| This paper from the United States
Environmental Protection Agency is widely respected as a resource for persons who need or
wish to use Low-Flow purging and sampling as a method of sampling the groundwater at their
site. Section: 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.
Section: 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-flow purging has the advantage of minimizing mixing between the overlying stagnant
casing water and water within the screened interval.
Section: III. A. Low-Flow Purging & 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 1.0 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 re-establishment of
natural flow paths in both the vertical and horizontal directions are 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.
Section: III.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 oxidation-reduction 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 through 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), 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 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). For
a full copy of this paper, please contact EnviroEquip. |
EnviroEquip
Product Links
|
