Long-term sedimentation rate in Black Lake, Alaska

NOTICE OF INTENT TO AWARD SOLE SOURCE: U. S. Army Corps of Engineers Alaska District, Elmendorf AFB, AK intends to award a sole source purchase order to Northern Arizona University, to determine the sedimentation rate in Blake Lake, Located near Chignik, Alaska. Water depth in the lake has decreased significantly during the last several decades, with major impacts to salmon production. Quantifying the rates of infilling is needed to inform management decisions and design specifications for any future water-control structure. Dated marker horizons in sediment cores will be used to quantify the amount of sediment that has accumulated at spatially distributed sites across the lake. This purchase order will be made under Other Than Full and Open Competition as authorized by FAR 6.302-1(a)(2)(ii). A suite of four to six sediment cores from 0.5 to 1.5 m long will be taken from sites across Black Lake. The cores will be split in the field and stabilized for shipment to NAU's Sedimentary Records of Environmental Change Laboratory. They will be photographed and logged at centimeter scale for their macro-stratigraphy, including the location of volcanic ash (tephra) beds. Magnetic susceptibility (MS) will be measured on all split-core surfaces at intervals of 0.5 cm using a Bartington meter and probe. The tops of two cores will be sampled for Pu content to locate the depth of the onset of nuclear weapons testing (1953) and the peak of Pu production (1963). The analyses will be by inductively coupled plasma mass spectroscopy (IPCMS) at NAU's Geochemistry Laboratory. One or two cores will also be analyzed for radiocarbon ( 14 C by accelerator mass spectrometry at University of California, Irvine) to generate a sedimentation-rate model that extends centuries to millennia, depending on core length and sedimentation rate. Prominent tephra layers will be analyzed petrographically and geochemically by the USGS-AVO Tephra Laboratory to indentify tephras produced by known eruptions. Sediment bulk density will be measured at 1 cm intervals to quantify sedimentation rates in terms of mass flux (which accounts for changes in porosity) as well as thickness (which does not). The cores will be stored under refrigeration at NAU for at least five years, where they will be made available for additional analyses on request. The marker horizons (MS peaks, tephras, and other distinctive sedimentary layers) will be used to construct the stratigraphic framework from across the lake. The age control will be mapped onto this framework to produce a sedimentation-rate model by fitting the age-depth control points with a spline curve, as documented in the literature. The age model will be integrated with site-to-site correlations reconstruct changes in sedimentation rates through time at the core sites. Sediment traps will be deployed in the lake to quantify sediment accumulation between July 2010 and the time of retrieval. The traps will be installed on mooring lines strung between a rock anchor and a buoy positioned below the likely depth of lake ice. Each mooring will include plastic sediment traps with different focusing factors to passively collect sediment near the lake floor, two temperature loggers (HOBO Water Temp Pro or Tidbit positioned at top and bottom of the water column), and a radio transmitter with battery life of 10 years (Advanced Telemetry Systems) to aid in retrieval. Two such moorings will be installed in the lake and can be used to attach any other instrumentation supplied by COE or UW (e.g., water-level logger). GPS coordinates will be recorded upon deployment. A final report will document the field and laboratory procedures and to present the results. The primary product will be quantities estimates of sedimentation rates (mass and thickness) at several sites and over intervals ranging from decades to centuries. The anticipated award date is June 15, 2010. This notice is neither a request for competitive proposals nor a solicitation of offers.