Abstract
Shallow aquifers can cross-contaminate deeper aquifers through penetration of an intervening aquitard, via sandy intervals in the aquitard, along well casings, across long well screens, or around aquitard pinchouts. Distinguishing among these potential pathways requires the use of evaluation tools that may support or eliminate certain pathways. These tools include groundwater gradient and aquitard penetration calculations, aquifer pumping test interpretation techniques, chemical concentration plots and statistical methods, hydraulic conductivity-based travel time calculations, pumping-concentration tests, methods for evaluating leaky wells, and methods for evaluating aquitard continuity. Based on analysis of several of these techniques at three sites experiencing aquifer cross-contamination, the authors conclude that calculation of flow rates for various pathways is the single most useful method to confirm or refute specific pathways. Evaluation of leaky wells and aquitard continuity generally must be coupled with other tools to adequately test specific pathways. While fingerprinting, statistical analysis or ratio analysis of contaminants from various sources and receptors was not completed for any of the evaluated sites, the authors believe that these techniques have strong potential for confirming or eliminating pathways. Future research in this area is suggested. Furthermore, the interpretation of pumping-concentration tests is not well developed and needs further assessment.
Résumé
Les aquifères de surface peuvent contaminer les aquifères plus profonds à travers un aquitard, un intersection sableuse dans l’aquitard, le long des forages et de leur crépines, ou de la terminaison biseautée des aquitards. Distinguer et sélectionner ces différents modes de contamination nécessite l’utilisation d’outils d’évaluation. Ces outils incluent le gradient d’écoulement des eaux souterraines et le calcul de pénétration des aquitards, les techniques d’interprétation des essais de pompage, les méthodes d’interprétation chimiques et de traitement statistique, les calculs des temps de transferts sur base de la conductivité, les tests de pompage couplé aux analyses de concentration, les méthodes pour évaluer l’infiltration directe alimentant les forages, et les méthodes permettant d’évaluer la continuité des aquitards. Basé sur l’analyse de plusieurs de ces techniques sur trois sites expérimentales présentant des contaminations transversale, les auteurs concluent que le calcul des rapports d’écoulement est la méthode la plus simple pour confirmer ou refuser les voies spécifiques. Alors que le traçage, les analyses statistiques et les analyses de la contamination provenant de différentes sources et récepteurs n’était pas complètes pour la plus part des différents sites évalués n’étaient pas compléments certains, les auteurs croient que ces techniques ont un potentiel fort pour confirmer ou éliminer ces techniques. Des recherches futures dans cette zone est suggérée. Par ailleurs, l’interprétation des tests de pompage couplés à l’analyse des concentrations n’est pas développée et nécessite de meilleurs estimations.
Resumen
Acuíferos someros pueden ocasionar contaminación transversal de acuíferos profundos mediante la penetración de un acuitardo intermedio, a través de intervalos arenosos en el acuitardo, a lo largo del revestimiento de pozos, o en las inmediaciones de lentes de acuitardos. Para distinguir entre estas trayectorias potenciales se requiere el uso de herramientas de evaluación que pueden apoyar o eliminar ciertas trayectorias. Estas herramientas incluyen cálculos de gradientes de agua subterránea y penetración de acuitardos, técnicas de interpretación de pruebas de bombeo de acuíferos, diagramas de concentración química y métodos estadísticos, estimaciones de conductividad hidráulica en base al tiempo de viaje, pruebas de concentración de bombeo, métodos para evaluar pozos con fugas, y métodos para evaluar la continuidad de acuitardos. Basándose en el análisis de varias de estas técnicas en tres sitios que experimentan contaminación transversal de acuíferos, los autores concluyen que la estimación de ritmos de flujo para varias trayectorias es el único método más útil para confirmar o rechazar trayectorias de flujo específicas. La evaluación de pozos con fugas y continuidad de acuitardos generalmente debe estar acompañada con otras herramientas para probar adecuadamente trayectorias específicas. Aunque no se completó para ninguno de los sitios evaluados técnicas de huellas, análisis estadístico o análisis de relaciones de contaminantes de varias fuentes y receptores, los autores creen que estas técnicas tienen fuerte potencial en la confirmación o eliminación de trayectorias. Se sugiere investigación futura en esta área. Además, la interpretación de pruebas de concentración de bombeo no se ha desarrollado bien y necesita evaluación posterior.
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Acknowledgments
Brian Aubry (currently of Geologica, Inc.) and Allison Remple developed the conceptual groundwater flow model at the Ft. Ord site. Julio Guerra of the City of Merced generously shared technical information on their regional PCE problem.
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Santi, P.M., McCray, J.E. & Martens, J.L. Investigating cross-contamination of aquifers. Hydrogeol J 14, 51–68 (2006). https://doi.org/10.1007/s10040-004-0403-8
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DOI: https://doi.org/10.1007/s10040-004-0403-8