Abstract
Management of groundwater resources can be improved by using groundwater models to perform risk analyses and to improve development strategies, but a lack of extensive basic data often limits the implementation of sophisticated models. Dar es Salaam in Tanzania is an example of a city where increasing groundwater use in a Pleistocene aquifer is causing groundwater-related problems such as saline intrusion along the coastline, lowering of water-table levels, and contamination of pumping wells. The lack of a water-level monitoring network introduces a problem for basic data collection and model calibration and validation. As a replacement, local water-supply wells were used for measuring groundwater depth, and well-top heights were estimated from a regional digital elevation model to recalculate water depths to hydraulic heads. These were used to draw a regional piezometric map. Hydraulic parameters were estimated from short-time pumping tests in the local wells, but variation in hydraulic conductivity was attributed to uncertainty in well characteristics (information often unavailable) and not to aquifer heterogeneity. A MODFLOW model was calibrated with a homogeneous hydraulic conductivity field and a sensitivity analysis between the conductivity and aquifer recharge showed that average annual recharge will likely be in the range 80–100 mm/year.
Résumé
La gestion des ressources en eau souterraine peut être améliorée en utilisant des modèles de nappe pour réaliser des analyses de risque et améliorer les stratégies de développement, mais un manque de données de bases suffisantes limite souvent l’utilisation de modèles sophistiqués. Le manque de réseau de surveillance du niveau phréatique introduit un problème de collecte des données, de tarage et de validation du modèle. En remplacement, des puits ont été utilisés localement pour mesurer la profondeur de la nappe, et la charge des têtes de puits a été estimée à partir d’un modèle altimétrique régional digital pour recalculer profondeurs et hauteurs d’eau. Ceux-ci ont été utilisés pour tracer une carte piézométrique régionale. Les paramètres hydrauliques ont été estimés à partir de pompages courte durée dans les puits locaux, mais la variation de conductivité hydraulique a été attribuée à l’incertitude des caractéristiques des puits (information souvent indisponible) et non à l’hétérogénéité de l’aquifère. Un modèle MODFLOW a été étalonné dans une champ à conductivité hydraulique homogène et une analyse de sensibilité de la conductivité à la recharge de l’aquifère a montré que la recharge annuelle moyenne a vraisemblablement une amplitude de 80–100 mm/an.
Resumen
El manejo de los recursos de aguas subterráneas puede ser mejorado usando modelos de agua subterránea para realizar análisis de riesgo y mejorar el desarrollo de estrategias, pero una falta de extensión de los datos básicos a menudo limita la implementación de modelos sofisticados. Dar es Salaam en Tanzania es un ejemplo de una ciudad donde el creciente uso de agua subterránea proveniente de un acuífero Pleistoceno está causando problemas relacionados con la intrusión salina a lo largo de la línea de costa, el descenso de los niveles freáticos, y la contaminación de pozos de bombeo. La falta de una red de monitoreo de niveles de agua introduce un problema para la recolección de datos básicos y la calibración y validación del modelo. Como un reemplazo se usaron los pozos de abastecimiento de agua local para la medición de la profundidad del agua subterránea y se estimaron las alturas de la parte superior de los pozos a partir de un modelo digital de elevación regional del terreno para recalcular las profundidades del agua a las cargas hidráulicas. Estas fueron usadas para confeccionar un mapa piezométrico regional. Los parámetros hidráulicos fueron estimados a partir de ensayos de bombeo de corta duración en los pozos locales, pero la variación en la conductividad hidráulica fue atribuida a la incertidumbre en las características de los pozos (información a menudo no disponible) y no a la heterogeneidad del acuífero. Se calibró un modelo MODFLOW con un campo de conductividad hidráulica homogéneo y un análisis de sensibilidad entre la conductividad y la recarga del acuífero mostró que el promedio anual de la recarga está probablemente en el intervalo de 80–100 mm/año.
摘要
地下水模型可用于风险分析,改善开发策略,从而加强地下水资源管理. 但是大量基础数据的缺乏通常限制了复杂模型的实现. 以坦桑尼亚达累斯萨拉姆为例,更新世含水层的地下水用量增加,导致咸水沿海岸带入侵、地下水位下降和抽水井污染等相关的地下水问题. 地下水位监测网引起基础数据收集和模型识别验证上的问题. 但作为替代,当地供水井可用于测量地下水水埋深,井口高程通过区域数字地面高程估计, 从而重新估算相对于水头的深度, 并用于绘制区域水压图. 水文参数通过当地井短期抽水试验获得, 但是水力传导系数的变化归因于井特征的不确定性(信息通常缺失)而不是含水层的各向异性. MODFLOW模型通过均质的水力传导系数场进行识别,传导性和含水层补给之间的敏感性分析表明多年平均补给量为80–100 mm/a.
Resumo
A gestão dos recursos hídricos subterrâneos pode ser melhorada através do uso de modelos de água subterrânea para realizar análises de risco e para melhorar as estratégias de desenvolvimento, mas a falta de uma grande quantidade de dados básicos limita, muitas vezes, a implementação de modelos sofisticados. Dar es Salaam, na Tanzânia, é um exemplo de uma cidade onde o aumento da utilização de águas subterrâneas dum aquífero do Plistocénico está a causar problemas, como a intrusão salina ao longo do litoral, diminuindo os níveis da água subterrânea, e contaminando os furos de bombagem. A falta de uma rede de monitorização dos níveis da água subterrânea representa um problema para a colheita de dados básicos e para a calibração e validação do modelo. Em substituição, foram utilizados furos de abastecimento de água locais para medir a profundidade das águas subterrâneas, e foi estimada a altitude dos furos, a partir de um modelo digital da elevação regional, para recalcular a profundidade da água em relação ao nível piezométrico. Estes níveis foram usados para desenhar um mapa piezométrico regional. Os parâmetros hidráulicos foram estimados a partir de ensaios de bombagem de curta duração nos furos locais, mas a variação na condutividade hidráulica foi atribuída à incerteza nas características dos furos (informação muitas vezes não disponível) e não à heterogeneidade do aquífero. Um modelo MODFLOW foi calibrado com um campo homogéneo de condutividade hidráulica e uma análise de sensibilidade entre a condutividade e a recarga do aquífero mostrou que a recarga anual média será provavelmente na ordem dos 80–100 mm/ano.
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Acknowledgements
The authors wish to express their sincere thanks to the Flemish Interuniversity Council (VLIR)-Institutional Co-operation Program and the VLIR- Interuniversity Collaboration Programme (IUC) with Sokoine University of Agriculture (SUA-VLIR Programme) for funding this study. The authors highly appreciate the help from all persons that provided information and/or assistance during data collection. They are grateful to three anonymous reviewers, whose constructive comments were helpful in improving the report.
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Van Camp, M., Mjemah, I.C., Al Farrah , N. et al. Modeling approaches and strategies for data-scarce aquifers: example of the Dar es Salaam aquifer in Tanzania. Hydrogeol J 21, 341–356 (2013). https://doi.org/10.1007/s10040-012-0908-5
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DOI: https://doi.org/10.1007/s10040-012-0908-5