United States Geological Survey

About: United States Geological Survey is a government organization based out in Reston, Virginia, United States. It is known for research contribution in the topics: Population & Groundwater. The organization has 17899 authors who have published 51097 publications receiving 2479125 citations. The organization is also known as: USGS & US Geological Survey.

Subsurface Valleys and Geoarcheology of the Eastern Sahara Revealed by Shuttle Radar

Abstract: The shuttle imaging radar (SIR-A) carried on the space shuttle Columbia in November 1981 penetrated the extremely dry Selima Sand Sheet, dunes, and drift sand of the eastern Sahara, revealing previously unknown buried valleys, geologic structures, and possible Stone Age occupation sites. Radar responses from bedrock and gravel surfaces beneath windblown sand several centimeters to possibly meters thick delineate sand- and alluvium-filled valleys, some nearly as wide as the Nile Valley and perhaps as old as middle Tertiary. The now-vanished major river systems that carved these large valleys probably accomplished most of the erosional stripping of this extraordinarily flat, hyperarid region. Underfit and incised dry wadis, many superimposed on the large valleys, represent erosion by intermittent running water, probably during Quaternary pluvials. Stone Age artifacts associated with soils in the alluvium suggest that areas near the wadis may have been sites of early human occupation. The presence of old drainage networks beneath the sand sheet provides a geologic explanation for the locations of many playas and present-day oases which have been centers of episodic human habitation. Radar penetration of dry sand and soils varies with the wavelength of the incident signals (24 centimeters for the SIR-A system), incidence angle, and the electrical properties of the materials, which are largely determined by moisture content. The calculated depth of radar penetration of dry sand and granules, based on laboratory measurements of the electrical properties of samples from the Selima Sand Sheet, is at least 5 meters. Recent (September 1982) field studies in Egypt verified SIR-A signal penetration depths of at least 1 meter in the Selima Sand Sheet and in drift sand and 2 or more meters in sand dunes.

Quantitative x-ray diffraction analysis of clay-bearing rocks from random preparations

Abstract: An internal standard X-ray diffraction (XRD) analysis technique permits reproducible and accurate calculation of the mineral contents of rocks, including the major clay mineral families: Fe-rich chlorites 1 berthierine, Mg-rich chlorites, Fe-rich dioctahedral 2:1 clays and micas, Al-rich dioctahedral 2:1 clays and micas, and kaolinites. A single XRD pattern from an air-dried random specimen is used. Clays are quantified from their 060 reflections, which are well-resolved and insensitive to structural defects. Zincite is used as the internal standard instead of rorundum, because its reflections are more conveniently located and stronger, allowing for a smaller amount of spike (10%). The grinding technique used produces powders free of grains coarser than 20 mm and suitable for obtaining random and rigid specimens. Errors in accuracy are low, ,2 wt % deviation from actual values for individual minerals, as tested on artificial shale mixtures. No normalization is applied and thus, for natural rocks, the analysis is tested by the departure of the sum of the measured components from 100%. Our approach compares favorably with other quantitative analysis techniques, including a Rietveld-based technique. Key Words—Clay Minerals, Marls, Quantitative Analysis, Shales, X-ray Diffraction.

Plate tectonic emplacement of upper mantle peridotites along continental edges

Abstract: Recently developed ideas of global tectonics haye provided a new framework within which to consider the origin of alpine-type peridotites In plate theory, compressional zones associated with island arcs are considered to represent plate boundaries where oceanic lithosphere is subducted The subduction zones are characterized by lithospheric underthrusting, andesitic volcanoes, and deep seismic activity that generally dips under the continental edge (the Benioff zone) The presence of large oceanic-mantle crustal slabs thrust over or into continental edges contemporaneously with blueschist metamorphism in New Caledonia and New Guinea establishes an important variant of plate tectonics in the zones of compression The ‘obduction’ zones are characterized by a complete lack of volcanic activity and by high-pressure metamorphism During formation, they can be represented by shallow seismic zones dipping oceanward The common association of peridotites and blueschists in these orogenic belts may result from the initial stage of compressional impact (or orogeny) between an oceanic and a continental lithospheric plate Disturbed zones combined with a lack of high-temperature contacts at boundaries between cold mantle-peridotite slabs and trench sediments provide geologic evidence of emplacement by obduction (tectonic overriding) Internal subsolidus plastic deformation of these peridotites can be attributed to deep-seated strain within the upper mantle during spreading Serpentinites represent alteration developed during tectonic emplacement into wet sediments of the continental plate, which produces a less dense and plastic envelope that facilitates further tectonic movement in these compressional zones Recognition of these peridotite-serpentinite-blueschist belts within exhumed subduction or obduction zones will allow delineation of ancient compressional impacts between moving lithospheric plates

Entrainment of gravel from naturally sorted riverbed material

Abstract: The critical dimensionless shear stress, τ*ci, required to entrain a given size particle, di, was computed from extensive bedload-transport measurements in three self-formed rivers that have naturally sorted gravel and cobble bed material. The value of τ*ci was determined to be significantly affected by the size distribution of the riverbed material. For bed particles between 0.3 to 4.2 times the median diameter of the subsurface bed material, d50, the average critical dimensionless shear stress, τ*ci, is equal to τ*ci = 0.0834 (di/d50)-0.872 with a coefficient of determination of 0.980. For bed particles larger than 4.2 times d50, τ*ci appears to approach a constant value of approximately 0.020 in a noneroding channel. Thus, τ*ci varies from 0.25 to 0.020 for a given particle, depending on the ratio of di to d50. Previous investigations of the hydraulic conditions required to entrain coarse riverbed material have reported critical shear stress values ranging from 0.25 to 0.020 for a given particle size. This analysis indicates that virtually all of the variation is due to differences in the subsurface bed-material size distribution. This analysis shows that τ*ci varies almost inversely proportional to the particle diameter for a nonuniform bed material; therefore, bed particles between 0.3 to 4.2 times d50 are entrained at nearly the same discharge. An investigation of the hydraulic characteristics and bed-material mobility of 24 self-formed gravel-bed rivers in Colorado determined that particles as large as the median diameter of the bed surface were entrained by discharges equal to the bankfull stage or less. Particles as large as the 90th-percentile fraction were entrained by the bankfull discharge in nine of the rivers studied. Consequently, in these rivers, a significant fraction of the riverbed was entrained rather frequently over a period of years, although the transport rate was very small.

Presence and distribution of wastewater-derived pharmaceuticals in soil irrigated with reclaimed water

Abstract: Three sites in the Front Range of Colorado, USA, were monitored from May through September 2003 to assess the presence and distribution of pharmaceuticals in soil irrigated with reclaimed water derived from urban wastewater. Soil cores were collected monthly, and 19 pharmaceuticals, all of which were detected during the present study, were measured in 5-cm increments of the 30-cm cores. Samples of reclaimed water were analyzed three times during the study to assess the input of pharmaceuticals. Samples collected before the onset of irrigation in 2003 contained numerous pharmaceuticals, likely resulting from the previous year's irrigation. Several of the selected pharmaceuticals increased in total soil concentration at one or more of the sites. The four most commonly detected pharmaceuticals were erythromycin, carbamazepine, fluoxetine, and diphenhydramine. Typical concentrations of the individual pharmaceuticals observed were low (0.02-15 microg/kg dry soil). The existence of subsurface maximum concentrations and detectable concentrations at the lowest sampled soil depth might indicate interactions of soil components with pharmaceuticals during leaching through the vadose zone. Nevertheless, the present study demonstrates that reclaimed-water irrigation results in soil pharmaceutical concentrations that vary through the irrigation season and that some compounds persist for months after irrigation.