Low-Flow Purging and Sampling
B. Specific LFPS Considerations (cont.)
Previous
Section
- Pump Decontamination
The pump forms one of the two key elements of sampling equipment (tubing
is the other). The importance of proper pump decontamination is especially
true when pumps are rented and utilized on a well-to-well basis. Never
assume that rented pumps have been thoroughly cleaned. Pumps
constructed with plastic parts, or sealed inner workings that are inaccessible
to direct handling are not an option for LFPS well-to-well consideration
because of their limited ability to be decontaminated thoroughly.
Most bladder pumps can not be easily decontaminated in the field due
to their unique construction. For that reason, bladder pumps are not
employed on a well-to-well basis unless they are constructed
with easy to clean parts and disposable bladders. Bladder pumps
are best suited for dedicated (permanently installed) scenarios. Another
popular pump, the variable-speed, 2-inch diameter submersible, is more
adaptable for well-to-well sampling; however, close attention to decontamination
is warranted. One manufacturer, Grundfos®, clearly states
in the operational handbook that the pump must be completely disassembled,
including removal of the motor shaft from the stator housing, and all
components within the impeller housing. Care must be taken upon reassembly
to insure that the cavity housing the motor shaft is completely
refilled with distilled/deionized water. Care must also be taken with
this pump during periods of cold weather to avoid freezing of the coolant
water. Proper decontamination not only helps to ensure more reliable
data; it also prolongs the life of any pump.
- Field Blank Collection
When employing LFPS techniques, collection of the field blank must follow
the same general rules for all groundwater sampling equipment. This
includes the requirement that "all" sampling equipment, which
comes in contact with the sample, must also come into contact with the
field blank water. To overcome some of the difficulties that manual
field blank collection through the inside of a pumping system creates,
the following procedure is strongly recommended. Fill a 1000-ml decontaminated,
graduated glass cylinder with method blank water supplied by the laboratory
performing the analysis. Place a properly decontaminated pump into the
graduated cylinder with sample tubing and plumbing fittings attached.
Activate the pump and collect the required field blank samples. As the
water is removed from the cylinder, replace it with additional method
blank water. This procedure will require that the laboratory supply
larger volumes of field blank water i.e., bulk water in liter or 4-liter
containers. The traditional requirement that field blank water be supplied
in the same identical containers as the sample being collected can not
be practically satisfied when using LFSP. The identical bottle-to-bottle
field blank requirement is waived for this sampling technique procedure
only.
- Tips
- Temperature Measurement and Submersible Pumps
Variable-speed submersible pumps such as the Grundfos® Redi
Flo 2 pump use water to cool the motor during operation. Sometimes,
reduced flow rates may result in insufficient cooling of the motor
and may elevate the temperature of the water to a point where it
may begin to affect sample integrity. If the pump is used in low-yielding,
two (2)- or four (4)- inch-diameter wells, temperature increases
that do not stabilize may result. If this is observed, a field decision
must be made to either discontinue or continue with LFPS. If all
other WQIPs have stabilized, then collecting the sample and qualifying
the water-quality data accordingly may be acceptable. If the temperature
increase continues and eventually exceeds 40% of the initial recorded
temperature (Celsius) and other WQIPs have not stabilized, sampling
should be discontinued. Turning the pump off and on to control overheating
is not acceptable. Always keep in mind that elevated temperature
has a direct relationship with dissolved oxygen, specific conductance
and, to a lesser degree, pH measurement. Higher temperatures may
also reduce the concentrations of volatile organic compounds in
groundwater samples due to their relatively high Henry’s Law constants.
If sampling with submersible pumps continues to result in elevated
water temperature, other sampling alternatives should be discussed
with the appropriate regulatory program.
When using some submersible pumps in large-diameter wells (six inch
and greater), overheating of the motor, followed by mechanical shutdown
and possible motor damage, may occur. This is the result of water
being drawn to the pump intake in a more horizontal flow pattern
which diminishes the design feature that normally moves cool water
vertically across the motor (stator) housing. The use of specially
designed shrouds may overcome this condition.
- Control of Pump Speed
In order to achieve the high turning speeds, low-speed startup torque
is generally lacking in some submersible pumps including the Grundfos®
Redi Flo 2 pump. When attempting to control initial drawdown and/or
sample flow rates, it is possible for the pump to cease pumping.
Then, if a check valve has been installed, the pump may not have
enough torque to overcome the head pressure when attempting to restart
it. Sometimes, turning the pump to the highest speeds will overcome
this situation or sometimes the pump may have to be pulled from
the well and reinstalled. Neither of these corrective measures is
conducive to LFPS. To avoid this scenario, make sure the control
box comes equipped with a "ten turn pot" frequency adjustment knob.
This will allow significantly greater control over pump speeds and
the risk of losing pump flow will be reduced.
- pH
Monitoring for stabilization of pH in groundwater is relatively
straightforward and rarely requires serious troubleshooting. When
calibrating for pH, do a two-point calibration, at a minimum. The
calibration range should bracket the anticipated pH. If the pH is
unknown, then a three-point calibration must be made. The temperature
of the buffer solutions should be as close to the temperature of
the groundwater as possible. If the probe does not calibrate properly,
check to make sure that the probe's electrical contact points are
dry. As with preventative maintenance of any probe, make sure that
the pH probe is rinsed with distilled/deionized water between use
and cleaned periodically per the manufacturer's specifications.
Overnight storage generally requires placement of the probe into
a 2-molar (M) solution of potassium chloride. This solution may
cause an unwanted build up of salt, therefore, frequent rinsing
is necessary.
- Temperature of Calibration Solutions
Correct field measurement of dissolved oxygen, conductivity and
pH requires tight control over calibration solution temperature.
Proper calibration calls for solution temperatures of these parameters
to be the same as the groundwater being measured. This may be difficult
to achieve when field sampling well-to-well as groundwater temperature
can vary between wells based on depth, local setting (asphalt vs.
open field) and other atmospheric and hydrogeological factors. In
addition, it is logistically difficult to bring solutions to groundwater
temperature at the point of pump intake without first installing
the pump, collecting purge water and allowing sufficient time to
bring calibration solutions to appropriate temperatures.
For the purposes of LFPS in New Jersey, calibration solution temperatures
and the flow-through cell itself must be maintained at approximately
54° F (12° C ± 2° C) during calibration. When ambient conditions
warrant, this will require the suspension of the solutions and flow-through
cell in a container/bucket of water at the aforementioned temperature.
When calibrating for dissolved oxygen, always make sure the cell
is vented to the atmosphere by attaching short pieces of tubing
to the inlet and outlet fittings while the cell is submerged.
During the purge phase, record the difference between the stabilized
temperature and the temperature of the calibration solutions. This
information must be presented to the end user. If the sampling event
is extended for two or more days, appropriate adjustments can then
be made to more accurately reflect the groundwater temperature during
calibration.
- Low Flow Purging and Sampling for Low Yielding Wells
The principal focus of water supply well installation is well yield.
In contrast, the principal focus of monitor well installation is water
quality; well yield is of secondary importance. In an attempt to locate
and delineate groundwater contamination, monitor wells are frequently
installed in low-yielding water-bearing zones.
Low-yield wells present challenges with respect to representative groundwater
sample collection. The removal of water by bailers draws down the water
level in the well by slug- type increments. Peristaltic pumps draw water
out of the well by vacuum (negative pressure) which may result in degassing
and VOC loss. The operation of variable-speed, submersible pumps at
low flow rates may result in heating of the sample as it flows around
and through the pump, which may also lead to degassing and VOC loss.
Wells that yield less than 0.1 l/min (100ml/min) frequently incur significant
drawdown during well purging. If drawdown occurs across the screened
interval or open borehole of a well, VOC loss may result. The increased
stress on a well caused by significant drawdown may also result in an
increase in water turbidity. In an effort to facilitate the collection
of a representative groundwater sample from low-yielding wells, NJDEP
will allow special sampling procedures to be used. This may include
sample collection without regard to monitoring WQIPs associated with
well stabilization.
At a minimum, water-quality data, well-construction data, water-level
data, and accurate well-yield data for each low-yielding well will need
to be submitted to the Department prior to the development of an acceptable
sampling procedure. Since sample collection may begin almost as soon
as purging is initiated, it is imperative that the exact interval where
the sample will be collected along the screen be predetermined. Aside
from the considerations for monitoring drawdown and WQIPs, all other
LFPS considerations discussed above apply here as well. The owner of
the well shall also propose possible explanations for the low yield
of the well(s). Once the aforementioned information has been received,
the Department will work with the well owner to formulate an acceptable
sampling plan. The sampling plans will be approved on a case-by-case
basis and will be well-specific. Implementation of any special sampling
procedure or use of any special sampling equipment shall not be performed
without prior NJDEP approval.
Next Section
|
 |
