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Guidance
Documents Low-Flow Purging & Sampling Guidance (December 2003)Part 4
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Low-Flow Purging and Sampling
B. Specific LFPS Considerations (cont.)
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Section
- Water Level Measurements
The depth to the top of the water column must be recorded prior to pump
installation and/or prior to purging. If the total
depth of the well needs to be determined (e.g., to verify the correct
well designation and/or to determine if silt has accumulated in the
bottom of a well), it should be measured at least 48 hours prior to
sample collection or after the sample has been collected and the pump
removed. Total depth measurements must never be taken immediately before
purging as this may cause the re-suspension of solids in the well and
prolong the purge time.
Once the initial water-level measurement has been recorded and the pump
installed, suspend the water-level probe in the well at the point at
which drawdown is equivalent to a 0.3-foot drop. Record water levels
simultaneously with WQIP measurements once every five minutes.
Water-level-measurement devices, which may impart some disturbance to
the water column (i.e., stainless steel "popper" or coated
tape), are not acceptable.
- Pump Installation
LFPS pump installation can be divided into two general collection scenarios:
permanent and portable (well-to-well). Permanent pump installation is
the most desirable. Among other advantages are improved consistency
in data acquisition and reduced long-term labor, preparation and material
costs. However, permanent installation is more typically associated
with long-term monitoring due to the high initial capital investment
required.
The more common practice is to use a pump on a portable or well-to-well
basis. While initial capital investment is comparatively less than that
of a permanent installation, this practice requires close attention
to quality control aspects of pump selection, preparation and decontamination.
Once pumps have been properly decontaminated and fitted with appropriate
tubing, installation of the pump can begin. Ideally, pumps should be
installed 24 to 48 hours prior to initiation of purging. However, this
is not always practical, especially when site security can not be guaranteed.
In addition, wells constructed with flush-mount casing are difficult
to protect from storm water or infiltration of other contaminants during
the extended period monitor wells are open.
Pumps must be installed in such a manner as to insure any disturbance
in the well is kept to an absolute minimum. Once pumps reach the top
of the water column, their descent should proceed very slowly through
the water column. The actual level where the pump intake is to be suspended
must be predetermined. Under no circumstance should the pump make contact
with, or be "bounced" off, the bottom of the well.
One helpful method to insure proper intake location is to accurately
measure and pre-cut the tubing for each individual well prior to site
activity. A mark can be made on the tubing, which coincides with the
top of the well. Cutting the tubing off-site in a controlled setting
is most desirable. Tubing can be wiped down with paper towels, moistened
with distilled/ deionized water, labeled and then sealed into plastic
bags until needed. If this practice is used, be sure to allow enough
tubing to account for the distance from the top of the well casing to
the flow cell.
- Purge Rates
Control over the purge rate is one of the most critical aspects of this
technique. Once the pump is set within the screened interval at the
desired location, a clean electronic water-level-monitoring device is
lowered approximately 0.3 ft into the water column. Start the pump at
a speed that results in a flow rate in the range of 100 to 500 ml/min.
Pump the initial purge water to waste in order to prevent any fouling
of the flow-through cell. With the pump running, connect the
tubing to the cell. Make sure that all air is purged from the tubing
and flow cell as the system fills with purge water. For LFPS, the pump
speed must remain constant such that flow rates never exceed 500 ml/min
and, once stabilized, the flow rate must not be varied, even during
sample collection. If drawdown continues to exceed 0.3 ft., reduce the
pump speed until the drawdown has stabilized but do not adjust pump
speed to a flow rate below 100 ml/min. Flow rates below this level may
induce pump stalling and undo the effort to reach stabilization. If
drawdown does not come under control at 100 ml/min, then a field decision
should be rendered as to how far to allow drawdown to continue until
sample collection. At no time should evacuation allow any portion of
the well screen to be exposed (for wells screened below the water table)
or bring the well to dryness.
Adjustments to pump speed are best made during the first 15 minutes.
Once a "feel" for the purge rate is obtained, begin recording
well stabilization indicators. Any significant change to purge rates
after this time may negatively impact well stabilization measurements.
Purge rates are best monitored by measuring the flow from the discharge
side of the flow cell with a graduated cylinder. Record all of the required
WQIPs once every 5 minutes. Once stability has been attained and recorded,
begin sample collection
- Sampling
Once WQIPs have stabilized, or a 4-hour time decision has been rendered,
sampling can proceed. Do not adjust the flow rate; maintain the same
pumping rate during sampling that was used to purge the well. Collect
the sample directly from the needle valve at the sample port. The needle
valve allows for sample collection with significantly reduced backpressure
and turbulence and offers the best means for sample collection without
affecting water quality. It also allows for monitoring using the flow-through
cell during sample collection, thereby allowing a final WQIP measurement
to be recorded immediately after sample collection. This is the preferred
method, especially if volatile organic compounds are the parameters
of concern. Any exceptions to this technique must first be approved
in writing from the NJDEP on a case-by-case basis before commencing
sampling operations.
If higher than expected water temperatures are being observed, evaluate
whether the submersible pump is overheating. If the pump motor is not
suspected, check the system for any exposure to direct sunlight, especially
during warmer periods of the year.
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