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31.
Linking organic matter deposition and iron mineral transformations to groundwater arsenic levels in the Mekong delta,Cambodia 总被引：1，自引：0，他引：1
Enriched As in drinking water wells in south and Southeast Asia has increased the risk of cancer for nearly 100 million people. This enrichment is generally attributed to the reductive dissolution of Fe oxides; however, the complex expression of As enrichment in these areas is not yet well understood. Here, the coupled sedimentological and geochemical factors that contribute to the extent and spatial distribution of groundwater As concentrations in the Mekong River delta, Cambodia in an avulsed scroll bar sequence are examined. X-Ray absorption spectroscopy (XAS) was used to determine Fe and As speciation in redox preserved sediment collected from drilled cores. Dissolved As, Fe and S solution concentrations in existing and newly drilled wells (cores) differed considerably depending on their source sedimentology. The rapid burial of organic matter in the scroll bar sequence facilitated the development of extensive Fe-reducing conditions, and As release into the aquifer. In older features organic C levels are high enough to sustain extensive Fe reduction and provide ample SO4 which is reduced to sulfide. This S reduction impacts As levels; As is sequestered in sulfide minerals outside of the scrollbar sequence, decreasing pore water concentrations. In contrast, As is depleted in sediments from the scroll sequence, and associated with elevated pore water aqueous concentrations. The concentration and form of organic C in the scrollbar sequence is related to depositional environment, and can facilitate Fe and S mineral transformations, distinct sedimentary environments explain a portion of the inherent heterogeneity of aquifer As concentrations. 相似文献
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Influence of vertical channel change associated with wood accumulations on delineating channel migration zones, Washington, USA 总被引：1，自引：1，他引：1
Chris J. Brummer Tim B. Abbe Jennifer R. Sampson David R. Montgomery 《Geomorphology》2006,80(3-4):295-309
We combine hydraulic modeling and field investigations of logjams to evaluate linkages between wood-mediated fluctuations in channel-bed-and water-surface elevations and the potential for lateral channel migration in forest rivers of Washington state. In the eleven unconfined rivers we investigated, logjams were associated with reduced channel gradient and bank height. Detailed river gauging and hydraulic modeling document significant increases in the water-surface elevation upstream of channel-spanning wood accumulations. Logjams initiated lateral channel migration by increasing bed-or water-surface elevations above adjacent banks. Because the potential for a channel to avulse and migrate across its floodplain increases with the size and volume of instream wood, the area of the valley bottom potentially occupied by a channel over a specified timeframe — the channel migration zone (CMZ) — is dependent on the state of riparian forests. The return of riparian forests afforded by current land management practices will increase the volume and caliber of wood entering Washington rivers to a degree unprecedented since widespread clearing of wood from forests and rivers nearly 150 years ago. A greater supply of wood from maturing riparian forests will increase the frequency and spatial extent of channel migration relative to observations from wood-poor channels in the period of post-European settlement. We propose conceptual guidelines for the delineation of the CMZs that include allowances for vertical fluctuations in channel elevation caused by accumulations of large woody debris. 相似文献
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Previous studies have identified unpaved roads as the primary source of erosion on St John in the US Virgin Islands, but these studies estimated road erosion rates only as annual averages based primarily on road rill measurements. The goal of this project was to quantify the effect of unpaved roads on runoff and sediment production on St John, and to better understand the key controlling factors. To this end runoff and sediment yields were measured from July 1996 to March 1997 from three plots on naturally vegetated hillslopes, four plots on unpaved road surfaces and two cutslope plots. Sediment yields were also measured from seven road segments with contributing areas ranging from 90 to 700 m2. With respect to the vegetated plots, only the two largest storm events generated runoff and there was no measurable sediment yield. Runoff from the road surface plots generally occurred when storm precipitation exceeded 6 mm. Sediment yields from the four road surface plots ranged from 0·9 to 15 kg m−2 a−1, and sediment concentrations were typically 20–80 kg m−3. Differences in runoff between the two cutslope plots were consistent with the difference in upslope contributing area. A sprinkler experiment confirmed that cross‐slope roads intercept shallow subsurface stormflow and convert this into surface runoff. At the road segment scale the estimated sediment yields were 0·1 to 7·4 kg m−2 a−1. Road surface runoff was best predicted by storm precipitation, while sediment yields for at least three of the four road surface plots were significantly correlated with storm rainfall, storm intensity and storm runoff. Sediment yields at the road segment scale were best predicted by road surface area, and sediment yields per unit area were most strongly correlated with road segment slope. The one road segment subjected to heavy traffic and more frequent regrading produced more than twice as much sediment per unit area than comparable segments with no truck traffic. Particle‐size analyses indicate a preferential erosion of fine particles from the road surface and a rapid surface coarsening of new roads. Published in 2001 by John Wiley & Sons, Ltd. 相似文献
35.
R. D. Smith K. L. Cameron F. W. McDowell S. Niemeyer D. E. Sampson 《Contributions to Mineralogy and Petrology》1996,123(4):375-389
Isotopic and trace element data from mantle and granulite xenoliths are used to estimate the relative contributions of mantle
and crustal components to a large ignimbrite, referred to as the upper ignimbrite, that is representative of the voluminous
mid-Cenozoic rhyolites of northwestern Mexico. The study also uses data from the volcanic rocks to identify deep crustal xenoliths
that are samples of new crust created by the Tertiary magmatism. The isotopic composition of the mantle component is defined
by mantle-derived pyroxenites that are interpreted to have precipitated from mid-Cenozoic basaltic magmas. This component
has ɛNd≈+1.5, 87Sr/86Sr≈0.7043 and 206Pb/204Pb≈18.6. Within the upper ignimbrite and associated andesitic and dacitic lavas, initial 87Sr/86Sr is positively correlated with SiO2, reaching 0.7164 in the ignimbrite. Initial 206Pb/204Pb ratios also show a positive correlation with silica, whereas ɛNd values have a crude negative correlation, reaching values as low as −2. Of the four isotopically distinct crustal components
identified from studies of granulite xenoliths, only the sedimentary protolith of the paragneiss xenoliths can be responsible
for the high initial 87Sr/86Sr of the upper ignimbrite. The Nd, Sr, and Pb isotopic compositions of the upper ignimbrite can be modeled with relatively
modest assimilation (≤20%) of the sedimentary component ± Proterozoic granulite. Gabbroic composition granulite xenoliths
have distinctive Nd, Sr, and Pb isotope ratios that cluster closely within the range of compositions found in the andesitic
and dacitic lavas. These mafic granulites are cumulates, and their protoliths are interpreted to have precipitated from the
intermediate to silicic magmas at 32–31 Ma. These mafic cumulate rocks are probably representative of much of the deep crust
that formed during mid-Cenozoic magmatism in Mexico. Worldwide xenolith studies suggest that the relatively great depth (≤20
km) at which assimilation-fractional crystallization took place in the intermediate to silicic magma systems of the La Olivina
region is the rule rather than the exception. Oligocene ignimbrites of the southwestern United States (SWUS) have substantially
lower ɛNd values (e.g. <−6) than the upper ignimbrite and other rhyolites from Mexico. This difference appears to reflect a greater
crustal contribution to ignimbrites of the SWUS, perhaps due to a higher temperature of the lower crust prior to the emplacement
of the Oligocene basaltic magmas.
Received: 16 December 1994 / Accepted: 13 September 1995 相似文献
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Naturally occurring As found in groundwaters has been identified to be a problem in at least 10 provinces of Cambodia with Kandal being one of the most heavily impacted. Estimates, using groundwater quality and population data for Kandal Province of Cambodia, suggest that over 100,000 people are at high risk of chronic As exposure. Levels in some areas approach 3500 μg/L, against the Cambodian Standard of 50 μg/L. Considerable work remains to adequately characterize the extent of As hazard and its possible health effects in Cambodia and the region. It is likely that additional populations will develop health problems attributed to As, of particular concern is arsenicosis. The symptoms of arsenicosis have been generally assumed to develop after 8–10 years of consumption of water with elevated As levels, however, new cases discovered in Cambodia have been identified with exposure times as short as 3 years. The rapid onset of arsenicosis may be attributed to contributing risk factors related to socioeconomic status, including malnutrition. It is thus imperative to develop strategies to rapidly identify possible regions of enrichment, to minimize exposure to As-rich waters, and to educate affected populations. To date the response to the As hazard has been led by the Ministry of Rural Development in cooperation with international organizations and NGOs, to identify at risk areas, and educate communities of the risk of As-rich water. However better coordination between government bodies, NGOs and donor agencies active in the field of water supply and treatment is essential to minimize future As exposure. 相似文献
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Miguel Cervantes Solano Avto Goguitchaichvili Leda Sánchez Bettucci Ruben Cejudo Ruiz Manuel Calvo-Rathert Vicente Carlos Ruiz-Martinez Ruth Soto Luis M. Alva-Valdivia 《Studia Geophysica et Geodaetica》2010,54(4):533-546
We report detailed rock-magnetic and paleomagnetic directional data from 35 lava flows (302 standard paleomagnetic cores)
sampled in the Central-Northern region of Uruguay in order to contribute to the study of the paleosecular variation of the
Earth’s magnetic field during early Cretaceous and to obtain precise Cretaceous paleomagnetic pole positions for stable South
America. The average unit direction is rather precisely determined from 29 out of 35 sites. All A95 confidence angles are less than 8°, which points to small within-site dispersion and high directional stability. Normal polarity
magnetizations are revealed for 19 sites and 10 are reversely magnetized. Two other sites yield well defined intermediate
polarities. The mean direction, supported by a positive reversal test is in reasonably good agreement with the expected paleodirection
for Early Cretaceous stable South America and in disagreement with a 10° clockwise rotation found in the previous studies.
On the other hand, paleomagnetic poles are significantly different from the pole position suggested by hotspot reconstructions,
which may be due to true polar wander or the hotspot motion. Our data suggest a different style of secular variation during
(and just before) the Cretaceous Normal Superchron and the last 5 Ma, supporting a link between paleosecular variation and
reversal frequency. 相似文献
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In this study, historical landscape dynamics were investigated to (i) map the land use/cover types for the years 1972, 1987, 2000 and 2014; (ii) determine the types and processes of landscape dynamics; and (iii) assess the landscape fragmentation and habitat loss over time. Supervised classification of multi-temporal Landsat images was used through a pixel-based approach. Post–classification methods included systematic and random change detection, trajectories analysis and landscape fragmentation assessment. The overall accuracies (and Kappa statistics) were of 68.86% (0.63), 91.32% (0.79), 90.66% (0.88) and 91.88% (0.89) for 1972, 1987, 2000 and 2014, respectively. The spatio-temporal analyses indicated that forests, woodlands and savannahs dominated the landscapes during the four dates, though constant areal decreases were observed. The most important dynamic process was the decline of woodlands with an average annual net loss rate of–2%. Meanwhile, the most important land transformation occurred during the transition 2000–2014, due to anthropogenic pressures. Though the most important loss of vegetation greenness occurred in the unprotected areas, the overall analyses of change indicated a declining trend of land cover quality and an increasing landscape fragmentation. Sustainable conservation strategies should be promoted while focusing restoration attention on degraded lands and fragmented ecosystems in order to support rural livelihood and biodiversity conservation. 相似文献
39.
Eliot?A.?AtekwanaEmail author Lenore?P.?Tedesco Leda?R.?Jackson 《Estuaries and Coasts》2003,26(6):1391-1400
Physical and chemical parameters were measured in a subtropical estuary with a blind river source in southwest Florida, United States, to assess seasonal discharge of overland flow and groundwater in hydrologic mixing. Water temperature, pH, salinity, alkalinity, dissolved inorganic carbon (DIC), δ18O, and δ13CDIC varied significantly due to seasonal rainfall and climate. Axial distribution of the physical and chemical parameters constrained by tidal conditions during sampling showed that river water at low tide was a mixture of freshwater from overland flow and saline ground-water in the wet season and mostly saline groundwater in the dry season. Relationships between salinity and temperature, δ18O, and DIC for both the dry and wet seasons showed that DIC was most sensitive to seawater mixing in the estuary as DIC changed in concentration between values measured in river water at the tidal front to the most seaward station. A salinity-δ13CDIC model was able to describe seawater mixing in the estuary for the wet season but not for the dry season because river water salinity was higher than that of seawater and the salinity gradient between seawater and river water was small. A DIC-δ13CDIC mixing model was able to describe mixing of carbon from sheet flow and river water at low tide, and river water and seawater at high tide for both wet and dry seasons. The DIC-δ13CDIC model was able to predict the seawater end member DIC for the wet season. The model was not able to predict the seawater end member DIC for the dry season data due to secondary physical and biogeochemical processes that altered estuarine DIC prior to mixing with seawater. The results of this study suggest that DIC and δ13CDIC can provide additional insights into mixing of river water and seawater in estuaries during periods where small salinity gradients between river water and seawater and higher river water salinities preclude the use of salinity-carbon models. 相似文献
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