Abstract
A two-dimensional numerical groundwater flow model was established and calibrated for the hyperarid Najd region in southern Oman. The results indicate that recent recharge rates are required to sustain the observed groundwater heads in the Najd. The model was also used to estimate possible ranges of past recharge rates and the effective porosity of the main aquifer unit. Recharge rates during past humid periods were estimated to be no more than 1–3 times modern rates. The effective porosity was estimated to be between 0.06 and 0.093. Insight into the nature of the long-term transport within the aquifer was gained by using transient model runs over the last 350 ka and (1) varying the recharge intensity (from 0.1 to 2.5 times modern), and (2) the timing and duration of humid and dry periods. Finally, results indicate that although recharge rates and the flow conditions have likely changed over time, a steady-state model is capable of reproducing the observed groundwater residence times in the Najd based on carbon-14, helium and chlorine-36 dating.
Résumé
Un modèle numérique bidimensionnel de l’écoulement des eaux souterraines de la région hyperaride du Nadj, dans le Sud de l’Oman, a été réalisé et calé. Les résultats indiquent que les taux de recharge récents sont nécessaires pour corroborer les charges hydrauliques observées de l’aquifère de Nadj. Le modèle a été également utilisé pour estimer l’éventail possible des taux de recharge du passé et la porosité efficace de la principale unité aquifère. Les taux de recharge des périodes humides passées n’ont pas été estimés à plus de 1–3 fois les taux actuels. La porosité efficace a été estimée entre 0.06 et 0.093. Un aperçu de la nature du transport à long terme au sein de l’aquifère a été obtenu en utilisant les flux d’un modèle transitoire durant les derniers 350,000 ans et en faisant varier (1) l’intensité de la recharge (de 0.1 à 2.5 fois l’actuelle) et (2) la succession et la durée des périodes humides et sèches. Finalement, les résultats indiquent que, bien que les taux de recharge et les conditions d’écoulement aient vraisemblablement changé au cours du temps, un modèle en régime permanent est. capable de reproduire les temps de résidence observés dans l’aquifère de Nadj, tels que déduits des datations au carbone-14, à l’hélium et au chlore-36.
Resumen
Se probó y calibró un modelo numérico bidimensional de flujo de agua subterránea para la región de Najd en el sur de Omán. Los resultados indican que se requieren tasas de recarga recientes para sostener las alturas hidráulicas del agua subterránea observadas en el Najd. El modelo también se utilizó para estimar posibles rangos de tasas de recarga pasadas y la porosidad efectiva de la unidad del acuífero principal. Las tasas de recarga durante los períodos húmedos pasados se estimaron en no más de 1–3 veces las tasas actuales. La porosidad efectiva se estimó entre 0.06 y 0.093. La comprensión de la naturaleza del transporte a largo plazo dentro del acuífero se obtuvo mediante el uso de un modelo transitorio en los últimos 350 ka y (1) variando la intensidad de recarga (de 0.1 a 2.5 veces de la actual), y (2) el tiempo y la duración de periodos húmedos y secos. Finalmente, los resultados indican que aunque las tasas de recarga y las condiciones de flujo probablemente han cambiado con el tiempo, un modelo de estado estable es capaz de reproducir los tiempos de residencia observados en el agua subterránea en el Najd basándose en la datación de carbono-14, helio y cloro-36.
摘要
建立了安曼南部高度干旱的Najd地区二维地下水流数值模型,并对此进行了校准。结果表明,需要近代的补给数量以维持所观测到的Najd地区水头。模型还用来估算过去补给量的可能范围和主要含水层单元的有效孔隙度。估算结果显示,过去湿润期补给量不超过当代补给量的1-3倍。估算的有效孔隙度为0.06至0.093。利用瞬时模型模拟了(1)不同补给密度(0.1到2.5倍的当代补给密度)和(2)湿润和干旱期的时间设定和持续时间不同条件下过去350 ka以来的各种情况,对含水层内部长期运移的特性有了新的认识。最终,结果表明,尽管补给量和水流条件过去一直在发生变化,但根据碳14、氦和氯36测年,稳定态模型能够再现Najd地区观测到的地下水滞留时间。
Resumo
Um modelo numérico bidimensional de fluxo de águas subterrâneas foi estabelecido e calibrado para a região hiperárida de Najd, no sul de Omã. Os resultados indicam que taxas de recarga recentes são necessárias para sustentar as nascentes observadas em Najd. O modelo também foi usado para estimar possíveis faixas de taxas de recargas passadas e a porosidade efetiva da unidade aquífera principal. As taxas de recarga durante períodos úmidos passados foram estimadas em não mais que 1–3 vezes as taxas modernas. A porosidade efetiva foi estimada entre 0.06 e 0.093. A compreensão da natureza do transporte de longo prazo, dentro do aquífero, foi obtida usando modelos transientes nos últimos 350 mil anos e (1) variando a intensidade de recarga (de 0.1 a 2.5 vezes a moderna), e (2) o tempo e a duração de períodos úmidos e secos. Finalmente, os resultados indicam que, embora as taxas de recarga e as condições de fluxo provavelmente tenham mudado com o tempo, um modelo de estado estacionário é capaz de reproduzir os tempos de residência observados em Najd, com base na datação de carbono-14, hélio e cloro-36.
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Acknowledgements
The authors would like to thank the Ministry of Regional Municipalities and Water Resources (MRMWR) of the Sultanate of Oman for providing data and their steady support.
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Müller, T., Sanford, W. Calibration of regional hydraulic and transport properties of an arid-region aquifer under modern and paleorecharge conditions using water levels and environmental tracers. Hydrogeol J 27, 685–701 (2019). https://doi.org/10.1007/s10040-018-1894-z
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DOI: https://doi.org/10.1007/s10040-018-1894-z