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Using geochemical indicators to investigate groundwater mixing and residence time in the aquifer system of Djeffara of Medenine (southeastern Tunisia)

Utilisation d’indicateurs géochimiques pour l’étude du mélange et du temps de résidence dans le système aquifère de Djeffara de Medenine (Tunisie du Sud-Est)

El uso de indicadores geoquímicos para investigar la mezcla de agua subterránea y el tiempo de residencia del sistema acuífero de Djeffara de Medenine (Sudeste de Túnez)

استخدام التقانات الجيوكيميائية لدراسة عملية اختلاط المياه الجوفية و مدة إقامتها في المنظومة المائية بجفارة مدنين (الجنوب التونسي)

Utilização de indicadores geoquímicos para investigar os processos de mistura de água subterrânea e os tempos de residência no sistema aquífero de Djeffara de Medenine (sudeste da Tunísia)

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Abstract

Stable isotopes (δ2H, δ18O and δ13C) and radiocarbon (14C) have been used in conjunction with chemical data to evaluate recharge mechanisms and groundwater residence time, and to identify inter-aquifer mixing in the Djeffara multi-aquifer in semi-arid southeastern Tunisia. The southern part of this basin, the Djeffara of Medenine aquifer system, is comprised of two main aquifers of Triassic and Miocene sandstone. The Triassic aquifer presents two compartments; the first one (west of the Medenine fault system) is unconfined with a well-defined isotope fingerprint; the second compartment is deeper and confined. Multi-tracer results show groundwater of different origins, ages and salinities, and that tectonic features control groundwater flows. Fresh and brackish groundwater from the unconfined part of the Triassic aquifer was mostly recharged during the Holocene. The recharge rates of this aquifer, inferred by 14C ages, are variable and could reach 3.5 mm/year. Brackish water of the deep confined part of the Triassic aquifer has stable isotope composition and 14C content that indicates earlier recharge during late Pleistocene cold periods. Brackish to saline water of the Miocene aquifer presents variable isotope composition. Groundwater flowing through the Medenine fault system is mainly feeding the Miocene aquifer rather than the deep confined part of the Triassic aquifer.

Résumé

Les isotopes stables (δ2H, δ18O and δ13C) et le radiocarbone (14C) ont été utilisés parallèlement à des données chimiques pour évaluer les mécanismes de recharge et temps de résidence, et pour identifier le mélange inter-aquifères de l’aquifère multi-couches de Djeffara dans le Sud-Est tunisien semi-aride. La partie Sud de ce bassin, le système aquifère de Djeffara de Medenine, présente deux aquifères principaux constitués de grès triasique et miocène. L’aquifère triasique présente deux compartiments; le premier (Ouest du système faille de Medenine), est libre avec une bonne marque isotopique; le second compartiment est plus profond et captif. Les multi-traçages montrent des aquifères de différentes origines, âges et salinités, et le contrôle des écoulements souterrains par la fracturation. L’eau douce et saumâtre de la partie libre de l’aquifère triasique a été rechargée prinicipalement durant l’Holocène. Les taux de recharge de cet aquifère, inférés par les âges 14C, sont variables et peuvent atteindre 3.5 mm/an. L’eau saumâtre de la partie profonde captive de l’aquifère triasique a une composition isotopique stable et une teneur en 14C indiquant une recharge plus ancienne, datant des périodes froides du Pléistocène tardif. L’eau, saumâtre à salée, de l’aquifère miocène présente une composition isotopique variable. L’écoulement de nappe à travers le système fracturé de Medenine alimente principalement l’aquifère miocène, plus que la partie profonde captive de l’aquifère triasique.

Resumen

Se utilizaron los isótopos estables (δ2H, δ18O y δ13C) y radiocarbono (14C) en conjunción con datos químicos para evaluar los mecanismos de recarga y tiempo de residencia del agua subterránea, y para identificar la mezcla interacuífero en el multiacuífero Djeffara en el sudeste semiárido de Túnez. La parte sur de esta cuenca, el sistema acuífero Djeffara de Medenine, está compuesto de dos acuíferos principales de areniscas del Triásico y del Mioceno. El acuífero Triásico presenta dos compartimentos; el primero de ellos (al oeste del sistema de falla de Medenine) es no confinado con una bien definida marca isotópica; el segundo compartimento es más profundo y confinado. Los resultados de multitrazadores muestran agua subterránea de diferentes orígenes, edades y salinidades, y que los aspectos tectónicos controlan el flujo subterráneo. El agua subterránea dulce y salobre de la parte no confinada del acuífero Triásico fue mayormente recargada durante el Holoceno. Los ritmos de recarga de este acuífero, deducido por edades de 14C, son variables y podrían alcanzar 3.5 mm/año. El agua salobre de la parte confinada profunda del acuífero Triásico tiene una composición de isótopos estables y un contenido de 14C que indica una recarga más temprana durante los períodos fríos del Pleistoceno tardío. Las aguas salobres a salinas del acuífero Mioceno presentan una composición isotópica variable. El agua subterránea que fluye a través del sistema de falla de Medenine es alimentada principalmente por el acuífero Mioceno mas que por la parte confinada profunda del acuífero Triásico.

الخلاصة

استخدمت النظائر المستقرة (δ2H ، δ18O وδ13C) والكربون المشع (C14) بالإضافة إلى البيانات الكيميائية لتقييم آليات التغذية ومدة إقامة المياه الجوفية ، وللتعرف على عمليات الاختلاط بين طبقات المياه الجوفية في المنظومة المائية المتعددة الطبقات بحوض الجفارة في جنوب شرق تونس الشبه جاف. وتتكون المنظومة المائية بمدنين، في الجزء الجنوبي من هذا الحوض ، من طبقتين مائية رئيسيتين برسوبيات العصر الترياسي و بالحجر الرملي للعصر الميوسيني. و تحتوي طبقة المياه الجوفية الترياسي على جزئين ، الأول (غرب نظام فوالق مدنين) هو سطحي يتميز بتركيبة نظائرية واضحة المعالم ، أما الجزء الثاني فهو أعمق وغير متجدد . تظهر النتائج المتعددة الاختصاصات مختلف أعمار المياه الجوفية ، ومصادر الملوحة ، ودور الفوالق التكتونية في السيطرة على سيلان المياه الجوفية. إذ تم شحن المياه الجوفية العذبة ومتوسطة الملوحة من الطبقة المائية السطحية للترياسي خلال العصر الهولوسين.وتتفاوت نسب تغذية هذه الطبقة المائية بالاعتماد على الأعمار المتحصل عليها عن طربق الكربون 14C ، ويمكن ان تصل الى 3.5 ملم / سنة. و تشير التركيبة النظائرية ومحتوى C14 بالمياه القليلة الملوحة في الجزء العميق من الطبقة المائية للترياسي على أن الشحن قد تم في وقت سابق خلال فترات أواخر العصر الحديث الباردة. وتتميز المياه المالحة ومتوسطة الملوحة لطبقة المياه الجوفية بالميوسين بمحتوى نظائري متفاوت. وتغذي المياه الجوفية التي تتدفق من خلال فوالق مدنين الطبقة المائية بالميوسين بصورة رئيسية بدلا من الجزء العميق من طبقة المياه الجوفية بالترياس.

Resumo

Os isótopos estáveis (δ2H, δ18O e δ13C) e de radiocarbono (14C) têm sido usados em conjunto com os dados químicos para avaliar os mecanismos de recarga e o tempo de residência das águas subterrâneas, e para identificar os processos de mistura entre unidades aquíferas do sistema multi-camada de Djeffara, localizado na região semi-árida do sudeste da Tunísia. Na parte sul desta bacia, o sistema aquífero Djeffara de Medenine é composto por dois aquíferos principais, os arenitos do Triássico e os arenitos do Miocénico. O aquífero de idade triássica apresenta dois sectores, em que o primeiro (a oeste do sistema de falhas Medenine) é livre, com uma assinatura isotópica bem definida, e o segundo, mais profundo, é confinado. Dos resultados obtidos por análise de traçadores múltiplos pode concluir-se que existem águas de diferentes origens, idades e salinidades, e que o seu fluxo subterrâneo é condicionado pela tectónica da região. As águas subterrâneas doces e salobras do aquífero livre do Triássico foram principalmente recarregadas durante o período holocénico. As taxas de recarga deste aquífero, inferidas pela análise de idade de 14C, são variáveis, podendo atingir valores da ordem dos 3.5 mm/ano. A água salobra da parte profunda do aquífero confinado triássico tem composição isotópica estável e um teor de 14C que indicia que o início da recarga tenha ocorrido durante os períodos frios do final do Plistocénico. As águas salobras e salinas do aquífero miocénico apresentam uma composição isotópica variável. As águas subterrâneas que fluem através do sistema de falhas de Medenine alimentam principalmente o aquífero miocénico, ao invés da parte profunda do aquífero Triássico.

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Notes

  1. see Fontes (1976); Huneau et al. (2001)

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Acknowledgements

This study was carried out within the framework of the regional IAEA project RAF/8/035.

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  1. Laboratory of Radio-Analysis and Environment, National School of Engineers of Sfax, BP 1173, 3038, Sfax, Tunisia

    Kamel Zouari, Rim Trabelsi & Najiba Chkir

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  1. Kamel Zouari
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Correspondence to Kamel Zouari.

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Zouari, K., Trabelsi, R. & Chkir, N. Using geochemical indicators to investigate groundwater mixing and residence time in the aquifer system of Djeffara of Medenine (southeastern Tunisia). Hydrogeol J 19, 209–219 (2011). https://doi.org/10.1007/s10040-010-0673-2

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