USGS Open-File Report 2004-1443, Operation Manual: Time-Series, Storm-Activated Suspended Sediment Sampler Deployed in the Coastal Ocean
ConclusionsThis report summarizes the operational and testing procedures for two models of aquatic in-situ suspended matter-sampling devices manufactured by McLane Laboratories. The first model of the instrument is called an action triggered Water Transfer System that is activated by a transmissometer signal WTS-Tr. The second model (WTS-P) triggers on output from a pressure sensor that monitors changing sea wave climate. In both models, the software is designed to evaluate the changing magnitude of signals from its external sensor and take samples before, during, and after the height of a storm that is resuspending bottom sediments. Each unit also collects samples on a programmable time schedule. The samples of particulate matter are collected by drawing ambient water through one of several filter assemblies. The rigorous maintenance and testing procedures described in this manual are used to confirm peak performance of the instruments prior to long-term field deployments. The procedures include testing of: batteries under a specified resistance load, O-ring integrity, gear pump operation, vacuum seal integrity in sample lines, and diagnostic tests of circuit boards. One important discovery from the laboratory tests was that the top grated frit of the filter holder was pre-filtering particulate material before it reached the filter. This frit has subsequently been removed from all the filter holders for test and field deployments. Field tests were conducted at the WHOI dock at depths of at least 30 feet. This provided sufficient hydrostatic pressure to prevent cavitation when the pump created a partial vacuum as the sample filters became clogged. In this setting a personal computer and a specially designed communication cable could be used to monitor and evaluate the real-time pumping and software responses from the instrument during the submerged phase of testing. While the WHOI dock test site was ideal with respect to winches and accessibility to deep water, the location is complicated for calibrations of the instruments because of the natural variability in the concentration and composition of suspended matter as tidal conditions changed while tests were in progress. We compared the concentrations of suspended matter determined by the WTS systems with those determined using a Niskin Bottle and laboratory filtration using a conventional open glass chimney and glass frit filter holder made by Millipore. In direct comparisons between suspended matter concentrations determined by the standard Niskin bottle and the WTS-Tr system at the dock, the correlation of results were reasonable (r² values equaled 0.98 and 0.72 for two WTS-Tr units, known as BOB and TED). However, on average, the values determined using the WTS units were about 20% lower than those measured using the Niskin bottle followed by filtration in the laboratory. In dock tests of the WTS-P Water Transfer System, the suspended matter concentrations were an average of 39% lower than the values measured with a Niskin bottle and conventional laboratory filtration. The reasons for the consistently lower values of suspended matter concentration determined with the WTS systems have not been fully explored. One possible explanation is that filamentous organic matter from plankton in surficial coastal water may clog the small orifices of the WTS systems to some degree or adsorb to the tubing and essentially prevent some fraction of the suspended matter from reaching the filter. The minimum internal diameter in the plumbing is 0.172 inches in the WTS-Tr and 0.100 in the WTS-P. The WTS-Tr has about 5 in of ¼ in ID tubing between each entry ports and the filter whereas the WTS-P has about 20 in of 1/8 in ID tubing from a single entry port to a manifold that directs the flow to individual filters. In contrast, the Niskin bottle has a ¼ in drain port and no tubing in the filtration apparatus. This potential problem of filamentous suspended matter is likely to be less significant at the Massachusetts Bay deployment site where the instrument is moored in bottom water at 30 m depth and where the composition of suspended matter is made up primarily of resuspended lithogenic particles, particularly during storms. Resuspended sediments from a sediment trap in Massachusetts Bay were used in laboratory tests to evaluate the WTS-P performance (Appendix 6). This sediment type (10% sand (made up largely of thin carbonate fragments), 40% silt, and 50% clay) passed through the WTS-P system with 100% yield at both flow rates tested (100 ml/min and 50 ml/min). Suspension of bottom sediment from the Continental Shelf (7% sand (made up largely of lithogenic fragments), 67 % silt, and 26% clay) had yields of 88% and 71% at flow rates of 125 ml/min and 50 ml/min, respectively. Suspensions of fine sand sieved (62-125 micron grain diameter) from a local beach had a yield of only 29% at 125 ml/min. The suspension of very fine sand (62-125 micron) is not representative of natural sediment. It is used in these tests to confirm that undersampling by the WTS increases as sediment grain size increases. Additional calibration studies with the WTS are necessary and should be conducted in a tank that is at least 30 ft deep, and that has the capability for introducing known composition and concentrations of suspended matter and for keeping the system uniformly mixed. Future calibration studies should compare the grain size distribution in the original suspension with that on the filter in order to evaluate any biasing of the sample size by sampling under different conditions of flow rate and sample type. A potential application for well-calibrated in-situ filtration devices is to provide information on concentration and composition of resuspended bottom sediment that will assist the development of sediment transport models.
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