Abstract
This study presents a modeling framework for quantifying human impacts and for partitioning the sources of contamination related to water quality in the mixed-use landscape of a small tropical volcanic island. On Tutuila, the main island of American Samoa, production wells in the most populated region (the Tafuna-Leone Plain) produce most of the island’s drinking water. However, much of this water has been deemed unsafe to drink since 2009. Tutuila has three predominant anthropogenic non-point-groundwater-pollution sources of concern: on-site disposal systems (OSDS), agricultural chemicals, and pig manure. These sources are broadly distributed throughout the landscape and are located near many drinking-water wells. Water quality analyses show a link between elevated levels of total dissolved groundwater nitrogen (TN) and areas with high non-point-source pollution density, suggesting that TN can be used as a tracer of groundwater contamination from these sources. The modeling framework used in this study integrates land-use information, hydrological data, and water quality analyses with nitrogen loading and transport models. The approach utilizes a numerical groundwater flow model, a nitrogen-loading model, and a multi-species contaminant transport model. Nitrogen from each source is modeled as an independent component in order to trace the impact from individual land-use activities. Model results are calibrated and validated with dissolved groundwater TN concentrations and inorganic δ15N values, respectively. Results indicate that OSDS contribute significantly more TN to Tutuila’s aquifers than other sources, and thus should be prioritized in future water-quality management efforts.
Résumé
Cette étude présente un cadre de modélisation pour quantifier les impacts humains et classer les sources de contamination de la qualité de l’eau dans un contexte d’usages multiples de petites iles volcaniques tropicales. Sur Tutuila, la principale île des Samoa Américaines, les puits de production dans la région la plus peuplée (la plaine de Tafuna-Leone) fournissent l’essentiel de l’eau potable pour les îles. Cependant, une grande partie de cette eau est. considérée comme non potable depuis 2009.Tutuila a trois sources anthropiques prédominantes de pollution diffuse des eaux souterraines: les systèmes de décharge sur site (OSDS), les engrais agricoles chimiques, et le lisier des élevages de porcs. Ces sources sont largement distribuées à travers la région et sont situées près de nombreux puits d’eau potable. Les analyses de qualité de l’eau montrent une relation entre les niveaux élevés d’azote total (TN) dissout dans les eaux souterraines et les zones présentant une forte pollution diffuse, suggérant que le TN peut être utilisé comme traceur de la contamination des eaux souterraines provenant de ces zones. Le cadre de modélisation utilisé dans cette étude intègre l’information sur les usages du sol, les données hydrologiques, et les analyses de qualité de l’eau avec la charge en azote et les modalités de transfert. L’approche utilize un modèle numérique d’écoulement d’eaux souterraines, un modèle de transport de l’azote, et un modèle de transport de contaminants multi-espèces. L’azote de chaque source est. modélisé en tant que composant indépendant afin de retracer l’impact des activités individuelles de l’occupation du sol. Les résultats du modèle sont calibrés et validés respectivement avec des concentrations TN et δ15N inorganiques dans les eaux souterraines. Les résultats indiquent que les OSDS contribuent de manière significative plus importante en TN aux aquifères de Tutuila que les autres sources, et qu’ils devraient ainsi être prioritaires dans les futurs efforts de gestion de la qualité de l’eau.
Resumen
Este estudio presenta un marco de modelización para cuantificar los impactos humanos y para separar las fuentes de contaminación relacionadas con la calidad del agua en los paisajes de uso mixto de pequeñas islas volcánicas tropicales. En Tutuila, la isla principal de Samoa Americana, los pozos de producción de agua en la región más poblada (la llanura de Tafuna-Leone) producen la mayor parte del agua potable de la isla. Sin embargo, gran parte de esta agua se ha considerado insegura para beber desde 2009. Tutuila tiene tres fuentes antropogénicas predominantes de contaminación del agua subterráneas no puntuales de preocupación: sistemas de disposición in situ (OSDS), productos químicos agrícolas y estiércol de cerdos. Estas fuentes están ampliamente distribuidas en todo el paisaje y se encuentran cerca de muchos pozos de agua potable. Los análisis de la calidad del agua muestran un vínculo entre niveles elevados de nitrógeno disuelto total del agua subterránea (TN) y áreas con alta densidad de contaminación no puntual, lo que sugiere que la TN puede usarse como trazador de contaminación de aguas subterráneas de estas fuentes. El marco de modelización utilizado en este estudio integra información de uso de la tierra, datos hidrológicos y análisis de calidad de agua con modelos de carga y transporte de nitrógeno. El enfoque utiliza un modelo numérico de flujo de agua subterránea, un modelo de carga de nitrógeno y un modelo de transporte de contaminantes multi-especies. El nitrógeno de cada fuente es modelado como un componente independiente con el fin de rastrear el impacto de las actividades individuales de uso del suelo. Los resultados del modelo se calibran y validan con concentraciones disueltas de TN de agua subterránea y valores inorgánicos de δ15N, respectivamente. Los resultados indican que los OSDS contribuyen significativamente más a la TN de los acuíferos de Tutuila que otras fuentes, y por lo tanto deben ser priorizados en futuros esfuerzos para el manejo de la calidad del agua.
لملخص
الصرف تقدم هذه الدراسة إطارا للنمذجة لقياس الآثار البشرية للتلوث وتقسيم مصادر التلوث المتصلة بنوعية المياه في المناطق ذات الاستخدامات المختلطة للجزر البركانية الاستوائية الصغيرة. في جزيرة توتويلا، الجزيرة الرئيسية في ساموا الأمريكية. تنتج آبار الإنتاج في معظم المناطق المأهولة بالسكان (سهل تافونا - ليون) معظم مياه الشرب في الجزيرة. ومع ذلك، فقد اعتبر جزء كبير من هذه المياه غير آمن للشرب منذ عام 2009. وتوتويلا لديها ثلاثة مصادر سائدة من مصادر المياه الجوفية المسببة للانبعاثات البشرية المنشأ: غير العضوية. وتشير النتائج إلى أن الصرف15الصحي ، المواد الكيميائية الزراعية، وسماد الخنازير. وتوزع هذه المصادر على نطاق واسع في جميع أنحاء المناطق وتقع بالقرب من العديد من آبار مياه الشرب. وتظهر تحاليل نوعية المياه وجود صلة بين المستويات المرتفعة من مجموع نتروجين المياه الجوفية المذاب (NT) والمناطق ذات الكثافة العالية من التلوث غير احادي المصدر، مما يشير إلى أن NT يمكن أن تستخدم كمتتبع لتلوث المياه الجوفية من هذه المصادر. ويدمج إطار النمذجة المستخدم في هذه الدراسة معلومات استخدام الأراضي والبيانات الهيدرولوجية وتحليلات نوعية المياه مع نماذج تحميل النتروجين والنقل. ويستخدم النهج نموذجا لتدفق المياه الجوفية العددية، ونموذج تحميل النيتروجين، ونموذجا لنقل الملوثات المتعددة الأنواع. ويتم تصميم النتروجين من كل مصدر كمكون مستقل من أجل تتبع أثر الأنشطة الفردية لاستخدام الأراضي. يتم معايرة النتائج النموذجية والتحقق من صحتها مع تركيزات NT الذائبة وقيم Nδ الصحي يساهم بشكل كبير في وصول NT إلى طبقات المياه الجوفية أكثر من المصادر الأخرى، وبالتالي يجب أن تكون ذات أولوية في الجهود المستقبلية لإدارة نوعية المياه.
摘要
本研究展示了量化人类影响及分隔与小型热带火山岛屿混合用途景观中水质相关的污染源的模拟框架。在美属萨摩亚的主要岛屿图图伊拉岛上,人口最多的地区(Tafuna-Leone平原)生产井为岛提供大部分饮用水。然而,自从2009年以来,这些水的大部分水被认为饮用不安全。图图伊拉岛有三个主要的源自人类的非点源地下水污染源:现场处理系统、农业化肥及猪的粪便。这些源广泛分布于整个景观中,位于许多饮水井附近。水质分析显示,地下水中总溶解的氮的升高和非点源污染密度高的地区有关联,表明地下水中总溶解的氮可用作由于这些源造成地下水污染的示踪剂。本研究中使用的模拟框架把土地利用信息、水文数据和水质分析与氮承载量和运移模型结合在一起。该方法利用了一个数值地下水流模型、一个氮承载量模型和一个多种类污染物运移模型。来自每个源的氮被模拟为独立的成分,以便追寻各自土地利用活动的影响。模型结果分别用溶解地下水氮浓度和无机的δ15N值校正和验证。结果显示,现场处理系统贡献到图图伊拉含水层的地下水中总溶解的氮比其他源的要多,因此,在将来水质管理实践中应该得到优先考虑。
Resumo
Este estudo apresenta um sistema de modelagem para quantificar os impactos e distinguir as fontes de contaminação relacionadas a qualidade da água em ambientes onde se desenvolvem atividades diversificadas como em pequenas ilhas vulcânicas tropicais. Em Tutuila, a principal ilha da Samoa Americana, poços em produção na região mais populosa (a Planície de Tafuna-Leone) produzem a maior parte da água que abastece a ilha. Contudo, uma grande parte dessa água foi considerada imprópria para consumo desde 2009. Tutuila possui três fontes de poluição antropogênica não pontuais predominantes: sistemas de disposição de resíduos locais (SDRL), agroquímicos, e estrume de porco. Essas fontes são amplamente distribuídas por toda paisagem e estão localizadas nas proximidades de muitos poços de abastecimento. Análises de qualidade da água mostram uma conexão entre níveis elevados de nitrogênio dissolvido total e áreas com alta densidade de fontes de poluição não pontuais, sugerindo que o nitrogênio total pode ser usado como um traçador de contaminação de água subterrânea por essas fontes. O sistema de modelagem usado nesse estudo integra informação de ocupação da terra, dados hidrológicos, e análises de qualidade da água com carga de nitrogênio e modelos de transporte. O enfoque utiliza um modelo numério de fluxo de água subterrânea, modelo de carga de nitrogênio, e um modelo de transporte de contaminantes multi-espécies. O nitrogênio de cada fonte é modelado como um componente independente com o intuito de traçar o impacto de cada atividade de uso da terra. Os resultados dos modelos são calibrados e validados com concentrações totais de nitrogênio dissolvido na água subterrânea e valores de δ15N inorgânico, respectivamente. Resultados indicam que SDRL contribuem de forma significativa, mais do que outras fontes, para o total de nitrogênio dissolvido nos aquíferos de Tutuila, e que os esforços para o gerenciamento das fontes poluidoras devem ser direcionados à essa fonte de contaminação.
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Acknowledgements
The authors express sincere appreciation to American Samoa Power Authority, American Samoa EPA, and American Samoa Department of Commerce staff and collaborators, including Utu Abe Malae, William Spitzenberg, Katrina Mariner, Danielle Mauga, Tim Bodell, and Mia Comeros for essential data and support. This project relied heavily on the groundwater model initially developed by Scot Izuka and his assistance throughout our modeling process was invaluable. We also thank Dr. Randy DeWees, Hugh Fuimaono, and all of the hard-working student interns at American Samoa Community College. We would also like to thank two anonymous reviewers for their time and comments, which improved the manuscript greatly. This project was made possible through support provided by NOAA’s Pacific Regional Integrated Sciences and Assessments Program (PacRISA) and the USGS Water Resources Research Institute Program (WRRIP). This is contributed paper WRRC-CP-2017-04 of the Water Resources Research Center, University of Hawaii at Manoa, Honolulu, Hawaii, SOEST Publication No. 10014.
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Shuler, C.K., El-Kadi, A.I., Dulai, H. et al. Source partitioning of anthropogenic groundwater nitrogen in a mixed-use landscape, Tutuila, American Samoa. Hydrogeol J 25, 2419–2434 (2017). https://doi.org/10.1007/s10040-017-1617-x
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DOI: https://doi.org/10.1007/s10040-017-1617-x