1

EPA/540/S-95/504

April 1996

United States

Environmental Protection

Agency

Office of Solid Waste

and Emergency

Response

Office of

Research and

Development

LOW-FLOW (MINIMAL DRAWDOWN)

GROUND-WATER SAMPLING PROCEDURES

by Robert W. Puls1 and Michael J. Barcelona2

Technology Innovation Office

Office of Solid Waste and Emergency

Response, US EPA, Washington, DC

Walter W. Kovalick, Jr., Ph.D.

Director

Ground Water Issue

National Risk Management Research Laboratory

Subsurface Protection and Remediation Division

Robert S. Kerr Environmental Research Center

Ada, Oklahoma

Superfund Technology Support Center for

Ground Water

Background

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

sampling.

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 awareness 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 generalizations 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 regarding 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,

1National Risk Management Research Laboratory, U.S. EPA

2University of Michigan
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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 processes 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 investigations 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 increasingly 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 transport 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 overestimation 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 undesirable 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

techniques.

Current subsurface conceptual models have undergone 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 recommended. With this information (together with other site

characterization data) and a clear understanding of sampling
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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 alteration 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 metalloids) or organic compounds.

II. Monitoring Objectives and Design

Considerations

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 siteassessments for property transfers and water availability

investigations. Monitoring objectives may change as contamination or water quality problems are discovered. However,

there are a number of common components of monitoring

programs which should be recognized as important regardless 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 reproducible techniques; and

 3) Refinement of the conceptual model based on

supplementary data collection and analysis.

These fundamental components serve many types of monitoring 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. Accuracy 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 properties, 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 Figure 1, provides a systematic approach to the goal of consistent data collection.

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.
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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 interpretation 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 determine 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 characterization 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 sampling 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. Placement 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 sampling point and adjacent formation to remove fines created

during emplacement will shorten this water quality recovery

period.

III. 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 representative 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 dedicated 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-
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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 indication 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, oxidation-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, temperature, and specific conductance, followed by oxidationreduction potential, dissolved oxygen and turbidity. Temperature 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 determination of stabilization should be based on water-level drawdown, pumping rate and equipment specifications for measuring indicator parameters. Instruments are available which

utilize in-line flow cells to continuously measure the above

parameters.

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 diameter, 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 recommended at wells which will undergo routine sampling over

time.

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

include:

 • samples which are representative of the mobile load of

contaminants present (dissolved and colloid-associated);

 • minimal disturbance of the sampling point thereby

minimizing sampling artifacts;

 • less operator variability, greater operator control;
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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

well;

 • maximize tubing wall thickness, minimize tubing

length;

 • 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

system.

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 elevation.

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.

 • reduced stress on the formation (minimal drawdown);

 • less mixing of stagnant casing water with formation

water;

 • 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

variability.

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

site,

 • increased training needs,

 • resistance to change on the part of sampling practitioners,

 • concern that new data will indicate a change in

conditions and trigger an action.

IV. Low-Flow (Minimal Drawdown) Sampling

Protocols

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) experiences 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). Highquality 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 groundwater 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 inappropriate 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 monitoring 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
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1) General Considerations

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 low 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 lowflow (minimal drawdown) purging and sampling and include

peristaltic pumps, bladder pumps, electrical submersible

pumps, and gas-driven pumps. Devices which lend themselves to both dedication and consistent operation at definable 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 illsuited 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).

E. 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

casing.

F. Filtration

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 μm filters]) concentrations of major ions and trace metals, 0.1 μm filters are

recommended although 0.45 μm filters are normally used for

most regulatory programs. Alkalinity samples must also be

filtered if significant particulate calcium carbonate is suspected, 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

samples.

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 nondisposable (in-line filter holder, flat membrane filters) formats

and various filter pore sizes (0.1-5.0 μm). 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 recommendations. 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
8

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 TeflonTM (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 4o

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 requirements.

(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

Rock

The overall sampling program goals or sampling

objectives will drive how the sampling points are located,

installed, and choice of sampling device. Likewise, sitespecific hydrogeologic factors will affect these decisions.

Sites with very low permeability formations or fractures

causing discrete flow channels may require a unique monitoring 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 lowflow 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

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 stabilization guidelines are provided for rough estimates based on

experience.

H. Sampling, Sample Containers, Preservation and

Decontamination

 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 established 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, H2

S/HS-, alkalinity) parameters should be sampled

first. The sequence in which samples for most inorganic

parameters are collected is immaterial unless filtered (dissolved) 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
9

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 contaminant 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 techniques 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

recharge)

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 regarding monitoring water quality parameters for stabilization, 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 “representative” 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

fractures.

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 recommendation for use.

VIII. References

Backhus, D,A., J.N. Ryan, D.M. Groher, J.K. McFarlane, and

P.M. Gschwend. 1993. Sampling Colloids and ColloidAssociated Contaminants in Ground Water. Ground Water,

31(3):466-479.

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.
10

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-

22.

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 Distinguishing Between Dissolved and Particulate Fractions in Natural

Waters. Water Research, 16:179.

Enfield, C.G. and G. Bengtsson. 1988. Macromolecular

Transport of Hydrophobic Contaminants in Aqueous Environments. 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 Hydrol., 1: 309-327.

Herzog, B., J. Pennino, and G. Nielsen. 1991. Ground-Water

Sampling. In Practical Handbook of Ground-Water Monitoring (D.M. Nielsen, ed.). Lewis Publ., Chelsea, MI, pp. 449-

499.

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, MI, 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 Americium 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. ColloidalFacilitated Transport of Inorganic Contaminants in Ground

Water: Part I. Sampling Considerations. EPA/600/M-90/023,

NTIS PB 91-168419.

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 Representative Ground Water Quality Samples for Metals. Ground Water

Monitoring Review, 12(3):167-176.

Puls, R.W., D.A. Clark, B.Bledsoe, 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,

15(1):116-123.

Ryan, J.N. and P.M. Gschwend. 1990. Colloid Mobilization in

Two Atlantic Coastal Plain Aquifers. Water Resour. Res., 26:

307-322.

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.

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Physical/Chemical Methods, EPA SW-846. Office of Solid

Waste and Emergency Response, Washington, D.C.
11

Figure 2. Ground Water Sampling Log

Project _______________ Site _______________ Well No. _____________ Date _________________________

Well Depth ____________ Screen Length __________ Well Diameter _________ Casing Type ____________

Sampling Device _______________ Tubing type _____________________ Water Level __________________

Measuring Point ___________________ Other Infor ________________________________________________

____________________________________________________________________________________________

Sampling Personnel__________________________________________________________________________

Type of Samples Collected

_______________________________________________________________________________________________

Information: 2 in = 617 ml/ft, 4 in = 2470 ml/ft: Volcyl = Br2h, Volsphere = 4/3B r3

Time pH Temp Cond. Dis.O Turb. [ ]Conc Notes 2
12

Figure 3. Ground Water Sampling Log (with automatic data logging for most water quality

parameters)

Project _______________ Site _______________ Well No. _____________ Date ________________________

Well Depth ____________ Screen Length __________ Well Diameter _________ Casing Type ___________

Sampling Device _______________ Tubing type _____________________ Water Level _________________

Measuring Point ___________________ Other Infor _______________________________________________

___________________________________________________________________________________________

Sampling Personnel_________________________________________________________________________

Type of Samples Collected

_______________________________________________________________________________________________

Information: 2 in = 617 ml/ft, 4 in = 2470 ml/ft: Volcyl = Br2h, Volsphere = 4/3B r3

Time Pump Rate Turbidity Alkalinity [ ] Conc Notes