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<br />Because of the limits of existing samplers to collect an isokinetic sample, considerable independent <br />judgement is necessary to collect a sample representative of the stream chemistry. The entire stream cross <br />section must be represented in the fmal sample to avoid the potential loss of pollutants that might be <br />seeping from the streambanks or streambed. Therefore, it might be necessary to increase the vertical <br />transit rate or use a smaller nozzle to avoid overfilling the sampler when representing the entire stream <br />depth. A weighted-bottle or D-77 bag sampler also might be used. Figure 1 illustrates the recommended <br />sampler options for the NA WQA program based on stream depth and velocity. <br /> <br />Intermittent streams require special consideration because little opportunity exists to study conditions <br />or sample in detail prior to a flow event. Rapidly changing stage, discharge, and constituent <br />concentrations dictate that abbreviated sampling schemes and techniques be planned carefully in advance <br />to ensure that the most representative samples possible are obtained. <br /> <br />STREAM VELOCITY, IN FEET PER SECOND <br />o 2 8 <br /> <br />I <br />I OPEN BOTTLE <br />I <br /> <br />DH - 81 <br /> <br />DH - 81 <br /> <br />DH - 81 <br /> <br />I- 3 <br />w <br />w <br />u. <br />Z <br /> <br />J: <br />I- <br />a.. <br />w <br />c <br />:E <br /><( <br />w <br />a: <br />I- <br />en <br /> <br />WEIGHTED <br />BOTTLE <br /> <br />D - 77 TM <br /> <br />DH - 81 <br /> <br />D - 77 TM <br /> <br />D -77 <br />BAG <br /> <br />D - 77 TM <br /> <br />15 ------------------------------------- <br /> <br />D - 77 TM <br /> <br />D - 77 TM <br /> <br />D -77 <br />BAG <br /> <br />KEMMERER <br />SAMPLER <br /> <br />D - 77 BAG <br /> <br />Figure 1. Schematic of suggested water-quality samplers for various stream regimes. <br /> <br />Collection Methods 15 <br />