Manual Compositing Methods for Urban Storm-Runoff Samples In Reply Refer To: October 20, 1980 EGS-Mail Stop 412 Quality of Water Branch Technical Memorandum No. 81.03 Subject: Manual Compositing Methods for Urban Storm-Runoff Samples One of the objectives of the Urban Hydrology Studies Program (UHSP) is to "determine the magnitude and frequency of storm- runoff loads of water-quality constituents from typical urban watersheds". In order to meet this objective, it is obvious that a number of samples must be collected throughout each storm-runoff event. This is particularly true if loads are to be determined for those constituents associated with the sediment being transported. Pumping samplers usually must be utilized to collect enough samples to meet this objective. Because of high analytical costs, it will not be practical to determine all constituents of interest for each of the samples collected during all storm events. Individual samples for several runoff events at each site will be analyzed, however, for the other events the individual samples will have to be composited and the single representative composite sample analyzed for the required constituents to determine the runoff-event loads. The problem is to determine the volume of each individual sample to include in a composite sample that represents the quality of the flow during the entire event. We will assume that whenever a pumping sampler is installed, the pumping system and intake location are designed such that a sample pumped into a container is representative of the discharge during the pumping cycle. This means that the sample of the water-sediment mixture is truly a sample of the flow in the cross section during those few seconds when the sample is being pumped into the container. The recommended method of compositing is based on the following assumptions: . A continuous record of stage is available during the event, 2. A stage-discharge relation exists (rating curve), 3. A stage-triggered pumping sampler is operating properly; samples can be collected at equal time intervals or at preset stages, or at equal increments of flow, 4. Pumping-sampler efficiency is 100 percent; that is, all sediment entering the intake is delivered to the container at the same concentration, 5. The exact time of collection of each sample is known (recorded), 6. The exact gage height at the time of collection can be recorded or determined, 7. The total volume of the composite sample required by the laboratory is known, 8. The sampler pumps virtually the same volume for each sample, (not critical). The discharge-weighted volume of each sample to be used in the storm-runoff composite sample is computed as follows: Vi = qi ti x VT (l) _qi ti where Vi = volume of sample to be added to composite. qi = instantaneous discharge (cfs) at the time of sample collection. ti = time interval (min)--equal to one-half the time since the previous sample plus one-half the time to the next sample. For the first sample, the time interval is from start of storm-runoff event to one-half the time to the second sample. For the last sample, the time interval is from one-half the time to the previous sample to the end of the event. VT = volume of composite sample required by laboratory. The example illustrated below (Fig. l) is an 18-hour storm-runoff event. The sampler was programmed to start pumping the first sample at a gage height of 1.1 ft, and to pump a sample at O.l-ft increments of gage-height change (rising or falling) until the gage height falls below 1.1 ft. This sampling scheme resulted in the collection of 10 samples. Table 1 is an example computation of the volume of each sample required to make up a 2000-mL discharge weighted composite sample. This method is based on the mid- interval method of subdividing a day to determine the suspended- sediment discharge for a day, which is described in detail in TWRI Book 3, chapter C3, pp 49-50. Figure 1.--Hydrograph of 18-hr runoff event showing times of sample collection. The method described above for determining subsample volumes for compositing probably is entirely adequate for this type of sampling program. It should not he used, however, to determine the final mean discharge for the runoff event. For instance, the final mean discharge computed by conventional methods for this 18-hr event is 10.3 or 10.4 cfs, depending on whether l5-min, 30-min, or 6O-min gage-heights are used. Sampling according to change in stage (discharge) or, if possible at equal increments of flow, is the most accurate method of sampling runoff events such as those encountered in the Urban Hydrology Studies Program. This is true regardless of whether samples are to be composited or analyzed individually. Use of the term "discharge-weighted" may infer that discharge is the only variable to consider when compositing. In fact, the method described above is based entirely on the distribution of flow during the runoff event. Keep in mind that we are proportioning water-sediment mixtures. Representativeness of composite sample--How should the "accuracy" or representativeness of a composite sample be judged? The program objective being addressed here is the determination of "loads of water-quality constituents. This means that a constituent load for a runoff event can be accurately computed by multiplying the concentration of the composite sample times the mean discharge times a conversion factor (0.0027 for 24 hours). In order to verify the representativeness of a composite sample, the following steps should be taken: 1. Use the method illustrated above (mid-interval method) for determining volumes of subsamples to use in the composite. ?. Use the cone splitter to obtain the volumes required (see Quality of Water Branch Technical Memorandum No. 80.17). 3. Analyze a portion of each sample for sediment concentration (0.1 part should be sufficient). (Missing--Table 1. Example of computation of subsample volumes to be composited using the mid-interval method of subdividing.) 4. Analyze a portion of each sample for dissolved-solids concentration (lOO-mL aliquot from 0.1 part). 5. Plot these concentrations on the hydrograph at the correct sampling times and draw a smooth curve through the points. 6. Use one of the two recommended methods of subdividing to determine the discharge-weighted loads for the runoff event (TWRI Book 3, chapter C3, pp 49-52) 7. Divide the loads by the mean discharge and by the conversion factor (0.0027 x hours/24) to determine the mean discharge- weighted concentrations for the event. 8. Compare the concentrations of the composite sample with those determined in step 7. Step 1 has already been illustrated in Table 1. Assume that steps 2, 3, and 4 have been completed. Figure 2 illustrates step 5. Figure 2.--Sediment and dissolved-solids graphs based on 10 samples Typically, the peak suspended-sediment concentration as well as the peak dissolved-solids concentration occurs before the discharge peak (see TWRI Book 3, chapter C3). During the first minutes of the passage of a typical urban storm-runoff event, the quality of the mixture of water and sediments is changing rapidly. The sediments that are entering the stream from various sources and distances upstream are mixing with those sediments that are already in the stream. As the flow increases, more sediments are being scoured from the streambed and banks and brought into suspension in the flowing mixture. The distribution of various sizes of sediments is continuously changing with the changes in turbulence and depth of flow. Although not completely documented, there apparently are relatively rapid transfers of some chemical constituents from solution to sorption on sediments; particularly, the metals and organic compounds. Therefore, it is important that samples he collected more frequently during the period from beginning of flow event to the peak flow. Based on the concentration graphs and the hydrographs (Fig 2), the mean-interval method of subdivision was used to determine the most accurate sediment and dissolved-solids loads for the 13-hr event (step 6). The loads were 13.1 tons of sediment and 4.5 tons of dissolved solids. The mean discharge-weighted sediment concentration (step 7), 13.1 tons C = _______________ 10.3 cfs x 0.00202 = 630 mg/L suspended sediment The mean dissolved-solids concentration, 4.5 tons C = ______________ 10.3 cfs x 0.00202 = 216 mg/L dissolved solids This means that the sediment concentration of the composite sample should be equal to 630 mg/L and the dissolved-solids concentration equal to 216 mg/L in order to determine the correct loads. If the sediment concentration of the composite sample is not close to 630 mg/L the computed sediment load will he incorrect, and all of the "total" concentrations (and loads) of those chemical constituents associated with sediment will also be incorrect. Pumping samplers must always be programed to sample according to some preset schedule. The example illustrated above (Table 1) is one of the most commonly used sampling schemes. Three other sampling schemes are described below along with the scheme illustrated in Table 1. The major differences are either in the method of triggering the initiation of sampling for the flow event, or in the time interval between samples collected during the flow event. Sampling Schemes 1. First sample at Ght of l.l ft and a sample collected at 0.l-ft increments of gage-height change (rising or falling) until gage height falls below 1.1 ft. (See Table 1)--samples collected. 2 First sample at Ght of 1.1 ft and a sample collected at l-hr intervals until gage height falls below 1.1 ft--samples collected. 3. First sample collected at start of event (triggered at Ght = l.0l ft) and a sample collected at l-hour intervals until same Ght reached--l9 samples collected (first and last samples not used in composite). 4. First sample collected when discharge reaches a point 25 percent above base-flow discharge (for this station base flow was assumed to be 5 cfs, Ght = 1.0 ft, so the sampler was triggered to start sampling at a Ght = 1.12 ft, when the rated Q = 6.25cfs) and a sample collected at l-hr intervals; last sample collected at same Ght on recession--l3 samples collected. Three methods of compositing are described below: Compositing Methods A. Mean-discharge-weighted--each sample is assumed to represent the mean discharge for it's time interval, ti. The volume of each sample required for the composite is computed according tn equation (1), except that mean discharge for the time interval is used instead of the instantaneous discharge. B. Mid-interval--same as described in equation (1). C. Short-cut--time is not used in the computation. The instantaneous discharges at the times of sample collection are summed and the ratios of each instantaneous discharge to the sum of the instantaneous discharges are used to compute volume of each sample to be used in the composite . Composite concentrations for sediment and dissolved solids were computed using each of these compositing methods and the four sampling schemes and the results shown in Table 2. All of the four sampling schemes gave good results when the samples were composited using either method A or B. The shortcut method gave erratic results and therefore is not recommended. (Missing--Table 2. Comparison of mean concentrations of composite samples for storm-runoff event using various sampling schemes and compositing methods) Summary and Conclusions The representativeness of any composite sample is dependent upon many factors, but is most heavily dependent upon l) the spacing of samples collected relative to the hydrograph, and 2) the number of samples collected during the event. Generally, relatively fewer samples can be collected provided they represent the significant periods of the event; the rising stage and the peak(s). This is true whether the samples are analyzed individually or are composited. Regardless of whether samples are to be composited or not, the selection of an appropriate sampling scheme for the types of runoff events that occur at a data collection site is extremely important. No method of compositing can yield correct concentrations of constituents if the samples are not collected at the proper times during the event. For instance, as shown in Table 2, sampling scheme No. 3 resulted in the collection of 17 samples, but it was not statistically any more accurate than sampling scheme No. 1 where only 10 samples were collected. Compositing method C, the shortcut method, obviously yields such erratic results that it should not be used. It is obvious, too, that this method yields better results as more samples are collected. If samples were collected at 15-min or 30-min intervals, this method may prove to be adequate at some sites. Inasmuch as both instantaneous gage-heights and times of sample collection must be known before any compositing can be done, compositing methods A or B could easily be used in every instance. For the example shown, method B (equation l) gave good results for both sediment and dissolved solids, and it is a simple and fast way of determining volumes of samples needed for the composite samples. The following information is needed before compositing can be done. It should be obtained as soon after the runoff event as possible so that the compositing can be completed and the sample(s) split and preserved as required. l. Begin time of each sampling cycle. ~. Gage height associated with each sampling cycle. 3. Begin time and end time for the event. 4. Composite volume required by laboratory. R. J. Pickering Chief, Quality of Water Branch Distribution: A, B, S, FO, PO Key words: water-quality, sampling, sediment, sample compositing, urban hydrology. This does not supersede any previous QW Technical Memorandum.