Nature and Composition of Newly Formed Precipitates in Relationship to Characteristics of Groundwater in Arid Environment

Abstract

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 fromthe 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. Shallowaquifer 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 H 4 SiO 4 , soluble Fe and Mn ranged from 3.15 to 18.82, 0.10 to 17.30, and 0.01 to 4 0.32 mg L-¹, respectively. Calculation of saturation indices from water composition at initial, closed and equilibriumconditions 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. XRD data confirmed that aragonite was the dominant carbonate mineral in the precipitates formed fromthe 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.