We examined the chemical reactions influencing dissolved concentrations, speciation, and transport of naturally occurring arsenic (As) in a shallow, sand and gravel aquifer with distinct geochemical zones resulting from land disposal of dilute sewage effluent. The principal geochemical zones were: (1) the uncontaminated zone above the sewage plume [350 μM dissolved oxygen (DO), pH 5.9]; (2) the suboxic zone (5 μM DO, pH 6.2, elevated concentrations of sewage-derived phosphate and nitrate); and (3) the anoxic zone [dissolved iron(II) 100–300 μM, pH 6.5–6.9, elevated concentrations of sewage-derived phosphate]. Sediments are comprised of greater than 90% quartz but the surfaces of quartz and other mineral grains are coated with nanometer-size iron (Fe) and aluminum (Al) oxides and/or silicates, which control the adsorption properties of the sediments. Uncontaminated groundwater with added phosphate (620 μM) was pumped into the uncontaminated zone while samples were collected 0.3 m above the injection point. Concentrations of As(V) increased from below detection (0.005 μM) to a maximum of 0.07 μM during breakthrough of phosphate at the sampling port; As(III) concentrations remained below detection. These results are consistent with the hypothesis that naturally occurring As(V) adsorbed to constituents of the coatings on grain surfaces was desorbed by phosphate in the injected groundwater. Also consistent with this hypothesis, vertical profiles of groundwater chemistry measured prior to the tracer test showed that dissolved As(V) concentrations increased along with dissolved phosphate from below detection in the uncontaminated zone to approximately 0.07 and 70 μM, respectively, in the suboxic zone. Concentrations of As(III) were below detection in both zones. The anoxic zone had approximately 0.07 μM As(V) but also had As(III) concentrations of 0.07–0.14 μM, suggesting that release of As bound to sediment grains occurred by desorption by phosphate, reductive dissolution of Fe oxides, and reduction of As(V) to As(III), which adsorbs only weakly to the Fe-oxide-depleted material in the coatings. Results of reductive extractions of the sediments suggest that As associated with the coatings was relatively uniformly distributed at approximately 1 nmol/g of sediment (equivalent to 0.075 ppm As) and comprised 20%-50% of the total As in the sediments, determined from oxidative extractions. Quartz sand aquifers provide high-quality drinking water but can become contaminated when naturally occurring arsenic bound to Fe and Al oxides or silicates on sediment surfaces is released by desorption and dissolution of Fe oxides in response to changing chemical conditions.
This paper presents statistical aspects related to the calibration process and a comparison of different regression approaches of relevance to almost all analytical techniques. The models for ordinary least-squares (OLS), weighted least-squares (WLS), and maximum likelihood fitting (MLF) were evaluated and, as a case study, X-ray fluorescence (XRF) calibration curves for major elements in geochemical reference materials were used. The results showed that WLS and MLF models were statistically more consistent in comparison with the usually applied OLS approach. The use of uncertainty on independent and dependent variables during the calibration process and the calculation of final uncertainty on individual results using error propagation equations are the novel aspects of our work. 相似文献
The third and final article in this series about employment conditions in geography addresses the issue of future demand in both academic and nonacademic settings. To gain an understanding of future demand conditions in colleges and universities, we projected the retirement of AAG members by topical specialty and then matched these retirement trends with a profile of new faculty searches as reported by geography department chairs. We assessed the likely future demand for geography teachers at the precollegiate level through a survey of Geography Alliance Coordinators about teacher certification requirements and the education environments in their respective states. We speculated on how the kinds of jobs geographers do will be affected by changes now underway in the national and global economies. And finally, we conducted a small telephone survey of AAG corporate sponsors to determine how future business trends will affect the demand for geographers. 相似文献
The calanoid copepod community was surveyed semi-monthly, from May to July 1992, at three stations in the Navesink-Shrewsbury rivers system, the southernmost branch of the Hudson-Raritan estuary (New York-New Jersey). The dominant species collected during the survey wasAcartia hudsonica, followed byA. tonsa. A comparison of this survey with three earlier surveys suggests that the calanoid copepod community and relative abundance of dominant species have not changed substantially since the 1960s. Findings from a 1972 study, which noted the absence ofA. hudsonica andPseudodiaptomus coronatus as dominant species, were probably reflecting a temporary situation. The variations may have been related to a change in water quality, caused by an upgrade in sewage treatment, completed just prior to the 1972 survey, and/or resulted from the residual effects of Tropical Storm Agnes on this estuary. 相似文献
The granites orthogneisses, hydrothermal albitities and rocks which have suffered uranium mineralization from the Lagoa Real District (South State of Bahia, Brazil) have been investigated by U-Pb, Rb-Sr and Sm-Nd techniques. U-Pb values on zircons from the granitic protolith give an age of 1725 Ma; U-Pb on U-mineralization dates the primary mineralization at 1395 Ma and indicates a reworking at 480 Ma, which may represent the age of the thrusting of the Lagoa Real complex over Espinhaço metasediments during the Brazilian orogeny. These two dates are given by Rb-Sr on albitites, but from sparse information and are not supported by unequivocal arguments. The uranium deposition and sodium metasomatism, however, cannot be linked either with the thermal history of the granite or with Brazilian thrusting. Sm-Nd gives scattered results which are suggestive of autochtonous reworkings of REEs. These results lead to the following suggestions (1) magmatic activity of subalkaline affinity existed within the Sao Francisco craton at about 1.7 Ga (2) such subalkaline plutons are likely sources for U-mineralization (3) at about 1.4 Ga an unknown event caused hydrothermal activity leading to U-deposition at Lagoa Real (4) Brazilian overthrusting at about 480 Ma did not play any genetic role in the hydrothermal activity. 相似文献
The REE content and isotopic composition of Sr and Nd have been determined in fish teeth ranging in age from the Trias to the present and from various localities mostly around the Atlantic. These measurements have been carried out on Selachian and Teleost remains from the same locality in Togo and show no appreciable difference, which suggests, with the help of a mass balance calculation of the Ce anomaly, that diagenetic effects are not responsible for the REE enrichment of biogenic phosphates.One group of fossil teeth has about 3 times the REE abundances of shale and a shale-normalized pattern with a minimum at Sm: it is thought to reflect deposition in the open-sea environment. A second group has REE concentration about 10 times higher than the first group with either a regular light REE enrichment or, more frequently, a maximum in the middle REE, both being probably indicative of deposition in estuarine or near-shore conditions. The shape of the REE spectra and the size of the Ce anomaly can be used semi-quantitatively to determine the depth of deposition. The results presented here on Late Cretaceous/Eocene fish teeth samples from Morocco reflect an increasing influx of deep waters with a lowLa/Yb ratio and strong negative Ce anomaly, which agrees well with the evolution of sediment chemistry and microfauna associations.In contrast, Nd is typical of the water mass in which the fish debris decayed. Examples of nearly isolated basins identified with Nd isotopes include the South Atlantic prior to the Lutetian (Nd ≈ −13.5), the Miocene Persian Gulf (Nd = −3.1), and Bolivia during the Late Cretaceous (Nd = −12.8). Togo and Guinea-Bissau results suggest that, in the South Atlantic, the meridional oceanic circulation had not started before 45 Ma ago.Combination of REE andNd data suggests that the assignment of Jurassic-Cretaceous samples measured so far to open-sea water masses is still ambiguous. 相似文献
The intraplate Ancestral Rocky Mountains of western North America extend from British Columbia, Canada, to Chihuahua, Mexico, and formed during Early Carboniferous through Early Permian time in response to continent–continent collision of Laurentia with Gondwana—the conjoined masses of Africa and South America, including Yucatán and Florida. Uplifts and flanking basins also formed within the Laurentian Midcontinent. On the Gondwanan continent, well inboard from the marginal fold belts, a counterpart structural array developed during the same period. Intraplate deformation began when full collisional plate coupling had been achieved along the continental margin; the intervening ocean had been closed and subduction had ceased—that is, the distinction between upper versus lower plates became moot. Ancestral Rockies deformation was not accompanied by volcanism. Basement shear zones that formed during Mesoproterozoic rifting of Laurentia were reactivated and exerted significant control on the locations, orientations, and modes of displacement on late Paleozoic faults.Ancestral Rocky Mountain uplifts extend as far south as Chihuahua and west Texas (28° to 33°N, 102° to 109°W) and include the Florida-Moyotes, Placer de Guadalupe–Carrizalillo, Ojinaga–Tascotal and Hueco Mountain blocks, as well as the Diablo and Central Basin Platforms. All are cored with Laurentian Proterozoic crystalline basement rocks and host correlative Paleozoic stratigraphic successions. Pre-late Paleozoic deformational, thermal, and metamorphic histories are similar as well. Southern Ancestral Rocky Mountain structures terminate along a line that trends approximately N 40°E (present coordinates), a common orientation for Mesoproterozoic extensional structures throughout southern to central North America.Continuing Tien Shan intraplate deformation (Central Asia) has created an analogous array of uplifts and basins in response to the collision of India with Eurasia, beginning in late Miocene time when full coupling of the colliding plates had occurred. As in the Laurentia–Gondwana case, structures of similar magnitude and spacing to those in Eurasia have developed in the Indian plate. Within the present orogen two ancient suture zones have been reactivated—the early Paleozoic Terskey zone and the late Paleozoic Turkestan suture between the Siberian and East Gondwanan cratons. Inverted Proterozoic to early Paleozoic rift structures and passive-margin deposits are exposed north of the Terskey zone. In the Alay and Tarim complexes, Vendian to mid-Carboniferous passive-margin strata and the subjacent Proterozoic crystalline basement have been uplifted. Data on Tien Shan uplifts, basins, structural arrays, and deformation rates guide paleotectonic interpretations of ancient intraplate mountain belts. Similarly, exhumed deep crustal shear zones in the Ancestral Rockies offer insight into partitioning and reorientation of strain during contemporary intraplate deformation. 相似文献
