Rim Trabelsi

Groundwater salinization processes in shallow coastal aquifer of Djeffara plain of Medenine, Southeastern Tunisia

Urban and industrial development and the expansion of irrigated agriculture have led to a drastic increase in the exploitation of groundwater resources. The over-exploitation of coastal aquifers has caused a seawater intrusion and has seriously degraded groundwater quality. The shallow coastal aquifer of the Djeffara plain, southeastern Tunisia constitutes an example of water resource suffering an intensive and uncontrolled pumping for irrigation. Intensive exploitation of the aquifer and climate aridity caused a decrease in piezometric level and an increase in salinity. According to the hydrochemical data (Cl−, SO42−, NO3−, HCO3−, Br−, Ca2+, Mg2+, Na+, K+) and the stable isotope composition (oxygen-18 and deuterium content), groundwater salinization in the investigated system is caused by three main processes: (i) salts dissolution especially in the central part of Jerba and around Medenine plain; (ii) evaporation process; and (iii) seawater intrusion which caused the increase in salinity in the peninsula of El Jorf, in Jerba and in the North of Ben Gardane.

Integrated socio-hydrogeological approach to tackle nitrate contamination in groundwater resources. The case of Grombalia Basin (Tunisia)

Nitrate contamination still remains one of the main groundwater quality issues in several aquifers worldwide, despite the perduring efforts of the international scientific community to effectively tackle this problem. The classical hydrogeological and isotopic investigations are obviously of paramount importance for the characterization of contaminant sources, but are clearly not sufficient for the correct and long-term protection of groundwater resources. This paper aims at demonstrating the effectiveness of the socio-hydrogeological approach as the best tool to tackle groundwater quality issues, while contributing bridging the gap between science and society. An integrated survey, including land use, hydrochemical (physicochemical parameters and major ions) and isotopic (δ15NNO3 and δ18ONO3) analyses, coupled to capacity building and participatory activities was carried out to correctly attribute the nitrate origin in groundwater from the Grombalia Basin (North Tunisia), a region where only synthetic fertilizers have been generally identified as the main source of such pollution. Results demonstrates that the basin is characterized by high nitrate concentrations, often exceeding the statutory limits for drinking water, in both the shallow and deep aquifers, whereas sources are associated to both agricultural and urban activities. The public participation of local actors proved to be a fundamental element for the development of the hydrogeological investigation, as it permitted to obtain relevant information to support data interpretation, and eventually guaranteed the correct assessment of contaminant sources in the studied area. In addition, such activity, if adequately transferred to regulators, will ensure the effective adoption of management practices based on the research outcomes and tailored on the real needs of the local population, proving the added value to include it in any integrated investigation.

Hydro-geochemical behaviour of two coastal aquifers under severe climatic and human constraints: comparative study between Essaouira basin in Morocco and Jeffara basin in Tunisia

Since surface waters are mostly irregular and rare, groundwater in arid and semi-arid regions are submitted to high human constraints enhanced by severe climatic conditions. Multiple isotope analyses and chemical tracing of groundwater from the Essaouira basin (South-Western Morocco) and from the Jeffara basin (South-Eastern Tunisia) reveal that salinisation processes are mainly related to natural conditions such as dissolution of evaporate rocks (gypsum and halite minerals) along the recharging outcrops. However, since these basins are coastal, seawater intrusion is one of multiple salinity sources that could affect the quality of groundwater if intense exploitation goes on. The comparison of hydro-geochemical and isotopic data available for the two basins provide a framework for a comprehensive diagnostic in which different states of risks could be defined according to climatic and human constraints. We argued that these heavily exploited coastal aquifers have exacerbated vulnerability given relatively low current recharge and salinisation processes.

Deciphering the interaction between quaternary and continental Sabkhas aquifers in Central Tunisia using hydrochemical and isotopic tools

Climate aridity and intensive exploitation due to uncontrolled pumping for irrigation have caused a drastic decrease in the piezometric level of the shallow aquifer of Chougafiya plain, central Tunisia, and have seriously degraded groundwater quality. According to the hydrochemical data (Cl−, SO42−, NO3−, HCO3−, Br−, Na+, Mg2+, K+, Ca2+, Sr2+) and the stable isotopes (18O and 2H content), groundwater salinization in the investigated aquifer is caused by four main processes: (1) evaporite dissolution (2) cation exchange reactions (3) evaporation processes and (4) mixing with Sabkhas salt water causing salinity to increase in the central and southern parts of the basin. The radiogenic (3H) isotope data provided insight into the presence of significant contemporaneous recharge waters in the western part of the shallow aquifer. The movement of the tritiated water may have occurred according to the general flow path (NW–SE). When tritium was used in conjunction with the stable isotopes and chloride, the mixing process could be clearly identified, especially in the central part of the study area.

Application of environmental tracers to study groundwater recharge in a semi-arid area of Central Tunisia

Abstract Groundwater of the Tertiary-Quaternary Formations in the Jeloula basin (Central Tunisia), together with rain and surface waters, were analysed to investigate the mineralization processes, the origin of the water and its recharge sources. The water samples present a large spatial variability of chemical facies which is related to their interaction with the geological formations. The main sources of the water mineralization are the dissolution of evaporitic and carbonate minerals and cation exchange reactions. Stable isotopes indicate that most groundwater samples originate from infiltration of modern precipitation. Surface water samples from small dam reservoirs show a 18O/2H enrichment, which is typical of water exposed to open-surface evaporation in a semi-arid region. Considerable data of 3H and 14C allow the qualitative identification of the present-day recharge that is probably supplied by infiltration of recent flood waters in the Wadi El Hamra valley, and by direct infiltration of meteoric water through the local carbonate outcrops. Editor D. Koutsoyiannis; Associate editor S. Faye

Hydrogeological and hydrochemical investigation of groundwater using environmental isotopes ( 18 O, 2 H, 3 H, 14 C) and chemical tracers: a case study of the intermediate aquifer, Sfax, southeastern Tunisia

Major element concentrations and stable (δ18O and δ2H) and radiogenic (3H and 14C) isotopes in groundwater have proved useful tracers for understanding the geochemical processes that control groundwater mineralization and for identifying recharge sources in the semi-arid region of Sfax (southeastern Tunisia). Major-ion chemical data indicate that the origins of the salinity in the groundwater are the water–rock interactions, mainly the dissolution of evaporitic minerals, as well as the cation exchange with clay minerals. The δ18O and δ2H relationships suggest variations in groundwater recharge mechanisms. Strong evaporation during recharge with limited rapid water infiltration is evident in the groundwater of the intermediate aquifer. The mixing with old groundwater in some areas explains the low stable isotope values of some groundwater samples. Groundwaters from the intermediate aquifer are classified into two main water types: Ca-Na-SO4 and Ca-Na-Cl-SO4. The high nitrate concentrations suggest an anthropogenic source of nitrogen contamination caused by intensive agricultural activities in the area. The stable isotopic signatures reveal three water groups: non-evaporated waters that indicate recharge by recent infiltrated water; evaporated waters that are characterized by relatively enriched δ18O and δ2H contents; and mixed groundwater (old/recent) or ancient groundwater, characterized by their depleted isotopic composition. Tritium data support the existence of recent limited recharge; however, other low tritium values are indicative of pre-nuclear recharge and/or mixing between pre-nuclear and contemporaneous recharge. The carbon-14 activities indicate that the groundwaters were mostly recharged under different climatic conditions during the cooler periods of the late Pleistocene and Holocene.RésuméLes concentrations en éléments majeurs et les teneurs en isotopes stables (δ18O and δ2H) et radiogéniques (3H et 14C) dans les eaux souterraines se sont avérés utiles comme traceurs pour comprendre les processus géochimiques qui contrôlent la minéralisation des eaux souterraines et pour identifier les sources de recharge dans la région semi-aride de Sfax (Sud-Est de la Tunisie). Les données chimiques en ions majeurs indiquent que la salinité des eaux souterraines provient des interactions eau-roche, principalement de la dissolution des minéraux évaporitiques, ainsi que des échanges cationiques avec les minéraux argileux. Les relations δ18O et δ2H suggèrent des variations dans les mécanismes de recharge des eaux souterraines. Une forte évaporation pendant la recharge et une infiltration rapide limitée sont mises en évidence dans les eaux souterraines de l’aquifère intermédiaire. Le mélange avec des eaux anciennes dans certaines régions explique les valeurs appauvries en isotopes stables de certains échantillons d’eau souterraine. Les eaux souterraines de l’aquifère intermédiaire sont classées en deux principaux types: Ca-Na-SO4 et Ca-Na-Cl-SO4. Les concentrations élevées en nitrates suggèrent une origine anthropique pour la contamination azotée due à des activités agricoles intensives dans la zone. Les signatures isotopes révèlent trois groupes d’eaux: eaux non évaporées qui indiquent une recharge par des eaux récemment infiltrées; eaux évaporées qui sont caractérisées par des teneurs relativement enrichies en δ18O et δ2H; et eaux souterraines mélangées (anciennes/récentes) ou eaux souterraines anciennes, caractérisées par une composition isotopique appauvrie. Les données de tritium attestent de l’existence d’une recharge récente limitée. Cependant, d’autres valeurs faibles en tritium indiquent l’existence d’une recharge pré-nucléaire et/ou d’un mélange entre une recharge pré-nucléaire et contemporaine. Les activités carbone-14 indiquent que les eaux souterraines sont principalement rechargées sous des conditions climatiques différentes, pendant les périodes plus froides du Pléistocène supérieur et de l’Holocène.ResumenLas concentraciones de elementos mayoritarios y isótopos estables (δ18O y δ2H) y radiogénicos (3H y 14C) en el agua subterránea han demostrado ser trazadores útiles para comprender los procesos geoquímicos que controlan la mineralización del agua subterránea y para identificar fuentes de recarga en la región semiárida de Sfax (sureste de Túnez). Los datos químicos de iones mayoritarios indican que los orígenes de la salinidad en el agua subterránea son las interacciones agua-roca, principalmente la disolución de minerales evaporíticos, así como el intercambio de cationes con minerales de arcilla. Las relaciones δ18O y δ2H sugieren variaciones en los mecanismos de recarga del agua subterránea. La fuerte evaporación durante la recarga con una infiltración de agua rápida y limitada es evidente en el agua subterránea del acuífero intermedio. La mezcla con agua subterránea antigua en algunas áreas explica los bajos valores de isótopos estables de algunas muestras de agua subterránea. El agua subterránea del acuífero intermedio se clasifica en dos tipos principales de agua: Ca-Na-SO4 y Ca-Na-Cl-SO4. Las altas concentraciones de nitratos sugieren una fuente antropogénica de contaminación por nitrógeno causada por actividades agrícolas intensivas en el área. Las firmas isotópicas estables revelan tres grupos de agua: aguas no evaporadas que indican la recarga por agua recientemente infiltrada; aguas evaporadas que se caracterizan por contenidos relativamente enriquecidos de δ18O y δ2H; y agua subterránea mixta (antigua/reciente) o agua subterránea antigua, caracterizada por su composición isotópica empobrecida. Los datos de tritio respaldan la existencia de una recarga reciente limitada. Sin embargo, otros valores bajos de tritio son indicativos de una recarga pre-nuclear y/o una mezcla entre la recarga pre-nuclear y contemporánea. Las actividades de carbono 14 indican que el agua subterránea se recargó principalmente bajo diferentes condiciones climáticas durante los períodos más fríos del Pleistoceno tardío y el Holoceno.摘要了解控制(突尼斯)斯法克斯市半干旱地区地下水矿化度的地球化学过程和确定补给源,地下水中主要元素含量和稳定同位素(δ18O 和 δ2H)及放射性同位素(3H和 14C)证明是非常有用的示踪剂。主要离子化学数据表明,地下水中的盐分的来源是水岩互相作用,主要是蒸发岩矿物的溶解,以及与粘土矿物的阳离子交换。δ18O和 δ2H的相互关系表明地下水补给机理会发生变化。在中间含水层的地下水中,补给期间强烈蒸发伴随着有限的快速水入渗非常明显。一些地下水水样中稳定同位素值低是由于一些地区与老的地下水混合造成的。中间含水层的地下水分为两个主要类型:Ca-Na-SO4 和 Ca-Na-Cl-SO4。硝酸盐含量高表明该地区强烈的农业活动造成硝酸盐污染。稳定同位素印记揭示了三组水:表明由近期入渗的水补给的非蒸发水;δ18O 和 δ2H含量相对丰富的蒸发水;及同位素组分含量所剩无几的混合地下水(老的/近期的)或古老地下水。氚数据支持存在着近期有限补给的论点。然而,其它很低的氚值表明有核武器出现之前的补给及/或核武器出现之前的补给和同时期补给发生过混合。碳14活动表明,地下水主要是在晚更新世和全新世较凉时期不同气候条件下得到补给。ResumoAs principais concentrações dos isótopos estáveis (δ18O e δ2H) e isótopos radiogênicos (3H e 14C) em águas subterrâneas, tem provado serem traçadores úteis para a compreensão dos processos geoquímicos que controlam a mineralização das águas subterrâneas e na identificação das fontes de recarga na região semiáridas de Sfax (sudeste da Tunísia). Os dados dos íons maiores indicam que a origem da salinidade nas águas subterrâneas provém da interação água-rocha, principalmente da dissolução dos minerais evaporíticos, bem como da troca de cátions com minerais de argila. A relação de δ18O e δ2H sugere variação nos mecanismos de recarga subterrânea. A forte evaporação durante a recarga, que limita rapidamente a infiltração da água, é evidente nas águas subterrâneas dos aquíferos intermediários. A mistura das águas antigas em algumas áreas explica os valores baixos dos isótopos estáveis em algumas amostras de águas subterrâneas. As águas subterrâneas dos aquíferos intermediários são classificadas em dois tipos principais: Ca-Na-SO4 e Ca-Na-Cl-SO4. As altas concentrações de nitrato sugerem fontes antrópicas de contaminação de nitrato, causada por intensivas atividades agrícolas na área. A assinatura isotópica revela três diferentes grupos de água: águas não evaporadas, infiltradas recentemente; águas evaporadas, que são caracterizadas pelo relativo enriquecimento dos valores de δ18O e δ2H; e mistura de águas subterrâneas (antigas/recentes) ou águas muito antigas, caracterizadas pela composição isotópica depletada. Os resultados do trítio mostram a existência de uma recarga recente. Entretanto, valores baixos de trítio podem ser indicativos de recargas pré-nucleares e/ou misturas entre pré-nucleares e recarga contemporânea. As atividades de carbono-14 indicam que as águas subterrâneas foram recarregadas, principalmente, sob diferentes condições climáticas durante os períodos mais frios do último Pleistoceno e do Holoceno.

Nitrate contamination assessment in the Grombalia shallow aquifer (Tunisia) using a combined geostatistical and hydrogeochemical approach

The study concerns part of the plain sector of Aosta Valley (NW Italy). The investigated area is located between the cities of Sarre-Gressan and Pollein-Saint Christophe and is characterized by the presence of hexavalent chromium in the main shallow unconfined aquifer. The pollution is probably related to the negative environmental effects induced by the industrial steel production that since 1915 is present over the area. Since 1990 the industrial area was subjected to a number of direct investigations aimed to assess the contamination intensity. A preliminary remediation activity has been developed in last decades but the contaminants have been still observed in the groundwater monitoring network. This study highlights the set-up of a groundwater conceptual and numerical model of the shallow aquifer aimed to better understand and analyze the transport dynamics of hexavalent chromium in the local aquifer. The simulation is performed using the specific finite element software Feflow for groundwater flow and mass transport modeling. The hydrogeological setting of this area is related to the different sedimentary glacial, lacustrine and fluvial processes which characterized the bottom of the Aosta valley during the Quaternary. The shallow 80m-width aquifer is constituted by sandy to gravelly deposits and presents rare silty lens while its bottom is characterized by a decametric lacustrine silty level. The main aims of the numerical model are to give a more unequivocal explanation of the origin of the contamination and to support the predictive analyses in order to design an efficient site remediation for soil and groundwater. This represents a fundamental task in order to preserve the safety of the public water uses supplied by the aquifer. The preliminary hypothesis about the source of contaminations are still uncertain and referred to different scenarios that have to be further investigated by comparing monitoring data and transient flow simulation conditions.