the particu-
Groundwater
Within the saturated zone VOCs exist princi-
late loading in the water column. A submersible
pally as a dissolved or undissolved (sorbed or
pump capable of reduced flows of between 0.1 and
nonaqueous) state. The collection of a groundwa-
0.4 L/minute and able to push a column of water
ter sample, while less susceptible than a porous
from the sampling depth to the surface (i.e., stain-
medium to volatilization losses, still must be per-
less steel centrifugal or bladder pumps) should be
formed in a timely well-orchestrated fashion. With
slowly lowered into the water column until the
respect to groundwater characterization, a recent
desired sampling zone is reached. Before starting
concern has been focused at how to obtain samples
the pump, the water level should be measured to
that are representative of the formation (ambient
establish the nonpumping elevation. Well purg-
flow conditions at a given depth within the satu-
ing is initiated by running the pump at its lowest
rated zone). This concern has led to the develop-
speed and increasing slowly until water is dis-
ment of a low stress (e.g., low flow, < 1 L/min.) or
charged from the transfer tubing. While water is
limited disruption approach to monitoring well
slowly being discharged from the well, the water
purging and sampling that requires that stable
level should be checked for drawdown. The pump
water chemistry exists prior to initiating sample
speed should be set at a minimum, and if possible,
collection. Past groundwater sampling guidance
should not cause more than a 9-cm drawdown
often specified the removal of five or more well
from the prepumping level. Throughout purging,
volumes of water prior to sample collection. This
the flow rate and well level should be monitored
practice is no longer recommended since it is ar-
at least every 5 minutes.
bitrary relative to the chemistry of groundwater.
Once the flow rate and drawdown criteria have
For example, rapid purging tends to create a draw-
been meet, a transparent flow-through cell should
down condition that heavily favors the chemistry
be put in line capable of monitoring DO (dissolved
related to the zone of greatest permeability (fast-
oxygen), specific conductance, temperature, pH,
est recharge), which may or may not be represen-
and ORP/Eh (redox potential). In addition, the
tative of the formation over the screened interval.
turbidity should also be monitored. Stable ground-
Furthermore, rapid purging also has a tendency
water chemistry is achieved and sampling can start
to increase particulate loading (Puls and Paul
when (1) three consecutive readings taken at 3- to
1995).
5-minute intervals fall within the range given for
4
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