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The Low-Down on Low Flow
By: Greg Giles and Jeff Story
Bureau of Ground Water Pollution Abatement
In recent years there has been much discussion of
alternative purging and sampling methods in the technical
literature, particularly where low flow rates are used. These
methods have been termed low-flow, low-stress, minimum-drawdown
and micropurging, among others. These terms
usually refer to the use of very low ground water extraction
rates (pumping rates below 1 liter/minute) during well
purging and sampling. Since this method is not addressed
in the Department's Field Sampling Procedures Manual (1),
this article has been written to discuss this procedure so that
people who write and review low-flow sampling plans will
be better able to assess when the use of this procedure is
History and Significance
Regulators and the regulated community have been at
odds over the issue of filtering water samples collected for
metals analysis. The regulated community has argued that
the incorporation of aquifer matrix material (i.e., silt &
clay) into ground water samples result in the generation of
ground water metals data that are biased high. Metals are
naturally occurring and exist in all subsurface geologic
material; the required acid preservation of water samples
may release metals previously bound to the surface of
aquifer material included in the water sample or dissolve
some aquifer material altogether (e.g., metal oxides &
hydroxides). Therefore, it is reasonable to assume that the
incorporation of aquifer matrix material into water samples,
which increases turbidity of the water samples, will result in
higher metals concentrations.
The cause(s) of turbidity in a given ground water
sample may be from any or all of the following:
Natural turbidity (i.e., mobile colloids) in the aquifer;
The well may have been improperly constructed.
Considering the nature of the aquifer material the well
is installed in, the screen slot size chosen for the well
may be too large or the sandpack placed in the annular
space around the well screen may be too coarse;
The well may have been improperly or inadequately
The well may have been pumped at too high a flow rate
during the purging or sampling of the well. Anytime
that a well is pumped at a rate that is higher than the
well's recharge rate, or higher than the rate at which the
well was previously developed at, sediment may be
stirred up within the well casing or released from the
aquifer into the well, resulting in the water becoming
The procedures used during well sampling may result
in increased well turbidity. The sounding of wells with
probes to determine total well depth may stir up
sediment that has accumulated at the bottom of the
well. The sediment will be pulled up the water column
as the probe is reeled up to the surface. The contact of
sampling equipment with the well casing or well screen
may release material into the well, and the quick
movement of sampling equipment within the well (e.g.,
bailers) may produce turbidity due to hydrostatic
stresses between the well and the surrounding aquifer.
A 0.45-micron filter is the industry standard to filter
water samples. Puls and Barcelona (2) concluded that the
use of a 0.45-micron filter was not useful, appropriate or
reproducible in providing information on metals mobility in
ground water systems, nor was it appropriate for the
determination of truly dissolved constituents in ground
water. The argument that well samples should not be
filtered for metals analyses is also supported by the
Department's Field Sampling Procedures Manual (1) which
states (on page 178) that the Department requires "metals
analysis to be performed on unfiltered ground water
samples pursuant to the requirements of the Safe Drinking
Water Act and the Clean Water Act."
With the requirement that water samples analyzed for
metals not be filtered, the technique of low-flow purging/
sampling appears to have originated as a means of reducing
turbidity in ground water samples. As stated in the April
1996 EPA Ground Water Issue (3), "Sampling-induced
turbidity problems can often be mitigated by using low-flow
purging and sampling techniques."
While the principal goal of the low-flow sampling
technique is the collection of representative ground water
samples, one of the major benefits of this technique is the
potential cost savings due to the generation of less purge
water requiring storage, transportation and disposal. The
Department has received an increasing number of requests
to allow low-flow sampling, and the potential reduction in
the amount of purge water generated appears to be largely
responsible for the increase in such requests.
The underlying principle of the low-flow technique is
that at low pumping rates (less than 1 liter/minute with 0.1
to 0.5 liter/minute being typical), the ground water flow in
the area of the sampling device intake approaches horizon-tal
linearity (i.e., the ground water that is being drawn into
the sampling device should be limited to the sandpack and
aquifer in the immediate area of the sampling device
intake). According to the EPA document (3) on page 5,
low-flow "refers to the velocity with which water enters the
pump intake and is imparted to the formation pore water in
the immediate vicinity of the well screen....The objective is
to pump at a rate that minimizes stress (drawdown) to the
Where there is no (or minimal) drawdown during
purging and sampling the well is recharged by the aquifer at
the same rate the well is being pumped. Under low-flow
sampling conditions, suspended solids (e.g., colloids) are considered to
be mobile in the aquifer, representative of natural conditions, and not
an artifact of sampling or well construction. For this reason,
analytical results for metals testing using low-flow sampling are
considered to be representative of the total mobile contaminant load in
the aquifer (4). Thus, sampling for both "total" and
"dissolved" metals is usually not needed.
Using the low-flow sampling technique, wells are
purged and sampled at flow rates at or below 1 liter/minute.
There is generally no required volume of water to purge
from the well before collecting the ground water sample.
Instead, the decision on when the ground water sample can
be collected is based on the stabilization of ground water-quality
parameters (e.g., temperature, pH, Eh, specific
conductance, dissolved oxygen (DO) and turbidity).
Accordingly, it is important that the stabilization parameters
be measured accurately, and that the purging be conducted
in such a manner that the sampling procedure does not
affect the field parameter values (e.g., some submersible
pumps generate a lot of heat when pumping at very low
rates; this may cause the temperature reading to drift);
sampling devices that operate under negative pressure (e.g.,
peristaltic pumps) cause degassing of the water which may
cause drift in the pH and dissolved oxygen readings.
During conventional purging and sampling the sample
can often represent an average of the entire screened
interval/bedrock borehole. In contrast, low-flow sampling
conceptually results in the collection of a sample drawn
from a discrete interval in the well. As such, the collection
of a ground water sample using low-flow techniques may be
considered somewhat analogous to collecting discrete
ground water samples using equipment such as "Geoprobe"
and "Hydropunch" samplers and temporary well points
which typically have short intakes.
With respect to metals analysis, low-flow samples are
typically considered to be representative of the total
mobile contaminant load (i.e., dissolved and colloid-associated).
This reduces the need for sample filtration. Samples collected
using low-flow methods usually contain less turbidity.
Using low-flow procedures, the volume of water
purged from the well may be significantly reduced.
Costs associated with the storage, transport and
disposal of the purge water may be reduced and the
amount of time needed to purge the well may be
Because the same equipment is used for well purging
and well sampling, less equipment may need to be used
in the field.
Because the low-flow technique generates water
samples obtained from very discrete zones, if the
contaminant distribution in the section of the aquifer
screened by the well is heterogeneous, the sample
collected by low-flow procedures may show signifi-cantly
higher contaminant concentrations than samples
collected at higher flow rates (i.e., using traditional
The reduction in the amount of fine-grained material
flowing into the well can increase well life and reduce
the need for well re-development.
Not all sampling equipment can be used for low-flow
sampling. Pumps used for low-flow sampling should be
variable in speed and designed to operate at very low
pumping rates. Pumps should preferably operate under
Because the measurement of DO and Eh must be made
before the ground water comes in contact with the
atmosphere, a flow-through cell must be used to
measure these parameters in the field.
The zone sampled within the well by low-flow methods
is conceptually limited. If the contaminant distribution
in the screened section of the aquifer is heterogeneous,
which may be the case in most wells, the sample results
obtained by low-flow sampling may be significantly
biased low if the sampling device intake is not placed
at the same depth as that of the highest contaminant
concentration entering the well. Accordingly, for wells
contaminated with DNAPL or LNAPL type contamina-tion,
the sampling device intake depth could signifi-cantly
affect the sampling results. For wells con-structed
with long screens, vertical flow gradients
within the well may cause mixing of ground water and
the samples will not be depth-discrete.
The method requires higher initial capital costs and
longer set-up time in the field.
EPA's issue paper (3) provides a very thorough
evaluation of the method and its advantages and disadvan-
tages. This paper is considered recommended reading by
anyone involved in low-flow sampling.
At this time the Department does not have a formalized
low-flow sampling procedure/policy in place. However, the
Department is currently working to develop a formalized
low-flow sampling guidance document. Currently, low-flow
sampling can be approved only on a case-by-case basis until
the Department develops appropriate guidance. A low-flow
sampling plan must be provided to the Department for
review and approval. Low-flow sampling plans should be
as detailed as possible and take into consideration well
construction, contaminant type and distribution in the
aquifer, and local hydrogeology.
NJDEP. 1992. Field sampling procedures manual
Puls, R.W. and M.J. Barcelona. 1989. Ground water
sampling for metals analyses; Superfund ground water
issue. EPA/540/4-89/001. Ada, Oklahoma, R.S. Kerr Lab.
Puls, R.W. and M.J. Barcelona. April 1996. Ground
water issue. Low-flow (minimal drawdown) ground
water sampling procedures. EPA/540/S-95/504. Ada,
Oklahoma, R.S. Kerr Lab.
The Nielsen Environmental Field School. Micropurge
low-flow purging and sampling, Edison, New Jersey,
May 22, 1997.