Modeling groundwater recharge mechanisms in semi-arid regions: integration of hydrochemical and isotopic data

Groundwater resources management is of strategic importance, particularly in arid and semi-arid regions, mainly associated to climatic environment and human pressures. Southeastern Tunisia, particularly Gabès region, is considered as one of the most promising areas for groundwater resources. Groundwater is the main water source used on agriculture and human consumption associated to the Continental Intercalary (CI) and Djeffara aquifers but to the interlayers from Ceno-Turonian and Miocene-Pliocene aquifer systems. The Cretaceous and Quaternary aquifer systems are the most exploited on the region. The present study aims to assess the hydrochemistry of the different aquifer systems as a contribution to the recharge mechanisms. 44 groundwater samples were analyzed for major ions, oxygen-18, and deuterium isotopes to assess the hydrochemistry and isotopic characteristics. Results highlighted that Total Dissolved Solids concentration ranges from 300 to 7200 mg/L jointed with a temperature variation from normal (19° C) to extremely high (>70° C). However, the cluster analysis reveals two main groups (A and B) with different chemical and isotopic characteristics. Isotope ratios of oxygen (δ18O) and hydrogen (δ2H) were applied to identify the recharge mechanism, which showed a relatively depleted 18O and 2H isotopic ratios and a mixing of groundwater from the Djeffara aquifer with deep thermal groundwater from CI. However, groundwater depth and local geologic structures also contribute to the mixing pattern. The obtained results suggest a more detailed structural characterization in the future to detect the spatial location and dominant properties of fractures, taking into account their relevance and contribution to regional groundwater recharge. Integration of hydrogeochemical data and isotopic groundwater composition will provide a better understanding of the geochemical processes that control the groundwater hydrochemistry and recharge of the different aquifer systems in the Gabès region and in areas with similar hydrogeological and structural conditions.The authors are grateful to the staf of the Applied Hydrosciences Laboratory for their efort and support during laboratory analysis. This research was developed under the program FCT–Fundação para a Ciência e a Tecnologia, I.P., through the project’s references UIDB/04683/2020 and UIDP/04683/2020

Evaluation of groundwater quality for irrigation purpose in the south-eastern of Tunisia (Menzel Habib area)

Groundwater is considered an important water source for agricultural uses in many regions of the world including Menzel Habib area, south-eastern Tunisia. Indeed, groundwater availability and quality became crucial in agricultural activities. Thus, pH, Electrical Conductivity (EC), Total Dissolved Solids, SO4, Cl, HCO3, Na, Ca, Mg and K were determined in 39 selected groundwater samples. The obtained results indicate that sulfate is the dominant anion, while sodium is the dominant cation. The groundwater samples from Menzel Habib aquifer system are characterized by Na-Cl, Na-SO4 and mixed water types. The suitability of groundwater for irrigation was determined on the basis of various parameters such as: sodium adsorption ratio (SAR), % Na, Kelly ratio (KR), magnesium adsorption ratio (MAR) and permeability index (PI). The Kelly ratio results indicate that 61.5% of groundwater samples are unsuitable to irrigation due to surplus (> 1) sodium content. The MAR ratio demonstrates that only 8% of groundwater samples fall in bad category. However, relatively to the % Na, 18% of groundwater samples are classified as good, 49% as permissible and 33% as doubtful for irrigation purposes. Groundwater from Menzel Habib aquifer were plotted on the USSL classification based on SAR and EC and are distributed as: 8% on the area of C4S2 (high salinity and medium alkalinity), 2.5% on the C4S3 (High salinity and high alkalinity) field, 23% on the C5S2 (very high salinity and medium alkalinity) area, 10% on the C5S3 (very high salinity and high alkalinity) area and 56.5% is on the unclassified group (EC > 10000µS/cm and SAR > 32)

Groundwater quality for irrigation in an arid region-application of fuzzy logic techniques

Groundwater is the main source to answer the irrigation supply in several arid and semi-arid areas. In the present work, groundwater quality for irrigation purposes in the arid region of Menzel Habib (Tunisia) for thirty-six groundwater samples is assessed considering the application of different conventional water quality indicators, particularly, electrical conductivity (EC), sodium absorption ratio (SAR), soluble sodium percentage (SSP), magnesium adsorption ratio (MAR), Kelly ratio (KR), and permeability index (PI). The results obtained indicate a variability for EC: 3.06 to 14.98 mS.cm-1; SAR: 4.08 to 19.30; SSP: 35.78 to 71.53%; MAR: 34.19 to 56.01; PI: 38.47 to 72.74; and KR: 0.56 to 2.47. These results suggest that groundwater from Menzel Habib aquifer system is classified between excellent to unsuitable according to the applied water quality indices. Furthermore, the groundwater samples are also plotted in the Richards diagram classification system, based on the relation between SAR and EC, suggesting that almost groundwater samples present a harmful quality. Moreover, fuzzy logic model has been proposed and created to assess groundwater quality for irrigation. The membership functions are constructed for six significant parameters such as EC, SAR, SSP, MAR, KR, and PI and the rules are, then, fired to get a simple Fuzzy Irrigation Water Quality Index (FIWQI). The obtained groundwater quality results suggest that 3% of the samples from Menzel Habib region are considered as "good" for irrigation, 3% are classified as "good to permissible", 33% with a "permissible" quality, 36% "permissible to unsuitable", while 25% of groundwater present an "unsuitable" quality. Thus, the use of fuzzy logic techniques has more reliable and robust results by overcoming the uncertainties in the decision-making attributed to the conventional methods by the creation of new classes (excellent to good, good to permissible, and permissible to unsuitable) in addition to the classes proposed by Richards diagram classification (excellent, good, permissible, and unsuitable) to assess the groundwater quality suitability for irrigation purposes.This research was developed under the FCT–Fundação para a Ciência e a Tecnologia, I.P. program, through the project’s reference UIDB/04683/2020 and UIDP/04683/2020

Drought intensity and spatial variability in Gabes Watershed, south-eastern Tunisia

Chronological series of monthly and annual precipitation data recorded in Gabes Watershed, south-eastern Tunisia, were analyzed. The study is based on the standardized precipitation index (SPI) values, computed for 10 rainfall stations over the period 1987–2012, which corresponds to an observatory period of 25 hydrologic years (from September to August). The results obtained show a great variability in SPI values. The historical evolution of the SPI made it possible to define the periods of excess and deficit, corresponding to wet and dry periods respectively. The wet years were found to be 1989–1990, 1995–1996 and 2006–2007 while the dry years were 1987–1988, 1996–1997, 2000–2001, 2001–2002, 2007–2008, 2008–2009 and 2009–2010. This clearly shows alternating wet and dry periods, but with drought episodes taking prevalence over rainy fronts throughout the study period. Indeed, a high tendency towards a drop in precipitation and important sequences of drought were observed. Spatial variability of drought throughout Gabes Watershed was examined by geostatistical analysis of SPI, as drought and rainfall distribution vary with latitude, longitude, topography and proximity to the Mediterranean Sea. The results obtained showed that, compared to coastal and southern areas, drought was observed to be more important in the West and the North of Gabes Watershed. The SPI showed that moderate droughts are generally more frequent than severe or extreme droughts in most of the Watershed

Sustainability and management of the Menzel Habib Aquifer System, Southeastern Tunisia

In arid and semi-arid areas, the water quantity and quality is a great problem. Salinzation is the major threat in the region of Menzel Habib (north-western Gabès, southeastern Tunisia). The region is a large basin which is essentially represented by sandy-clay formations and bordered by cretaceous reliefs. Geochemical and statistical approaches are reported in the Menzel Habib Aquifer system to examine groundwater salinization processes and factors controlling its mineralization. Geochemical studies were developed in 25 groundwater samples from the shallow aquifer to identify the origin of groundwater salinization. Groundwater geochemistry shows a high correlation between salinity and Na, Cl, Ca, Mg and SO4. These elements are mainly associated to the evaporitic Triassic by dissolution of halite, anhydrite and gypsum which occur on the area and is related to the tectonic context of the region. Additionally, bivariate diagram beween Na and Cl, and Ca and SO4 have also provided a comprehensive understanding of other salinization processes that are involving in Menzel Habib shallow aquifer such as cation exchange and reverse cation exchange

Groundwater recharge mechanism in a semi-arid region from southern Tunisia – a hydrogeochemical and isotopic contribution

Groundwater resources management is of strategic importance, particularly in arid and semi-arid regions, mainly associated to climatic environment and human pressures. Southeastern Tunisia, particularly Gabès region, is considered as one of the most promising areas for groundwater resources. Groundwater is the main water source used on agriculture and human consumption associated to the Continental Intercalary (CI) and Complex Terminal aquifers but to the interlayers from Ceno-Turonian and Miocene-Pliocene aquifer systems. The Cretaceous and Quaternary aquifer systems are the most exploited on the region. The present study aims to assess the hydrochemistry of the different aquifer systems as a contribution to the recharge mechanisms. Fifteen six groundwater samples were analyzed for major ions, oxygen-18, and deuterium isotopes to assess the hydrochemistry and isotopic characteristics. Results highlighted that Total Dissolved Solids concentration ranges from 300 to 7200 mg/L. However, the cluster analysis reveals two main groups (A and B) with different chemical and isotopic characteristics. Isotope ratios of oxygen (18O) and hydrogen (2H) were applied to identify the recharge mechanism, which showed a relatively depleted 18O and 2H isotopic ratios and a mixing of groundwater from the Djeffara aquifer with deep thermal groundwater from CI. However, depth to groundwater and local geologic structures also contribute to the mixing pattern. A future detailed structural study to detect the location and properties of fractures will be a relevant contribution. Integration of hydrogeochemical and isotopic data provide a better understanding of the geochemical processes that control the groundwater hydrochemistry and recharging of the different aquifer systems in Gabès region and other areas with similar conditions

Geochemical processes of groundwater salinization in an arid area, southeastern Tunisia

Groundwater salinization is a major problem throughout arid and semi-arid areas due to different natural processes and anthropogenic activities and has caused irreparable environmental and economic effects. Groundwater vulnerability of Menzel Habib has been firstly assessed using multiple methods such as DRASTIC, DRASTIC pesticide, SINTACS and SI models. These indices are based on combination of intrinsic and specific characteristics of the aquifer. Almost the area presents a low to moderate vulnerability with the highest vulnerability on the western region, associated to lower deep of groundwater and evaporation processes, with consequent salinity increase. Total Dissolved solids (TDS), chloride, sodium, sulfate, calcium and magnesium water contents were determined in a total of twenty-five groundwater samples from Menzel Habib aquifer. The accuracy of the best robust model was evaluated by the correlation between the different vulnerability indices and contamination water indicators. On the Menzel Habib aquifer area, a modified vulnerability index called DRASTIC_Sal was applied including total dissolved solids water content. This index is a simple and transparent approach for salinization vulnerability assessment of aquifers, particularly inland aquifers from arid and semi-arid regions with associated agricultural activities.This work is co-funded by the national Funds provided by FCT - Fundacao para a Ciencia e a Tecnologia, I. P., with the projects UIDB/04683/2020 and UIDP/04683/2020

Groundwater vulnerability assessment in Menzel Habiba area, South-Eastern Tunisia

Groundwater vulnerability assessment has become a useful tool for groundwater management. Hydrogeological characteristics influence the natural (intrinsic) vulnerability of an aquifer system. However, the natural vulnerability can be severely compromised by anthropogenic influence (extrinsic vulnerability). The physical structure and material composition of aquifers show resistance to contaminants transport from surface to groundwater. Currently, numerous methods have been posited evaluating aquifer's vulnerability. Similarly, the DRASTIC and DRASTIC pesticides models utilize computer algorithms and hydro-geological data within a Geographical Information System (GIS) to compute spatial aquifer vulnerability. The DRASTIC and DRASTIC pesticides models are constructed using combined spatial datasets on: Depth to groundwater (D), Aquifer Recharge (R), Aquifer media (A), Soil media (S), Topography (T), Impact of the Vadose Zone (I) and Hydraulic Conductivity (C) of the aquifer. The degree of vulnerability of the aquifer system can be evaluated by computing sensitivity analysis of DRASTIC index using GIS, showing the contribution of each parameter to vulnerability sensitivity. The GIS was used to develop a vulnerability map for Menzel Habib aquifer area. The obtained results indicated that moderately vulnerable areas are of 5%, while areas of no risk correspond to 95% using DRASTIC index. Otherwise, DRASTIC pesticide index indicated that 15% area of low vulnerability, 84% moderately vulnerable and 1% high vulnerability. The central area of Menzel Habib aquifer showed a low vulnerability due to dense human settlement and a deeper water level. However, agricultural areas recorded high vulnerability risk. Menzel Habib's environmental and socio-economic development is dependent on policy makers and planner's ability to use information effectively for decision making. The obtained groundwater vulnerability maps provide a basis for this aimed at protecting the aquifer from pollutants. Additionally, land use and development activities can be informed by mapping variables, showing that agriculture areas are highly vulnerable as compare to settlement areas

Management of groundwater salinization under a climate change scenario in an aridarea

Most future scenarios for water resources are predicting water scarcity, with a decrease in the amount of precipitation and limitation on groundwater recharge for the next five decades. In arid and semi- arid areas, the water quality is a great problem and groundwater salinization is one of the principal causes of degradation of water resources worldwide. Menzel Habib aquifer is located in the northwest of Gabès region (southeastern Tunisia), included in the arid Mediterranean bioclimatic area, with dry hot summers and relatively warm winters. Groundwater geochemistry from the study area shows a Na-Cl and Ca-Mg-Cl-SO 4 dominant facies. The high groundwater mineralization and its correlation between total dissolved solids and major ions suggest a contribution of SO4, Cl, Na, Ca, and Mg in groundwater salinization processes. The salinization of groundwater is mainly associated with the Triassic evaporites, with the dissolution of halite, anhydrite and gypsum, occurring in the area, and related to the tectonic context of the region. Additionally, other geochemical processes occurred, such as the cation exchange mechanisms. Changes in precipitation patterns and intensity, with water scarcity, low recharge, and excessive pumping have affected groundwater quantity and quality. Nowadays, the occurrence of climate change scenarios is a major drawback for water use for irrigation and drinking water supply in arid and semi-arid regions, such as the Menzel Habib aquifer