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