WATER QUALITY--Sampling mixtures of water and suspended sediment in streams
May 12, 1976
QUALITY OF WATER BRANCH TECHNICAL MEMORANDUM NO. 76.17
Subject: WATER QUALITY--Sampling mixtures of water and suspended
sediment in streams
As a result of District reviews, the Quality o£ Water Branch has
become more aware that principles and methods for sampling
mixtures of water and suspended sediment in streams are not well
understood and, therefore, are not always followed properly.
Although proper methods generally are used in collecting
representative samples for determination of suspended-sediment
concentration and particle-size distribution, they often are
ignored when it comes to collecting representative samples for
chemical analyses of mixtures of water and suspended sediment (so-
called whole-water, or unfiltered samples).
Several districts have issued field instructions dealing with
methods for collecting samples for analysis of mixtures of water
and suspended sediment. The subject will be covered fully in a
TWRI chapter on collection of samples for water-quality
determinations (now in preparation). In the meantime, I ask that
all Districts review their field instructions and practices to see
that they conform to the following guidelines.
PRINCIPLE
All samples to be analyzed for determination of "total"
concentrations of constituents in the mixture of water and
suspended sediment are to be collected in such a manner that they
will best represent the water being transported by the stream.
METHODS WHERE SAND-SIZE SEDIMENTS ARE IN TRANSPORT
When turbulence and mean stream velocities are great enough to
support the transport of sand in suspension [generally greater
than about 2 ft/s (0.6m/s)], sampling for mixtures of water and
suspended sediment must be done using appropriate sediment-
sampling equipment and techniques. In descending order of
accuracy, these methods are:
1. Equal-transit-rate (ETR),
2. Equal-discharge-increment (EDI),
3. Multi-vertical, and
4. Single-vertical.
Equal-transit-rate method.--This method yields the most accurate
sample of the streamflow. It requires the collection of depth-
integrated samples at 10 to 25 verticals in the cross section
(F.I.A.S.P., 1963, p. 41). Guy and Norman (1970, p. 40) discuss
this method and the conditions under which fewer than ten
verticals may be sampled. For example, for smaller streams or when
the ratio of velocity squared to depth, V2/D, is less than about
1.2 ft/s2 (0.37 m/s2) as few as three verticals may yield an
acceptable sample.
Equal-discharge-increment-method.--The EDI method is more complex,
and generally requires a bettter knowledge of the flow conditions
prior to sampling. Two to ten verticals generally are sampled in
this method (F.I.A.S.P., 1963, p. 41). Guy and Norman (I970,
p. 31-32) describe in detail how centroids of equal-discharge
increments of flow may be determined . Sampling by this method may
yield samples equal in accuracy to the ETR method if samples of
equal volume are collected at each centroid of flow.
Multi-vertical method.--The basis for selection of the number of
verticals to be sampled is mainly intuition. Generally, two to
five depth-integrated samples are collected in the cross section--
usually, only three verticals. Discussion and guidelines for this
method are found in the manual by Guy and Norman (1970, p. 30) and
in F.I.A.S.P. (1963, p. 39-40) Report 14.
Single vertical method.--Next to automatic pumping-type samplers,
this method is the least accurate. It is the last resort, but its
use may be necessary under extreme conditions such as rapidly
changing stage. Guy and Norman (1970, p. 27-30) discuss this
method and its many shortcomings in relation to the more accurate
ETR and EDI methods for sampling of streamflow transporting sand-
size sediments.
Remember that standard depth-integrating sediment samplers should
not be used to sample depths greater than about 15 ft (4.6 m) (Guy
and Norman., 1970, p. 24; F.I.A.S.P., 1963, p. 44). Point-
integrating samplers should be used to sample depths greater than
15 ft (4.6 m). All district water-quality specialists should be
thoroughly familiar with the two references referred to in this
discussion.
CONDITI0NS OF LOW VELOCITY
When mean stream velocities are low [less than about 2 ft/s
(0.6m/s)] and the flow is tranquil, generally fine silt- and clay-
sized particles are in suspension, and sediment concentrations do
not vary greatly either vertically or laterally. Furthermore,
standard suspended-sediment samplers do not fill properly at
velocities less than about l.5 ft/s (O.5m/s) (OWDC, p. III-18).
When such conditions are documented at a site by discharge
measurements, it usually is acceptable to collect depth-integrated
samples using open-mouth bottles. The open-mouth bottle commonly
used is a narrow-mouth bottle, usually one litre or more in size.
It should be weighted so that it will sink readily to the bottom,
taking in sample on the trip from the surface to near the bottom
and back to the surface (Brown and others, 1970, fig. 2; Beam,
l973). There still is a need for a more suitable sampler for deep,
slowly moving rivers.
Open-mouth bottles used as samplers should be filled by lowering
and raising at several verticals in the cross-section in order to
best sample the vertical and lateral variations in water quality
that frequently exist in slowly moving waters. Here again, the
number of verticals sampled is largely a matter of intuition,
realizing that large variations in the water quality in the cross
section will require sampling at more verticals than if little
variation exists.
COMPOSITING
Samples from several verticals should be composited and then split
into fractions for various types of field treatment and for
different laboratory analyses. This is easily accomplished using a
large clean jug or bottle. Care should be taken to try to assure
uniform mixing and withdrawal of representative aliquots. A new
churn-type sample splitter has been developed, tested, and
ordered; a limited number will be available to field offices
within the next few months. Samples for bacterial determination or
for analysis of sediment concentration and particle size analyses
should not be composited, but should be left in the original
collecting bottles. Obviously, samples for chemical analyses
should not be composited if separate analyses at different points
or verticals in the cross-section are desired.
SAMPLING FOR DETERMINATION OF PESTICIDES
Special l-pint, borosilicate, screw-cap, glass bottles must be
used to collect samples of mixtures of water and suspended
sediment for determination of pesticides and organics. These
bottles are streamlined versions of the standard sediment (milk)
bottles. Use only freshly cleaned bottles that have been supplied
by the Central laboratory or EPA pesticide laboratory. These
bottles are to be used in standard depth-integrating or point-
integrating suspended sediment samplers fitted with metal or
teflon/nylon nozzles and silicone rubber gaskets. Bottles are to
be filled to the shoulders (standard sediment practice; Guy and
Norman, l970, p. 28-29) and are not to be composited or
transferred. The bottles should be labeled to indicate the
vertical (station) in the cross-section where the sample was
collected. For low-velocity streams where open-mouth sampling can
be conducted, samples may be collected as described in the earlier
section of this memorandum on "conditions of low velocity." Care
should be taken to avoid touching the lip of the sample container,
and to collect the sample upstream from the body when wading.
A NOTE REGARDING FILTERFD SAMLES
Samples for determinations of "dissolved" constituents are
filtered in the field. Usually it is assumed that solutes are well
mixed throughout a cross-section, and therefore, require somewhat
less care in sampling than do mixtures of water and suspended
sediment. However, non-uniform chemical quality frequently exists
in places such as estuaries, slack water pools, below dams, and
below tributaries, and an assumption of a well mixed system should
be used with caution. As a general rule, unless one has
measurements to show that chemical quality of the constituents
being sampled is relatively uniform (range in variation of
specific conductance no more than 10 percent) throughout a cross-
section for flow conditions experienced, it is best to use caution
and apply the same practices that are recommended for mixtures of
water and suspended sediment.
CHOICES OF CONTAINER MATERIALS
Instructions issued by the Central laboratories ("Central
Laboratory Parameter List") specify the types of containers to be
used for the shipment of samples to the laboratories. The types of
container materials specified are intended to prevent
contamination of the samples by their shipping containers. In
general, specifications call for the use of containers made of
materials different from those of the particular constituent for
which the lab is to analyze. For example, plastics are used for
inorganic samples (special acid rinsing for trace metals), and
glass is used for organics.
These same principles of selection of non-contaminating materials
apply also to every piece of equipment that touches the sample,
including the sampler nozzle and gasket, and the compositing
container. Plastic (teflon or nylon) nozzles should be used when
sampling for determination of inorganics; metal nozzles are
preferable for determination of organics, although contamination
from teflon or nylon probably is minimal if the nozzles have been
carefully cleaned. Care should be taken to use silicon rubber
gaskets when sampling for determination of trace metals or
organics. Samples collected for inorganic analyses should be
composited in clean plastic containers (acid-rinsed for trace
metals). Samples collected for organic analyses should be
composited in clean glass containers..
REFERENCES
Beam, A. C., 1973, Depth-integrating sampler for four bottles: WRD
Bulletin, Jan.-Dec. 1973, p. 41.
Brown, Eugene, Skougstad, M. W., and Fishman, M. J., 1970, Methods
for collection and analysis of water samples for dissolved
minerals and gases: U.S. Geol. Survey Techniques of Water
Resources Inv., bk. 5, chap. Al, 160 p.
Guy, H. P., and Norman, V. W., 1970, Field methods for measurement
of fluvial sediment: U.S. Geol. Survey Techniques of Water
Resources Inv., bk. 3, chap. C2, 59 p.
Federal Inter-Agency Sedimentation Project of the Inter-Agency
Committee on Water Resources, 1963, Determination of fluvial
sediment discharge--Inter-Agency Report 14: Minneapolis, Minn.,
St. Anthony Falls Hydraulics Lab., 151 p.
Office of Water Data Coordination, 1972, Recommended Methods for
Water-Data Acquisition, Chap. III, 50 p.
COMMENTS INVITED
I hope that every District will review the practices being used by
all of its field people, and will bring this memorandum to the
attention of every field person. Your comments regarding the
recommendations and instructions contained in this memorandum are
invited. Please contact your Regional Water Quality Specialist or
write to me through him.
R. J. Pickering
Chief, Quality of Water Branch
WRD Distribution: A,B,S,FO,PO