Rachida Bouhlila

Characterization of mechanisms and processes of groundwater salinization in irrigated coastal area using statistics, GIS, and hydrogeochemical investigations

Coastal aquifers are at threat of salinization in most parts of the world. This study was carried out in coastal shallow aquifers of Aousja-Ghar El Melh and Kalâat el Andalous, northeastern of Tunisia with an objective to identify sources and processes of groundwater salinization. Groundwater samples were collected from 42 shallow dug wells during July and September 2007. Chemical parameters such as Na+, Ca2+, Mg2+, K+, Cl−, SO42−, HCO3−, NO3−, Br−, and F− were analyzed. The combination of hydrogeochemical, statistical, and GIS approaches was used to understand and to identify the main sources of salinization and contamination of these shallow coastal aquifers as follows: (i) water-rock interaction, (ii) evapotranspiration, (iii) saltwater is started to intrude before 1972 and it is still intruding continuously, (iv) irrigation return flow, (v) sea aerosol spray, and finally, (vi) agricultural fertilizers. During 2005/2006, the overexploitation of the renewable water resources of aquifers caused saline water intrusion. In 2007, the freshening of a brackish-saline groundwater occurred under natural recharge conditions by Ca-HCO3 meteoric freshwater. The cationic exchange processes are occurred at fresh-saline interfaces of mixtures along the hydraulic gradient. The sulfate reduction process and the neo-formation of clays minerals characterize the hypersaline coastal Sebkha environments. Evaporation tends to increase the concentrations of solutes in groundwater from the recharge areas to the discharge areas and leads to precipitate carbonate and sulfate minerals.

Application of multivariate statistical analysis and hydrochemical and isotopic investigations for evaluation of groundwater quality and its suitability for drinking and agriculture purposes: case of Oum Ali-Thelepte aquifer, central Tunisia.

Groundwater plays a dominant role in arid regions; it is among the most available water resources in Tunisia. Located in northwestern Tunisia, Oum Ali-Thelepte is a deep Miocene sedimentary aquifer, where groundwater is the most important source of water supply. The aim of the study is to investigate the hydrochemical processes leading to mineralization and to assess water quality with respect to agriculture and drinking for a better management of groundwater resources. To achieve such objectives, water analysis was carried out on 16 groundwater samples collected during January–February 2014. Stable isotopes and 26 hydrochemical parameters were examined. The interpretation of these analytical data showed that the concentrations of major and trace elements were within the permissible level for human use. The distribution of mineral processes in this aquifer was identified using conventional classification techniques, suggesting that the water facies gradually changes from Ca–HCO3 to Mg–SO4 type and are controlled by water–rock interaction. These results were endorsed using multivariate statistical methods such as principal component analysis and cluster analysis. The sustainability of groundwater for drinking and irrigation was assessed based on the water quality index (WQI) and on Wilcox and Richards’s diagrams. This aquifer has been classified as “excellent water” serving good irrigation in the area. As for the stable isotope, the measurements showed that groundwater samples lay between global meteoric water line (GMWL) and LMWL; hence, this arrangement signifies that the recharge of the Oum Ali-Thelepte aquifer is ensured by rainwater infiltration through mountains in the border of the aquifer without evaporation effects.

Modelling nonpoint source pollution by nitrate of soil in the Mateur plain, northeast of Tunisia

In order to quantify groundwater contamination by nitrate diffuse pollution in Mateur plain (northeast of Tunisia), it is necessary to evaluate the amount of pollutants, rejected at soil surface as fertilizer or as farm discharge, that passed through the unsaturated zone. For this purpose, we used two methods: the first one is to determine a vulnerability map of Mateur aquifer through the empirical DRASTIC method. The second method consists of determining the average water flows of percolation to the aquifer and the amount of nitrate lixiviation by using Leaching Estimation Chemistry Mode (LEACHM): a one-dimensional difference element model. In order to obtain annual average values, we have been simulating many profiles with the same average daily data to obtain a constant annual recharge and nitrate mass reaching the aquifer. Integrating these results on the whole plain allowed us to produce maps of the groundwater recharge and their nitrate concentrations. The results of nonpoint agricultural pollution modelling are compared to those of index method DRASTIC. We noticed a good correlation between LEACHM simulation and DRASTIC index results. Water table depths, surface soil permeability, and soil texture are the most important factors governing nitrate leaching to groundwater.

Nonstationary porosity evolution in mixing zone in coastal carbonate aquifer using an alternative modeling approach

In the last few decades, hydrogeochemical problems have benefited from the strong interest in numerical modeling. One of the most recognized hydrogeochemical problems is the dissolution of the calcite in the mixing zone below limestone coastal aquifer. In many works, this problem has been modeled using a coupling algorithm between a density-dependent flow model and a geochemical model. A related difficulty is that, because of the high nonlinearity of the coupled set of equations, high computational effort is needed. During calcite dissolution, an increase in permeability can be identified, which can induce an increase in the penetration of the seawater into the aquifer. The majority of the previous studies used a fully coupled reactive transport model in order to model such problem. Romanov and Dreybrodt (J Hydrol 329:661–673, 2006) have used an alternative approach to quantify the porosity evolution in mixing zone below coastal carbonate aquifer at steady state. This approach is based on the analytic solution presented by Phillips (1991) in his book Flow and Reactions in Permeable Rock, which shows that it is possible to decouple the complex set of equation. This equation is proportional to the square of the salinity gradient, which can be calculated using a density driven flow code and to the reaction rate that can be calculated using a geochemical code. In this work, this equation is used in nonstationary step-by-step regime. At each time step, the quantity of the dissolved calcite is quantified, the change of porosity is calculated, and the permeability is updated. The reaction rate, which is the second derivate of the calcium equilibrium concentration in the equation, is calculated using the PHREEQC code (Parkhurst and Apello 1999). This result is used in GEODENS (Bouhlila 1999; Bouhlila and Laabidi 2008) to calculate change of the porosity after calculating the salinity gradient. For the next time step, the same protocol is used but using the updated porosity and permeability distributions.

Impact of mixing induced calcite precipitation on the flow and transport

In several hydrogeochemical situations, there is an interaction between the solid matrix and the carbonate species according to calcite dissolution and precipitation reactions (evaporate deposition in Sebkha and Chotts, calcite dissolution in seawater–freshwater mixing zone). The precipitation of such rocks can easily induce a development of the porosity and permeability as a result of the mixing processes of two different solutions. The purpose of this work is to present a relatively complete modeling tool to evaluate the porosity decrease during calcite precipitation and their impacts on the flow and transport. The modeling of such problem requires a set of highly nonlinearly coupled equations. GEODENS code, used in this work, can solve these equations by a finite element procedure. It can handle geochemical reactions such as mineral dissolution-precipitation reaction. The coupled model iteratively calculates the fluid pressure, the ion concentrations and the quantities of the different salts that may precipitate or dissolve in the domain over time. The porosity is calculated by considering the pore volume invaded during calcite precipitation in every node and each time step. The new porosity value is used to update the permeability using empirical model. The new hydrodynamic distribution is the new inputs for the next time step to solve the flow and reactive transport problem.

Reactive Henry problem: effect of calcite dissolution on seawater intrusion

Freshwater stored in a coastal aquifer is extensively extracted through pumping wells due to high water demand in coastal area (touristic, industrial, and public use). To enhance freshwater security and to avoid contamination of water reserves, seawater intrusion becomes a topic of great interest for hydrogeologists. During the last few decades, hydrogeologists have provided a deeper understanding of the prediction, processes, investigative tools, and management of such systems. The majority of the studies quantifies these hydrogeological systems using traditional density-dependent flow and transport models and does not consider the effect of the chemical reactions. Interdependence of density-dependent flow and chemical reactions and their effects on the porosity and permeability is the most important key toward a reliable modeling of these complex systems. Seawater intrusion can increase by the solid matrix dissolution processes in the saltwater–freshwater mixing zone in the case of a coastal carbonate aquifer. The dissolution of such rocks can easily induce a development of porosity and permeability as a result of the mixing processes. The increase of permeability would enhance further seawater flux to the freshwater side. In this work, a relatively complete modeling scheme is presented to quantify and predict this risk. The modeling of such a problem requires a set of highly nonlinearly coupled equations. In this regard, GEODENS code used in this work can solve these equations by a finite element procedure; it can handle density-dependent flow, transport, and geochemical reactions in porous media. Its main purpose is to represent the physicochemical processes in the subsurface system. The code is used to simulate the effect of calcite dissolution during seawater intrusion in a coastal carbonate homogeneous aquifer.

Hydrogeochemical and Isotopic Investigations for Evaluation of the Impact of Climate Change on Groundwater Quality, a Case Study of the Plaine of Kasserine, Central Tunisia

Located in arid region in Central Tunisia, the plaine of Kasserine is a deep Plio-Mio-Quaternary aquifer representing the most available source of water supply in the region. The increase of water demand with the impact of climate change caused a significance decline in the groundwater quality and quantity. The challenge of this study is to investigate hydrochemical and isotopic data for a better understanding of the groundwater mineralization mechanism and to highlight the link between the impact of global change and hydrochemical aspects in the plaine of Kasserine. To achieve these goals, 19 wells were sampled and several physico-chemical parameters (Temperature, pH, Salinity, Na, K, Ca, Mg, Cl, HCO3, SO4 and ∂2H and ∂18O) were analyzed. Conventional hydrogeochemical techniques and multivariate statistical analyses were performed. The water type of the plaine of Kasserine gradually evolved from Ca–HCO3 to Ca–Na–SO4 suggesting two possible main processes: dedolomization and cation exchange generated by the dissolution of gypsum and dolomite and the precipitation of calcite. Furthermore, data inferred from stables isotopes in groundwater samples indicated that direct infiltration principally ensure the recharge of this aquifer. However, as an arid region, the plaine of Kasserine is threatened by climate change, due to the low and the irregularities of precipitation, leading to the decrease of water table level and deterioration of its water quality. This study help understand the hydrochemical processes of the plaine of Kasserine and assess the impacts of climate change in this arid region for a better monitoring of these valuable resources.