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Golden, CO, United States

Spiller D.E.,Colorado School of Mines | Norgren C.A.,TetraTech Inc.
Mineral Processing and Extractive Metallurgy: 100 Years of Innovation | Year: 2014

The separation and upgrading of heavy mineral sands is accomplished by a combination of wet and dry processing. The wet side comprises liberation by scrubbing, size classification, and various techniques of gravity separation to produce a bulk (total) heavy mineral concentrate. The dry side, treating size classified total heavy mineral concentrate, principally involves the use of magnetic and electrostatic separation to produce marketable concentrates of rutile, ilmenite, zircon, monazite, and other heavy minerals. These various separating unit operations were invented, developed and/or adopted as the result of issues experienced at the operational level but ultimately driven by market forces. Source

Van Gaalen J.F.,University of South Florida | Kruse S.,University of South Florida | Lafrenz W.B.,TetraTech Inc. | Burroughs S.M.,The University of Tampa
GroundWater | Year: 2013

A rise in water table in response to a rainfall event is a complex function of permeability, specific yield, antecedent soil-water conditions, water table level, evapotranspiration, vegetation, lateral groundwater flow, and rainfall volume and intensity. Predictions of water table response, however, commonly assume a linear relationship between response and rainfall based on cumulative analysis of water level and rainfall logs. By identifying individual rainfall events and responses, we examine how the response/rainfall ratio varies as a function of antecedent water table level (stage) and rainfall event size. For wells in wetlands and uplands in central Florida, incorporating stage and event size improves forecasting of water table rise by more than 30%, based on 10 years of data. At the 11 sites studied, the water table is generally least responsive to rainfall at smallest and largest rainfall event sizes and at lower stages. At most sites the minimum amount of rainfall required to induce a rise in water table is fairly uniform when the water table is within 50 to 100 cm of land surface. Below this depth, the minimum typically gradually increases with depth. These observations can be qualitatively explained by unsaturated zone flow processes. Overall, response/rainfall ratios are higher in wetlands and lower in uplands, presumably reflecting lower specific yields and greater lateral influx in wetland sites. Pronounced depth variations in rainfall/response ratios appear to correlate with soil layer boundaries, where corroborating data are available. © 2012, The Author(s). Groundwater © 2012, National Ground Water Association. Source

Deverel S.J.,HydroFocus Inc. | Lucero C.E.,HydroFocus Inc. | Bachand S.,TetraTech Inc.
San Francisco Estuary and Watershed Science | Year: 2015

We used available data to estimate changes in land use and wet, non-farmable, and marginally farmable (WNMF) areas in the Delta from 1984 to 2012, and developed a conceptual model for processes that affect the changes observed. We analyzed aerial photography, groundwater levels, land-surface elevation data, well and boring logs, and surface water elevations. We used estimates for sea level rise and future subsidence to assess future vulnerability for the development of WNMF areas. The cumulative WNMF area increased linearly about 10-fold, from about 274 hectares (ha) in 1984 to about 2,800 ha in 2012. Moreover, several islands have experienced land use changes associated with reduced ability to drain the land. These have occurred primarily in the western and central Delta where organic soils have thinned; there are thin underlying mud deposits, and drainage ditches have not been maintained. Subsidence is the key process that will contribute to future increased likelihood of WNMF areas by reducing the thickness of organic soils and increasing hydraulic gradients onto the islands. To a lesser extent, sea level rise will also contribute to increased seepage onto islands by increasing groundwater levels in the aquifer under the organic soil and tidal mud, and increasing the hydraulic gradient onto islands from adjacent channels. WNMF develop from increased seepage under levees, which is caused by changing flow paths as organic soil thickness has decreased. This process is exacerbated by thin tidal mud deposits. Based primarily on projected reduced organic soil thickness and land-surface elevations, we delineated an additional area of about 3,450 ha that will be vulnerable to reduced arability and increased wetness by 2050. Source

Etienne E.,U.S. Federal Aviation Administration | Hinton V.,U.S. Federal Aviation Administration | Frodge S.,U.S. Federal Aviation Administration | Achanta R.,TetraTech Inc.
26th International Technical Meeting of the Satellite Division of the Institute of Navigation, ION GNSS 2013 | Year: 2013

DME service volumes are currently described as being identical to the service volume of the VOR with which they are co-located. This has been a sensible arrangement as the VOR/DME associated NAVAIDS each provides aircraft with the ability to navigate an established airway - the VOR providing azimuth information and the DME providing the distance to or from VOR. The VOR service volume is often described as an "upside-down wedding cake" because of its unique shape (See figure 1 below). The shape reflects the regulatory availability of navigation services and not how Radionavigation signals actually propagate. The current VOR service volume expands in radius at the system imposed transitions between terminal, low en-route, and high en-route airspace. This arrangement has served the NAS well for many decades, supporting point-to-point navigation and the safety of flight. However, when aircraft began to use multiple DMEs to support RNAV, it quickly became apparent that disassociating the DME and the VOR and changing the service volume of DMEs to better align with the laws of physics would be most prudent to provide the desired RNAV services. Many DMEs only officially serve terminal and low en-route airspace. Several have antenna patterns that may limit propagation at low take off angles, and in some cases broadcast at less than 1000 watts. Therefore to provide the NAS with a robust and reliable DME infrastructure to support RNAV operations, test and analyses of the current regulatory structure and actual propagation aspects of the various DMEs in operation is required. The International Civil Aviation Organization (ICAO) Annex 10 recommendation for co-channel DME protection limits is 8 dB; however since 1982 the FAA has historically required this protection limit to be 11 dB, to account for peak power deterioration in DME transponders. To determine whether the additional 3 dB of protection over the ICAO recommended level needs to be continued in determining the new service volume of an unassociated DME - FAA's Navigation Program Engineering Team, Spectrum Engineering Services and test personnel at the William J. Hughes Technical Center conducted a series of flight tests and completed the engineering analysis necessary to demonstrate that, for the current generation of DME transponders, the additional 3 dB of interference protection for peak power deterioration is not required and that the ICAO recommended 8 dB protection limit will provide ample interference protection for the DME systems operating in the NAS The team tested and compared the individual performance of eight different DME antennae: DB Systems LPTA, DB Systems 510A, and DB Systems 5100A (Omni), DB Systems 5100A-BD (bidirectional), DB Systems 5100A-D (directional), ITT/WILCOX RTA-2 (TACAN), Cardion, and the Montek. This paper focuses on the flight test setup, procedures and engineering analyses. The results of the testing and other necessary activities will be used in part to support procurement and installation determinations leading to operation of stand-alone DMEs to support the NextGen goal of moving to a performance-based NAS. Source

Elias E.,New Mexico State University | Rodriguez H.,TetraTech Inc. | Srivastava P.,Auburn University | Dougherty M.,Auburn University | And 2 more authors.
Forests | Year: 2016

We used coupled watershed and reservoir models to evaluate the impacts of deforestation and l Niño Southern Oscillation (ENSO) phase on drinking water quality. Source water total organic carbon (TOC) is especially important due to the potential for production of carcinogenic disinfection byproducts (DBPs). The Environmental Fluid Dynamics Code (EFDC) reservoir model is used to evaluate the difference between daily pre- and post- urbanization nutrients and TOC concentration. Post-disturbance (future) reservoir total nitrogen (TN), total phosphorus (TP), TOC and chlorophyll-a concentrations were found to be higher than pre-urbanization (base) concentrations (p < 0.05). Predicted future median TOC concentration was 1.1 mg· L-1 (41% higher than base TOC concentration) at the source water intake. Simulations show that prior to urbanization, additional water treatment was necessary on 47% of the days between May and October. However, following simulated urbanization, additional drinking water treatment might be continuously necessary between May and October. One of six ENSO indices is weakly negatively correlated with the measured reservoir TOC indicating there may be higher TOC concentrations in times of lower streamflow (La Niña). There is a positive significant correlation between simulated TN and TP concentrations with ENSO suggesting higher concentrations during El Niño. © 2016 by the authors. Source

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