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Technische Universität Dresden (2013)

Recharge and residence times in an arid area aquifer

Müller, Thomas

Titre : Recharge and residence times in an arid area aquifer

Auteur : Müller, Thomas

Université de soutenance : Technische Universität Dresden

Grade : PhD, Fakultät Umweltwissenschaften, 2013

Résumé
Deteriorating water quality in the face of a rising demand for agricultural products triggered interest in the groundwater resources of the Najd dessert, an arid region of southern Oman. Groundwater in this area usually is abstracted from one of the largest aquifers on the Arabian Peninsula, the Umm Er Radhuma aquifer. Increased discharge stands in contrast to limited precipitation : the monsoon is an annual event but it is regionally limited ; cyclones infrequently occur within the range of three to seven years. Both are possible sources for groundwater recharge in the Najd.
With these preconditions in mind, the present study investigates recharge to the Najd groundwaters as part of an active flow system and evaluates the mean residence time in the deep groundwaters. The tools of choice are a groundwater flow model combined with environmental isotope tracer data. The two-dimensional flow model replicates the characteristics of the aquifer system from the potential recharge area in the south (Dhofar Mountains) to the discharge area in the north (Sabkha Umm as Sammim). The south-to-north gradients and the observed artesian heads in the confined aquifer are reproduced. Simulation results indicate that changes between wet and dry periods caused transient responses in heads and head gradients lasting for several thousand years. Based on the used parameters the model calibration indicated, that a recharge rate of around 4 mm a−1 is sufficient to reproduce current groundwater levels. Since rising groundwater levels were documented after cyclone Keila in November 2011, modern recharge evidently occurs. 36-Cl concentrations and dissolved-helium concentrations indicate that the deep groundwaters in the central Najd are up to 550,000 years old. Thus, radiocarbon values indicating groundwater residence times for the central Najd up to 20,000 years and the northern Najd up to 35,000 years underestimate the groundwater residence times and seem to have been strongly affected by mixing during sampling. Decreasing 36-Cl and increasing 4-He concentrations confirm the expected trend in the direction of groundwater flow and prove to be more robust tracers for age dating of Najd groundwaters. Backward pathline tracking was used to simulate the groundwater ages. The tracking results show that a total porosity value between 15 and 20 % is consistent with the range of the observed chlorine-36 and heliumbased ages. The results and parameters obtained in the present study provide the basis for future 3D-groundwater models designed to evaluate the water resources available to the Najd’s agricultural complex. In addition, the developed 2D-model allows for studies of paleoclimate scenarios and their influence on the groundwater regime.

Mots clés : arid area, groundwater recharge, groundwater residence times, environmental isotopes, groundwater modeling — Arides Gebiet, Grundwasser, Modellierung, Grundwasserneubildung, Grundwasseraufenthaltszeiten, Umweltisotope

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Page publiée le 19 septembre 2013, mise à jour le 16 janvier 2020