Abstract
The efficacy of different proportions of silt-loam/bentonite mixtures overlying a vadose zone in controlling solute leaching to groundwater was quantified. Laboratory experiments were carried out using three large soil columns, each packed with 200-cm-thick riverbed soil covered by a 2-cm-thick bentonite/silt-loam mixture as the low-permeability layer (with bentonite mass accounting for 12, 16 and 19 % of the total mass of the mixture). Reclaimed water containing ammonium (NH4 +), nitrate (NO3 −), organic matter (OM), various types of phosphorus and other inorganic salts was applied as inflow. A one-dimensional mobile–immobile multi-species reactive transport model was used to predict the preferential flow and transport of typical pollutants through the soil columns. The simulated results show that the model is able to predict the solute transport in such conditions. Increasing the amount of bentonite in the low-permeability layer improves the removal of NH4 + and total phosphorous (TP) because of the longer contact time and increased adsorption capacity. The removal of NH4 + and OM is mainly attributed to adsorption and biodegradation. The increase of TP and NO3 − concentration mainly results from discharge and nitrification in riverbed soils, respectively. This study underscores the role of low-permeability layers as barriers in groundwater protection. Neglect of fingers or preferential flow may cause underestimation of pollution risk.
Résumé
L’efficacité de différentes proportions de mélanges de silts limons/bentonites surmontant une zone non saturée dans le contrôle du transport d’un soluté vers l’eau souterraine a été quantifiée. Des expériences de laboratoire ont été menées en utilisant trois grandes colonnes de sol constituées chacune de 200 cm d’épaisseur de sédiments de rivière recouverts par une couche de 2 cm d’un mélange de bentonite et de limons silteux comme couche de faible conductivité hydraulique (avec une quantité de bentonite de 12, 16 et 19 % de la masse totale du mélange). L’eau usée traitée utilisée contenant de l’ammonium (NH4 +), des nitrates (NO3 −), de la manière organique (MO), différents types de phosphore et d’autres els inorganiques a été utilisée comme flux d’entrée. Un modèle uni-dimensionnel de transport réactif multi-espèces et intégrant les phases mobiles et immobiles a été utilisé pour prédire le flux préférentiel et le transport des contaminants typiques au travers des colonnes de sol. Les résultats simulés montrent que le modèle est capable de prédire le transport du soluté dans de telles conditions. L’augmentation de la quantité de la bentonite dans le niveau de faible conductivité hydraulique améliore l’élimination de NH4 + et du phosphore total (PT) en raison du temps de contact plus grand et d’une plus grande capacité d’adsorption. L’élimination de NH4 + et de la MO est principalement attribuée à l’adsorption et à la biodégradation. L’augmentation des concentrations de PT et de NO3 − résulte principalement de la libération et de la nitrification dans les sédiments du lit de rivière respectivement. Cette étude souligne le rôle des couches de faible conductivité hydraulique en tant que barrière pour la protection des eaux souterraines. La négligence des écoulements préférentiels peut causer une sous-estimation du risque de pollution.
Resumen
Se cuantificó la eficacia de distintas proporciones de mezclas de limo-suelo franco/bentonita suprayacente a una zona vadosa para controlar la lixiviación de solutos a las aguas subterráneas. Se llevaron a cabo experimentos de laboratorio usando tres grandes columnas de suelo, cada una de ellas empaquetada con un espesor de suelo del lecho del río de 200-cm cubierto por una mezcla de bentonita/limo-suelo franco de 2-cm de espesor como una capa de baja permeabilidad (con una masa de bentonita del 12, 16 y 19 % de la masa total de la mezcla). Se aplicó como entrada el agua reciclada conteniendo amonio (NH4 +), nitrato (NO3 −), material orgánico (OM), varios tipos de fósforo y otras sales inorgánicas. Se usó un modelo unidimensional de transporte reactivo de multispecies móviles e inmóviles para predecir los flujos preferenciales y el transporte de típicos contaminantes a través de las columnas de suelo. Los resultados simulados muestran que el modelo es capaz de predecir el transporte de soluto en tales condiciones. Incrementando la cantidad de bentonita en las capas de baja permeabilidad se mejora la remoción del NH4 + y del fósforo total (TP) debido al mayor tiempo de contacto y al incremento de la capacidad de adsorción. La remoción de NH4 + y OM está atribuida principalmente a la adsorción y a la biodegradación. El incremento de de la concentración de TP y NO3 − resulta principalmente de la descarga y nitrificación en el suelo de lecho fluvial, respectivamente. Este estudio resalta el rol de las capas de baja permeabilidad como barreras para la protección del agua subterránea. El descuido del flujo preferencial puede causar una subestimación del riesgo de contaminación.
摘要
针对包气带上部不同比例粉砂壤土/膨润土的混合物在控制溶质向地下水淋滤过程的功效,本文进行了定量研究。开展的室内大型土柱试验中,三个土柱分别填装200cm厚的河床土,其上覆盖2cm厚的膨润土/粉砂壤土混合物作为低渗透性土层(膨润土所占混合物总质量的比例分别为12,16和19 %)。采用再生水作为入流溶液,其中含有铵态氮(NH4 +)、硝态氮(NO3 −)、有机物(OM)、不同形态的磷酸盐以及其它无机盐。利用一维可动区-不可动区多组分溶质的反应运移模型模拟了土柱中的优先流以及典型污染物的运移过程。模拟结果表明该模型可以预测试验条件下的溶质运移过程。增加低渗透性土层中的膨润土含量延长了接触时间并且增强了吸附能力,因此可以提高NH4 +和总磷(TP)的去除效果。NH4 +和OM的去除主要是由于吸附和生物降解作用。TP和NO3 −浓度的增加分别由于河床土中的释放和硝化作用所致。该研究表明低渗透性土层作为地下水保护屏障具有重要作用。忽略指流或者优先流可能低估污染风险。
Resumo
Foi quantificada a eficácia de diferentes proporções de mistura de areias siltosas e bentonite sobrepostas à zona vadosa no controle da lixiviação de solutos para as águas subterrâneas. Foram realizadas experiências laboratoriais com três grandes colunas de solo, cada uma preenchida com solo de leito fluvial com 200 cm de espessura coberto com uma mistura de bentonite/solo areno siltoso com 2 cm de espessura como camada de baixa permeabilidade (com a bentonite nas proporções mássicas para o total da mistura de 12, 16 e 19 %). Como entrada, foi aplicada água tratada contendo amónio (NH4 +), nitrato (NO3 −), matéria orgânica (MO), vários tipos de fósforo e outros sais inorgânicos. Foi utilizado um modelo de transporte reativo unidimensional multi-espécies móveis–imóveis para prever o fluxo preferencial e o transporte de poluentes típicos através da coluna de solo. Os resultados simulados mostram que o modelo é capaz de prever o transporte de solutos em tais condições. O aumento da quantidade de bentonite na camada de baixa permeabilidade melhora a remoção de NH4 + e de fósforo total (FT) em razão do maior tempo de contacto e da crescente capacidade de adsorção. A remoção de NH4 + e da MO é atribuída principalmente à adsorção e à biodegradação. O aumento das concentrações de FT e de NO3 − resultam principalmente da descarga e da nitrificação nos solos de leito fluvial, respetivamente. Este estudo ressalta o papel das camadas de baixa permeabilidade como barreiras na proteção das águas subterrâneas. Não tomar em conta os macroporos ou os caminhos de fluxo preferencial pode causar subestimação do risco de poluição.
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Acknowledgements
This study was funded by Projects of National Natural Science Foundation of China (Grant No. 51379207), Innovation Fund for Graduate Student of China Agricultural University (Grant No. KYCX2011087) and Beijing Municipal Science and Technology Project (Grant No. D090409004009004). The authors would like to thank the editor Elizabeth Screaton, the associate editor and anonymous reviewers for their detailed and thorough reviews.
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Hu, H., Mao, X., Barry, D.A. et al. Modeling reactive transport of reclaimed water through large soil columns with different low-permeability layers. Hydrogeol J 23, 351–364 (2015). https://doi.org/10.1007/s10040-014-1187-0
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DOI: https://doi.org/10.1007/s10040-014-1187-0