Water environment

About: Water environment is a research topic. Over the lifetime, 13384 publications have been published within this topic receiving 125138 citations.

Chapter 6 Origin and Occurrence of Dolostones

Abstract: Summary This chapter begins with a historical review of the dolomite literature and traces progressive advances in the search for a solution of the “dolomite problem”. The “dolomite question” defied solution until the recent advent of the X-ray revolution, when dolomite identification became simple, fast, and efficient. The general framework of the “question” now appears to have been largely resolved as a result of intensive field studies of Quaternary carbonate sediments and the application of X-ray and stable isotope (carbon and oxygen) analysis to both Quaternary carbonate sediments and ancient limestones and dolostones. Many problems, however, are still outstanding, but they are on a different level. In the opinion of the authors, most dolostone deposits in the geologic record owe their origin to hypersaline brines. They must, therefore, be considered as evaporite deposits. Exceptions are uncommon and include those that are related to soil-forming and bacterial processes or have resulted from recycling of pre-existing dolomite. Hypersaline brines are formed by “capillary concentration” or by “refluxion” in the depositional environment in areas where evaporation exceeds precipitation plus run-off and by as yet unexplained processes in the subsurface. In “capillary concentration”, interstitial waters in the sediments transpire upward through porous sea-marginal sediments and evaporate at the sediment-air interface, a process similar to that which is responsible for caliche formation. This process leads to dolomite formation in supratidal and intertidal environments on broad shallow shelves with interfingering gypsum and/or anhydrite (landward) and marine carbonates (seaward). Under conditions of humid climate, however, anhydrite and gypsum may not develop. Supratidal to intertidal dolostones display many or most of the following structures: mud cracks, “birdseye” structures, burrow mottles, boundinage-like structures, scour-and-fill structures, channels, ripple marks, cross-stratification, and algal structures; these dolostones are commonly unfossiliferous and laminated and are texturally structureless or pellet muds. In “refluxion”, evaporation increases the concentration and density of the water in a restricted lagoon or intermontane basin. This produces a brine that sinks and migrates to the lowest possible topographic depressions where it may seep slowly through the underlying sediments, which are progressively dolomitized. Alternately in refluxion, dolomite may form directly from the brine with aragonite as a possible transitional phase, and with the deposition of layered dolomite mud at the bottom of the basin. The Mg/Ca ratio of the brine must be increased from that of sea water to a ratio larger than that which should be in equilibrium with both calcite and dolomite for dolomite to form. This ratio can be raised by the removal of calcium from the brine to form aragonite or gypsum or both; however, gypsum is only deposited in very shallow water where a supply of oxygen is ample, because it tends to be degraded to HzS and iron sulfide by bacteria where the oxygen supply is less. Hence layered dolomite formed under these conditions is indicative of a deeper water environment, of unspecified depth, with gypsum deposited in correlative positions along shore. Precaution is advised, therefore, in interpreting all syngenetic dolomite of supratidal origin. Both “capillary concentration” and “refluxion” may operate in the same basin; thus, at Salt Flat Graben, West Texas, an intermontane basin, the former is responsible for dolomite formation along the margin of the basin and the latter, in its center. Dolomite formed after burial, which includes most diagenetic and all epigenetic (fault and fracture-related) dolomite, owes its origin to subsurface brines. The origin of these brines is unknown, but their salinity is very close to that under which dolomite is formed in the depositional environment. Subsurface waters are normally depleted with respect to both magnesium and sulfate ions which were withdrawn from the brines during dolomitization. Dolostones occur in a variety of stratigraphic associations which can be summarized in the following main types: (1) Syngenetic dolomite, a dolomite which is formed penecontemporaneously in its environment of deposition as a micrite or as fine-grained crystals, and which may also: (a) interfinger with both marine and nonmarine evaporites, with or without associated terrigenous sediments; (b) interfinger with limestones, both marine and nonmarine, with or without terrigenous sediments; (c) interfinger with terrigenous sediments; (d) be in the form of dolomite crystals disseminated in terrigenous sediments; (e) be formed by biological agents, such as bacteria; and (f) occur in nonmarine environments as a weathering product in soils and caliche. (2) Detrital dolostone and detrital dolomite is formed by recycling of preexisting dolomite sediments. (3) Diagenetic dolostone is formed by replacement of limestone following consolidation of the sediment or coincident with the consolidation; this type of dolostone may also form by penecontemporaneous replacement, Diagenetic dolostone may form within individual beds or along surfaces of stratigraphic dis-continuities. (4) Epigenetic dolostone is formed by replacement of limestones with the replacement localized by post-depositional structural elements, such as faults and fractures. Most dolostones are formed by repIacement of pre-existing calcium carbonate sediments, but dolostones may revert back to calcium carbonate rocks by the process of dedolomitization.

Effect of bias voltage on microstructure and properties of Ti-doped graphite-like carbon films synthesized by magnetron sputtering

Abstract: Ti-doped graphite-like carbon (GLC) films with different microstructures and compositions were fabricated using magnetron sputtering technique. The influence of bias voltages on microstructure, hardness, internal stress, adhesion strength and tribological properties of the as-deposited GLC films were systemically investigated. The results showed that with increasing bias voltage, the graphite-like structure component (sp2 bond) in the GLC films increased, and the films gradually became much smoother and denser. The nanohardness and compressive internal stress increased significantly with the increase of bias voltage up to ?300 V and were constant after ?400 V. GLC films deposited with bias voltages in the range of -300–-400 V exhibited optimum adhesion strength with the substrates. Both the friction coefficients and the wear rates of GLC films in ambient air and water decreased with increasing voltages in the lower bias range (0–-300 V), however, they were constant for higher bias values (beyond ?300 V). In addition, the wear rate of GLC films under water-lubricated condition was significantly higher for voltages below ?300 V but lower at high voltage than that under dry friction condition. The excellent tribological performance of Ti-doped GLC films prepared at higher bias voltages of ?300–-400 V are attributed to their high hardness, tribo-induced lubricating top-layers and planar (2D) graphite-like structure.

Nitrate Accumulation and Leaching in Surface and Ground Water Based on Simulated Rainfall Experiments

Abstract: To evaluate the process of nitrate accumulation and leaching in surface and ground water, we conducted simulated rainfall experiments. The experiments were performed in areas of 5.3 m2 with bare slopes of 3° that were treated with two nitrogen fertilizer inputs, high (22.5 g/m2 NH4NO3) and control (no fertilizer), and subjected to 2 hours of rainfall, with. From the 1st to the 7th experiments, the same content of fertilizer mixed with soil was uniformly applied to the soil surface at 10 minutes before rainfall, and no fertilizer was applied for the 8th through 12th experiments. Initially, the time-series nitrate concentration in the surface flow quickly increased, and then it rapidly decreased and gradually stabilized at a low level during the fertilizer experiments. The nitrogen loss in the surface flow primarily occurred during the first 18.6 minutes of rainfall. For the continuous fertilizer experiments, the mean nitrate concentrations in the groundwater flow remained at less than 10 mg/L before the 5th experiment, and after the 7th experiment, these nitrate concentrations were greater than 10 mg/L throughout the process. The time-series process of the changing concentration in the groundwater flow exhibited the same parabolic trend for each fertilizer experiment. However, the time at which the nitrate concentration began to change lagged behind the start time of groundwater flow by approximately 0.94 hours on average. The experiments were also performed with no fertilizer. In these experiments, the mean nitrate concentration of groundwater initially increased continuously, and then, the process exhibited the same parabolic trend as the results of the fertilization experiments. The nitrate concentration decreased in the subsequent experiments. Eight days after the 12 rainfall experiments, 50.53% of the total nitrate applied remained in the experimental soil. Nitrate residues mainly existed at the surface and in the bottom soil layers, which represents a potentially more dangerous pollution scenario for surface and ground water. The surface and subsurface flow would enter into and contaminate water bodies, thus threatening the water environment.

Distribution of organophosphoric acid triesters between water and sediment at a sea-based solid waste disposal site

Abstract: Organophosphoric acid triester (OPE) concentration levels in water and bottom sediment at the Osaka North Port Sea-Based Solid Waste Disposal Site were investigated, and the behavior of OPEs in the water environment of the waste disposal site was examined. The more highly water-soluble OPEs were frequently detected in raw water. Of the OPEs detected, TCEP and TCPP showed very high concentrations (1.0–90 μg/l), followed by TEP (0.3–10 μg/l) > TBXP (0.8–6.3 μg/l) > TDCPP (0.6–6.2 μg/l) > TBP (0.2–1.5 μg/l) > TPP (<0.1 μg/l). Most OPEs detected in water were eluted from the disposal waste to the water phase immediately and behaved as dissolved forms with no distribution in suspended solids (SS). On the other hand, the less water-soluble OPEs, such as TCP or TEHP, were detected in bottom sediment but hardly at all in water samples. All OPEs were detected at the waste disposal site, within which their concentration levels were uniform. It appeared that the less water-soluble OPEs were present as SS-associated forms and behaved in line with the floating surface sludge at the bottom.

Some Aspects of Speciation of Mercury in a Water Environment

Abstract: Extreme toxicity of some species of mercury, the ability of this element to bioaccumulate in particular in fish meat, and the known cases of lethal poisoning by mercury have drawn particular attention to this element's presence in the natural environment. Due to the relatively long time of its presence in the air, elemental mercury can be transported over large distances, hence the presence of mercury of anthropogenic origin is detected practically all over the world. Apart from the elemental mercury, the main species of mercury in water are Hg(II) and mercury-organic species, in particular methylmercury. The latter undergoes strong bioaccumulation in living organisms and concentration in the trophic chains. That is why the relative concentration of mercury in organisms is determined by its presence in water. The concentration of mercury in water is related to the processes of methylation and demethylation, influenced by biotic and abiotic factors such as the activity of microogranisms, access to oxygen, illumination, temperature, pH and others. Despite intense studies, full and reliable recognition of the ecological and health effects of pollution by this toxic metal is still impossible. The aim of this paper is to present the problems related to speciation of mercury, and describe some conversion and migration processes of mercury in the water environment.