Nature and composition of newly formed precipitates in relationship to characteristics of groundwater in arid environment

Chemical characteristics of groundwater and the composition of newly formed precipitates are critical in the management of modern irrigated agricultur e in arid regions. Water samples representing the main aquifers in Riyadh r egion, Saudi Arabia, and solid samples from the newly formed precipitates w ere studied. Results showed that water temperature varied between 30.3-69 d egrees C, being higher in the deep aquifer and low in the relatively shallo w 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 su lfate-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 H4SiO4, 0.32 mg L-1, respectively. Calculation of saturation indices from water composition at i nitial, closed and equilibrium conditions indicated that changes in pH and water temperature cause major chemical changes in the water favoring the pr ecipitation 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 domin ant fractions in the surface crust precipitates, while crystalline Fe compo unds was the dominant form in the precipitates in irrigation tubes and in t he cooling reservoir. XRD data confirmed that aragonite was the dominant ca rbonate mineral in the precipitates formed from the deep aquifer water. Cal cite and Mg-calcite were detected in considerably low quantities. Poorly cr ystalline Fe-oxide minerals were present in all the precipitate samples. Re sults suggested that formation of precipitates either in cooling reservoirs or in irrigation systems can be minimized by controlling the degassing thr ough keeping the system closed, lowering the pH through the injection of in organic acids in the system or both.