United States
Environmental Protection
Office of
Research and
Office of Solid Waste
and Emergency
EPA/540/S-95/504 ,
April 1996
&EPA      Ground  Water   Issue
                        LOW-FLOW (MINIMAL DRAWDOWN)
                       by Robert W. Puls1 and Michael J. Barcelona2

       The Regional Superfund Ground Water Forum is a
group of ground-water scientists, representing EPA's
Regional Superfund Offices, organized to exchange
information related to ground-water remediation at Superfund
sites. One of the major concerns of the Forum is the
sampling of ground water to support site assessment and
remedial performance monitoring objectives. This paper is
intended to provide background information on the
development of low-flow sampling procedures and its
application under a variety of hydrogeologic settings. It is
hoped that the paper will support the production of standard
operating procedures for use by EPA Regional personnel and
other environmental professionals engaged in ground-water

For further information contact: Robert Puls, 405-436-8543,
Subsurface Remediation and Protection Division, NRMRL,
Ada, Oklahoma.
I. Introduction

       The methods and objectives of ground-water
sampling to assess water quality have evolved over time.
Initially the emphasis was on the assessment of water quality
of aquifers as sources of drinking water. Large water-bearing
units were identified and sampled in keeping with that
objective.  These were highly productive aquifers that
supplied drinking water via private wells or through public
water supply systems. Gradually, with the increasing aware-
ness of subsurface pollution of these water resources, the
understanding of complex hydrogeochemical processes
which govern the fate and transport of contaminants in the
subsurface increased. This increase in understanding was
also due to advances in a number of scientific disciplines and
improvements in tools used for site characterization and
ground-water sampling. Ground-water quality investigations
where pollution was detected initially borrowed ideas,
methods, and materials for site characterization from the
water supply field and water analysis from public health
practices.  This included the materials and manner in which
monitoring wells were installed and the way in which water
was brought to the surface, treated, preserved and analyzed.
The prevailing conceptual ideas included convenient generali-
zations of  ground-water resources in terms of large and
relatively homogeneous hydrologic units.  With time it became
apparent that conventional water supply generalizations of
homogeneity did not adequately represent field data regard-
ing pollution of these subsurface resources. The important
role of heterogeneity became increasingly clear not only in
geologic terms, but also in terms of complex physical,

'National Risk Management Research Laboratory, U.S. EPA
'University of Michigan
                       Superfund Technology Support Center for
                       Ground Water
                       National Risk Management Research Laboratory
                       Subsurface Protection and Remediation Division
                       Robert S. Kerr Environmental Research Center
                       Ada, Oklahoma
                     Response, US EPA, Washington, DC

                     Walter W. Kovattck, Jr., Ph.D.
                                                                                Printed on Recycled Paper

 chemical and biological subsurface processes. With greater
 appreciation of the role of heterogeneity, it became evident
 that subsurface pollution was ubiquitous and encompassed
 the unsaturated zone to the deep subsurface and included
 unconsolidated sediments, fractured rock, and aquitards or
 low-yielding or impermeable formations. Small-scale pro-
 cesses and heterogeneities were shown to be important in
 identifying contaminant distributions and in controlling water
 and contaminant flow paths.

         It is beyond the scope of this paper to summarize all
 the advances in the field of ground-water quality investiga-
 tions and remediation, but two particular issues have bearing
 on ground-water sampling today: aquifer heterogeneity and
 colloidal transport. Aquifer heterogeneities affect contaminant
 flow paths and include variations in geology, geochemistry,
 hydrology and microbiology.  As methods and the tools
 available for subsurface investigations have become increas-
 ingly sophisticated and understanding of the subsurface
 environment has advanced, there is an awareness that in
 most cases a primary concern for site investigations is
 characterization of contaminant flow paths rather than entire
 aquifers. In fact, in many cases, plume thickness can be less
 than well screen  lengths (e.g., 3-6 m) typically installed at
 hazardous waste sites to detect and monitor plume movement
 over time. Small-scale differences have increasingly been
 shown to be important and there is a general trend toward
 smaller diameter wells and shorter screens.

        The hydrogeochemical significance of colloidal-size
 particles in subsurface systems has been realized during the
 past several years (Gschwend and Reynolds, 1987; McCarthy
 and Zachara, 1989; Puls, 1990; Ryan and Gschwend, 1990).
 This realization resulted from both field and laboratory studies
 that showed faster contaminant migration over greater
 distances and at  higher concentrations than flow and trans-
 port model predictions would suggest (Buddemeier and Hunt,
 1988; Enfield and Bengtsson, 1988; Penrose et al., 1990).
 Such models typically account for interaction between the
 mobile aqueous and immobile solid phases, but do not allow
for a mobile, reactive solid phase. It is recognition of this third
phase as a possible means of contaminant transport that has
brought increasing attention to the manner in which samples
are collected and processed for analysis (Puls et al., 1990;
 McCarthy and Degueldre, 1993; Backhus  et al., 1993;  U. S.
 EPA, 1995). If such a phase is present in sufficient mass,
 possesses high sorption reactivity, large surface area, and
 remains stable in suspension, it can serve as an  important
 mechanism to facilitate contaminant transport in many types
 of subsurface systems.

        Colloids are particles that are sufficiently small so
that the surface free energy of the particle dominates the bulk
free energy. Typically, in ground water, this includes particles
with diameters between 1 and 1000 nm. The most commonly
observed mobile  particles include: secondary clay minerals;
hydrous iron, aluminum, and manganese oxides; dissolved
and particulate organic materials, and viruses and bacteria.
These reactive particles have been shown to be mobile under
a variety of conditions in both field studies and laboratory
column experiments, and as such need to be included in
monitoring programs where identification of the total mobile
contaminant loading  (dissolved + naturally suspended
particles) at a site is an objective. To that end, sampling
methodologies must be used which do not artificially bias
naturally suspended  particle concentrations.

         Currently the most common ground-water purging
and sampling methodology is to purge a well using bailers or
high speed pumps to remove 3 to 5 casing volumes followed
by sample collection. This method can cause adverse impacts
on sample quality through collection of samples with high
levels of turbidity. This results in the inclusion of otherwise
immobile artifactual particles which produce  an overestima-
tion of certain analytes of interest (e.g., metals or hydrophobic
organic compounds).  Numerous documented problems
associated with filtration (Danielsson,  1982; Laxen and
Chandler, 1982; Horowitz et al.,  1992) make this an undesir-
able method of rectifying the turbidity  problem, and include
the removal of potentially mobile (contaminant-associated)
particles during filtration, thus artificially biasing contaminant
concentrations low.  Sampling-induced turbidity problems can
often be mitigated by using low-flow purging  and sampling

        Current subsurface conceptual models have under-
gone considerable refinement due to the recent development
and increased use of field screening tools. So-called
hydraulic push technologies (e.g., cone penetrometer,
Geoprobe®, QED HydroPunch®) enable relatively fast
screening site characterization which can then be used to
design and install a monitoring well  network.   Indeed,
alternatives to conventional monitoring wells are now being
considered for some hydrogeologic settings.  The ultimate
design of any monitoring system should however be based
upon adequate site characterization and  be consistent with
established monitoring objectives.

        If the sampling program objectives include accurate
assessment of the magnitude and extent of subsurface
contamination over time and/or accurate  assessment of
subsequent remedial  performance, then some information
regarding plume delineation in three-dimensional space is
necessary prior to monitoring well network design and
installation. This can be accomplished with a variety of
different tools and equipment ranging  from hand-operated
augers to screening tools mentioned above and large drilling
rigs. Detailed information on ground-water flow velocity,
direction, and horizontal and  vertical variability are essential
baseline data requirements.  Detailed  soil and geologic data
are required prior to and during the installation of sampling
points.  This includes historical as well as detailed soil and
geologic logs which accumulate during the site investigation.
The use of borehole geophysical techniques  is also recom-
mended. With this information (together with  other site
characterization data) and a clear understanding of sampling

 objectives, then appropriate location, screen length, well
 diameter, slot size, etc. for the monitoring well network can be
 decided. This is especially critical for new in situ remedial
 approaches or natural attenuation assessments at hazardous
 waste sites.

         In general, the overall goal of any ground-water
 sampling program is to collect water samples with no alter-
 ation in water chemistry; analytical data thus obtained may be
 used for a variety of specific monitoring programs depending
 on the regulatory requirements.  The sampling methodology
 described in this paper assumes that the  monitoring goal is to
 sample monitoring wells for the presence of contaminants and
 it is applicable whether mobile colloids are a concern or not
 and whether the analytes of concern are metals (and metal-
 loids) or organic compounds.
 II.  Monitoring Objectives and Design

         The following issues are important to consider prior
 to the design and implementation of any ground-water
 monitoring program, including those which anticipate using
 low-flow purging and sampling procedures.

 A. Data Quality Objectives (DQOs)

         Monitoring objectives include four main types:
 detection, assessment, corrective-action evaluation and
 resource evaluation, along with hybrid variations such as site-
 assessments for property transfers and water availability
 investigations.  Monitoring objectives may change as contami-
 nation or water quality problems are discovered.  However,
 there are a number of common components of monitoring
 programs which should be recognized as important regard-
 less of initial objectives. These components include:

     1) Development of a conceptual model that incorporates
       elements of the regional geology to the local geologic
       framework. The conceptual model development also
       includes initial site characterization efforts to identify
       hydrostratigraphic units and likely flow-paths using a
       minimum number of borings and well completions;

    2) Cost-effective and well documented collection of high
      quality data utilizing simple, accurate, and reproduc-
       ible techniques; and

    3) Refinement of the conceptual model based on
      supplementary data collection and analysis.

These fundamental components serve many types of monitor-
ing programs and provide a basis for future efforts that evolve
in complexity and level of spatial detail as purposes and
objectives expand. High quality, reproducible data collection
is a common goal regardless of program objectives.
         High quality data collection implies data of sufficient
 accuracy,  precision, and completeness (i.e., ratio of valid
 analytical  results to the minimum sample number called for by
 the program design) to meet the program objectives. Accu-
 racy depends on the correct choice of monitoring tools  and
 procedures to minimize sample and subsurface disturbance
 from collection to analysis.  Precision depends on the
 repeatability of sampling and analytical protocols. It can be
 assured or improved by replication of sample analyses
 including blanks, field/lab standards and reference standards.
 B. Sample Representativeness

         An important goal of any monitoring program is
 collection of data that is truly representative of conditions at
 the site. The term representativeness applies to chemical and
 hydrogeologic data collected via wells,  borings, piezometers,
 geophysical and soil gas measurements, lysimeters, and
 temporary sampling points. It involves a recognition of the
 statistical variability of individual subsurface physical proper-
 ties, and contaminant or major ion concentration levels, while
 explaining extreme values. Subsurface temporal and spatial
 variability are facts. Good professional practice seeks to
 maximize representativeness by using proven accurate and
 reproducible techniques to define limits on the distribution of
 measurements collected at a site. However, measures of
 representativeness are dynamic and are controlled by
 evolving site characterization and monitoring objectives. An
 evolutionary site characterization model, as shown in  Fig-
 ure 1, provides a systematic approach  to the goal of consis-
 tent data collection.
         I"  ~~ H^ Define Program Objectives

                Establish Data Quality

     —  —   ]>• Define Sampling and
Evolutionary Site     Analytical Protocols

                      Apply Protocols
              __ __   Refine Protocols «_  _ —> Make Site Decisions
Figure 1.  Evolutionary Site Characterization Model
The model emphasizes a recognition of the causes of the
variability (e.g., use of inappropriate technology such as using
bailers to purge wells; imprecise or operator-dependent
methods) and the need to control avoidable errors.

1) Questions of Scale

        A sampling plan designed to collect representative
samples must take into account the potential scale of
changes in site conditions through space and time as well as
the chemical associations and behavior of the parameters
that are targeted for investigation. In subsurface systems,
physical (i.e., aquifer) and chemical properties over time or
space are not statistically independent. In fact, samples
taken in close proximity (i.e., within distances of a few meters)
or within short time periods (i.e., more frequently than
monthly) are highly auto-correlated.  This means that designs
employing high-sampling frequency (e.g., monthly) or dense
spatial monitoring designs run the risk of redundant data
collection and misleading inferences regarding trends in
values that aren't statistically valid. In practice, contaminant
detection and assessment monitoring programs rarely suffer
these over-sampling concerns. In corrective-action evaluation
programs, it is also possible that too  little data may be
collected over space or time. In these cases, false interpreta-
tion of the spatial extent of contamination or underestimation
of temporal concentration variability may result.

2) Target Parameters

        Parameter selection in monitoring program design is
most often dictated by the regulatory status of the site.
However, background water quality constituents, purging
indicator parameters, and contaminants, all represent targets
for data collection programs.  The tools and procedures used
in these programs should be equally rigorous and applicable
to all categories of data, since all  may be  needed to deter-
mine or support regulatory action.

C. Sampling Point Design and  Construction

        Detailed site characterization is central to all
decision-making  purposes and the basis for this characteriza-
tion resides in identification of the geologic framework and
major hydro-stratigraphic units. Fundamental data for sample
point location include:  subsurface lithology, head-differences
and background  geochemical conditions.  Each sampling point
has a proper use or uses which should be documented at a
level which is appropriate for the program's data quality
objectives. Individual sampling points may not always be
able to fulfill multiple monitoring objectives (e.g., detection,
assessment, corrective action).

1) Compatibility with Monitoring Program and Data
    Quality Objectives

        Specifics of sampling point location and design will
be dictated by the complexity of subsurface lithology and
variability in contaminant and/or geochemical  conditions. It
should be noted that, regardless of the ground-water sam-
pling approach, few sampling points  (e.g., wells, drive-points,
screened augers) have zones of influence in excess of a few
feet. Therefore, the spatial frequency of sampling points
should be carefully selected and designed.

2)  Flexibility of Sampling Point Design

        In most cases well-point diameters in excess of 1 7/8
inches will permit the use of most types of submersible
pumping devices for low-flow  (minimal drawdown) sampling.
It is suggested that short (e.g., less than 1.6 m) screens be
incorporated into the monitoring design where possible so
that comparable results from one device to another might be
expected.  Short, of course, is relative to the degree of vertical
water quality variability expected at a site.

3)  Equilibration of Sampling Point

        Time should be allowed for equilibration of the well
or sampling point with the formation after installation.  Place-
ment of well or sampling points in the subsurface produces
some disturbance of ambient conditions. Drilling techniques
(e.g., auger, rotary, etc.) are generally considered to cause
more disturbance than direct-push technologies. In either
case, there may be a period (i.e., days to months) during
which water quality near the point may be distinctly different
from that in the formation. Proper development of the sam-
pling point and adjacent formation to remove fines created
during emplacement will shorten this water quality recovery
ill.  Definition of Low-Flow Purging and Sampling

        It is generally accepted that water in the well casing
is non-representative of the formation water and needs to be
purged prior to collection of ground-water samples.  However,
the water in the screened interval may indeed be representa-
tive of the formation, depending upon well construction and
site hydrogeology.  Wells are purged to some extent for the
following reasons: the presence of the air interface at the top
of the water column resulting in an oxygen concentration
gradient with depth, loss of volatiles up the water column,
leaching from or sorption to the casing or filter pack, chemical
changes due to clay seals or backfill, and surface infiltration.

        Low-flow purging, whether using portable or dedi-
cated systems, should be done using pump-intake located in
the middle or slightly above the middle of the  screened
interval. Placement of the pump too close to the bottom  of the
well will cause increased entrainment of solids which have
collected in the well over time.  These particles are present as
a result of well development, prior purging and sampling
events, and natural colloidal transport and deposition.
Therefore, placement of the pump in the middle or toward the
top of the screened interval is suggested.  Placement of the
pump at the top of the water column for sampling is only
recommended in unconfined aquifers, screened across the
water table, where this is the desired sampling point. Low-

flow purging has the advantage of minimizing mixing between
the overlying stagnant casing water and water within the
screened interval.

A.  Low-Flow Purging and Sampling

        Low-flow refers to the velocity with which water
enters the pump intake and that is imparted to the formation
pore water in the immediate vicinity of the well screen. It
does not necessarily refer to the flow rate of water discharged
at the surface which can be affected by flow regulators or
restrictions.  Water level drawdown provides the best indica-
tion of the stress imparted by a given flow-rate for a given
hydrological situation.  The objective is to pump in a manner
that minimizes stress (drawdown) to the system to the extent
practical taking into account established site sampling
objectives. Typically, flow rates on the order of 0.1 - 0.5 L/min
are used, however this is dependent on site-specific
hydrogeology.   Some extremely coarse-textured formations
have been successfully sampled in this manner at flow rates
to 1 L/min. The effectiveness  of using low-flow purging  is
intimately linked with proper screen location, screen  length,
and well construction and development techniques.  The
reestablishment of natural flow paths in both the vertical and
horizontal  directions is important for correct interpretation of
the data. For high resolution sampling needs, screens less
than 1 m should be used. Most of the need for purging has
been found to be due to passing the sampling device through
the overlying casing water which causes mixing of these
stagnant waters and the dynamic waters within the screened
interval. Additionally, there is  disturbance to suspended
sediment collected in the bottom of the casing and the
displacement of water out into the formation immediately
adjacent to the well screen.  These disturbances and impacts
can be avoided using dedicated sampling equipment, which
precludes  the need to  insert the sampling device prior to
purging  and  sampling.

        Isolation of the screened interval water from the
overlying stagnant casing water  may be accomplished using
low-flow minimal drawdown techniques. If the pump intake is
located within the screened interval, most of the water
pumped will  be drawn  in directly from the formation with little
mixing of casing water or disturbance to the sampling zone.
However, if the wells are not constructed and  developed
properly, zones other than those intended may be sampled.
At some sites where geologic  heterogeneities are sufficiently
different within the  screened interval, higher conductivity
zones may be preferentially sampled. This is another reason
to use shorter screened intervals, especially where high
spatial resolution is a sampling objective.

B.  Water Quality Indicator Parameters

        It is recommended that water quality  indicator
parameters be used to determine  purging needs prior to
sample collection in each well.  Stabilization of parameters
such as pH,  specific conductance, dissolved oxygen, oxida-
tion-reduction potential, temperature and turbidity should be
used to determine when formation water is accessed during
purging.  In general, the order of stabilization is pH, tempera-
ture, and specific conductance, followed by oxidation-
reduction potential, dissolved oxygen and turbidity. Tempera-
ture and pH, while commonly used as purging indicators, are
actually quite insensitive in distinguishing between formation
water and stagnant casing water; nevertheless, these are
important parameters for data interpretation purposes and
should also be measured.  Performance criteria for determi-
nation of stabilization should be based on water-level draw-
down, pumping rate and equipment specifications for measur-
ing indicator parameters.  Instruments are available which
utilize in-line flow cells to continuously measure the above

        It is important to establish specific well stabilization
criteria and then consistently follow the same methods
thereafter, particularly with respect to drawdown,  flow rate
and sampling device. Generally, the time or purge volume
required for parameter stabilization is independent of well
depth or well volumes.  Dependent variables are  well diam-
eter, sampling device, hydrogeochemistry, pump  flow rate,
and whether the devices are used in a portable or dedicated
manner. If the sampling device is already in place (i.e.,
dedicated sampling systems),  then the time and purge
volume needed for stabilization is much  shorter. Other
advantages of dedicated equipment include less  purge water
for waste disposal, much less  decontamination of equipment,
less time spent in preparation of sampling as well as time in
the field, and  more consistency in the sampling approach
which probably will translate into less variability in sampling
results. The use of dedicated  equipment is strongly recom-
mended at wells which will undergo routine sampling  over

        If parameter stabilization criteria are too  stringent,
then minor oscillations in indicator parameters may cause
purging operations to become unnecessarily protracted. It
should also be noted that turbidity is a very conservative
parameter in terms of stabilization. Turbidity is always the
last parameter to stabilize. Excessive purge times are
invariably related to the establishment of too stringent turbidity
stabilization criteria.  It should  be noted that natural turbidity
levels in ground water may exceed 10 nephelometric turbidity
units (NTU).

C. Advantages and Disadvantages of Low-Flow
    (Minimum Drawdown) Purging
         In general, the advantages of low-flow purging
      samples which are representative of the mobile load of
      contaminants present (dissolved and colloid-associ-
      minimal disturbance of the sampling point thereby
      minimizing sampling artifacts;
      less operator variability, greater operator control;

     •  reduced stress on the formation (minimal drawdown);
     •  less mixing of stagnant casing water with formation
     •  reduced need for filtration and, therefore, less time
       required for sampling;
     •  smaller purging volume which decreases waste
       disposal costs and sampling time;
     •  better sample consistency; reduced artificial sample

Some disadvantages of low-flow purging are:
     •  higher initial capital costs,
     •  greater set-up time in the field,
     •  need to transport additional equipment to and from the
     •  increased training needs,
     •  resistance to change on the part of sampling practitio-
     •  concern that new data will indicate a change in
       conditions and trigger an action.
IV. Low-Flow (Minimal Drawdown) Sampling

        The following ground-water sampling procedure has
evolved over many years of experience in ground-water
sampling for organic and inorganic compound determinations
and as such summarizes the authors' (and others) experi-
ences to date (Barcelona et al., 1984, 1994; Barcelona and
Helfrich, 1986; Puls and Barcelona, 1989; Puls et. al. 1990,
1992; Puls and Powell, 1992; Puls and Paul, 1995). High-
quality chemical data collection is essential in ground-water
monitoring and site characterization. The primary limitations
to the collection of representative ground-water samples
include: mixing of the stagnant casing and fresh screen
waters during insertion of the sampling device or ground-
water level measurement device; disturbance and
resuspension of settled solids at the bottom of the well when
using high pumping rates or raising and lowering a pump or
bailer; introduction of atmospheric gases or degassing from
the water during sample handling  and transfer, or inappropri-
ate use of vacuum sampling device, etc.

A. Sampling Recommendations

        Water samples should not be taken immediately
following well development. Sufficient time should be allowed
for the ground-water flow regime in the vicinity of the monitor-
ing well to stabilize and to approach chemical equilibrium with
the well construction materials.  This lag time will depend on
site conditions and methods of installation but often  exceeds
one week.

        Well purging is nearly always necessary to obtain
samples of water flowing through the geologic formations in
the screened interval.  Rather than using a general but
arbitrary guideline of purging three casing volumes prior to
 sampling, it is recommended that an in-line water quality
 measurement device (e.g., flow-through cell) be used to
 establish the stabilization time for several parameters (e.g. ,
 pH, specific conductance, redox, dissolved oxygen, turbidity)
 on a well-specific basis. Data on pumping  rate, drawdown,
 and volume required for parameter stabilization can be used
 as a guide for conducting subsequent sampling activities.

        The following are recommendations to be considered
 before, during and after sampling:
    •  use low-flow rates (<0.5 L/min), during both purging
       and sampling to maintain minimal drawdown in the
    •  maximize tubing wall thickness, minimize tubing
    •  place  the sampling device intake at the desired
       sampling point;
    •  minimize disturbances of the stagnant water column
       above the screened interval during  water level
       measurement and sampling device insertion;
    •  make  proper adjustments to stabilize the flow rate as
       soon as possible;
    •  monitor water quality indicators during purging;
    •  collect unfiltered samples to estimate contaminant
       loading and transport  potential in the subsurface

 B. Equipment Calibration

        Prior to sampling, all sampling device and monitoring
 equipment should be calibrated according to manufacturer's
 recommendations and the site Quality Assurance Project Plan
 (QAPP) and Field Sampling Plan (FSP). Calibration of pH
 should be performed with at least two buffers which bracket
 the expected  range. Dissolved oxygen calibration must be
 corrected for  local barometric pressure readings and eleva-

 C. Water Level Measurement and Monitoring

        It is recommended that a device be used which will
 least disturb the water surface in the casing. Well depth
 should be obtained  from the well logs. Measuring to the
 bottom of the well casing will  only cause resuspension of
 settled solids  from the formation and  require longer purging
times for turbidity equilibration. Measure well depth after
 sampling is completed. The water level measurement  should
 be taken from a permanent reference point which is surveyed
 relative to ground elevation.

 D. Pump Type

        The  use of low-flow (e.g., 0.1-0.5 L/min) pumps is
suggested for purging and sampling all types of analytes. All
pumps have some limitation and these should be investigated
with respect to application at a particular site.  Bailers  are
 inappropriate  devices for low-flow sampling.

1) General Considerations
F. Filtration
        There are no unusual requirements for ground-water
sampling devices when using low-flow, minimal drawdown
techniques.  The major concern is that the device give
consistent results and minimal disturbance of the sample
across a range of tow flow rates (i.e., < 0.5 L/min).  Clearly,
pumping rates that cause minimal to no drawdown in one  well
could easily cause  significant drawdown in another well
finished in a less transmissive formation.  In this sense, the
pump should not cause undue pressure or temperature
changes or physical disturbance on the water sample over a
reasonable sampling range. Consistency  in operation is
critical to meet accuracy and precision goals.

2) Advantages and Disadvantages of Sampling Devices

        A variety of sampling devices are available for low-
flow (minimal drawdown) purging and sampling and include
peristaltic pumps, bladder pumps, electrical submersible
pumps, and gas-driven pumps. Devices which  lend them-
selves to both dedication and consistent operation at defin-
able low-flow rates are preferred. It is desirable that the pump
be easily adjustable and operate  reliably at these lower flow
rates. The peristaltic pump is limited to shallow applications
and can cause degassing resulting in alteration of pH,
alkalinity, and some volatiles loss. Gas-driven pumps should
be of a type that does not allow the gas to be in direct  contact
with the sampled fluid.

        Clearly, bailers  and other grab type samplers are ill-
suited for low-flow sampling since they will cause repeated
disturbance and mixing of stagnant water in the casing and
the dynamic water  in the screened interval. Similarly, the use
of inertial lift foot-valve type samplers may cause too much
disturbance at the point of sampling. Use  of these devices
also tends to introduce uncontrolled and unacceptable
operator variability.

        Summaries of advantages and disadvantages of
various sampling devices are listed in Herzog et al. (1991),
U. S. EPA (1992), Parker (1994) and Thurnblad (1994).

£.  Pump Installation

        Dedicated sampling devices (left  in the well) capable
of pumping and sampling are preferred over any other type of
device. Any portable sampling device should be slowly and
carefully lowered to the middle of the screened interval or
slightly above the middle (e.g., 1-1.5 m below the top of a 3  m
screen). This is to minimize excessive mixing  of the stagnant
water in the casing above the screen with  the screened
interval zone water, and to minimize resuspension of solids
which will have  collected at the bottom of the well.  These two
disturbance effects have been shown to directly affect the
time required for purging. There  also appears to be a direct
correlation between size of portable sampling devices relative
to the well bore and resulting purge volumes and times. The
key is to minimize disturbance of water and solids in the well
        Decisions to filter samples should be dictated by
sampling objectives rather than as a fix for poor sampling
practices, and field-filtering of certain constituents should not
be the default. Consideration should be given as to what the
application of field-filtration is trying to accomplish. For
assessment of truly dissolved (as opposed to operationally
dissolved [i.e., samples filtered with  0.45 urn filters]) concen-
trations of major ions and trace metals, 0.1 urn filters are
recommended although 0.45 urn filters are normally used for
most regulatory programs. Alkalinity samples must also be
filtered if significant paniculate calcium carbonate is sus-
pected, since this material is likely to impact alkalinity titration
results (although filtration itself may alter the CO2 composition
of the sample and, therefore, affect the results).

        Although filtration may be appropriate, filtration of a
sample may cause a number of unintended changes to occur
(e.g. oxidation, aeration) possibly leading to filtration-induced
artifacts during sample analysis and uncertainty in the results.
Some of these unintended changes  may be unavoidable but
the factors leading to them must be recognized.  Deleterious
effects can be minimized by consistent application of certain
filtration guidelines. Guidelines should address selection of
filter type, media, pore size, etc. in order to identify and
minimize potential sources of uncertainty when filtering

        In-line filtration is recommended because it provides
better consistency through less sample handling, and
minimizes sample exposure to the atmosphere.  In-line filters
are available in both disposable  (barrel filters) and non-
disposable  (in-line filter holder, flat membrane filters) formats
and various filter pore sizes (0.1-5.0 urn). Disposable filter
cartridges have the advantage of greater sediment handling
capacity when compared to traditional membrane filters.
Filters must be pre-rinsed following manufacturer's recom-
mendations.  If there are no recommendations for rinsing,
pass through a minimum of  1 L of ground water following
purging and prior to sampling. Once filtration has begun, a
filter cake may develop as particles larger than the pore size
accumulate on the filter membrane.  The result is that the
effective pore diameter of the membrane is reduced and
particles smaller than the stated pore size are excluded from
the filtrate.  Possible  corrective measures include prefiltering
(with larger pore size filters), minimizing particle loads to
begin with, and reducing sample volume.

G.  Monitoring of Water Level and Water Quality
    Indicator Parameters

        Check water level periodically to  monitor drawdown
in the well as a guide to flow rate adjustment. The goal is
minimal drawdown (<0.1 m) during purging.  This goal may be
difficult to achieve under some circumstances due to geologic
heterogeneities within the screened  interval, and may  require
adjustment based on site-specific conditions and personal
experience. In-line water quality indicator parameters should
be continuously monitored during purging.  The water  quality

indicator parameters monitored can include pH, redox
potential, conductivity, dissolved oxygen (DO) and turbidity.
The last three parameters are often most sensitive. Pumping
rate, drawdown, and the time or volume required to obtain
stabilization of parameter readings can be used as a future
guide to purge the well. Measurements should be taken
every three to five minutes if the above suggested rates are
used. Stabilization is achieved after all parameters have
stabilized for three successive readings. In lieu of measuring
all five parameters, a minimum  subset would include pH,
conductivity, and turbidity or DO.  Three successive readings
should be within ± 0.1  for pH, ± 3% for conductivity, ± 10  mv
for redox potential, and ± 10% for turbidity and DO. Stabilized
purge indicator parameter trends are generally obvious and
follow either an exponential or asymptotic change to stable
values during purging.  Dissolved oxygen and turbidity usually
require the longest time for stabilization. The above stabiliza-
tion guidelines are provided for rough estimates based on

H.  Sampling, Sample Containers, Preservation and

        Upon parameter stabilization, sampling can be
initiated. If an  in-line device is used to monitor water quality
parameters, it should be disconnected or bypassed during
sample collection. Sampling flow rate may remain at estab-
lished purge rate or  may be adjusted slightly to minimize
aeration, bubble formation, turbulent filling of sample bottles,
or loss of volatiles due to extended residence time in tubing.
Typically, flow rates  less than 0.5 L/min are appropriate.  The
same device should be used for sampling as was used for
purging. Sampling should occur in a progression from least to
most contaminated well, if this is known. Generally, volatile
(e.g., solvents and fuel constituents) and gas sensitive (e.g.,
Fe2+, CH4, H2S/HS', alkalinity) parameters should be sampled
first. The sequence  in which samples for most inorganic
parameters are collected is immaterial unless filtered (dis-
solved) samples are desired. Filtering should be done last
and in-line filters should be used as discussed above. During
both well purging and sampling, proper protective clothing
and equipment must be used based upon the type and level
of contaminants present.

        The appropriate sample container will be prepared in
advance of actual sample collection for the analytes of
interest and include  sample preservative where necessary.
Water samples should be collected directly into this container
from the pump tubing.

        Immediately after a sample bottle has been filled, it
must be preserved as specified in the site (QAPP).  Sample
preservation requirements are based on the analyses being
performed (use site  QAPP, FSP, RCRA guidance document
[U. S. EPA, 1992] or EPA SW-846 [U. S. EPA, 1982]). It
may be advisable to add preservatives to sample bottles in a
controlled setting prior to entering the field in order to reduce
the chances of improperly preserving sample bottles or
introducing field contaminants into a sample bottle while
adding the preservatives.

        The preservatives should be transferred from the
chemical bottle to the sample container using a disposable
polyethylene pipet and the disposable pipet should be used
only once and  then discarded.

        After a sample container has been filled with ground
water, a Teflon™ (or tin)-lined cap is screwed on tightly to
prevent the container from leaking.  A sample label is filled
out as specified in the FSP.  The samples should be stored
inverted at 4°C.

        Specific decontamination protocols for sampling
devices are dependent to some extent on the type of device
used and the type of contaminants encountered.  Refer to the
site QAPP and FSP for specific requirements.

I.  Blanks

        The following blanks should be collected:

    (1) field blank: one field blank should be collected from
       each source water (distilled/deionized water) used for
       sampling equipment decontamination or for assisting
       well development procedures.

    (2) equipment blank: one equipment blank should be
       taken prior to the commencement of field work, from
       each set of sampling equipment to be used for that
       day. Refer to site QAPP or FSP for specific require-

    (3) trip blank: a trip blank is required to accompany each
       volatile  sample shipment.  These blanks are prepared
       in the laboratory by filling a 40-mL volatile organic
       analysis (VOA) bottle with  distilled/deionized water.
V. Low-Permeability Formations and Fractured

        The overall sampling program goals or sampling
objectives will drive how the sampling points are located,
installed, and choice of sampling device.  Likewise, site-
specific hydrogeologic factors will affect these decisions.
Sites with very low permeability formations or fractures
causing discrete flow channels may require a unique monitor-
ing approach. Unlike water supply wells, wells installed for
ground-water quality assessment and restoration programs
are often installed in low water-yielding settings (e.g., clays,
silts).  Alternative types of sampling points and sampling
methods are often needed in these types of environments,
because low-permeability  settings may require extremely low-
flow purging (<0.1 L/min) and may be technology-limited.
Where devices are not readily available to pump at such low
flow rates, the primary consideration is  to avoid dewatering of

the well screen. This may require repeated recovery of the
water during purging while leaving the pump in place within
the well screen.

        Use of low-flow techniques may be impractical in
these settings, depending upon the water recharge rates.
The sampler and the end-user of data collected from such
wells need to understand the limitations of the data collected;
i.e., a strong potential for underestimation of actual contami-
nant concentrations for volatile organics, potential false
negatives  for filtered metals and potential false positives for
unfiltered metals.  It is suggested that comparisons be made
between samples recovered using low-flow purging tech-
niques and samples recovered using passive sampling
techniques (i.e., two sets of samples). Passive sample
collection would essentially entail acquisition of the sample
with no or very little purging using a dedicated sampling
system installed within the screened interval or a passive
sample collection device.

A.  Low-Permeability Formations (<0.1 L/min

1. Low-Flow Purging and Sampling with Pumps

    a.  "portable or non-dedicated mode" - Lower the pump
       (one capable of pumping at <0.1 L/min) to mid-screen
       or slightly above and set in place for minimum of 48
       hours (to lessen purge volume requirements). After 48
       hours, use procedures listed in Part IV  above regard-
       ing monitoring water quality parameters for stabiliza-
       tion, etc., but do not dewater the screen. If excessive
       drawdown and  slow recovery is a problem, then
       alternate approaches such as those listed below may
       be  better.

    b.  "dedicated mode" - Set the pump as above at least a
       week prior to sampling; that is, operate in a dedicated
       pump mode. With this approach significant reductions
       in purge volume should be realized. Water quality
       parameters should stabilize quite rapidly due to less
       disturbance of the sampling zone.

2. Passive Sample Collection

        Passive sampling collection requires insertion of the
device into the  screened interval for a sufficient time period to
allow flow  and sample  equilibration before extraction for
analysis. Conceptually, the extraction of water from  low
yielding formations seems more akin to the collection of water
from the unsaturated zone and passive sampling techniques
may be more appropriate in terms of obtaining "representa-
tive" samples.  Satisfying usual sample volume requirements
is typically a problem with  this approach and some latitude will
be needed on the part  of regulatory entities to  achieve
sampling objectives.
B.  Fractured Rock

        In fractured rock formations, a low-flow to zero
purging approach using pumps in conjunction with packers to
isolate the sampling zone in the borehole is suggested.
Passive multi-layer sampling devices may also  provide the
most "representative" samples. It is imperative in these
settings to identify flow paths or water-producing fractures
prior to sampling using tools such as borehole flowmeters
and/or other geophysical tools.

        After identification of water-bearing fractures, install
packer(s) and pump assembly for sample collection using
low-flow sampling in "dedicated mode" or use a passive
sampling device which can isolate the identified water-bearing
VI. Documentation

        The usual practices for documenting the sampling
event should be used for low-flow purging and sampling
techniques. This should include, at a minimum:  information
on the conduct of purging operations (flow-rate, drawdown,
water-quality parameter values, volumes extracted and times
for measurements), field instrument calibration data, water
sampling forms and chain of custody forms. See Figures 2
and 3 and "Ground Water Sampling Workshop - A Workshop
Summary" (U. S. EPA, 1995) for example forms and other
documentation suggestions and information. This information
coupled with laboratory analytical data and validation data are
needed  to judge the "useability" of the sampling data.
VII. Notice

The U.S. Environmental Protection Agency through its Office
of Research and Development funded and managed the
research described herein as part of its in-house research
program and under Contract No. 68-C4-0031 to Dynamac
Corporation. It has been subjected to the Agency's peer and
administrative review and has been approved for publication
as an EPA document.  Mention of trade names or commercial
products does not constitute endorsement or recommenda-
tion for use.
VIII. References

Backhus, D,A., J.N. Ryan, D.M. Groher, J.K. McFarlane, and
P.M. Gschwend. 1993. Sampling Colloids and Colloid-
Associated Contaminants in Ground Water. Ground Water,

Barcelona, M.J., J.A. Helfrich, E.E. Garske, and J.P. Gibb.
1984. A laboratory evaluation of groundwater sampling
mechanisms. Ground Water Monitoring Review, 4(2):32-41.

Barcelona, M.J. and J.A. Helfrich. 1986. Well construction and
purging effects on ground-water samples. Environ. Sci.
Technol., 20(11 ):1179-1184.

Barcelona, M.J., H.A. Wehrmann, and M.D. Varljen. 1994.
Reproducible well purging procedures and VOC stabilization
criteria for ground-water sampling. Ground Water, 32(1 ):12-

Buddemeier, R.W. and J.R. Hunt. 1988. Transport of Colloidal
Contaminants in Ground Water: Radionuclide Migration at the
Nevada Test Site. Applied Geochemistry, 3:  535-548.

Danielsson, L.G. 1982. On the Use of Filters for Distinguish-
ing Between Dissolved and Paniculate Fractions in Natural
Waters. Water Research, 16:179.

Enfield, C.G. and G. Bengtsson. 1988. Macromolecular
Transport of Hydrophobic Contaminants in Aqueous Environ-
ments. Ground Water, 26(1): 64-70.

Gschwend, P.M. and M.D. Reynolds.  1987. Monodisperse
Ferrous Phosphate Colloids in an Anoxic Groundwater
Plume,  J. of Contaminant Hydro/., 1: 309-327.

Herzog, B., J. Pennine, and G. Nielsen. 1991. Ground-Water
Sampling.  In Practical Handbook of Ground-Water Moni-
toring (D.M. Nielsen, ed.). Lewis Publ., Chelsea, Ml, pp. 449-

Horowitz, A.J., K.A. Elrick, and M.R. Colberg. 1992. The effect
of membrane filtration artifacts on dissolved trace element
concentrations. Water Res., 26(6):753-763.

Laxen, D.P.H. and I.M. Chandler. 1982. Comparison of
Filtration Techniques for Size Distribution in Freshwaters.
Analytical Chemistry, 54(8): 1350.

McCarthy, J.F. and J.M. Zachara. 1989. Subsurface Transport
of Contaminants, Environ. Sci. Technol., 5(23):496-502.

McCarthy, J.F. and C. Degueldre. 1993. Sampling and
Characterization of Colloids and Ground Water for Studying
Their Role in Contaminant Transport.  In: Environmental
Particles (J. Buffle and H.P. van Leeuwen, eds.), Lewis Publ.,
Chelsea, Ml, pp. 247-315.

Parker, L.V. 1994. The Effects of Ground Water Sampling
Devices on Water Quality: A Literature Review.  Ground
Water Monitoring and Remediation, 14(2):130-141.

Penrose, W.R.,  W.L. Polzer, E.H. Essington, D.M. Nelson,
and K.A. Orlandini. 1990. Mobility of Plutonium and Ameri-
cium through a Shallow Aquifer in a Semiarid Region,
Environ. Sci. Technol., 24:228-234.

Puls, R.W. and M.J.  Barcelona. 1989. Filtration of Ground
Water Samples for Metals Analyses. Hazardous Waste and
Hazardous Materials, 6(4):385-393.
Puls, R.W., J.H. Eychaner, and R.M. Powell. 1990. Colloidal-
Facilitated Transport of Inorganic Contaminants in Ground
Water: Part I. Sampling Considerations. EPA/600/M-90/023,

Puls, R.W. 1990. Colloidal Considerations in Groundwater
Sampling and Contaminant Transport Predictions. Nuclear
Safety, 31(1):58-65.

Puls, R.W. and R.M. Powell. 1992. Acquisition of Representa-
tive Ground Water Quality Samples for Metals. Ground Water
Monitoring Review, 12(3):167-176.

Puls, R.W., D.A. Clark, B.BIedsoe, R.M. Powell, and C.J.
Paul. 1992. Metals in Ground Water: Sampling Artifacts and
Reproducibility. Hazardous Waste and Hazardous Materials,
9(2): 149-162.

Puls, R.W. and C.J. Paul. 1995. Low-Flow Purging and
Sampling of Ground-Water Monitoring Wells with Dedicated
Systems. Ground Water Monitoring and Remediation,
Ryan, J.N. and P.M. Gschwend. 1990. Colloid Mobilization in
Two Atlantic Coastal Plain Aquifers. Water Resour. Res.., 26:

Thurnblad, T.  1994. Ground Water Sampling Guidance:
Development of Sampling Plans, Sampling Protocols, and
Sampling Reports. Minnesota Pollution Control Agency.

U. S. EPA. 1992. RCRA Ground-Water Monitoring: Draft
Technical Guidance. Office of Solid Waste, Washington, DC
EPA/530/R-93/001 , NTIS PB 93-139350.

U. S. EPA. 1995. Ground Water Sampling Workshop - A
Workshop Summary, Dallas, TX, November 30 - December 2,
1993. EPA/600/R-94/205, NTIS PB 95-193249, 126 pp.

U. S. EPA. 1982. Test Methods for Evaluating Solid Waste,
Physical/Chemical Methods,  EPA SW-846. Office of Solid
Waste and Emergency Response, Washington, D.C.

Figure 2.  Ground Water Sampling Log
Well Depth	
Sampling Device.
Measuring Point_
          _ Screen Length.
                        Well No.
                             Well Diameter
                                           .Casing Type
                   .Tubing type.
                                             .Water Level
                      . Other Inf or.
Sampling Personnel.
[  JConc
Type of Samples Collected
Information: 2 in =617 ml/ft, 4 in = 2470 ml/ft: Vol  . = nr2h, Vo\^lm = 4/3n r3

Figure 3.  Ground Water Sampling Log (with automatic data logging for most water quality
>roject Site Well No. Date
Veil Depth Screen Lenath Well Diameter Casino Tvoe
Sampling Device
/leasuring Point
Sampling Person

Tubing type Water Level
Other Infor


Pump Rate



[ ] Cone


Type of Samples Collected
Information: 2 in = 617 ml/ft, 4 in = 2470 ml/ft: Vol.,, = nr'h, Vol.^ = 4/3n r3

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