Field processing begins with the removal of large rocks and organic debris, such as leaves, twigs, and roots, from the sample. These materials are discarded after checking to ensure that all attached invertebrates have been removed. The remaining material is quickly examined for large, rare organisms that could be lost during subsequent sample splitting. These large-rare organisms are removed and placed in a separate, labeled container that is identified as the "large-rare" sample component. All organ isms that are picked from the sample by hand prior to sample splitting are added to the large-rare sample component.
The remaining sample material is elutriated onto an appropriately sized sieve (425-µm mesh for semi-quantitative samples and 212-µm mesh for qualitative samples) to separate the lighter organic material from the heavier sand and gravel. Elutriation is usually accomplished by placing the sample in a deep bucket (a 19-L plastic bucket works well for this purpose) filled about one-fourth to one-half with water. The contents of the bucket are stirred by hand to suspend as much material as possible. The buc ket is picked up, swirled, and then gently decanted onto an appropriate sieve while the advancing sediment front is carefully watched. Sieving effectiveness is increased and clogging is decreased if the sieve is kept in constant motion while the sample is decanted. An effective motion is created by holding the sieve in one hand with the thumb on the top lip of the sieve and the other fingers supporting the bottom of the sieve. The wrist is used to impart a back and forth motion perpendicular to the flow of water onto the sieve. Decanting stops when the sediment front reaches the lip of the bucket. A backup container, such as a wash tub or another 19-L bucket, is placed under the sieve to catch any material that is spilled over the edge of the sieve d uring the elutriation. If material is spilled into the backup container, the contents of the backup container are returned to the sample bucket and the elutriation is repeated. If no material is spilled, then the contents of the backup container are dis carded after each decantation. The elutriation process is repeated until it appears that only sand and gravel remain in the elutriation bucket.
The sand, gravel, and small pebbles remaining in the bucket are visually examined for invertebrates, particularly case-building caddisflies and small mollusks such as Corbicula. Visual inspections are usually easier and faster if only small amounts of the sample are examined at a time and(or) the material is sieved through nested large-meshed sieves (for example, 4-, 2-, and 1-mm mesh sizes). A shallow white pan or tray containing approximately 1 cm depth of water is ideal for visual inspections . Invertebrates that are removed during this process are added to the large-rare sample component. Sieving this material often speeds up the examination process. Once free of invertebrates, the left-over sand and gravel is retained as a quality-assuran ce check on the efficiency of elutriation. This "elutriate" sample component is split, if necessary, preserved in 10-percent buffered formalin, labeled, and shipped to the USGS Quality Management Group's Biological Quality-Assurance Unit (BQAU) located at the National Water-Quality Laboratory (NWQL) in Arvada, Colo., to determine the effectiveness of elutriation. The BQAU is responsible for handling quality assurance and quality control (QA/QC) for laboratory analysis of the samples.