首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 765 毫秒
1.
Processes driving carbonate diagenesis in islands of the northern Bahamas are investigated using major ion, dissolved oxygen and dissolved organic carbon analyses of water samples from surface and ground waters, and measurements of soil gas P. Meteoric waters equilibrate with aragonite, but reactions are water controlled rather than mineral‐controlled and drive dissolution rather than concurrent precipitation of calcite. Surface runoff waters equilibrate with atmospheric P and rapidly recharge the vadose zone, limiting subaerial bedrock dissolution to only 6·6–15 mg l?1 Ca. P of soil gas measured in the summer wet season ((7·4 ± 3·7) × 10?3 atm) is elevated compared with that of the atmosphere, despite the thin skeletal organic nature of the soil and the discontinuous soil cover. Soil waters retained in surface pockets are equilibrated with respect to aragonite and have dissolved 51 ± 19 mg l?1 Ca. This is substantially less than the 93 ± 18 mg l?1 Ca in samples from pumping boreholes that sample meteoric waters from the freshwater lens. The high P of the freshwater lens ((16 ± 8·3) × 10?3 atm for pumping boreholes) suggests that significant additional CO2 may be derived by oxidation of soil‐ and surface‐derived organic carbon within the lens. The suboxic nature of the majority of the freshwater lens and the observed depletion in sulphate support this suggestion, and indicate that both aerobic and anaerobic oxidation may take place. Shallow lens samples from observation boreholes are calcite supersaturated and have a lower P than deeper lens waters, indicating that CO2 degasses from the water table, driving precipitation of calcite cements. We suggest that the geochemical evolution of waters in the vadose zone and upper part of the freshwater lens may be determined by the presence of a body of ground air with P controlled by production in the freshwater lens and soil and by degassing to the atmosphere. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

2.
A study of the changes in the ionic loads of NO, NH, SO and H+ in a boreal forest snowpack at Lake Laflamme, Québec was carried out using hydrological and chemical data from field lysimeters. The results showed that depletion of the N-containing species occurs periodically in the snowpack during meltwater discharge. Rain-on-snow events led to in-pack losses of NO and NH at a rate of 130 μeq m?2 day?1 and 101·3 μeq m?2day?1 respectively. On dry days, however, dry deposition and deposition of organic debris from the canopy resulted in increases of 183·3 μeq m?2day?1 for NO and 4·5 μeq m?2day?1 for NH in the pack. In contrast, SO42? showed continual in-pack increases due to deposition of 5·0 μeq m?2day?1 for wet days and 92·6 μeq m?2day?1 for dry days. The depletion of NO and NH is due to microbiological uptake of these nutrients during periods when the free water content of the pack is high. Controlled melts in a laboratory snowmelt simulator containing snow and organic matter from the forest canopy at Lake Laflamme showed losses of NO and NH similar to those observed in the field. As the microbiological uptake proceeds at a rate comparable to that of ionic load increases in the pack by dry deposition, models of the chemical dynamics of snowmelt should take the former into account in any system where organic content of the snowpack is appreciable.  相似文献   

3.
High‐resolution measurements of rainfall, water level, pH, conductivity, temperature and carbonate chemistry parameters of groundwater at two adjacent locations within the peak cluster karst of the Guilin Karst Experimental Site in Guangxi Province, China, were made with different types of multiparameter sonde. The data were stored using data loggers recording with 2 min or 15 min resolution. Waters from a large, perennial spring represent the exit for the aquifer's conduit flow, and a nearby well measures water in the conduit‐adjacent, fractured media. During flood pulses, the pH of the conduit flow water rises as the conductivity falls. In contrast, and at the same time, the pH of groundwater in the fractures drops, as conductivity rises. As Ca2+ and HCO3? were the dominant (>90%) ions, we developed linear relationships (both r2 > 0·91) between conductivity and those ions, respectively, and in turn calculated variations in the calcite saturation index (SIC) and CO2 partial pressure (P) of water during flood pulses. Results indicate that the P of fracture water during flood periods is higher than that at lower flows, and its SIC is lower. Simultaneously, P of conduit water during the flood period is lower than that at lower flows, and its SIC also is lower. From these results we conclude that at least two key processes are controlling hydrochemical variations during flood periods: (i) dilution by precipitation and (ii) water–rock–gas interactions. To explain hydrochemical variations in the fracture water, the water–rock–gas interactions may be more important. For example, during flood periods, soil gas with high CO2 concentrations dissolves in water and enters the fracture system, the water, which in turn has become more highly undersaturated, dissolves more limestone, and the conductivity increases. Dilution of rainfall is more important in controlling hydrochemical variations of conduit water, because rainfall with higher pH (in this area apparently owing to interaction with limestone dust in the lower atmosphere) and low conductivity travels through the conduit system rapidly. These results illustrate that to understand the hydrochemical variations in karst systems, considering only water–rock interactions is not sufficient, and the variable effects of CO2 on the system should be evaluated. Consideration of water–rock–gas interactions is thus a must in understanding variations in karst hydrochemistry. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

4.
The reservoir quality of Jurassic and Triassic fluvial and lacustrine-deltaic sandstones of the Yanchang Oil Field in the Ordos Basin is strongly influenced by the burial history and facies-related diagenetic events. The fluvial sandstones have a higher average porosity (14.8%) and a higher permeability (12.7 × 10−3 μm2) than those of the deltaic sandstones (9.8% and 5.8 ×10−3) μm2, respectively). The burial compaction, which resulted in 15% and 20% porosity loss for Jurassic and Triassic sandstones, respectively, is the main factor causing the loss of porosity both for the Jurassic and Triassic sandstones. Among the cements, carbonate is the main one that reduced the reservoir quality of the sandstones. The organic acidic fluid derived from organic matter in the source rocks, the inorganic fluid from rock-water reaction during the late diagenesis, and meteoric waters during the epidiagenesis resulted in the formation of dissolution porosity, which is the main reason for the enhancement of reservoir-quality.  相似文献   

5.
Stone forest (‘Shilin’ in Chinese) is a unique karst landform with a complex evolution process. Based mainly on the characteristics and interrelationships of sub‐soil, soil and sub‐aerial erosion in Lunan karst area, the authors develop a triplex erosion model to describe the evolution of stone forest, and apply it to examine the current development stage and the prospect of the Lunan Stone Forest. The study shows that sub‐soil corrosion, a basic driving force for the vertical scope of a stone forest, usually occurs within 10 m below ground surface but is observed to be most active within the top 2 m, which constitutes the best development zone for stone forest. Under modern climatic conditions, the tip of the stone pillars in Lunan karst area is lowering at a rate of 10·4 mm ka?1, whereas the base of the stone pillars is deepening at 26·17 mm ka?1. Therefore, the height of stone pillars is increasing at a rate of 15·77 mm ka?1. Considering that soil erosion in the study area is as high as 650 mm ka?1, the visible height of the stone forest is actually increasing at a rate of 639·6 mm ka?1. However, the best evolution time for Lunan Stone Forest has already passed despite the fact that it is still growing taller at the present time. This is because the soil layer, which plays an extremely significant role in the heightening of stone pillars, is rapidly thinning at a rate of 623·83 mm ka?1. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

6.
Growing evidence suggests microbial respiration of dissolved organic carbon (DOC) may be a principal driver of subsurface dissolution and cave formation in eogenetic carbonate rock. Analyses of samples of vadose zone gasses, and geochemical and hydrological data collected from shallow, uncased wells on San Salvador Island, Bahamas, suggest tidally varying water tables may help fuel microbial respiration and dissolution through oxygenation. Respiration of soil organic carbon transported to water tables generates dysaerobic to anaerobic groundwater, limiting aerobic microbial processes. Positive correlations of carbon dioxide (CO2), radon-222 (222Rn) and water table elevation indicate, however, that tidal pumping of water tables pulls atmospheric air that is rich in oxygen, and low in CO2 and 222Rn, into contact with the tidal capillary fringe during falling tides. Ratios of CO2 and O2 in vadose gas relative to the atmosphere indicate this atmospheric oxygen fuels respiration within newly-exposed, wetted bedrock. Deficits of expected CO2 relative to O2 concentrations indicate some respired CO2 is likely removed by carbonate mineral dissolution. Tidal pumping also appears capable of transferring oxygen to the freshwater lens, where it could also contribute to respiration and dissolution; dissolved oxygen concentrations at the water table are at least 5% saturated and decline to anaerobic conditions 1–2 m below. Our results demonstrate how tidal pumping of air to vadose zones can drive mineral dissolution reactions that are focused near water tables and may contribute to the formation of laterally continuous vuggy horizons and potentially caves. © 2020 John Wiley & Sons, Ltd.  相似文献   

7.
In order to investigate the rate of dissolution of gypsiferous rocks under natural conditions in streams of the Ebro river basin, 55 dissolution trials were carried out with spheres of alabastrine gypsum placed in flowing water. Experimental conditions involved flow velocities between 0·3 and 1·2 m s?1, electrical conductivities between 0·3 and 1·9 dS m?1, and the saturation indices of gypsum between ?2·1 and ?0·33. Mean values of dissolution rate obtained vary from 104 to 226 gm?2 h?1. This loss of mass corresponded to a volume of gypsum of between 393 and 853mmm?2 year?1. The factors most affecting the dissolution of gypsum are in descending order of importance: flow velocity > electrical conductivity > gypsum saturation.  相似文献   

8.
In twelve flowing water (Q 1.04… 30.4 m3/s, 0.5… 1.2 m/s) the self-purification efficiency in 110 river sections (L 1.3… 26.4 km) is determined as the load difference in kg · d?1 COD-Mn as well as in g · m?3 referred to the daily passage. In the economical comparison, the self-purification efficiency is valued as substitution for wastewater treatment plants of the same capacity. At a mean specific capacity of 8.6 g · m?3, the self-purification efficiency of the investigated waters is equivalent to economical values of 124 · 103… 534 · 103 M km?1 investment costs, 11 · 103… 80 · 103 M · a?1km?1 operating costs and 5… 81 MWh · a?1 km?1 expenditure of energy. The specific capacity in g · m?3 COD-Mn shows an exponential regression to the degree of saprobity (L = 0.015 · exp (1.7358 · S)). From this empirical model the limits of the self-purification capacity of aerobic waters by oxygen input can be detected: in respective examples more than 50% of the required oxygen input are due to weirs.  相似文献   

9.
This paper explores the relationship between vadose zone hydrology and geochemical changes in mixed mineralogy carbonate sands from a Bahamian coastal dune of Holocene age. Cores were taken from two sites: at site A, a shallow humic Entisol is developed beneath open scrub vegetation, while at site B a deeper, more organic-rich Inceptisol has formed beneath a mature hardwood coppice. X-ray diffraction analysis reveals significant contrasts in mineralogy both within and between the two sites, with partial stabilization of high-Mg calcite and aragonite, to low-Mg calcite. Stabilization is greater at site B, and is accompanied by a significant increase in total porosity. Diagenetic changes in pore-size distribution have implications for residence times of percolating water, as determined using measurements of moisture retention characteristics using pressure plate apparatus, and hydrological models of unsaturated zone moisture flux. The diagenetically more mature sands from site B have a 50–100 per cent higher moisture retention, although unsaturated hydraulic conductivity is also higher, particularly at greater suctions. The increase in water retention is likely to enhance further rates of mineral-controlled reactions, while development of an organic-rich soil also enhances the geochemical drive for dissolution. Carbonate diagenesis thus appears to be strongly linked to vadose zone hydrology, and the interactions identified here have important consequences for the nature and long-term rates of mineral stabilization. © 1997 by John Wiley & Sons, Ltd.  相似文献   

10.
The reservoir quality of Jurassic and Triassic fluvial and lacustrine-deltaic sandstones of the Yanchang Oil Field in the Ordos Basin is strongly influenced by the burial history and facies-related diagenetic events. The fluvial sandstones have a higher average porosity (14.8%) and a higher permeability (12.7×10?3 ?m2) than those of the deltaic sandstones (9.8% and 5.8 ×10?3 ?m2, respectively). The burial compaction, which resulted in 15% and 20% porosity loss for Jurassic and Triassic sandstones, respectively, is the main factor causing the loss of porosity both for the Jurassic and Triassic sandstones. Among the cements, carbonate is the main one that reduced the reservoir quality of the sandstones. The organic acidic fluid derived from organic matter in the source rocks, the inorganic fluid from rock-water reaction during the late diagenesis, and meteoric waters during the epidiagenesis resulted in the formation of dissolution porosity, which is the main reason for the enhancement of reservoir-quality.  相似文献   

11.
Alkaline diagnesis is a diagenetic process that a reservoir undergoes under an alkaline environment. Because of the influence of alkaline formation water, the most typical characteristics of diagnesis is that quartz is obviously dissolved, feldspar is massively enlarged, and less late carbonate cement is formed in the evolution of carbonate minerals. With the decrease of the alkalinity of the formation water in diagenesis, the quartz overgrowths become common. The change in the chemical characteristics of the formation water leads to a more complex distribution of reservoir porosity at different depths than that of the secondary porosity formed by classical acidic water. It also makes the B stage of early diagenesis the important development period of secondary porosity.  相似文献   

12.
Impure reworked evaporitic sandstones, preserved on Meridiani Planum, Mars, are mixtures of roughly equal amounts of altered siliciclastic debris, of basaltic provenance (40 ± 10% by mass), and chemical constituents, dominated by evaporitic minerals (jarosite, Mg-, Ca-sulfates ± chlorides ± Fe-, Na-sulfates), hematite and possibly secondary silica (60 ± 10%). These chemical constituents and their relative abundances are not an equilibrium evaporite assemblage and to a substantial degree have been reworked by aeolian and subaqueous transport. Ultimately they formed by evaporation of acidic waters derived from interaction with olivine-bearing basalts and subsequent diagenetic alteration. The rocks experienced an extended diagenetic history, with at least two and up to four distinct episodes of cementation, including stratigraphically restricted zones of recrystallization and secondary porosity, non-randomly distributed, highly spherical millimeter-scale hematitic concretions, millimeter-scale crystal molds, interpreted to have resulted from dissolution of a highly soluble evaporite mineral, elongate to sheet-like vugs and evidence for minor synsedimentary deformation (convolute and contorted bedding, possible teepee structures or salt ridge features). Other features that may be diagenetic, but more likely are associated with relatively recent meteorite impact, are meter-scale fracture patterns, veins and polygonal fractures on rock surfaces that cut across bedding. Crystallization of minerals that originally filled the molds, early cement and sediment deformation occurred syndepositionally or during early diagenesis. All other diagenetic features are consistent with formation during later diagenesis in the phreatic (fluid saturated) zone or capillary fringe of a groundwater table under near isotropic hydrological conditions such as those expected during periodic groundwater recharge. Textural evidence suggests that rapidly formed hematitic concretions post-date the primary mineral now represented by crystal molds and early pore-filling cements but pre-date secondary moldic and vug porosity. The second generation of cements followed formation of secondary porosity. This paragenetic sequence is consistent with an extended history of syndepositional through post-depositional diagenesis in the presence of a slowly fluctuating, chemically evolving, but persistently high ionic strength groundwater system.  相似文献   

13.
The Krusné hory (Erzgebirge or Ore Mountains) has been heavily affected by high atmospheric pollutant deposition caused by fossil fuel combustion in an adjacent Tertiary coal basin. Long‐term routine sampling of bulk precipitation (1977–1996) and stream water (1977–1998) in a forested area on the south‐eastern slope of the mountains were used to evaluate trends and patterns in solute concentration and flux with respect to controlling processes. From 1977 to 1996, the annual volume‐weighted Ca2+ and SOconcentrations decreased in bulk precipitation. However, after 1989, when a pronounced and continuous decrease occurred in coal production, annual volume‐weighted concentrations decreased for most solutes, except H+. The concentration decreases were marked, with 1996 levels at or below 50% of those in 1989. The lack of a trend in H+ is attributed to similar decreases in both acid anions and neutralizing base cations. Stream water concentrations of most solutes, i.e. H+, Ca2+, Mg2+, SONOwere highest at the onset of sampling in 1977, decreased markedly from 1977 to 1983 and decreased more gradually from 1983 to 1998. The spruce forest die‐back and removal reduced dry deposition of these solutes by reducing the filtering action, which was provided by the forest canopy. A notable decrease in stream water Ca2+ concentrations occurred after 1995 and may be due to the depletion of Ca2+, which was provided by catchment liming in 1986, 1988 and 1989. Solute flux trends in bulk atmospheric deposition and stream water generally were not significant and the lack of trend is attributed to the large interannual variability in precipitation quantity and runoff, respectively. All solutes except Na+ varied seasonally. The average seasonal concentrations varied between the solutes, but for most solutes were highest in winter and spring and lowest in summer, correlating with the seasonal trend in runoff. For Ca2+, Mg2+ and SOthe concentration minimum occurs in September and the maximum occurs in February or March, correlating with the seasonal baseflow. These solutes are primarily controlled by the contribution of soil water and groundwater to stream flow. During snowmelt, the meltwater generally causes concentrations to decrease as soil water and groundwater are diluted. For NO3 , average minimum concentrations occur in August at the end of the growing season concurrent with the lowest stream flow, and the maximum occurs in February and March with high stream flow during snowmelt. Seasonal stream water NOconcentration variations are large compared with the long‐term decrease. Copyright © 1999 John Wiley & Sons, Ltd.  相似文献   

14.
Small bounded plots were used to investigate dissolved nutrient losses in overland flow in eucalyptus and pine forests in northern Portugal following understorey fires. Losses of NON, POCa2+, Mg2+ and K+ were measured over 19 months during the first 2–3 years after fire. Solute losses in overland flow increased after fire in eucalyptus and pine terrain due to the mineralization of litter and vegetation and increased overland flow. The data suggest that (i) elevated losses persist for at least 2 and 3 years at the pine and eucalyptus sites respectively and (ii) soluble losses of POK+ in a post‐fire cycle may exceed those adsorbed to eroded material. Losses of POalthough relatively small, are potentially the most detrimental to soil fertility and forest productivity because of the limited opportunity for P replenishment and correspondingly high P losses adsorbed to eroded sediment. Copyright © 2000 John Wiley & Sons, Ltd.  相似文献   

15.
Jordan is classified as an arid to semi‐arid country with a population according to 1999 estimates of 4·8 millions inhabitants and a growth rate of 3·4%. Efficient use of Jordan's scarce water is becoming increasingly important as the urban population grows. This study was carried out within the framework of the joint European Research project ‘Groundwater recharge in the eastern Mediterranean’ and describes a combined methodology for groundwater recharge estimation in Jordan, the chloride method, as well as isotopic and hydrochemical approaches. Recharge estimations using the chloride method range from 14 mm year?1 (mean annual precipitation of 500 mm) for a shallow and stony soil to values of 3·7 mm year?1 for a thick desert soil (mean annual precipitation of 100 mm) and values of well below 1 mm year?1 for thick alluvial deposits (mean annual rainfall of 250 mm). Isotopically, most of the groundwater in the Hammad basin, east Jordan, falls below the global meteoric water line and far away from the Mediterranean meteoric water line, suggesting that the waters are ancient and were recharged in a climate different than Mediterranean. Tritium levels in the groundwater of the Hammad basin are less than the detection limit (<1·3 TU). However, three samples in east Hammad, where the aquifer is unconfined, present tritium values between 1 and 4 TU. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

16.
Studies on the hydrogeological conditions of the Mesogea basin in east Attica reveal that the aquifers developed on the post‐alpine formations at the inner part of the coastal brackish zone exhibit positive hydraulic head. These Neogene and Quaternary deposits present high salt concentrations. Selected points were sampled (total 85: 51 wells and 34 boreholes) in order to obtain hydrogeological and hydrochemical data for a better understanding of the structure, operation and dynamics of the aquifer of the area. Statistical methods, R‐mode factor analysis and scatter‐plot diagrams were used for the hydrochemical analysis and presentation of the data. The groundwater resources are relatively weak and there is significant quality degradation due to the geological structure of the greater area, as well as the bad management of the aquifer and anthropogenic activities. Groundwater is characterized by high salt concentrations. Electrical conductivity values range between 260 and 6970 µS cm?1. High salt concentrations at the coastal aquifers are due to sea intrusion, whereas they are attributed to the dissolution of minerals of the geological environment in the inland area. The groundwaters of the study area can be classified into five water types: Ca–HCO3, Mg–HCO3, Na–HCO3, Na–Cl and Mg–Cl. They are saturated in dolomite and calcite, whereas they are unsaturated in anhydrite. High ion concentrations, e.g. ] (0‐221 mg l?1), ] (0·01‐1·88 mg l?1), ] (0·01‐6·75 mg l?1), as well as high heavy metals concentrations are attributed to anthropogenic impacts. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

17.
Vertical profiles of the streamwise mass flux of blown sand in the near-bed (< 17 mm) region are analysed from high-resolution measurements made using an optical sensor in a wind tunnel. This analysis is complemented by detailed measurements of mass flux and mean velocity profiles throughout the boundary layer depth (0·17 m) using passive, chambered sand traps of small dimensions and armoured thermal anemometers, respectively. The data permit a preliminary analysis of the relations between the observed forms of the profiles of near-bed fluid stress and horizontal mass flux within a carefully conditioned boundary layer. Profiles of mass flux density are found to be characterized by three regions of differing gradient with transitions at about 2 mm and 19 mm above the bed. The exponential decay of mass flux with height is confirmed for elevations above 19 mm, and when plotted as a function of u*2/g (a parameter of mean vertical trajectory height in saltation), the gradient of mass flux in this region scales with the wake-corrected friction velocity (u), where u > 0·30 m s−1. A separate near-bed region of more intense transport below 19 mm is identified which carries 80 per cent of the total mass flux. This region is evident in some previous field and wind tunnel data but not in profiles simulated by numerical models. Ventilated passive sand traps underestimate mass flux in this region by 37 per cent. At slow or moderate wind speeds a third significant region below 2 mm is observed. These regions are likely to be related to grain populations in successive saltation, low-energy ejections and intermittent bed contact, respectively. Optical measurements reveal locally high grain concentrations at some elevations below 5 mm; these heights scale with transport rate, mass flux gradient and wind speed. Copyright © 1999 John Wiley & Sons, Ltd.  相似文献   

18.
Cryoturbated Upper Chalk is a dichotomous porous medium wherein the intra‐fragment porosity provides water storage and the inter‐fragment porosity provides potential pathways for relatively rapid flow near saturation. Chloride tracer movement through 43 cm long and 45 cm diameter undisturbed chalk columns was studied at water application rates of 0·3, 1·0, and 1·5 cm h?1. Microscale heterogeneity in effluent was recorded using a grid collection system consisting of 98 funnel‐shaped cells each 3·5 cm in diameter. The total porosity of the columns was 0·47 ± 0·02 m3 m?3, approximately 13% of pores were ≥ 15 µm diameter, and the saturated hydraulic conductivity was 12·66 ± 1·31 m day?1. Although the column remained unsaturated during the leaching even at all application rates, proportionate flow through macropores increased as the application rate decreased. The number of dry cells (with 0 ml of effluent) increased as application rate decreased. Half of the leachate was collected from 15, 19 and 22 cells at 0·3, 1·0, 1·5 cm h?1 application rates respectively. Similar breakthrough curves (BTCs) were obtained at all three application rates when plotted as a function of cumulative drainage, but they were distinctly different when plotted as a function of time. The BTCs indicate that the columns have similar drainage requirement irrespective of application rates, as the rise to the maxima (C/Co) is almost similar. However, the time required to achieve that leaching requirement varies with application rates, and residence time was less in the case of a higher application rate. A two‐region convection–dispersion model was used to describe the BTCs and fitted well (r2 = 0·97–0·99). There was a linear relationship between dispersion coefficient and pore water velocity (correlation coefficient r = 0·95). The results demonstrate the microscale heterogeneity of hydrodynamic properties in the Upper Chalk. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

19.
The hydrology and nitrogen biogeochemistry of a riparian zone were compared before and after the construction of beaver dams along an agricultural stream in southern Ontario, Canada. The beaver dams increased surface flooding and raised the riparian water table by up to 1·0 m. Increased hydraulic gradients inland from the stream limited the entry of oxic nitrate‐rich subsurface water from adjacent cropland. Permeable riparian sediments overlying dense till remained saturated during the summer and autumn months, whereas before dam construction a large area of the riparian zone was unsaturated in these seasons each year. Beaver dam construction produced significant changes in riparian groundwater chemistry. Median dissolved oxygen concentrations were lower in riparian groundwater after dam construction (0·9–2·1 mg L?1) than in the pre‐dam period (2·3–3·9 mg L?1). Median NO3‐N concentrations in autumn and spring were also lower in the post‐dam (0·03–0·07 mg L?1) versus the pre‐dam period (0·1–0·3 mg L?1). In contrast, median NH4‐N concentrations in autumn and spring months were higher after dam construction (0·3–0·4 mg L?1) than before construction (0·13–0·14 mg L?1). Results suggest that beaver dams can increase stream inflow to riparian areas that limit water table declines and increase depths of saturated riparian soils which become more anaerobic. These changes in subsurface hydrology and chemistry have the potential to affect the transport and transformation of nitrate fluxes from adjacent cropland in agricultural landscapes. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

20.
In extremely arid regions, deeply buried phreatic water evaporates during the daytime from March to November in the northern hemisphere. It has been found that the earth–air undergoes ‘autonomous breathing’ and ‘passive breathing’, respectively caused by the changes of temperature and atmospheric pressure. In this paper, the effects of these breathing modes on phreatic evaporation (PhE) were investigated as well as the responsible mechanisms. Quantitative estimates suggest that the direct contribution from autonomous breathing is only 0.55 g·m−2·yr−1. Passive breathing pumps water vapour upwards from the deeply buried phreatic water table. Film water on the soil continuously migrates in pulsation from deep layers to the upper layer. Na2SO4 in the shallow soil absorbs moisture from the earth–air at night and decomposes during the day, forming water vapour, which is critical to the occurrence of PhE. The diurnal PhE process can be elucidated in detail by the bimodal variation in the atmospheric pressure. PhE occurs mainly from 10:00 to 17:00 during daytime from March to November, which correlates with passive breathing of the earth–air. The amplitude of atmospheric fluctuation determines the amount of earth–air that outflows, while temperature determines the water vapour concentration. In calculation, PhE is equal to the net absolute humidity (AH) times the amount of earth–air. There is 1.55 mm·year−1 of PhE caused by daily peak→valley differences, and about 2.97 mm·year−1 in estimation caused by numerous atmospheric fluctuations smaller than 2.84 hPa. The results coincide with the actual amount of PhE monitored of 4.52 mm·year−1. Therefore, the amount of PhE is proportional to the range and frequency of fluctuation in external atmospheric pressure, and is also positively related to soil temperature, salt content, water content, porosity, and vadose zone thickness.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号