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1.
 Land subsidence due to groundwater withdrawal combined with a global sea level rise creates a serious environmental problem in the coastal region. Groundwater withdrawal results in fluid pressure change in the layers. The pressure change in the layers induces both elastic and inelastic land compaction. The elastic compaction can be recovered if the water level rises again and inelastic compaction becomes permanent. Groundwater response to barometric pressure change is used to estimate the elastic compaction in this study. The storativity, specific storage and other layer and hydrological information are used to estimate the inelastic compaction of the layers due to fluid withdrawal. The discussed methods are applied to estimate and predict the subsidence potentials resulting from overdrafting of the groundwater in the southern New Jersey. The estimated subsidence is about 2–3 cm near the location of monitoring wells in Atlantic, Camden, Cumberland and Cape May Counties over the past 20 years. If the current trend of water-level drop continues, the average subsidence in southern New Jersey in the vicinity of some monitoring wells will be about 3 cm in the next 20 years. The rise of global sea level is about 2 mm/year on average. Because of the very gentle slope in southern NJ, the combination of subsidence and sea level rise will translate into a potentially substantial amount of land loss in the coastal region in each 20 year period. This combination will also accelerate the coastal flooding frequency and the erosion rate of the New Jersey coastal plain, and pose a serious threat to the coastal economy. Received: 15 December 1997 · Accepted: 30 June 1998  相似文献   

2.
 The study of water chemistry and sediment core samples from Udaisagar lake in Rajasthan has revealed high phosphate contents; 186 and 236 μg/l in surface and sub-surface waters and in core samples 0.157 wt % P2O5, low D.O. 4.61 and 3.50 mg/l on average in surface and depth waters and high E.C. 1316 and 1395 μs/cm and higher sedimentation rate 8.90 mm/year in this lake compared to other lakes in the vicinity. The enormous and wide-spread growth of algal bloom in lake water and Ahar river and occasional fish mortality are indications of eutrophic conditions prevailing in the lake. Such a situation is developing due to the discharge of pollutants from phosphorite mines, chemical factories, distillary, sewage and domestic waste from settlements, hotels throughout the length of River Ahar which during its journey receives and finally discharges pollution in this lake. This renders the water unhygenic for human consumption and deleterious to aquatic life. Received: 27 January 1998 · Accepted: 28 September 1998  相似文献   

3.
Summary Amphibolite-facies para- and orthogneisses near Dulan, in the southeast part of the North Qaidam terrane, enclose minor ultra-high pressure (UHP) eclogite and peridotite. Field relations and coesite inclusions in zircons from paragneiss suggest that felsic, mafic, and ultramafic rocks all experienced UHP metamorphism and a common amphibolite-facies retrogression. Ion microprobe U–Pb and REE analyses of zircons from two granitic orthogneisses indicate magmatic crystallization at 927 ± Ma and 921 ± 7 Ma. Zircon rims in one of these samples yield younger ages (397–618 Ma) compatible with partial zircon recrystallization during in-situ Ordovician-Silurian eclogite-facies metamorphism previously determined from eclogite and paragneiss in this area. The similarity between a 2496 ± 18 Ma xenocrystic core and 2.4–2.5 Ga zircon cores in the surrounding paragneiss suggests that the granites intruded the sediments or that the granite is a melt of the older basement which supplied detritus to the sediments. The magmatic ages of the granitic orthogneisses are similar to 920–930 Ma ages of (meta)granitoids described further northwest in the North Qaidam terrane and its correlative west of the Altyn Tagh fault, suggesting that these areas formed a coherent block prior to widespread Mid Proterozoic granitic magmatism.  相似文献   

4.
太湖流域降雨与太湖水位关系分析   总被引：1，自引：0，他引：1  
章杭惠  刘曙光  季同德 《水文》2021,41(1):85-89,41
太湖流域快速城镇化、水利工程建设等人类活动对流域水文过程产生了较大影响.基于1981～2018年太湖流域日降雨、太湖水位实测资料,分析了流域降雨与太湖水位涨幅关系,旨在揭示变化环境下该地区降雨与水位关系的变化特征及可能的驱动因素.研究表明,2000年以来太湖水位抬高趋势明显,但同等时段降雨量条件下对应的太湖水位涨幅有所...  相似文献   

5.
The time evolution of atmospheric parameters on intraseasonal time scale in the eastern Arabian Sea (EAS) is studied during the summer monsoon seasons of 1998–2003 using Tropical Rainfall Measuring Mission Microwave Imager (TMI) data. This is done using the spectral and wavelet analysis. Analysis shows that over EAS, total precipitable water vapour (TWV) and sea surface wind speed (SWS) have a periodicity of 8–15 days, 15–30 days and 30–60 days during the monsoon season. Significant power is seen in the 8–15-day time scale in TWV during onset and retreat of the summer monsoon. Analysis indicates that the timings of the intensification of 8–15, 15–30, and 30–60 days oscillations have a profound effect on the evolution of the daily rainfall over west coast of India. The positive and negative phases of these oscillations are directly related to the active and dry spells of rainfall along the west coast of India. The spectral analysis shows interannual variation of TWV and SWS. Heavy rainfall events generally occur over the west coast of India when positive phases of both 30–60 days and 15–30 days modes of TWV and SWS are simultaneously present.  相似文献   

6.
《Quaternary Science Reviews》2007,26(19-21):2438-2462
Curves for Holocene lake levels and salinity changes are presented for An Loch Mór, a small oligohaline lake on the Aran Islands off the west coast of Ireland, based on palaeoecological investigations of a 12 m long, lake-sediment core. New insights are also provided into Holocene sea-level change in the Galway Bay region. Particular emphasis has been placed on the ostracod fauna, both past and present. Salinity and lake-level changes were reconstructed from the fossil ostracod assemblages, based on the known tolerances of individual species and on the assemblages as a whole. Additional evidence was provided by other proxies including strontium–isotope ratios derived from ostracod shells and other carbonates, plant macrofossil and pollen analyses, and sedimentological changes. The early Holocene (pre-Boreal, i.e. 11.5–10 ka) was characterised by low lake levels and slightly elevated salinity values, probably the result of high evapotranspiration and low precipitation rather than elevated sea levels. Early Holocene plant and animal migration to the island does not seem to have been impeded but relative sea levels were not necessarily so low (below −40 m a.s.l.) that landbridges were present to the mainland. Between ca 10 and 8.5 ka, relatively high lake levels prevailed. At 8.3 and 7.5 ka, minor fluctuations (lowering) of the lake level occurred that are assumed to relate to early Holocene abrupt events. Beginning at 7.05 ka, lake levels declined sharply. A general trend towards rising lake levels started at ca 6.4 ka and accelerated at ca 5.6 ka as runoff increased as a result of Neolithic clearances. At ca 4.8 ka, lake levels began to rise once again, probably in response to changes in rainfall and/or evapotranspiration and runoff. Lower lake levels during the first half of the 1st millennium AD were probably a response to decreased runoff as a result of a drier climate coupled with regeneration of woody vegetation. The sharpest rise in both lake levels and salinity began during the ninth century AD, which is attributed to a rapid rise in relative sea level.  相似文献   

7.
Summary Silica-undersaturated phlogopite schists from the Cackleberry Metamorphics, Arunta Inlier, central Australia, preserve relatively low-temperature sapphirine-bearing parageneses that developed during low-pressure upper amphibolite facies metamorphism. Peak metamorphic phlogopite–cordierite–sapphirine assemblages are interpreted to have formed during the same event recorded in nearby metapelites, at c.3 kbar and 650–700 °C. Initial cooling of the terrain resulted in the breakdown of sapphirine to corundum–chlorite–phlogopite and corundum–spinel–chlorite assemblages. Further retrogression at greenschist facies conditions resulted in the replacement of sapphirine by diaspore–chlorite intergrowths. The reaction textures are consistent with a near-isobaric heating-cooling path at low-pressure, and provide evidence for the stability of sapphirine at c.700 °C at low pressures in rocks of an appropriate Mg- and Fe3+-rich bulk composition. Received August 15, 2001 accepted December 27, 2001  相似文献   

8.
 The Aracena metamorphic belt (AMB), southwest Iberian peninsula, is characterized by the following geological elements: (a) a high-temperature/low-pressure (HT/LP) metamorphic belt a few kilometres wide and more than 200 km long; (b) a linear belt of oceanic amphibolites with a low-pressure inverted metamorphic gradient; (c) crustal-scale ductile shear zones; and (d) mafic, noritic intrusions of high-Mg andesite (boninite) composition. The relationships between these elements led to the proposal of a model of ridge subduction for this sector of the Hercynian belt of Europe. This interpretation is supported by the age relationships displayed between the main rock units considered representative of the main tectonic and petrological processes responsible for the geological elements mentioned previously. The results of a geochronological study (Ar–Ar, Rb–Sr and Sm–Nd) clearly support a Late Paleozoic tectonic evolution at an active continental margin. The time evolution of the metamorphism in the oceanic domain, ranging from 342.6±0.6 Ma in the west to 328.4±1.2 Ma in the east, over a distance of 70 km along the metamorphic belt, support a tectonic model of triple-junction migration responsible for the creation at depth of a slab-free window with decisive consequences for the thermal evolution of the region. The origin of the linear metamorphic belt of HT/LP regime may be explained by the migration along a continental margin of a punctual thermal anomaly induced by the creation of a triple-junction at the continental margin. Received: 9 March 1998 / Accepted: 9 December 1998  相似文献   

9.
 The marine coastal sediments from Togo have been analysed for the trace elements Cd, Cr, Cu, Ni, Pb, Sr, V, Zn and Zr to ascertain the geo-ecological impact of dumping of phosphorite tailings into the sea. Trace element concentrations ranged from 2–44 ppm for Cd, 22–184 ppm for Cu, 19–281 ppm for Ni, 22–176 ppm for Pb, 179–643 ppm for Sr, 38–329 ppm for V, 60–632 ppm for Zn and 18–8928 ppm for Zr. Regional distribution of trace elements in the marine environment indicates that the concentrations of Cr, Cu, Ni, Pb, V, Sr and Zn increase seawards and along the coastal line outwards of the tailing outfall, whereas Cd and Zr showed reversed spatial patterns. Sorting and transport of phosphorite particles by coastal currents are the main factors controlling the distribution of particle-bound trace metals in the coastal environment. The Cd, Sr and Zn concentrations decrease with decreasing grain size in marine coastal sediments, whereas Cr, Cu, Ni and Zn concentrations increase with decreasing grain size. Percolation and shaking experiments were carried out in laboratory using raw phosphate material and artificial sea water. Enhanced mobilization of Cd from phosphorites by contact with the sea water was observed. Received: 11 May 1998 · Accepted: 20 October 1998  相似文献   

10.
 Bottom-water data and trace metal concentration of Cu, Cr, Ni, Pb, Co, Zn, and organic matter in surficial sediment samples from 13 sampling stations of Lake Chapala in Mexico were studied. The lake is turbid with a great amount of flocculated sediments as a result of wind mixing, sediment re-suspension, and Lerma River discharges. Al distribution pattern in sediments was used as an indicator of the Lerma River discharges into Lake Chapala. The highest values of Cu (33.27 ppm), Cr (81.94 ppm), Pb (99.8 ppm), and Zn (149.7 ppm) were detected in sediments near the lake outlet. The bioavailable metal fraction is low for all metals except Pb, which shows 65–93% of the total metal concentration in bioavailable form. The minimum energy zone in the lake was related to organic matter concentration and was located in the SE part of the lake. An analysis of the studied parameters shows two zones: eastern zone (fluvio-deltaic) and central-western zone (lacustrine). Received: 9 September 1998 · Accepted: 16 November 1998  相似文献   

11.
To examine the biogeochemistry of amino acids (AAs) in the sediment of Lake Taihu, surface sediments (0–3 cm) and deeper sediments (18–21 cm) were collected at 21 sites from different ecotype zones of the lake. AAs were extracted from the sediments, and the total hydrolyzable amino acids (THAA) were determined by high-performance liquid chromatography instrument. The THAA contents in Taihu sediment were much lower than that in marine sediments, ranging from 6.84 to 38.24 μmol g−1 in surface sediments and from 2.91 to 18.75 μmol g−1 in deeper sediments in Taihu, respectively. AAs were a major fraction of the organic matter (OM) and organic nitrogen in Taihu sediments. The AAs on average contributed 8.2% of organic carbon (OC) and 25.0% of total nitrogen (TN) from surface sediments, and 5.9% of OC and 20.5% of TN in deeper sediments, respectively. AA composition provided very useful information about the degradation of OM. Glycine (Gly) and lysine (Lys) were the predominant forms of AAs in the sediments, irrespective of lake regions, followed by alanine, glutamic acid, serine (Ser), and aspartic acid (Asp). The high concentrations of Gly, Lys, and Ser suggested that these forms of AAs were relatively refractory during OM degradation in sediments. The relationship between the Asp/Gly ratio and Ser + Thr [mol%] indicated that OM in surface sediment was relatively fresher than that in deeper sediments. The AAs-based degradation index (DI) gave a similar conclusion. The composition and DI of AAs in surface sediments are markedly different across different zones in Taihu. The percentages of AAs to organic carbon (AA-C%) and total nitrogen (AA-N%) were higher in phytoplankton-dominated zones than those in macrophyte-dominated zones. These results suggest that DI could provide useful information about the degradation of OM in shallow lakes such as Taihu.  相似文献   

12.
The hydroelectric reservoir of Petit Saut, French Guiana, was created in 1994–1995 by flooding 350 km2 of tropical forest. When sampled in 1999, the lake exhibited a permanent stratification separating the 3–5 m thick, oxygenated epilimnion from the anoxic hypolimnion. The rate of anaerobic organic carbon mineralization below the oxycline was on the order of 1 μmol C m−2 s−1 and did not show a pronounced difference between wet and dry seasons. Methanogenesis accounted for 76–83% of anaerobic carbon mineralization, with lesser contributions of sulfate reduction and dissimilatory iron reduction. Upward mixing of reduced inorganic solutes explained 90% of the water column O 2 demand during the dry season, while most O 2 consumption during the wet season was coupled to aerobic respiration of organic matter synthesized in the surface waters. Inorganic mercury species represented 10–40% of total dissolved mercury in the epilimnion, but were of relatively minor importance (≤10%) in the anoxic portion of the water column. Net production of soluble organic mercury compounds in the flooded soils and anoxic water column did not vary significantly between wet and dry seasons. Methylmercury accounted for about 15% of total dissolved mercury below the oxycline. Its estimated net production rate, 0.04 mg m−2 yr−1, is of the same order of magnitude as values reported for contaminated lakes and flooded terrestrial ecosystems.  相似文献   

13.
Lower Calcsilicate Unit metasediments and underlying migmatitic Napperby Gneiss metagranite at Conical Hill in the Reynolds Range, central Australia, underwent regional high-grade (∼680 to 720 °C), low-pressure/high-temperature metamorphism at 1594 ± 6 Ma. The Lower Calcsilicate Unit is extensively quartz veined and epidotised, and discordant grandite garnet + epidote quartz veins may be traced over tens of metres depth into pegmatites that pooled at the Lower Calcsilicate Unit-Napperby Gneiss contact. The quartz veins were probably precipitated by water-rich fluids that exsolved from partial melts derived from the Napperby Gneiss during cooling from the peak of regional metamorphism to the wet granite solidus. Pb stepwise leaching (PbSL) on garnet from three discordant quartz veins yielded comparable single mineral isochrons of 1566 ± 32 Ma, 1576 ± 3 Ma and 1577 ± 5 Ma, which are interpreted as the age of garnet growth in the veins. These dates are in good agreement with previous Sensitive High Resolution Ion Microprobe (SHRIMP) ages of zircon and monazite formed during high-temperature retrogression (1586 ± 5 to 1568 ± 4 Ma) elsewhere in the Reynolds Range. The relatively small age difference between peak metamorphism and retrograde veining suggests that partial melting and melt crystallisation controlled fluid recycling in the high-grade rocks. However, PbSL experiments on epidote intergrown with, and partially replacing, garnet in two of the veins yielded isochrons of 1454 ± 34 and 1469 ± 26 Ma. The ∼100–120 Ma age difference between intergrown garnet and late epidote from the same vein suggests that the vein systems may have experienced multiple episodes of fluid flow. Received: 24 April 1998 / Accepted: 17 December 1998  相似文献   

14.
 A geophysical survey was conducted to determine the depth of the base of the water-table aquifer in the southern part of Jackson Hole, Wyoming, USA. Audio-magnetotellurics (AMT) measurements at 77 sites in the study area yielded electrical-resistivity logs of the subsurface, and these were used to infer lithologic changes with depth. A 100–600 ohm-m geoelectric layer, designated the Jackson aquifer, was used to represent surficial saturated, unconsolidated deposits of Quaternary age. The median depth of the base of the Jackson aquifer is estimated to be 200 ft (61 m), based on 62 sites that had sufficient resistivity data. AMT-measured values were kriged to predict the depth to the base of the aquifer throughout the southern part of Jackson Hole. Contour maps of the kriging predictions indicate that the depth of the base of the Jackson aquifer is shallow in the central part of the study area near the East and West Gros Ventre Buttes, deeper in the west near the Teton fault system, and shallow at the southern edge of Jackson Hole. Predicted, contoured depths range from 100 ft (30 m) in the south, near the confluences of Spring Creek and Flat Creek with the Snake River, to 700 ft (210 m) in the west, near the town of Wilson, Wyoming. Received, May 1997 · Revised, February 1998 · Accepted, April 1998  相似文献   

15.
In deltaic marshes, mineral sediment promotes positive elevation change and counters subsidence and sea level rise. In many such marshes sediment deficits result in wetland loss. One new way to address sediment deficiency is to supply marshes with sediments in a slurry that deposits the sediment in a thin layer over a large area. The long-term effects of this strategy are poorly understood. In a rapidly submerging,Spartina alterniflora salt marsh, we tested how different amounts of sediment ameliorated the effects of sea level rise and subsidence over 7 yr (1992–1998). Sediment slurry enrichment likely affected plants and soils by two mechanisms. It increased elevation and soil bulk density, leading to increased plant vigor and soil condition. These effects were long lasting, such that by 1998 areas receiving moderate amounts of sediment (5–12 cm relative elevation) had better plant vigor and soil condition compared to areas not receiving sediment (55% cover versus 20%; bulk densities of 0.4–1.0 g cm−3 versus 0.2 g cm−3; 0 mM hydrogen sulfide versus > 1.0 mM). The sediment slurry also had high nutrient content, which resulted in a pulse of growth, especially in areas receiving the most sediment (areas > 12 cm relative elevation initially had >90% cover and canopy heights >1.6 m). This nutrient-induced growth spurt was short lived and faded after 3 yr, at which point the long lasting effects of increased elevation probably became the dominant factor promoting plant vigor and soil condition. Moderate levels of sediment generated the most beneficial and long lasting effects to the vegetation and soils. This degree of sediment slurry addition countered the effects of subsidence and sea level rise, but not so much as to surpass the intertidal position to whichS. alterniflora is best adapted.  相似文献   

16.
 Long Lake, located near Lake Michigan within the dune-complexes of Indiana Dunes National Lakeshore, USA, was formed some time during the Pleistocene and Holocene epochs. A surficial aquifer underlies Long Lake, which is either a source or sink for the later. The hydrologic processes in the lakeshore and surrounding environs have been significantly altered during the agricultural, municipal, and industrial development of the region. Limited data suggest that the organisms of Long Lake have elevated levels of several contaminants. This study attempts to quantify seepage within the lake to assess the potential threat to groundwater quality. Seepage measurements and minipiezometric tests were used to determine seepage within the lake. Seepage measurements and minipiezometric tests suggest that water seeps out of Long Lake, thus recharging the groundwater that flows southwest away from the lake. There is a great deal of variability in the seepage rate, with a mean of 11.5×10–4±11.2×10–4 m d–1. The mean seepage rate of 0.3 m yr–1 for Long Lake is greater than the 0.2 m yr–1 recharge rate estimated for the drainage basin area. The Long Lake recharge volume of 2.5×105 m3 yr–1 is approximately 22% of the volume of the lake and is significant when compared to the total surface recharge volume of 4.8×105 m3 yr–1 to the upper aquifer of the drainage area. There is a potential for contamination of the groundwater system through seepage from the lake from contaminants derived from aerial depositions. Received: 16 August 1995 · Accepted: 18 September 1995  相似文献   

17.
The Malter Reservoir is situated about 30 km south of Dresden (eastern Germany) in a historical mining area of the eastern Erzgebirge. It was built in 1913 for the protection from floodwaters, droughts and for generating electricity. The river Rote Wei?eritz is the main source of clastic input into the lake. Geochemical and sedimentological data of gravity-and piston-cores, recovered from the deepest point of the lake, document the environmental history of the drainage area since 1963. 137Cs dating gives an average sedimentation rate of ∼2.9 cm/year. Within the whole core, heavy metals are strongly enriched (parentheses refer to enrichment factors as compared with average shale): cadmium (290), silver (140), bismuth (90), antimony (25), lead (21), zinc (14), tin (13), uranium (9), tungsten (9), molybdenum (5), copper (4), thallium (3) and chromium (2). Enrichments are detectable for the whole registered time-period of 81 years. Peaks of up to 27 mg/kg silver, 37 mg/kg bismuth, 91 mg/kg cadmium, 410 mg/kg chromium, 240 mg/kg copper, 20 mg/kg molybdenum, 14000 mg/kg phosphorus, 740 mg/kg lead, 6,5 mg/kg antimony, 74 mg/kg tin, 52 mg/kg tungsten and 1900 mg/kg zinc reflect local events caused by human impact. Inputs from different pollution sources at different times are represented by highly variable elemental concentrations and ratios within the core. High pH values within the water and the sediment column, the large adsorption capacity of the fine-grained Corg.-rich sediment, and the presence of low Eh-values and sulphide ions in the sediment prevent the remobilisation of the toxic elements. Erosion of these contaminated sediments during floods, channel flows or resuspension during removal of the sediments may lead to a downstream transfer of pollutants. Contents of P and Corg., as well as diatom abundance, indicate a change from oligotrophic to eutrophic conditions in the lake during ∼1940–1950. This was mainly caused by high agricultural activity in the drainage area. Reduced contents of Cu, Zn, Cd and Cr since the reunification of East and West Germany are obviously caused by increasing environmental protection measures, such as wastewater purification and especially the closing of contaminating industries. Revision received: 23 September 1999 · Accepted: 17 December 1999  相似文献   

18.
 The accumulation of heavy metals and trace elements has been investigated in a well laminated sequence of Holocene and late Pleistocene lake sediments composed of diatomaceous gyttja, tuff and silt and clay sediments. Varve chronology of the annually deposited gyttja yielded a continuous high-resolution time sequence and allowed the absolute age dating of the sediment. Fluxes of elements remained largely uniform from the late Pleistocene into the Holocene (12 867–2 364 VT years ago; VT: varve time, years before 1950). Higher trace element and heavy metal fluxes occur from 2 322 to 862 VT years ago and reached their maxima in the uppermost sediments (<845 VT years ago). These increasing element fluxes correlate with increasing inputs of clastic material. The changing accumulation rates are the result of elevated soil erosion in the lake catchment caused by human settlement, deforestation and agricultural activities. Thus disturbances of the natural geochemical cycles of the Holzmaar region have occurred since the beginning of the Iron Age and especially since the beginning of the Middle Ages. Received: 29 May 1996 · Accepted: 19 August 1996  相似文献   

19.
 The San Antonio-El Triunfo mining district, located at a mountainous region 45 km south-east of La Paz, Baja California, has been worked since the late 1700s. Mine waste material produced during 200 years of mineral extraction area poses a risk of local groundwater pollution and eventually, regional pollution to the Carrizal (west basin) and the Los Planes (east basin) aquifers. There are different types of deposits in the mining area. These are dominated by epithermal veins, in which arsenopyrite is an important component. Carrillo and Drever (1998a) concluded that, even though the amount of mine waste is relatively small in comparison to the large scale area, significant As in groundwater derived from the mine waste piles is found locally in the groundwater. This paper shows the results of geochemical analyses of groundwater samples from the San Antonio-El Triunfo area and the Carrizal and Los Planes aquifers during several years of monitoring (1993–1997). The highest values of total dissolved solids (TDS) and As are in the mineralized area where the mining operations occurred (∼1500 ppm TDS and 0.41 ppm As). The lowest concentrations of TDS and As are, in general, away from the mineralized area (∼500 ppm TDS and 0.01 ppm As). Sulfate and bicarbonate (alkalinity) are, in general, high near the mineralized area and low away from it. The arsenic concentrations vary seasonally, especially after the heavy summer thunderstorms. Geochemical modeling (MINTEQA2 and NETPATH) and analysis of the regional geochemical evolution of the groundwater from the mining area towards the aquifer of Los Planes shows that the most likely hydrochemical processes include: dilution, precipitation of calcite, and adsorption of As onto surfaces of iron oxyhydroxides (ferrihydrite). These processes act as natural controls to the extent and amount of As pollution in the Carrizal and Los Planes aquifers. Received: 4 May 1999 · Accepted: 22 February 2000  相似文献   

20.
 A strong geochemical gradient was observed in the thick overburden aquifer of the Asa drainage basin. Different types of groundwater occur at different (downslope) locations and groundwater table depths. The following sequence was noticed with increasing distance downslope or with increasing groundwater table depth: 1. Ca–Mg–HCO3 water at about 390-m groundwater table elevations or upslope locations. 2. Ca–Mg–HCO3–Cl water at middle-slope locations or groundwater table elevations of about 350 m above sea level; 3. Ca–Mg–SO4–Cl water at downslope locations or groundwater table elevations of about 300 m above sea level. In this basin, changes in the type of water are expected at about every 40–50 m depth from the surface. Statistical analysis via the determination of the correlation coefficient (r) and regression analysis shows that about 80–99% of the variation in groundwater chemistry is accounted for by the topography, using the model presented in this paper. The rate of change in the sequence will depend on the permeability of the aquifer, which determines the rate of groundwater flow and the residence time, and the nature of recharge. Received: 4 February 1997 · Accepted: 22 July 1997  相似文献   

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