Low-Flow Purging and Sampling
B. Specific LFPS Considerations (cont.)
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Section
- Flow-Through Cell
Typical flow-through cell design is not complicated and almost all on
the market today have common shared features. Cells should be transparent
in order to "see" the physical condition of the purge water
or air bubbles passing through the system. Highly turbid or iron bacteria-laden
water can be visually monitored for change as the purge progresses.
The cell must be sealed against unwanted exposure to the atmosphere,
thus insuring accurate measurement of air-sensitive parameters (dissolved
oxygen, pH, etc.). The total capacity of the cell must be small (<300
ml) in order to maintain a desirable turnover rate of water coming into
the cell to ensure real-time data integrity. The in-line design must
allow for purge water to enter the flow cell from a bottom port and
exit at the top. The discharge may be fitted with a check valve.
Upon initial pump startup, it is good practice to not connect the pump
discharge line to the flow-through cell. This will allow the sampler
time to monitor drawdown, stabilize the flow rate and prevent fowling
of probes by bacteria, sediment, or NAPL. Once drawdown measurements
indicate that the flow rate has been controlled and a few minutes (<10)
have been allowed to clear any unwanted material, the pump discharge
line can then be connected to the flow cell.
Flow cell decontamination is important, not only to reduce the potential
for cross contamination, but also to ensure data integrity and consistent
instrument performance. The cell and probes should be rinsed with distilled/deionized
water between each monitor well as accumulation of suspended material
may impact probe performance. If they are exposed to contaminants, use
a mild detergent or laboratory glassware cleaning solution. Flow cell
exposure to high levels of contamination may damage probes and require
their repair by the manufacturer. Since LFPS is NOT normally a first-round
sampling option, knowledge of contaminant levels will generally be known
prior to the cell’s exposure to purge water.
The location of the flow cell or cells in relation to the sample port
is critical. Samples for turbidity measurement, general chemistry and
laboratory analysis must be collected ahead of the flow cell. When two
cells are used in series, the dissolved oxygen probe must be located
in the first cell.
Set up the flow-through cell in a location which will cause minimal
fluctuation of the flow rate due to elevation changes in the sample
tubing as the tubing is disconnected from the cell prior to sample collection.
It is also important to locate the flow-through cell as close as possible
to the well head in order to minimize the length of tubing needed between
the well head and flow-through cell. The flow-through cell must be protected
from ambient conditions and the ground surface.
- Pump Selection
Pumps used for monitoring WQIPs must be submersible, positive-displacement
pumps. Examples of acceptable positive-displacement pumps include bladder,
variable-speed submersible-centrifugal, reciprocating-piston, progressive-cavity,
and gear pumps. The pump discharge must be fitted appropriately to receive
either ¼- or 3/8-inch inside-diameter (ID) Teflon® or
Teflon®-lined polyethylene tubing.
Peristaltic pumps are suction-lift pumps which can create a negative
pressure gradient. Therefore, their use is not appropriate when collecting
groundwater samples for analysis of organic compounds. However, peristaltic
pumps may be used for the collection of groundwater samples for analysis
of inorganic compounds. It should be kept in mind, however, that sampling
with peristaltic pumps may affect the stabilization of some WQIPs including
dissolved oxygen, pH and redox potential. Since these WQIPs can be affected
by the peristaltic pump, this pump should not be used when these data
are to be used to evaluate the effectiveness of Monitored Natural Attenuation
of groundwater.
Two basic collection scenarios have a bearing on pump selection. These
include: 1) a permanently installed pump system, or 2) a portable (well-to-well)
pump installation. Bladder pumps can be used for either scenario, however,
only those with disposable bladders and easily cleaned parts are suitable
when sampling on a well-to-well basis. Variable-speed submersible-centrifugal
pumps, gear or progressive-cavity pumps can be used for either scenario
as long as they are constructed of easy to clean stainless steel/Teflon®
parts. Pumps constructed with impellers, helicoils, or gears, which
are difficult to clean or are constructed of unacceptable plastic parts,
are not suitable for sampling. In addition, when conducting LFPS on
a portable basis, the power or gas supply line should be isolated from
the sample tubing. Power supply and sample tubing lines that form a
single unit do not allow for easy decontamination and are not recommended.
- Plumbing Fittings
A check valve should be incorporated into the tubing train or flow cell
discharge to eliminate accidental drainage and subsequent aeration of
the flow cell. More importantly, a check valve will prevent a back-surge
of purged water being reintroduced at the screen interval of the well
should the power source or pump experience mechanical failure. A back-surge
of purge water into the screened interval of the well may result in
variability of the WQIPs and create analytical bias. In order to avoid
the need to decontaminate the check valve, it may be placed on the discharge
side of the flow cell or installed immediately above the pump discharge.
Some flow-through cells have check valves built into the unit. By design,
bladder pumps also have a check valve built into their construction.
A 1/4- or 3/8-inch ID barbed "T" or "Y" fitting, placed ahead
of the flow cell, may be used to establish the line which will receive
a needle valve for turbidity, general chemistry and analytical sample
collection. The "T" or "Y" fitting used should be
constructed of Teflon® or stainless steel and decontaminated
between each use, if used for analytical samples. The fitting may be
constructed of polyethylene and decontaminated between each use if it
is only used to sample for turbidity and general chemistry parameters.
If analytical samples are collected through the "T" or "Y"
fitting and needle valve, then those parts must be incorporated into
the field blank collection technique.
When collecting a sample at the port ahead of the flow cell, a flow
control valve (stainless-steel needle valve [preferred] or stainless
steel/Teflon ball valve [optional]) must be used to prevent backpressure
and air bubbles from forming in the tubing (see http://water.usgs.gov/owq/FieldManual/chap4_rpt.pdf,
page 84). The "needle valve" offers versatility as it can be used for
collection of turbidity, general chemistry and
analytical samples. It can be used with Teflon® tubing
and can be used to control sample flow rate because the design significantly
reduces any backpressure gradient. Like all other sampling equipment,
the "needle valve" must be decontaminated before use at any well.
- Calibration of Probes
| CALIBRATION OF THE PROBES USED TO MONITOR WATER QUALITY INDICATOR
PARAMETERS MUST TAKE PLACE IN THE FIELD PRIOR
TO THE DAY'S EVENTS. THE OFFICE OF QUALITY ASSURANCE MUST CERTIFY
THE PROBES USED FOR pH, DISSOLVED OXYGEN AND TEMPERATURE MEASUREMENT. |
There are no exceptions to these rules. Probe calibration is critical
to the accurate and precise measurement of WQIPs.
For warranty purposes, all manufacturers’ instructions
for proper care and calibration must be followed. Solutions for probe
calibration must be held to the temperature of the liquid (groundwater)
being measured as temperature correlation is critical in calculating
conductivity, dissolved oxygen and pH. Tables and equations to compensate
for the difference between ambient groundwater and calibration solution
temperature are sometimes provided in the operating manuals or with
the calibration solutions. Some instruments are designed with internal
features to compensate for this difference in temperature. The respective
difference between calibration of conductivity and specific conductivity
requires compensation for groundwater temperature at the time of calibration
vs. solution temperature adjusted to 25°C at the time of calibration.
For dissolved oxygen, the flow cell itself must be maintained at the
temperature of groundwater during calibration. All efforts made to account
for proper temperature control of solutions during calibration must
be reported to the end user. All steps must be recorded in the field
notes. No sampling shall commence until all instruments are calibrated
and operating properly. See the "Tips"
section below for further discussion on Temperature of Calibration Solutions.
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