David Kaminski – QED Environmental Systems,
Inc.
Ann Arbor, Michigan
Obtaining samples from wells set in low hydraulic conductivity formations
is a challenge. The limiting factor, of course, is the well itself or more
correctly, the low conductivity formation being sampled - no matter how
water is extracted, it will be altered from its natural state. The
traditional approach is to evacuate the well by bailing or pumping and
then sample upon sufficient recovery (typically the following day), since
these wells will not readily yield multiple "volumes" of the well even
when pumped at very low flow rates. This evacuation approach poses several
concerns:
The time required for sufficient recovery of the well may be excessive,
affecting sample chemistry through prolonged exposure to the atmosphere.
In some cases, the well may not recover sufficiently to produce the sample
volume required within a reasonable time period.
Purging below top of screen may cause "jetting" or cascading in well
screen as the well recovers, resulting in a change in dissolved gasses and
redox state and ultimately affecting the concentration of the analytes of
interest through the oxidation of dissolved metals and possible loss of
VOCs.
Draining water from the sand pack surrounding the screen can result in air
being trapped in the pore spaces, with lingering effects on dissolved gas
levels and redox state.
·Increased sample turbidity can result from the stress on the formation
and stirring up of any settled solids in the bottom of the well.
Continued dewatering over time can lead to well screen encrustation and
clogging due to precipitation and mineral deposits.
An alternative to the traditional evacuation approach is to use "passive"
sampling techniques (or more correctly, sampling without purging) to avoid
the pitfalls of well evacuation to obtain a better and probably more
representative sample. Sampling the water present in the screen zone
provides the greatest chance of obtaining samples without increased
turbidity and with minimal alteration of the sample chemistry. Although
the low movement rate of the ground water in the screen provides only a
limited exchange with the surrounding formation, avoiding the alteration
caused by the factors mentioned above is really the best alternative.
The passive sampling approach requires the removal of the smallest
possible volume of water prior to sampling, generally limited to the
volume of the sampling system. Since the volume of water stored in the
screen zone is often very small in low-yield wells, it is important to
minimize the water displaced by the sampling pump and tubing to ensure
adequate water for sample collection. Using very small diameter tubing and
the smallest possible pump chamber (bladder) volume minimizes the sampling
system volume and the water displaced by the equipment. After purging the
sampling system volume, samples are taken from the subsequent water
pumped. Since the sample volume required by the laboratory can often
exceed several liters of water for a suite of samples, it is advisable to
work with them in advance to reduce this volume to the minimum needed to
reduce drawdown and avoid running out of water.
The pumping rates used for passive sampling are much lower than for
low-flow/low-stress purging, generally 100 ml/minute or less. Drawdown is
expected, since it cannot be avoided; however, it is still advisable to
pump at the lowest possible rate to limit drawdown to the minimum
possible. As with low-flow/low-stress techniques, the water level in the
well should not be lowered below the top of the screen if possible. Where
this is unavoidable, it is still preferable to sample in this manner
rather than evacuate the well. Also, since passive sampling requires the
minimum possible disturbance to the water column and surrounding
formation, dedicated sampling systems are required for this approach.
Portable pumps and bailers will agitate and mix the water column,
increasing turbidity and affecting sample chemistry.
Monitoring indicator parameters for stability is not part of this
approach, since the intention is not to purge until stabilization of these
measurements. However, pumping through a flow cell is still the best way
to get accurate field measurements prior to sampling. Where the total
volume of water in the well is very small, field measurements can be
accomplished using a very small volume flow cell (50-200 ml), or grab
sampling and measurement can be used with the recognition that some
parameters (such as dissolved oxygen) may be affected by exposure to
atmospheric conditions.
References on passive sampling:
Powell, Robert M. and Robert W. Puls, 1997. "Hitting the Bull’s-Eye in
Groundwater Sampling." Pollution Engineering, June 1997, pp. 50-54.
Powell, Robert M. and Robert W. Puls, 1993. "Passive Sampling of
Groundwater Monitoring Wells Without Purging." Multilevel well chemistry
and tracer disappearance." Journal of Contaminant Hydrology. No. 12, 1993,
pp. 51-77.
Puls, R.W. and M.J. Barcelona, 1996. "Low-Flow (Minimal Drawdown)
Ground-water Sampling Procedures." U.S. EPA, Ground Water Issue,
Publication Number EPA/540/S-95/504, April 1996.
Sevee, John E., C.A. White and D.J. Maher, 2000. "An Analysis of
Low-Flow Ground Water Sampling Methodology." Ground Water Monitoring and
Remediation, Spring 2000, pp. 87-93.
Shanklin, D.E., W.C. Sidle, and M.E. Ferguson, 1995. "Micro-Purge
Low-Flow Sampling of Uranium-Contaminated Ground Water at the Fernald
Environmental Management Project", Ground Water Monitoring and
Remediation, Summer 1995, pp.168-176. |
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