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