Showing papers on "Vadose zone published in 1980"

Carbonate Diagenetic Textures from Nearsurface Diagenetic Environments

Abstract: Understanding the processes and products of carbonate diagenesis is essential to exploration for, and optimum development of, hydrocarbon reservoirs in carbonate rocks. Much (and perhaps most) cementation and formation of secondary porosity (except fractures) in carbonates occurs at relatively shallow depths in one of four major diagenetic environments: the vadose zone, meteoric phreatic zone, mixing zone, and marine phreatic zone. Each of these zones may be divided into several parts on the basis of rate of water movement and saturation of the water with respect to calcium carbonate. Most carbonates are deposited in and begin their diagenetic history in the marine phreatic environment. This zone may be divided into two end members of a continuous spectrum: a zone of relatively little water circulation in which micritization and minor intragranular cementation occur, and a zone of good water circulation near the sediment/water interface of shelf margins or the upper shoreface in which extensive intergranular and cavity-filling cementation occur. Fibrous aragonite and micritic Mg-calcite are the dominant cements. With subaerial exposure, fresh water will replace sea water in the pores of shallow-water carbonates, and a zone of mixed fresh and marine waters may form. In long-lived mixing zones, dolomite may form if the water is of relatively low salinity, whereas bladed Mg-calcite may form if the water is relatively marine. Active water circulation in the mixing zone, which may be caused by seasonal rainfall, is necessary for dolomitization or cementation. Diagenesis in the freshwater phreatic environment may involve leaching in the zone of solution, neomorphism of grains accompanied by extensive intergranular calcite cementation in the active saturated zone, or neomorphism of grains without cementation in the stagnant saturated zone. Syntaxial overgrowths on echinoderm fragments and interlocking crystals of equant calcite that coarsen toward pore centers are typical of cementation in the active freshwater phreatic zone. The freshwater vadose environment is the zone with both air and meteoric water in the pores and may be divided into the zone of solution and the zone of precipitation. CO2 from the atmosphere and soil contributes to solution which generally occurs near the soil zone and forms vugs, molds, and etched grains. When the water becomes saturated with respect to calcite, evaporation or CO2 loss may cause precipitation of fine equant calcite in the form of pendant and meniscus cements. Grains may be altered to calcite, particularly in humid climates, and caliche crusts may be produced by evaporation and/or biologic (generally algal) factors. Climate plays an important role in early diagenesis if subaerial exposure occurs. In arid climates, cementation in freshwater environments may be limited and primary intergranular porosity may be preserved. In humid climates, little primary porosity is likely to escape cementation, but significant amounts of secondary moldic and vuggy porosity may form. Interpretation of diagenesis in carbonates is complicated by the fact that diagenetic environments may change many times in the history of a carbonate rock. By recognizing the processes leading to the formation or preservation of porosity, and the distribution of diagenetic End_Page 461------------------------------ environments in which those processes acted, the distribution of porosity in subsurface carbonates can often be predicted.

Quaternary pedogenic calcretes from the Kalahari (southern Africa): mineralogy, genesis and diagenesis

Abstract: The calcretes of the Kalahari are amongst the thickest in the world representing pedogenic episodes in a semi-arid climate during Pliocene to Recent times. The descriptive morphological terminology of Netterberg is used to describe the calcrete types and a differentiation into simple and composite profiles is made. A pedogenic/diagenetic scheme has been constructed using all available data. Early calcite cementation is induced by two mechanisms. Slow evaporation and/or CO2 loss causes the precipitation of low-Mg calcite, whereas rapid evaporation and/or CO2 loss precipitates predominantly high-Mg calcite, in thermodynamic disequilibrium with the low Mg/Ca ratio vadose water, and minor aragonite (which transforms rapidly to low-Mg calcite). High-Mg calcite is also precipitated from high Mg/Ca ratio vadose waters in calcretes developed on Mg-rich host lithologies and by capillary rise from shallow groundwaters in saline depressions. Calcite precipitation may be passive (cement), displacive or replacive, in the latter released silica migrating down-profile to precipitate length-slow chalcedony, clinoptilolite (saline conditions), length-fast chalcedony and megaquartz (non-saline conditions). Displacive introduction of calcite takes place from highly supersaturated solutions due to rapid evaporation (with CO2 loss) of vadose waters. During low-Mg calcite precipitation (in a ‘closed’ system) the Mg concentration of the resulting solution increases. This, combined with Mg released during high-Mg to low-Mg calcite transformation, induces precipitation of authigenic palygorskite, sepiolite and minor dolomite. Vadose dolomite is often present whilst some dolomite may be precipitated at the mixing-zone of vadose and phreatic waters. The proposed model applies to the Kalahari calcretes, although it may perhaps be extrapolated to other areas. Further detailed studies, involving analyses of pore water chemistry, soil microclimate, and trace element and isotopic analyses of individual cements are necessary.

Nitrate‐reducing and ammonium‐oxidizing bacteria in the vadose zone of the chalk aquifer of England

Abstract: The vadose zone of the Chalk aquifer from two sites of different land use was found to contain large numbers of nitrate‐reducing and ammonium‐oxidizing bacteria. Relationships between the type of bacteria and nitrogen compounds produced showed that denitrification was occurring beneath the permanent grassland site, whereas the vadose zone beneath the fertilized arable site was essentially aerobic and little attenuation of nitrate by denitrification was occurring. The results show that nitrate is certain to be involved in biological reactions in the vadose zone of the Chalk, although quantification of the rate and the extent of these reactions is still not possible.

Vadose, Phreatic, and Marine Diagenesis of Pleistocene-Holocene Carbonates in a Borehole: Mediterranean Coast of Israel

Abstract: A 136 m section of Pleistocene-Holocene carbonate sediments in the Ashdod Yam 15/0 water well, which penetrates the coastal plain aquifer of Israel, was chosen for a study of carbonate diagenesis. The following diagenetic parameters were used to diagnose vadose, phreatic and marine diagenetic environments: original textures of aragonitic molluscan shells, kind and distribution of carbonate cements, trace-element (Sr and Mg) concentration and carbonate mineralogy. Comparison of the diagenetic parameters with the present hydrologic regime in the borehole indicates the following: 1) the preservation of vadose features well within the present fresh-water phreatic zone; 2) abundant intergranular and moldic porosity in a leached interval coinciding with a brackish-water mixing zone; 3) clear signs of fresh-water alteration, including leaching and recementation of molluscs and complete occlusion of pore space by drusy cement, in a permeable layer in a highly saline zone of sea-water intrusion; 4) the preservation of original metastable carbonate mineralogy, molluscan microstructure, and marine cements in the thin permeable layers shielded by impermeable shales; 5) extensive fresh-water alteration (coalescive neomorphic recrystallization, leaching, an occlusion of pore space by drusy cement) in thick permeable intervals.

The Role of the Limnologist in Ground Water Studies

Abstract: The following topics are discussed: the advantages of an interdisciplinary approach to ground water studies; the porous aquifer as an open system; the problem of the terminology used in ground water ecology; the interest of ecologists in hydrogeological terminology; biological activity in the unsaturated zone of porous aquifers; the distribution of ground water fauna in the saturated zone of porous aquifers; ground water animals as environmental indicators; the manipulation of ground water ecosystems; basic lines for the further development of ground water ecological studies.

Model for unsaturated flow above a shallow water-table, applied to a regional sub-surface flow problem

Abstract: A mathematical model is developed to simulate transient unsaturated flow above a shallow water-table The unsaturated zone, here extending from just below the phreatic level to soil surface, is schematized into a root zone and a subsoil In the root zone the gradient of the hydraulic potential is assumed equal to zero Vertical flow in the subsoil is described by a combination of steady-state situations corresponding to the upper and lower boundary flux, respectively Transient flow is solved by a sequence of steady-state situations, subject to boundary flux conditions at the soil surface and from below the water table The solution uses time increments of the order of days and is efficient in terms of computer costs To verify the model for an actual field situation, it is linked at the upper boundary to a model for evapotranspiration and at the lower boundary to a model for two-dimensional horizontal saturated flow The resulting quasi three-dimensional model is applied to a field-size flow problem Results agree closely with observed water-table elevations The composite model is further used to predict consequences of groundwater extraction

Assessment of recharge to aquifers / Evaluation de recharge d'aquifères

Abstract: This paper describes a research project based in Cyprus to investigate a number of different methods for determining quantitative recharge to aquifers in semiarid areas. Large lysimeters and geochemical profile techniques have been used in the southeastern Mesaoria area of Cyprus where there has already been a quantitative water resources investigation and where a groundwater model is in preparation. Further joint geochemical and lysimeter studies are also being undertaken in the Akrotiri peninsula (average annual rainfall 430 mm). Excavation and piling techniques have been used to install the lysimeters which are up to 100 m2 in area by 4 m deep. The lysimeters have been made as large as possible to minimize edge effects whilst maintaining the soil and aquifer material within them in an ‘undisturbed’ state. A dry drilling method has been developed to obtain sand samples from the unsaturated zone together with a method for processing the core material to obtain data on moisture and solutes. Chlor...

Monitoring and physical characterization of unsaturated zone transport. Laboratory analysis

Abstract: Batch Kd screening tests on Hanford sediments indicated that anionic radionuclide mobility is dependent upon specie and concentration. Technetium, as TcO/sub 4//sup -/, is not sorbed to any extent and will move as rapidly as water. Iodine, as I/sup -/, will also move at high concentrations (>5 ppB) but is retarded at very low concentrations (<1 ppB). Neutral species such as tritium and complexed nuclides such as cobalt-60 (EDTA) are only slightly retarded while uncomplexed cobalt-60 is strongly bound to the sediments under the conditions studied. Strontium sorption is controlled by ion exchange-like reactions and sorption is predictable from a knowledge of soil and solution characteristics. Cesium is strongly bound to the Hanford sediments under typical conditions and appears to be sorbed independent of salt concentration for the synthetic ground waters tested. The sorption of relatively mobile nuclides such as tritium, cobalt-EDTA, and technetium could be measured with greater precision using the unsaturated column tests than with batch methods. Little or no difference in sorption was observed when the flow rates were changed by more than an order of magnitude (2 to 50 cm/day). Changes in saturation percentage of the soil, from 56 to 31%, also had only a minormore » effect on nuclide sorption. For layered sediments, the effective sorption of a layered sequence is controlled by the sorption characteristics of the most sorbing layer.« less

Experimental assessment of pollutant migration in the unsaturated zone of the Lower Greensand

Abstract: Summary In Britain most groundwater resources are held in bedrock aquifers and the unsaturated zone forms an important buffer between landfill disposal sites and the water table. The processes which control the transfer of (1) persistent organic compounds, (2) inorganic anions and (3) heavy metals, in the unsaturated zone have been evaluated using experimental lysimeters constructed in the Lower Greensand at a site near Uffington, Oxfordshire. Complementary monolith and column experiments were conducted. Biodegradation is the primary mechanism for the attenuation of the inorganic anions and the organic species, but the migration of heavy metals is limited by interactions with the sediment. Sequential, selective chemical extraction indicates that the stability of the geochemical phases retaining heavy metals controls their rate of movement. Although the processes which regulate the partition of the heavy metals are complex, uptake by sesquioxides and clay minerals are particularly important.

Groundwater Chemistry of Some Selected Areas in Southeastern Norway

Abstract: Within three areas in Southeastern Norway, Lake Mjosa district, As and Moss - Jeloy, groundwater samples for chemical analysis were collected during the years 1971–77 from 98 drilled wells in bedrocks. The water was taken at depths ranging from 15 m to 110 m below the land surface. The groundwater surface is usually present well below the overlying unconsolidated deposits of glacial, glacifluvial or marine origin. The movement of groundwater within the aquifers investigated is so slow that regional changes in water quality is not only dependent on weathering in the unsaturated zone, but also dependent on the solution of reservoir rocks below the groundwater surface. Variations in specific electrical conductance (20°C) largely reflects the different reservoir rock types. The highest values, around 550 μS/cm, are typically found in dark calcareous shales, while sandstones and gneisses give values around 300 μS/cm. The areas As and Moss-Jeloy are situated below the Late-Postglacial marine limit. The groundwater is here more or less influenced by ancient sea salts, perhaps also by fossil sea water, left over in sediments or in rock fractures. Brackish groundwater was also found. The composition of groundwater is largely governed by mineral-water equilibria. Most investigated water samples have not reached equilibrium with their surrounding minerals.

Experimental Study on Determining Unsaturated Property of Soil

Abstract: This paper deals with the experimental study of hydraulic properties of unsaturated soil. In treating unsaturated zone, a great deal more data are required than are required for the saturated zone, but these properties of soils must be known to apply the finite element approach to actual groundwater flow problems. The purposes of this paper are to propose a rational basis of getting experimental relationships between prossure head() and hydraulic conductivity(K) and between pressure head() and volumetric moisture content(θ) with "the instantaneous profile method" in a laboratory. An apparatus was constructed and test procedures were developed to measure pressure head and volumetric moisture content by using pressure transducers and low-energy gamma ray attenuation. The technique of a low-energy gamma radiation apparatus does provide a means for accurate measurement of water content without disturbing the system into which water is moving. Furthermore rapid measurement of water content becomes possible at any position in a soil so that water content changes with time may easily be followed. The tensiometer~transducer system provides a most valuable means of measuring pressure head with rapid response and with provision of a complete record of the pressure head changes with time.

Water distribution and movement in an unsaturated Piedmont soil profile

Abstract: The purpose of the study was to further investigate the deep seepage - groundwater recharge phenomenon, to develop models of the deep seepage process and to examine moisture displacement in the profile as it relates to the transport of pollutants from the soil surface downward to the groundwater. The study consisted of field investigations and model development. Field investigations include soil moisture and water table measurements, observations of surface runoff and interflow, and a radioactive tracer study. Two computer programs were written using the IBM simulation language CSMP. In one, called the Root Zone Model, precipitation and temperature data are used to estimate runoff, evapotranspiration and storage in the root zone. Any excess moisture is considered to percolate below the root zone as deep seepage. The second program, called the Deep Seepage model, uses the deep seepage calculated in the Root Zone Model and simulates its movement through the unsaturated zone to the pheatic surface, at a depth of 20m.

A Consideration on Physics of Saturated-UnsaturatedGroundwater Motion

Abstract: The purposes of this paper are primarily to research on behavior of groundwater flow in saturated and unsaturated zone, and to present the fundamentals of the theory of groundwater flow. This paper discusses the physics of the saturated-unsaturated groundwater motion. Evaluations confirm the early belief that Darcy's law is of the nature of statistical result giving the empirical equivalent of Navier-Stokes equations. The governing equation of saturated-unsaturated flow in porous media is derived from the law of mass conservation and from the Darcy's law and Richard's equation of motion and is compared with the Klute's diffusion equation which has been widely used in the analysis of unsaturated flow. As a result; it is concluded that the governing equation has the advantage that can be applied for the whole flow region. Typical boundary conditions are enumerated.