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Modeling effects of nitrate from non-point sources on groundwater quality in an agricultural watershed in Prince Edward Island, Canada

Modélisation des effets des apports nitratés diffus sur la qualité des eaux souterraines dans un bassin agricole de l’île du Prince Edouard, Canada

Modelación de los efectos del nitrato de fuentes no puntuales en la calidad de las aguas subterráneas en una cuenca agrícola en Isla Prince Edward, Canadá

模拟非点源硝酸盐对加拿大爱德华王子岛某农业流域地下水水质的影响

Modelação da influência do nitrato de fontes não pontuais na qualidade da água subterrânea numa bacia hidrográfica de uso agrícola em Prince Edward Island, Canadá

Abstract

Intensification of potato farming has contaminated groundwater with nitrate in many cases in Prince Edward Island, Canada, which raises concerns for drinking water quality and associated ecosystem protection. Numerical models were developed to simulate nitrate-N transport in groundwater and enhance understanding of the impacts of farming on water quality in the Wilmot River watershed. Nitrate is assumed non-reactive based on δ15N and δ18O in nitrate and geochemical information. The source functions were reconstructed from tile drain measurements, N budget and historical land-use information. The transport model was calibrated to long-term nitrate-N observations in the Wilmot River and verified against nitrate-N measurements in two rivers from watersheds with similar physical conditions. Simulations show groundwater flow is stratified and vertical flux decreases exponentially with depth. While it would take several years to reduce the nitrate-N in the shallow portion of the aquifer, it would take several decades or even longer to restore water quality in the deeper portions of the aquifer. Elevated nitrate-N concentrations in base flow are positively correlated with potato cropping intensity and significant reductions in nitrate-N loading are required if the nitrate level of surface water is to recover to the standard in the Canadian Water Quality Guidelines.

Résumé

L’intensification de la culture de la pomme de terre a entraîné une contamination en nitrates des eaux souterraines dans plusieurs secteurs de l’île Edouard au Canada qui présentent maintenant des problèmes de qualité des eaux potables et de protection des écosystèmes associés. Des modèles numériques ont été développés pour simuler le transport de l’azote dans les aquifères et améliorer la compréhension de l’impact de la culture sur la qualité des eaux souterraines dans le bassin de la rivière Wilmot. D’après les données de δ15N et δ18O des nitrates et l’information géochimique, les nitrates sont considérés comme non-réactifs. Les fonctions d’origine sont construites à partir de mesures effectuées sur les drains agricoles, le bilan d’azote et l’historique des informations sur l’utilisation des terres. Le modèle de transport est calibré pour de longues chroniques de mesures d’azote de la rivière Wilmot et vérifié avec les mesures d’azote de deux rivières de bassins versants présentant des caractéristiques physiques similaires. Les simulations montrent que l’écoulement souterrain est stratifié avec des flux verticaux décroissant exponentiellement avec la profondeur. Alors que la diminution des concentrations en nitrates prendra plusieurs années dans les horizons superficiels de l’aquifère, il prendra plusieurs dizaines d’années ou plus pour restaurer la qualité des eaux souterraines profondes. Le flux de base présente des concentrations en nitrates élevées corrélées positivement avec l’intensité de la culture de la pomme de terre. Une réduction importante des apports azotés est nécessaire pour que les eaux de surface retrouvent des niveaux respectant les standards de qualité pour les eaux au Canada.

Resumen

La intensificación del cultivo de la papa ha contaminado las aguas subterráneas con nitrato en muchos casos en Isla Prince Edward, Canadá, lo cual ha suscitado preocupaciones con respecto a la calidad del agua potable y de la protección de los ecosistemas asociados. Se desarrollaron modelos numéricos para simular el transporte del nitrato-N en aguas subterráneas y para mejorar la comprensión de los cultivos de la agricultura sobre calidad del agua en la cuenca del río Wilmot. Se supone que el Nitrato es no-reactivo sobre la base de δ15N y δ18O en nitrato e información geoquímica. Las funciones fuentes fueron reconstruidas a partir de mediciones en canales de drenaje, balance de N e información de uso histórico de la tierra. El modelo de transporte fue calibrado a largo plazo con observaciones de nitrato-N en el río Wilmot y verificado con medidas de nitrato-N en dos ríos de cuencas hidrográficas con condiciones físicas similares. Las simulaciones mostraron que el flujo subterráneo es estratificado y el flujo vertical disminuye exponencialmente con la profundidad. Mientras que tomaría varios años reducir el nitrato-N en la parte somera del acuífero, llevaría varias décadas o incluso más tiempo para restaurar la calidad del agua en las porciones profundas del acuífero. Las elevadas concentraciones de nitrato-N en el flujo de base están correlacionadas positivamente con la intensidad de las cosechas de la papa y se requieren reducciones significativas en la carga de nitratos- N para recuperar el nivel de nitrato del agua superficial de acuerdo a las normas de calidad del agua canadiense.

摘要

马铃薯种植业的扩张导致加拿大爱德华王子岛发生多例地下水硝酸盐污染, 引起了对饮用水水质和相关生态系统保护的关注. 本文建立了地下水中硝态氮运移的数值模型, 从而加深了耕作对威尔莫特河盆地水质影响的理解. 基于硝酸根δ15N 和 δ18O 数据及地球化学信息, 认为硝酸盐不具有反应性. 基于瓦管排水测量、N均衡和土地利用的历史等信息重建了污染输入函数. 应用威尔莫特河硝态氮的长期观测数据对运移模型进行了标定, 并通过物理条件相似流域的两条河流的硝态氮测量数据对模型进行了验证. 模拟显示地下水流是分层的, 且垂向通量随深度呈指数衰减. 降低浅部含水层中的硝态氮需要几年的时间, 而恢复深部含水层的水质则需数十年或更长的时间. 基流中硝态氮浓度的升高与马铃薯种植密度正相关, 若要地表水中的硝酸盐含量恢复到加拿大水质规范中的标准, 必须显著减小硝态氮载荷.

Resumo

A intensificação da cultura da batata em Prince Edward Island, Canadá, em muitos casos contaminou a água subterrânea por nitrato, o que levanta questões sobre a qualidade da água para consumo humano e sobre a protecção dos ecossistemas associados. Foram desenvolvidos modelos numéricos para simular o transporte de nitrato-N na água subterrânea e aumentar o conhecimento sobre os impactos da agricultura na qualidade da água na bacia hidrográfica do rio Wilmot. Assumiu-se que o nitrato é não-reactivo com base nos valores de δ15N e δ18O do nitrato e na informação geoquímica. As funções de entrada foram reconstruídas a partir de medições em drenos agrícolas, balanços do azoto e informação histórica sobre o uso do solo. O modelo de transporte foi calibrado com séries temporais de nitrato-N no rio Wilmot e verificado contra medições de nitrato-N em dois rios de bacias hidrográficas com características físicas similares. As simulações demonstram que o escoamento subterrâneo é estratificado e que o fluxo vertical diminui exponencialmente com a profundidade. Enquanto seriam necessários vários anos para reduzir a concentração de nitrato-N nas zonas mais superficiais do aquífero, seriam precisas várias décadas, ou até mais tempo, para restaurar a qualidade da água nas zonas mais profundas do aquífero. As concentrações elevadas de nitrato-N no escoamento de base correlacionam-se positivamente com a intensidade do cultivo da batata e verifica-se a necessidade de reduções significativas na fertilização com nitrato-N para que os níveis de nitrato na água superficial recuperem para os teores admitidos nos Canadian Water Quality Guidelines.

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Acknowledgements

This research was mainly funded by the Prince Edward Island Department of Environment, Energy and Forestry (PEIEEF), and partially by the Geological Survey of Canada, Environment Canada and Agriculture and Agri-Food Canada. The authors are thankful for assistance provided by the staff of the Water Management Division with PEIEEF. The authors would like to express their thanks to B. Potter for his GIS assistance, and M. L. McCourt, B. Thompson and Dr. R. Coffin for land-use information, and Drs. John Macleod and Bernie Zebarth for information on the estimation of nitrate-N leaching. Drs. K. MacQuarrie and A. Rivera are thanked for their comments on the preliminary results. Dr. D. van Stempvoort is thanked for his comments on the manuscript, which improves the text. We are grateful to the two reviewers and associate editor for helpful suggestions.

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Correspondence to Yefang Jiang.

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Jiang, Y., Somers, G. Modeling effects of nitrate from non-point sources on groundwater quality in an agricultural watershed in Prince Edward Island, Canada. Hydrogeol J 17, 707–724 (2009). https://doi.org/10.1007/s10040-008-0390-2

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Keywords

  • Non-point sources
  • Nitrate
  • Solute transport
  • Numerical modeling
  • Canada