1,720,980 research outputs found
Polygonal deformation bands
No full textWe report for the first time the occurrence of polygonal faults in sandstone, which is compelling given that layer-bound polygonal fault systems have been observed so far only in fine-grained sediments such as clay and chalk. The polygonal faults are shear deformation bands that developed under shallow burial conditions via strain hardening in dm-wide zones. The edges of the polygons are 1-5 m long. The shear deformation bands are organized as conjugate faults along each edge of the polygon and form characteristic horst-like structures. The individual deformation bands have slip magnitudes ranging from a few mm to 1.5 cm; the cumulative average slip magnitude in a zone is up to 10 cm. The deformation bands heaves, in aggregate form, accommodate a small isotropic horizontal extension (strain <0.005). The individual shear deformation bands show abutting T-junctions, veering, curving, and merging where they mechanically interact. Crosscutting relationships are rare. The interactions of the deformation bands are similar to those of mode I opening fractures. The documented fault networks have important implications for evaluating the geometry of km-scale polygonal fault systems in the subsurface, top seal integrity, as well as constraining paleo-tectonic stress regimes
Water and (bio)chemical cycling in gravel pit lakes: A review and outlook
Full text linkThe world produces 1.7 × 108 metric tons of gravel and sand per year (USGS, 2015) creating many gravel pit lakes that change the morphology and drainage pattern of catchments. Gravel pit lakes abruptly intersect the geologic layering creating an environment where surface and groundwater will interact and where elaborate food webs can develop. Here we preview previous work on gravel pit lakes and compiled a comprehensive hydrochemical database to compare the chemistry of gravel pit lake water with other types of surface and groundwater. Water budget calculations confirm that gravel pit lakes cause freshwater loss in temperate and Mediterranean climates where surface water evaporation is larger than the actual evapotranspiration of vegetated land that was replaced by the gravel pit lakes. Groundwater fed gravel pit lakes where evaporated water is replaced by groundwater are especially sensitive to climate change.The gravel pit lakes included in this review have a relatively low acidity and high alkalinity most likely caused by weathering and leaching of carbonates in the catchment. The inflow of groundwater is a key process in gravel pit lakes with important consequences. The creation or presence of the gravel pit lakes may induce fluctuation of the up-stream water table which enhances groundwater flow and redox reactions in the soil. Groundwater rich in dissolved elements typically meets more alkaline water in gravel pit lakes enhancing the precipitation of metal oxides, calcite and other composite minerals including phosphorus (P), calcium (Ca) and carbon (C). Gravel pit lakes provide many different ecological habitats increasing the biodiversity in typically an agricultural or urban setting. Plant and animal species observed in gravel pit lakes consists of phytoplankton, zooplankton, micro plankton, macrophytes, fish and birds similar to natural lakes but the fact that gravel pit lakes may be only groundwater fed, or instead in open contact with rivers causes large variations between the ecosystem of different lakes. Plants and animal species take part in the chemical cycling of gravel pit lakes by, among others, uptake of atmospheric carbon dioxide (CO2) and nitrogen (N2), of dissolved compounds including bicarbonate (HCO3), iron (Fe) and manganese (Mn); of elements including phosphate (P) and Fe from lake sediments, and carbon mineralization and burial. Gravel pit lakes may contribute to denitrification of groundwater as N is consumed by plankton, but they may also enhance the mobilization of soil-bound compounds like potentially toxic (trace) metals released from aquifer sediments. The creation of gravel pit lakes provides more available sites for carbon burial but once deposited on the lake bottom, metals and other elements may be released again due to redox cycling, influenced by climatic or land use change. Gravel pit lakes are water bodies of recent formation and so far only a few different settings have been studied in detail compared to other types of natural- and man-made lakes. From this review it is evident that gravel pit lakes are hydrochemically most similar to so called 'marl lakes' or 'nutrient rich' lakes. Key areas for further research include the study of gravel pit lakes in other settings to better separate the similarities and differences between natural and gravel pit lakes. Also the feedback mechanisms between change in land use and climate, ground- and lake water chemistry ecological functioning and use of the gravel pit lakes need to be addressed
Reply to Comment by Trinchero et al. on “Application of Analytical Diffusion Models to Outcrop Observations: Implications for Mass Transport by Fluid Flow Through Fractures”
No full textThe effect of artificial recharge on hydrochemistry: A comparison of two fluvial gravel pit lakes with different post-excavation uses in The Netherlands
No full textGravel pit lakes form when gravel deposits are excavated below the water table. We studied two fluvial gravel pit lakes called De Lange Vlieter (DLV Lake) and the Boschmolen Plas (BP Lake), in the Meuse River valley (The Netherlands). Water from the Meuse River is pumped only into the DLV Lake that is used for drinking water production. The mean values, the linear trends and seasonal patterns of time series data (2003-2014), of temperature, pH, nitrate, phosphate and sulphate were compared using one-way tests of variance and tests of differences. The effects of river water infiltration on DLV Lake are (1) a change in lake water temperature; (2) an increase in nitrate concentration (3) an increase in phosphate concentration and (4) a decrease in sulphate concentration. The effects of the air blowers in DLV Lake are (1) mixing of lake water; (2) decreasing pH in spring and summer (3) water oxygenation. Linear regression analysis shows an initially increasing nitrate concentration in DLV Lake that can be explained by the input of nitrate rich Meuse river water. Instead decreasing nitrate and phosphate concentrations in BP Lake and Meuse River reflect a diminished use of fertilizers. The gravel pit lake water temperature does not reflect climatic changes but the use of DLV Lake for artificial recharge has an impact on the seasonal and long-term trends in hydrochemistry. This poses a challenge to lake managers to find the right balance between reduction of eutrophication and accumulation of nutrients and sulphate
Architecture and fluid flow properties of faults and associated structures in different rock types: examples from outcrops
No full textRock type strongly influences the way in which faults and other fractures develop and their resulting geometrical patterns. Consequently faults in sandstone have a different influence on fluid flow than faults in dolomite or carbonates. Knowing the mechanics of fault development helps to predict the distribution of faults and associated fractures in an aquifer, especially where geophysical or other data are lacking.
We compare the geometry, architecture and petrophysical properties of faults and other fractures documented in outcrops of porous sandstone (Utah, USA and Italy), tight sandstones (Bolivia), dolomite (Northern Italy) and carbonates (Central Italy).
We consider the fractured surface outcrops as a natural analog for faulted and fractured aquifers. We characterized the faults and fractures assemblages by type (fault, joint, compaction band, stylolite, and vein), length, structural position, stress state and their relationship to nearby structures.
Faults in porous sandstone typically consist of a zone of compacted, cataclastic material called deformation bands that have been shown to inhibit fluid flow. Some faults have a slip plane and open joints at their tips (tail cracks), especially when they are along bedding planes. Compaction bands occur in porous sandstone and are equivalent to stylolites in carbonates.
Faults in tight sandstones are associated with joint clusters, breccias zones and cataclasis and they form typically along preexisting discontinuities. Faults in Dolomite typically consist of breccias zones that formed in zones of joint localization. The Dolomite host rock is characterized by a very high density of joints. Carbonate outcrops show a variety of faulting mechanisms with many different types of fractures involved such as faults, veins and stylolites. Typically there are different generations of faults and fractures whereby faulting along pre-existing discontinuities such as older veins or older stylolite play an important role. Another important characteristic is the fact that the secondary fractures that typically form at the tip of faults play an important role in fragmenting the rock.
Whether faults are barriers or pathways for groundwater flow in an aquifer depends on their architecture, size and connectivity. Assuming that joints enhance fluid flow whereas cataclastic bands, cemented breccia zones and slip planes inhibit fluid flow we evaluate and quantify the effect of all structures observed in the outcrops in terms of fluid flow properties
Seasonal variation in natural recharge of coastal aquifers
No full textMany coastal zones around the world have irregular precipitation throughout the year. This results in discontinuous natural recharge of coastal aquifers, which affects the size of freshwater lenses present in sandy deposits. Temperature data for the period 1960–1990 from LocClim (local climate estimator) and those obtained from the Intergovernmental Panel on Climate Change (IPCC) SRES A1b scenario for 2070–2100, have been used to calculate the potential evapotranspiration with the Thornthwaite method. Potential recharge (difference between precipitation and potential evapotranspiration) was defined at 12 locations: Ameland (The Netherlands), Auckland and Wellington (New Zealand); Hong Kong (China); Ravenna (Italy), Mekong (Vietnam), Mumbai (India), New Jersey (USA), Nile Delta (Egypt), Kobe and Tokyo (Japan), and Singapore. The influence of variable/discontinuous recharge on the size of freshwater lenses was simulated with the SEAWAT model. The discrepancy between models with continuous and with discontinuous recharge is relatively small in areas where the total annual recharge is low (258–616 mm/year); but in places with Monsoon-dominated climate (e.g. Mumbai, with recharge up to 1,686 mm/year), the difference in freshwater-lens thickness between the discontinuous and the continuous model is larger (up to 5 m) and thus important to consider in numerical models that estimate freshwater availability
Theoretical Constraints on Fracture Aperture Based on Linear Elastic Fracture Mechanics: the importance of stress concentration and lithology
No full textOne of the major unknowns in fractured aquifer or fractured reservoir studies is the magnitude of fracture aperture. Since fluid flow through fractures scales with the third power of fracture aperture (cubic law), small variations in aperture have a large influence on the permeability. Fracture apertures, however, are difficult to measure in-situ since the very measurement itself often changes the stress state and therefore the fracture aperture. We used Linear Elastic Fracture Mechanical theory to calculate in-situ apertures for fractures with different dimensions, for a variety of rock types, under different tensional stress states. We calculated the displacement field around a penny-shaped discontinuity in a homogeneous, anisotropically stressed elastic material. We assume that penny-shaped discontinuities are present a priori in the homogeneous rock and that the cracks do not grow, they just open up in the direction of the minimum compressive stress in a way that the tensile stresses around the crack and especially at the crack’s tip do not exceed the tensile strength of the rock. The displacement field around these cracks depends on the position with respect to the centre of the crack, the magnitude of the tensile stress, and the material properties such as Young’s modulus and Poisson's ratio. The maximum displacement in the center of the crack can be regarded as the maximum theoretical aperture for fluid flow. The aperture tapers out towards the crack tip where aperture reduces to zero. The Poisson’s ratio is kept as 0.25 for all rock types and the Young’s modulus varies according to lithology. For shale, marl and chalk, anhydrite, sandstone, dolomite, limestone, gypsum, rock salt and conglomerate curves are constructed that show the relationship between fracture length and fracture aperture.
These calculated curves show that the longer the fractures, the larger the maximum possible aperture at its center. In limestone for example, a one meter long fracture loaded by 0.5 MPa tensional stress has a maximum aperture of 20 μm and a 10 meter long fracture has a maximum aperture of 150 μm.
Fractures in shale, marl and chalk tend to have larger apertures for the same fracture length than gypsum, rock salt and conglomerate which in turn have larger openings than limestone, dolomite sandstone and anhydrite. For example a fracture with a length of 1 m has a maximum aperture of 10 000 μm in shale but of only 20 μm in anhydrite
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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