Evaluation of Sentinel-1 data for flood mapping in the upstream of Sidi Salem dam (Northern Tunisia)

Flood mapping is a powerful asset that allows drawing better strategies to contain possible economic repercussions and to rescue the affected population. This work is directly unfolded after the rainfall events that occurred in the north of the country, in February 2015, during which certain cities located in the vicinity of the Tunisian basin of Medjerda were flooded by the overflow of the Medjerda river, causing important damage to the towns of Jendouba and Bou Salem. The present research illustrates the potentiality of Sentinel-1 sensor in detecting flood areas in the upstream of Medjerda river. The Medjerda is the most important river in Tunisia, with an annual water potential reaching 0.8 billion m³. We compared the signature of flood water in vertical transmit and horizontal received (VH) and vertical transmit and vertical received (VV) polarizations of radar data. The study proves that the segregation of land/water areas with a threshold technique is better observed in VH polarization rather than VV polarization.     

Efficient approximations for the simulation of density driven flow in porous media

Due to the non-linear coupling between flow and transport equations the simulation of real density driven flow problems requires a lot of computational time and/or heavy equipments. We suggest some approximations and numerical recipes to reduce the CPU costs for these strongly non-linear coupled equations without loss in accuracy.     

Electro-coagulation treatment of raw and autoclaved landfill leachate with aluminum electrodes: case study of Djebel Chakir (Tunisia)

Landfilling is a common practice worldwide for solid waste management. The leachate generated at landfill sites contains various organic and inorganic pollutants while it should be treated properly. In this study, the electrocoagulation (EC) process was recognized for its simplicity and effectiveness which was used for the treatment of leachate from the Djebel Chakir landfill site in northern Tunisia. In addition, we investigated the effect of microorganisms (e.g., bacteria, fungi, spore) on sludge production by the application of autoclaving treatment on raw leachate. The application of low current density (15 mA/cm²) within 2 h of treatment and using Al-Al electrodes revealed significant improvement of performance when autoclaving was applied. The chemical oxygen demand (COD) and nitrogen removal increased from 39 to 64% and from 13 to 30%, respectively. The sludge volume was reduced from 40 to 10%, and thus, its handling and disposal costs would be significantly decreased. The energy consumption rate was stable after 40 min of treatment at about 0.8 kWh/kg COD removed. Our study shows that removal of microorganisms by autoclaving prior to the EC process is promising for landfill leachate treatment. However, since autoclaving is far from being practical and cost-effective at full-scale plant, research on coupling EC with an alternative disinfecting process might be of great interest.     

Phytoremediation assessment of native plants growing on Pb–Zn mine site in Northern Tunisia

Pollution by heavy metals presents an environmental concern, and their toxicity threats soil, water, animals and human health. Phytoremediation can be used as a solution to remediate contaminated soils. The aim of this study was to identify native plants collected from tailings: material of Pb–Zn mine sites of Fedj Lahdoum and Jebel Ressas (two abandoned mines located, respectively, in the northwest of Tunisia and in the south of Tunis City). The tolerance of plant to heavy metals (lead, zinc and cadmium) is evaluated. Soil samples were collected and analyzed for Pb, Zn and Cd concentration. The total soil Pb, Zn and Cd are, respectively, reached 6132 mg kg^?1, 11,052 mg kg^?1 and it doesn’t exceed 479 mg kg^?1 for Cd. The highest content of Zn in plants was detected in shoots of Rumex bucephalophorus (1048 mg kg^?1), and the highest Pb concentration was detected in roots of Chrysopogon zizanioides (381 mg kg^?1), while for Cd Silene colorata it accumulated the highest content in roots (51 mg kg^?1). From all plants, only 12 have a translocation factor for Pb which is higher than one. Among all plants, only 17 have a translocation factor that is higher than one for Zn, while for Cd only 13 plants indicate TF > 1. As for the biological absorption coefficient, all samples indicate a rate which is lower than one. These plants can be primarily hyper accumulators and useful in remediation of lead- and zinc-contaminated soils after further biochemistry researches in mechanism of accumulation and translocation of heavy metals in plants.     

Use of a time-domain electromagnetic method with geochemical tracers to explore the salinity anomalies in a small coastal aquifer in north-eastern Tunisia

The study area is a small coastal plain in north-eastern Tunisia. It is drained by an ephemeral stream network and is subject to several pollutant discharges such as oilfield brine coming from a neighboring oil company and wastewater from Somâa city, located in the upstream of the plain. Furthermore, a hydraulic head near the coastal part of the aquifer is below sea level, suggesting that seawater intrusion may occur. A time-domain electromagnetic (TDEM) survey, based on 28 soundings, was conducted in Wadi Al Ayn and Daroufa plains to delineate the saline groundwater. Based on longitudinal and transversal resistivity two-dimensional pseudosections calibrated with boring data, the extent of saline water was identified. Geochemical tracers were combined with the resistivity dataset to differentiate the origin of groundwater salinization. In the upstream part of the plain, the infiltration of oilfield brine through the sandy bed of Wadi Al Ayn seems to have a considerable effect on groundwater salinization. However, in the coastal part of the aquifer, groundwater salinization is due to seawater intrusion and the saltwater is reaching an inland extent around 1.3 km from the shoreline. The contribution ratios of saline water bodies derived from the inverted chloride data vary for the oilfield brine from 1 to 13 % and for the seawater from 2 to 21 %.     

Computational issues of hybrid and multipoint mixed methods for groundwater flow in anisotropic media

In this work, lowest-order Raviart–Thomas and Brezzi–Douglas–Marini mixed methods are considered for groundwater flow simulations. Typically, mixed methods lead to a saddle-point problem, which is expensive to solve. Two approaches are numerically compared here to allow an explicit velocity elimination: (1) the well-known hybrid formulation leading to a symmetric positive definite system where the only unknowns are the Lagrange multipliers and (2) a more recent approach, inspired from the multipoint flux approximation method, reducing low-order mixed methods to cell-centered finite difference schemes. Selected groundwater flow scenarios are used for the comparison between hybrid and multipoint approaches. The simulations are performed in the bidimensional case with a general triangular discretization because of its practical interest for hydrogeologists.