A finite-volume ELLAM for three-dimensional solute-transport modeling

A three-dimensional finite-volume ELLAM method has been developed, tested, and successfully implemented as part of the U.S. Geological Survey (USGS) MODFLOW-2000 ground water modeling package. It is included as a solver option for the Ground Water Transport process. The FVELLAM uses space-time finite volumes oriented along the streamlines of the flow field to solve an integral form of the solute-transport equation, thus combining local and global mass conservation with the advantages of Eulerian-Lagrangian characteristic methods. The USGS FVELLAM code simulates solute transport in flowing ground water for a single dissolved solute constituent and represents the processes of advective transport, hydrodynamic dispersion, mixing from fluid sources, retardation, and decay. Implicit time discretization of the dispersive and source/sink terms is combined with a Lagrangian treatment of advection, in which forward tracking moves mass to the new time level, distributing mass among destination cells using approximate indicator functions. This allows the use of large transport time increments (large Courant numbers) with accurate results, even for advection-dominated systems (large Peclet numbers). Four test cases, including comparisons with analytical solutions and benchmarking against other numerical codes, are presented that indicate that the FVELLAM can usually yield excellent results, even if relatively few transport time steps are used, although the quality of the results is problem-dependent.     

Advection and removal of Pb and stable Pb isotopes in the oceans: a general circulation model study

Natural Pb-isotope variability in the oceans encodes information about the sources of continental material to the oceans, about ocean circulation, and about Pb removal. In order to use this information, we must understand the natural cycle of Pb in the oceans, which is overprinted by large anthropogenic input. In this study we use ²¹⁰Pb, which has not been significantly anthropogenically perturbed, to investigate oceanic Pb. GEOSECS ²²⁶Ra and model-derived atmospheric fluxes of ²¹⁰Pb are used to input ²¹⁰Pb into an ocean general circulation model. Irreversible scavenging of this ²¹⁰Pb onto settling biogenic particles and at the seafloor are tuned so that the model replicates the observed pattern of ²¹⁰Pb in the oceans. The best-fit model run provides a map of the variability of residence time for Pb. The global average residence time of Pb in this model is 48 yr, but there is over an order of magnitude variation between areas of high and low productivity. This is expected to enhance provinciality of Pb isotope ratios in the oceans. Because stable Pb isotopes are expected to behave in seawater in a similar fashion to ²¹⁰Pb, the pattern of removal of ²¹⁰Pb derived by the model can be used to investigate the behavior of stable Pb isotopes. We use a simplified input of Pb consisting of five point sources representing major rivers and a disseminated dust input. Although this simplified input scheme does not allow precise reconstruction of Pb concentration and isotopes in the oceans, it allows us to answer some first-order questions about the behavior of Pb as an ocean tracer. With a total Pb input of 6.3 × 10⁷ mol/yr (Chow T. J. and Patterson C. C., “The occurrence and significance of Pb isotopes in pelagic sediments,” Geochim. Cosmochim. Acta26, 263-308, 1962), the model predicts natural seawater Pb concentrations averaging 2.2 pmol/kg. Even in the absence of anthropogenic input, the model ocean exhibits a near-surface maximum in Pb concentration. And the model suggests natural Pb concentrations in the Northern Hemisphere an order of magnitude higher than in the Southern Hemisphere. A point source of Pb is suggested to dominate the seawater Pb inventory close to the source but is reduced to typically less than 10% of the inventory by the time that Pb is advected out of the originating ocean. This length scale of advection for Pb isotope signals confirms their potential as tracers of ocean circulation. Assuming an 8% dissolution of dust, their input to the open ocean are seen to be a significant portion of Pb inventory throughout the oceans and make up >50% of the Pb inventory in the model’s Southern Ocean. Finally, a preliminary investigation of the response of Pb-isotope distributions to changes in boundary conditions between glacial and interglacial times illustrates that significant variation in the Pb isotopes are expected in some regions, even for reasonably small changes in climate conditions.     

Marginal-water application in arid zones

The intensified efforts to improve agricultural economy in arid zones is associated with increased demand for water. Scarcity of potable water has hightened the importance of marginal water in dry regions. Marginal water includes saline water, wastewater, and runoff water. These water sources differ in location in regards to the application site, quality, quantity, and stability of supply. Previous experience proved that each of the above alternative water sources can be used separately or in combination, for the economic benefit of the farmers.     

Residence of silver in mineral deposits of the Thunder Mountain caldera complex,Central Idaho,U.S.A.

Summary Silver is an accessory element in gold, antimony, and tungsten deposits of the caldera complex. Most of the deposits are economically of low grade and genetically of xenothermal or epithermal character. Their gold- and silver-bearing minerals are usually disseminated, fine grained, and difficult to study. Sparsely disseminated pyrite and arsenoprite are common associates.Identified silver minerals are: native silver and electrum; the sulfides acanthite, argentite (the latter always inverted to acanthite), and members of the Silberkies group; the sulfosalts matildite, miargyrite, pyrargyrite, argentian tetrahedrite, and unnamed Ag-Sb-S and Ag-Fe-Sb-S minerals; the telluride hessite and the selenide naumannite; halides of the cerargyrite group; and the antimonate stetefeldtite. Suspected silver minerals include the sulfide uytenbogaardtite and the sulfosalts andorite, diaphorite, and polybasite. Electrum, acanthite, and argentian tetrahedrite are common, though nowhere abundant. The other silver minerals are rare.Silver is present as a minor element in the structure of some varieties of other minerals. These include arsenopyrite, chalcopyrite, chalcostibite, covelline, digenite, galena, sphalerite, and stibnite. The search for adventitious Ag in most of these minerals has been cursory. The results merely indicate that elemental silver is not confined to discrete silver minerals and is, therefore, an additional complication for the recovery of silver-bearing material from some deposits.Silver occurs cryptically in some plants of the region. At Red Mountain, for example, the ashed sapwood of Douglas-fir (Pseudotsuga menziesii) contains 2 to 300 ppm Ag. Silver in the ashed wood is roughly 100 times as abundant as it is in soil. The phenomenon, useful in biogeochemical exploration, deserves the attention of mineralogists.

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Das Vorkommen von Silber in den Erzlagerstätten des Thunder-Mountain-Caldera-Komplexes, Idaho, U.S.A.

Zusammenfassung Silber ist in den Gold-, Antimon- und Wolframlagerstätten des Caldera-Komplexes ein akzessorisches Element. Die Lagerstätten sind wirtschaftlich geringhaltig und genetisch als xeno- oder epithermal zu deuten. Gold- und silberführende Minerale sind meist feinkörnig und treten akzessorisch auf. Im einzelnen wurden folgende Minerale beobachtet: Gediegen Silber und Elektrum; die Sulfide Akanthit und Argentit (immer zu Akanthit umgewandelt) sowie Silberkiese; die Sulfosalze Matildit, Miargyrit, Pyrargyrit, silberhaltiger Tetraedrit und nicht näher identifizierte Ag-Sb-S- und Ag-FeSb-S-Verbindungen; das Tellurid Hessit und das Selenid Naumannit; Halogenide der Kerargyritgruppe; und das Antimonat Stetefeldit. Mögliche weitere Ag-Minerale schließen das Sulfid Uytenbogaardtit und die Sulfosalze Andorit, Diaphorit und Polybasit ein. Von diesen Mineralen kommen Elektrum, Akanthit und silberhaltiger Tetraedrit am häufigsten vor, während alle anderen selten sind. Die gewöhnlichen Nebengemengteile sind Pyrit und Arsenkies.Darüber hinaus wird vermutet, daß Spuren von Silber im Kristallgitter anderer Minerale eingebaut vorliegen; insbesondere in den Gittern von Arsenkies, Kupferkies, Chalcostibit, Covellin, Digenit, Bleiglanz, Zinkblende und Antimonit. Hierzu liegen noch keine umfassenden Untersuchungen vor, jedoch kann vermutet werden, daß Silber nicht ausschließlich auf Ag-Minerale sensu stricto beschränkt ist.Einige Pflanzen des Lagerstättenbezirkes enthalten beachtliche Mengen an Silber, so zum Beispiel die Asche der Douglas-FichtePseudotsuga menziesii. Der Silbergehalt beträgt 2 bis 300 g/t und ist damit gegenüber dem Boden etwa 100fach erhöht. Dieses Phänomen ist von Nutzen in der biogeochemischen Exploration und verdient das Interesse der Mineralogen.

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With 10 Figures

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Contribution to the Ore Mineralogy Symposium (IMA/COM) at the 14th General Meeting of the International Mineralogical Association, at Stanford, California, in July, 1986.     

Phytoplankton dynamics within Gulf Stream intrusions on the southeastern United States continental shelf during summer 1981

During July and August 1981 subsurface intrusion of upwelled nutrient-rich Gulf Stream water was the dominant process affecting temporal and spatial changes in phytoplankton biomass and productivity of the southeastern United States continental shelf between 29 and 32°N latitude. Intruded waters in the study area covered as much as 10¹ km including virtually all of the middle and outer shelf and approximately 50% of the inner shelf area.Within 2 weeks following a large intrusion event in late July, middle shelf primary production and Chl a reached 3 to 4 gC m⁻ d⁻¹ and 75 mg m⁻, respectively. At the peak of the bloom 80% of the water column primary production occurred below the surface mixed-layer, and new primary production (i.e., NO₃-supported) exceeded 90% of the total. Chl a-normalized photosynthetic rates were very high as evidenced by high mean assimilation number (15.5 mg C mg Chl a⁻¹ h⁻¹), high mean α (14 mg C mg Chl a⁻¹ Ein⁻¹ m), and no photoinhibition. As a result of the high photosynthetic rates, mean light-utilization index (Ψ) was 2 to 3 times higher than reported for temperature sub-arctic and arctic waters.The results imply a seasonal (June to August) middle shelf production of 150 g C m⁻¹, about 15% higher than previous estimates of annual production on the middle shelf. Intrusions of the scale we observed in 1981 may not occur every summer. However, when such events do occur, they are by far the most important processes controlling summer phytoplankton dynamics of the middle and outer shelf and of the inner shelf in the southern half of the study area.     

The joint effect of rank and grain size on the flotation of Australian bituminous coals

Laboratory batch flotation tests were carried out to determine the effect of grain size on the recovery of four Australian bituminous coals of high and low rank. The flotation concentrates, taken over successive time intervals, and the tailings, were sized and the recovery-size curves plotted. A method was developed for determining the proportions of liberated and composite grains in the various flotation products. Flotation tests were carried out in the presence of methylisobutylcarbinol (MIBC) and MIBC + kerosene. The shapes of the recovery-size curves depended on the rank of the coal and whether or not kerosene was present. The variation of the recovery with size was explained in terms of the joint effect of rank and size on the floatability of composite grains.     

Arecibo radar observations of Mars surface characteristics in the northern hemisphere

Mars surface characteristics at and near the Viking Chryse and Tritonis Lacus landing areas were determined by radio scatter using the new 12.6 cm radar at the Arecibo Observatory during 1975–1976. Interpretation of each power spectrum suggests rms surface tilts of 4° at the final A1WNW (47.9°W, 22.5°N) site, 5° near the original A1 site, and 6° between the two. At the back-up site (A2) surface roughness estimates were about 4°. Striking changes in surface texture have been found near the eastern bases of Tharsis Montes and Albor Tholus, each volcanic feature marking the western boundary of very smooth surface units. The roughness sensed at 1 to 100 m scales by radar appears to be relatively independent of the surface units defined at large scale lengths by photogeologists. Radar properties thus provide an additional means by which planetary surfaces may be characterized.     

United States' Nationwide Differential Global Positioning System

This article describes the Nationwide Differential Global Positioning System (NDGPS) service being developed in the United States and the enormous benefits to federal agencies, state governments, and the general public. ? 1999 John Wiley & Sons, Inc.     

The neighborhood effects of foreclosure

Neighborhood quality is an important attribute of housing yet its value is rarely known to researchers. We argue that changes in nearby foreclosures reveal changes in neighborhood quality. Thus estimates of the hedonic price of nearby foreclosures provide a glimpse of values that people hold for local neighborhood quality. The empirical models include controls for both spatial dependence in housing prices and in the errors. The estimates indicate that nearby foreclosures produce externalities that are capitalized into home prices—an additional foreclosure within 250 feet of a sale negatively impacts selling price by approximately $1,666, ceteris paribus.