Natural and human-induced sinkhole hazards in Saudi Arabia: distribution,investigation, causes and impacts

Approximately 60 % of the 2,150,000 km² area of Saudi Arabia is underlain by soluble sediments (carbonate and evaporite rock formations, salt diapirs, sabkha deposits). Despite its hyper-arid climate, a wide variety of recent sinkholes have been reported in numerous areas, involving significant property losses. Human activities, most notably groundwater extraction, have induced unstable conditions on pre-existing cavities. This work provides an overview of the sinkhole hazard in Saudi Arabia, a scarcely explored topic. It identifies the main karst formations and the distribution of the most problematic sinkhole areas, illustrated through several case studies covering the wide spectrum of subsidence mechanisms. Some of the main investigation methods are presented through selected examples, including remote sensing, trenching and geophysics. Based on the available data, the main causal factors are identified and further actions that should be undertaken to better assess and manage the risk are discussed.     

Heterogeneous C<Subscript>16</Subscript>Zn<Subscript>8</Subscript>O<Subscript>8</Subscript> nanocluster as a selective CO/NO nanosensor: computational investigation

Density functional theory (DFT) calculations were employed to investigate the effects of adsorption of toxic carbon monoxide (CO) and nitrogen monoxide (NO) molecules on heterogeneous C₁₆Zn₈O₈ nanocage. A detailed analysis of the energetic, geometry, and electronic structure of various CO and NO adsorptions on the cluster surface was performed. It has been shown that CO molecule was adsorbed on the surface of the cluster resulting in more stable complex system, while NO molecule adsorption led to less stable system. These processes also changed the electronic properties of the cluster by reducing the HOMO/LUMO energy gap after adsorption process. Since this phenomenon led to an increment in the electrical conductivity of the cluster at a definite temperature, the C₁₆Zn₈O₈ was transformed to a stronger semiconductor substance upon the CO and NO adsorption. We believe that this research may be helpful in the several fields study such as sensor and catalyst investigation.     

Analysis of karst spring recession curves,west of Iran

The recession hydrographs of karst springs provide important information about aquifer characteristics such as storage properties and drainage potential, karstification degree, and other hydrological features. The Faresban, Famaasb, Gonbad-e-Kabood, and Gian springs are the important karst springs which drain main karst aquifers in west of Iran. In this study, we evaluate hydrological characteristics of these four karst springs using different analytic equations. Almost, all the recession curves were well fitted by a function that consists of three main exponential terms. The initial portion of the recession curve represents the fast drainage of large fractures and conduits, ending with the slowly decreasing curve, where the drainage of rock matrix and small fissures or baseflow is dominant. The Gian spring has the largest storage volume but the lowest drained water (7%). Using Mangin’s equation, it is apparent that the baseflow supplies over 85% of the total drained water to the Gian spring, while a small quantity of total outflow is made up of quickflow periods. Of the water available to the Famaasb, Faresban, and Gonbad-e-Kabood springs, 85, 80, and 70% are drained during period of baseflow, respectively, whereas the quickflow periods have minor importance in some springs. Based on the Coutagne’s equation results, it can be concluded that the catchment of the Famaasb and Gonbad-e-Kabood springs have a karstified saturation zone with drainage channels that provide a rapid discharge with little storage capacity. At Gian catchment area, the initial discharge is not as rapid as karstification but is fairly homogeneous throughout the aquifer.     

Groundwater risk assessment based on optimization framework using DRASTIC method

Due to anthropogenic influences and large amounts of pollutant released into the groundwater, it is vital to investigate groundwater quality and to characterize susceptible areas to contamination. In this paper, a new optimization-based methodology is proposed for determining groundwater risk using DRASTIC model based on genetic algorithm optimization model and Wilcoxon test. The correlation coefficient between DRASTIC/modified DRASTIC indices and nitrate concentrations in monitoring wells is used as a criteria for evaluating the efficiency of the proposed models. In this regard, because of the unsatisfactory original DRASTIC’s result, sensitivity analysis, genetic algorithm (GA), and Wilcoxon test (1945) are carried out to tackle the subjectivity associated with the original DRASTIC model and obtain better and reliable results. The results indicate that application of Wilcoxon test and GA optimization outperforms the others. Consequently, the correlation coefficient increased remarkably as compared to the original DRASTIC model (from 0.57 to 0.82). The proposed optimization process is adaptable to be applied in different case studies; mainly since it has the ability to optimize the weights of the model based on hydrogeological characteristics of the aquifer. Finally, the risk maps of the models are prepared using ArcGIS® to determine the most vulnerable areas.     

Estimating width of the stable channels using multivariable mathematical models

Predicting behavior and the geometry of the channels and alluvial rivers in which the erosion and sediment transport are in equilibrium is one of the most important topics in river engineering. Various researchers have proposed empirical equations to estimate stable river width (W). In this research, empirical equations were examined and tested with a comprehensive available data set consisting of 1644 points collected from 29 stable rivers in various parts of the world. The data set covers a wide range of flow conditions, river geometry, and bed sediments. This data set is classified in two groups (W < 600 m and W ≥ 600 m) for presenting the new models. The new linear and nonlinear multivariable equations were fitted to these two groups, and the best models were selected by preliminary tests and diagnostic determined for each group. The determination coefficient of these models ranged from 0.87 to 0.96. The results show that the models presented in this paper are more accurate with respect to the previously presented models. In the second part, “Artificial neural networks,” perceptron was used and a new methodology for estimating stable channel width was developed. Comparison of the statistical methods presented in this paper and the results of perceptron neural network revealed preferential recent method.     

Textures and compositions of cobalt pentlandite and cobaltian mackinawite from the Madan-Kudan copper deposit,Khetri Copper Belt,Rajasthan, India

The Khetri Copper Belt (KCB), a part of the Proterozoic Delhi–Aravalli fold belt in western India, hosts several Cu deposits, which are known to contain considerable Au, Ag, Co and Ni. Although many Co-bearing phases have been reported from the KCB and adjacent areas, detailed textural and geochemical data are either unavailable or scant except for mackinawite. In this study, we describe the textures and compositions (determined by EPMA) of two very rare Co-rich phases, namely cobaltian mackinawite (containing up to 12.68 wt.% Co, 1.90 wt.% Ni and 2.52 wt.% Cu) and cobalt-pentlandite (containing up to 49.30 wt.% Co and 10.19 wt.% Ni), identified based on composition, from the Madan-Kudan deposit. To the best of our knowledge, neither cobalt-pentlandite nor such highly Co-rich mackinawite have previously been reported from this area. The common sulphide minerals viz. chalcopyrite, pyrrhotite and rare pyrite occur in chalcopyrite-pyrrhotite ± pyrite-magnetite-chlorite-blue amphibole (Cl-rich hastingsite-pargasite-sadanagaite) ± marialitic scapolite ± allanite ± uraninite veins in amphibole-bearing feldspathic quartzite and garnetiferous chlorite schist. Cobaltian mackinawite is invariably associated with chalcopyrite and occurs as exsolution and inclusion within chalcopyrite or outside, but at the contact of chalcopyrite. On the other hand, cobalt-pentlandite is invariably associated with pyrrhotite and shows similar textural relation with pyrrhotite as that of mackinawite with chalcopyrite. Mineralogically diverse undeformed sulphide veins comprising Cl-rich amphibole and locally Cl-rich marialitic scapolite suggests epigenetic hydrothermal mineralization involving Cl-rich saline fluid in the Madan-Kudan deposit. Transport of metals, derived from a mafic source rock with high intrinsic Ni:Co ratio, by Cl-rich fluid can suitably explain the high Co:Ni ratio of the studied ore minerals. Presence of such highly Co-rich phases and other circumstantial evidences, enumerated in this work, are consistent with variants of Fe oxide (–Cu–Au) (IOCG) style mineralization, at least for some stages of mineralization in the Madan-Kudan deposit.     

Geochemistry of groundwater from a rhyolite aquifer,Northwest Iran

Chemical data on groundwater composition in rhyolitic hard rock aquifers with limited global occurrence are rarely found. In this research geochemistry of Mahabad Rhyolite Aquifer, NW Iran, was studied considering major ions, silica and trace elements measured in wet and dry seasons. Based on the results, the mean silica content was 18 mg l^?1, less than the average of the rhyolitic waters. However, the relatively higher electrical conductivity (EC) of 418 µS cm^?1 was measured. Based on a PHREEQCI model, the weathering of the silicate minerals and dissolution of carbonated intercalations turns groundwater dominantly into Ca–HCO₃ type, enhancing EC, pH and silica concentration along the flow path. Trace elements of Sr, Ba and Pb were measured at highest concentrations, the later with an average value of 83 ppb exceeds the drinking guidelines. Cluster analysis confirms biotite weathering and barite dissolution as the main sources of the trace elements in the groundwater. The results signify geochemical features of rhyolitic groundwater which can be a useful tracer of mixing in flow systems containing variety of aquifers including rhyolites.