These investigations were conducted to provide basic soil chemical, physical, and mineralogical data for an area of Saudi Arabia that had not been studied and for which greatly increased agricultural development is anticipated. Fourteen soils were collected from two arid interior basin areas of Saudi Arabia. The silt and clay mineralogy was studied using x‐ray diffraction, differential thermal analysis, and electron microscopy. Quantitative estimation of clay‐sized minerals was based on a combination of chemical and x‐ray diffraction techniques. The calcareous soils of the central basins show minimal profile development. Particle‐size classes range from sandy‐skeletal to fine silty. Soils with shallow water tables are saline with large SAR values. The clay minerals in soils of the basins are primarily inherited. Soils developed in alluvium from mixed igneous rocks have clay fractions in which smectite > mica > kaolinite > chlorite, palygorskite, vermiculite. Soils formed from Permian sedimentary rocks have kaolinitic clay fractions. It is postulated that the central basins do not receive sufficient Si inputs for palygorskite neoformation and it is though that palygorskite is detrital, originating from easternregion Tertiary rocks. The soil clays of the central hasins have greater smectite contents than those of the kaolinitic western highland soils and palygorskite‐rich eastern‐region soils.
Chemical characteristics of groundwater and the composition of newly formed precipitates are critical in the management of modern irrigated agriculture in arid regions. Water samples representing the main aquifers in Riyadh region, Saudi Arabia, and solid samples from the newly formed precipitates were studied. Results showed that water temperature varied between 30.3-69¡C, being higher in the deep aquifer and low in the relatively shallow aquifers. Initial water pH ranged from 6.39-7.92, increasing to 7.65-8.20 at atmospheric conditions. Shallow aquifer waters were categorized into sulfate-type and Cl-or no-dominant type waters, while the deep aquifers were characterized as Cl-type and no-dominant type. Soluble soluble Fe and Mn ranged from 3.15 to 18.82, 0.10 to 17.30, and 0.01 to H 4 SiO 4 , 0.32 mg L ] 1 , respectively. Calculation of saturation indices from water composition at initial, closed and equilibrium conditions indicated that changes in pH and water temperature cause major chemical changes in the water favoring the precipitation of carbonates and Fe-silicates. Data of total chemical analysis indicated that carbonates are present in all samples and constituted up to 976 g kg ] 1 of the precipitates formed from the deep aquifer water in the irrigation tubes. Amorphous and/or crystalline Fe compounds were the dominant fractions in the surface crust precipitates, while crystalline Fe compounds was the dominant form in the precipitates in irrigation tubes and in the cooling reservoir. X RD data conÐrmed that aragonite was the dominant carbonate mineral in the precipitates formed from the deep aquifer water. Calcite and Mg-calcite were detected in considerably low quantities. Poorly crystalline Fe-oxide minerals were present in all the precipitate samples. Results suggested that formation of precipitates either in cooling reservoirs or in irrigation systems can be minimized by controlling the degassing through keeping the system closed, lowering the pH through the injection of inorganic acids in the system or both.
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