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Subsurface (ground) water
Published in A.C. McLean, C. D. Gribble, Geology for Civil Engineers, 2017
Evaporation is not restricted to surface water, but also takes place from the top of the water table. The phenomenon is particularly important in semi-arid regions. As a result, a layer of strongly cemented material forms in the otherwise unconsolidated sediment of the soil (see Section 3.3.2). This may be mistaken for bedrock. It may consist of lime (calcareous), chalcedony (siliceous) or iron hydroxides (ferruginous). It is commonly referred to as hard pan and occasionally as duricrust. Other terms are used for particular types (for example, iron pan) and the terms are further multiplied by regional usage. For example, calcareous hard pan is called caliche in Spanish American regions but kankar in India. Hard pan is relatively impermeable, and its presence affects percolation through the soil. In some areas it may be thick enough to interfere with seismic refraction surveys (Section 6.3.3) and give spurious values to rock head by acting as the refractor.
Surface Processes
Published in F.G.H. Blyth, M. H. de Freitas, A Geology for Engineers, 2017
F.G.H. Blyth, M. H. de Freitas
In previously hot, semi-arid regions, where evaporation from the ground had been rapid and nearly equal to the rainfall, chemical decomposition of the rocks will have proceeded to great depths and a hard, superficial crust formed by the deposition of mineral matter just below the soil. The water from the occasional rains carries dissolved salts only a short distance below the surface, where they are retained by capillarity, with the result that as evaporation proceeds a mineral deposit is built up. If solutions are saturated with calcium carbonate the deposit will be a calcareous one (calcrete or kankar), like that which covers large areas in India. With ferruginous solutions, such as would result from the decomposition of basic igneous rocks, a red concretionary deposit may be formed, as in many parts of Africa. Major accumulations of silica occur as silcrete duricrusts in Australia and S. Africa. They form very resistant horizons up to 5 m thick and are often found capping prominent plateaux and mesas (Fig. 3.11e). It is believed that the silcrete formed during a period of warm, humid climate that existed at the end of the Tertiary.
Management of Alkali Soils
Published in Ranbir Chhabra, Soil Salinity and Water Quality, 2017
Rice yield in the problem soils is affected by the method of its cultivation, (i.e., direct sowing through seeds or transplanting seedlings), nutritional disorders, adverse pH, drainage, and the genetic potential of the variety selected for the area. Some of the problems affecting rice yields are: Native low fertility or nutrient depletion or imbalances caused by intensive cultivation and injudicious fertiliser use.High salt contents leading to nutritional and osmotic stress and toxicities.Problems of low nutrient availability due to poor soil physical conditions and tissue injury associated with high pH.Soils with kankar or clay layers close to the surface obstructing root growth, salt removal, and drainage and causing nutritional problems.Low pH leading to nutritional problems and toxicities of iron and aluminium.Poorly drained high organic matter soils, with presence of toxic ions and oxygen stress.Highly permeable soils having low water and fertiliser use efficiency.Problems associated with topography and location, such as excessive water impounding or plant submergence, runoff, soil erosion, and poor drainage.
Geochemical assessment of fluoride enriched groundwater and health implications from a part of Yavtmal District, India
Published in Human and Ecological Risk Assessment: An International Journal, 2020
Deepali Marghade, Deepak B. Malpe, N. Subba Rao, B. Sunitha
The saturation indices (SI) with respect to the solid phases of calcite (CaCO3), dolomite (CaMg(CO3)2), fluorite (CaF2), and gypsum (CaSO4) to assess the impact of semi-arid climate and also to predict solubility conditions on groundwater system. The negative values of SI with respect to halite, fluorite, and gypsum in most groundwater samples (Table 4 and Figure 11) specify the dissolution of these minerals. The positive SI values of calcite and dolomite divulge equilibrium or precipitated state, in the decrease of Ca2+ concentration, concurrently increases the fluorite dissolution (Subba Rao 2017). The formation of kankar in soil zone of the present study area further authenticate a long history of evaporation due to a precipitation of CaCO3, under the semi-arid climatic condition. Calcite, dolomite, gypsum, and fluorite contain Ca2+ and however their solubility is interdependent. Therefore, conditions of low solubility of calcite, dolomite, and gypsum may lead to high F− concentration in the groundwater.
Geochemical and health risk evaluation of fluoride rich groundwater in Sattenapalle Region, Guntur district, Andhra Pradesh, India
Published in Human and Ecological Risk Assessment: An International Journal, 2020
N. Subba Rao, B. Ravindra, Jianhua Wu
The highest contour elevation is 80 m above mean sea level (amsl) at southwestern part, while the lowest contour elevation is 60 m amsl at northern and eastern parts (Figure 2). The sloping of the region, thus, follows the topography. The dominated soil type is black cotton soil (mostly clay variety), which is formed by highly weathering of upper surface rock. The occurrence of CaCO3 concretions, locally known as kankar in the soil zone, is a characteristic feature of the semi-arid climate.