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Open-File Report 2005-1001
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U.S. Geological Survey Open-File Report 2005-1001
USGS East-Coast Sediment Analysis: Procedures, Database, and GIS Data


Quality Assurance

Extensive textural analyses performed on standards have shown that the above methods, with the possible exception of the hydrometer, will produce results with an accuracy of better than plus or minus 10% (electro-resistance multichannel particle size analyzer: about 3%; sieves: about 5%; pipette and sand-fraction settling tube: about 5%) (Shideler, 1976; Syvitski and others, 1991). However, the optimum precision for the above analyses is realized only if the operator is conscientious in attention to detail and consistently follows established procedure. To monitor precision and accuracy, (1) the operator should run replicates on at least every tenth sample, and (2) the analyst, laboratory manager, and requesting scientist should independently review each dataset. It is also important to note that natural small-scale spatial changes in sea-floor sediments and the difficulty in getting a representative split of the sampled sediment for analysis can introduce even more variability than the analytical error discussed above.

As discussed earlier, the representativeness of any textural data set depends on the methods used to obtain the data. These methods must be chosen in accordance with the original purpose of the study. If the importance of knowing the actual size distribution outweighs the hydraulic equivalence, the sieve and the particle-counter analyses should be performed rather than the settling-tube and pipette analyses.

The comparability problems between sets of data generated by different devices are not as great as one might expect. For example, the data produced by sieve and settling tube analyses would be remarkably similar for a given sample because a settling tube is usually calibrated using natural sediments sieved to known fractions. On the other hand, pipette and hydrometer analyses will usually produce data indicating a given sample is slightly finer than the data produced on an EMPSA. This result occurs because fine-fraction particles are generally plate shaped and do not settle as fast as the quartz spheres upon which Stoke's Law, and therefore the pipette and hydrometer settling analyses, is based. And again, an EMPSA can not typically be calibrated to analyze the very fine clay (less than 0.6 microns) and colloidal-clay portions of the size distribution.

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