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Hydrogeology Journal

, Volume 25, Issue 4, pp 1003–1016 | Cite as

Transient bacterial contamination of the dual-porosity aquifer at Walkerton, Ontario, Canada

  • Stephen R. H. WorthingtonEmail author
  • C. Christopher Smart
Paper

Abstract

Contamination of the Paleozoic carbonate aquifer at Walkerton (Ontario, Canada) by pathogenic bacteria following heavy rain in May 2000 resulted in 2,300 illnesses and seven deaths. Subsequent tracer testing showed that there was rapid groundwater flow in the aquifer, and also rapid exchange between the aquifer and the ground surface. Electrical conductivity (EC) profiling during a 3-day pumping test showed that most flow was through bedding-plane fractures spaced about 10 m apart, that there were substantial contrasts in EC in the major fracture flows, and that there were rapid changes over time. Total coliform sampling revealed transient groundwater contamination, particularly after heavy rain and lasting up to a few days. These characteristics can be understood in terms of the dual-porosity nature of the aquifer. Most of the storage is in the matrix, but this can be considered to be static in the short term. Almost all transport is through the fracture network, which has rapid groundwater flow (∼100 m/day) and rapid transmission of pressure pulses due to the high hydraulic diffusivity. Rapid recharge can occur through thin and/or fractured overburden and at spring sites where flow is reversed by pumping during episodes of surface flooding. These characteristics facilitated the ingress of surface-derived bacteria into the aquifer, and their rapid transport within the aquifer to pumping wells. Bacterial presence is common in carbonate aquifers, and this can be explained by the well-connected, large-aperture fracture networks in these dual-porosity aquifers, even though many, such as at Walkerton, lack karst landforms.

Keywords

Dual porosity Canada Microbial processes Karst Health 

Contamination bactérienne transitoire d’un aquifère à double-porosité à Walkerton, Ontario, Canada

Résumé

La contamination de l’aquifère carbonaté paléozoïque à Walkerton (Ontario, Canada) par des bactéries pathogènes suite à un épisode pluvieux intense en mai 2000 a entrainé 2300 malades et sept décès. Des essais de traçage ultérieurs ont montré qu’il y avait un écoulement rapide des eaux souterraines au sein de l’aquifère, et également un transfert rapide entre l’aquifère et la surface du sol. Des profils de conductivité électrique lors d’un pompage d’essai de 3 jours ont montré que l’essentiel de l’écoulement se faisait à travers des fractures planes d’inter-bancs espacées d’environ 10 m les unes des autres, qu’il existait des contrastes substantiels de conductivité électrique au droit des principales fractures où se produisent les écoulements, et que des changements rapides apparaissaient au cours du temps. L’analyse des coliformes totaux a révélé une contamination transitoire des eaux souterraines, en particulier après des pluies intenses et durant les quelques jours qui suivent. Ces caractéristiques peuvent être appréhendées du fait de la double-porosité de l’aquifère. La majeure partie du stockage prend place dans la matrice, mais elle peut être considérée comme étant non mobile à court terme. La quasi-totalité du transport se fait à travers le réseau de fractures, qui présente un écoulement rapide des eaux souterraines (∼ 100 m/j) et une transmission des impulsions de pression du fait de la diffusivité hydraulique élevée. La recharge rapide peut se produire à travers une couverture peu épaisse et/ou fracturée et au niveau des sites des sources où l’écoulement est inversé à cause des pompages lors des épisodes d’inondation. Ces caractéristiques ont facilité la pénétration des bactéries issues de la surface dans l’aquifère, et leur transfert rapide au sein de l’aquifère vers les puits de pompage. La présence de bactéries est fréquente dans les aquifères carbonatés, et cela peut s’expliquer par les réseaux de fractures à grande ouverture et bien connectés dans ces aquifères à double porosité, même si nombre d’entre eux, comme à Walkerton, ne présentent pas de figures karstiques en surface.

Contaminación bacteriana transitoria de un acuífero de doble porosidad en Walkerton, Ontario, Canadá

Resumen

La contaminación del acuífero carbonático del Paleozoico en Walkerton (Ontario, Canadá) por bacterias patógenas después de fuertes lluvias en mayo de 2000 resultó en 2300 enfermedades y siete muertes. Las pruebas de trazabilidad posteriores mostraron que había un flujo rápido de agua subterránea en el acuífero y también un intercambio rápido entre el acuífero y la superficie del suelo. La evaluación del perfil de conductividad eléctrica (EC) durante un ensayo de bombeo de tres días mostró que la mayor parte del flujo se realizó a través de las fracturas de los planos de estratificación espaciadas a una distancia de aproximadamente 10 m entre sí, que tuvieron contrastes sustanciales en la EC en los flujos de las fracturas mayores, y cambios rápidos en el tiempo. El muestreo de coliformes totales reveló la contaminación transitoria del agua subterránea, especialmente después de lluvias intensas y con una duración de hasta unos pocos días. Estas características se pueden entender en términos de la naturaleza de la doble porosidad del acuífero. La mayor parte del almacenamiento está en la matriz, pero esto puede considerarse estático en el corto plazo. Casi todo el transporte se realiza a través de la red de fracturas, que tiene un flujo rápido de agua subterránea (∼ 100 m/día) y una transmisión rápida de los impulsos de presión debido a la alta difusividad hidráulica. La recarga rápida puede ocurrir a través de la cubierta delgada y/o fracturada y en sitios de manantiales donde se invierte el flujo por el bombeo durante los episodios de inundación superficial. Estas características facilitaron el ingreso de bacterias derivadas de la superficie en el acuífero, y su transporte rápido dentro del acuífero a los pozos de bombeo. La presencia de bacterias es común en los acuíferos carbonatados, lo que puede explicarse por las redes de fractura de gran abertura bien conectadas en estos acuíferos de doble porosidad, aunque muchos, como en Walkerton, carecen de formas de relieve kárstico.

加拿大安大略省沃克顿地区双重介质含水层瞬态细菌污染

摘要

2000年5月一场暴雨之后致病菌造成的(加拿大安大略省)沃克顿地区古生代碳酸盐岩含水层的污染致使2300人患病,7 人死亡。后来的示踪实验显示,含水层地下水流非常快,含水层的水和地表水交换也非常迅速。三天的抽水实验期间电导率显示,大部分水流通过间隔为10米的层理面断裂,主要断裂水流中的电导率有和大差别,并随着时间的过去变化迅速。总大肠杆菌采样揭示,存在着瞬态地下水污染,特别是在暴雨之后,并可持续几天。依据含水层的双重介质性质,可以了解这些特征。大部分储存的水在基质里,但被认为是短时间内是静止的。几乎所有的传输通过断裂网络,由于水力扩散系数很高,断裂网络中地下水流速度很快(∼ 100 m/day),压力脉冲传播也很快。通过薄的和/或者断裂的表层以及在地表洪水期间抽水导致水流反转的泉点可出现快速补给。这些特征促进了源自地表的细菌进入含水层,及含水层内快速运移到抽水井。碳酸盐岩含水层中存在细菌很常见,这可以通过这些双重介质含水层中联通良好、大孔断裂网络得到解释,尽管许多含水层,诸如沃克顿地区的含水层,缺乏岩溶地貌。

Contaminação bacteriana transiente do aquífero com porosidade dual em Wakerton, Ontário, Canadá

Resumo

A contaminação do aquífero carbonatico Paleozóico em Walkerton (Ontário, Canadá) por bactérias patogênicas após chuvas pesadas em Maio de 2000 resultou em 2300 doenças/enfermidades e sete mortes. Testes de traçadores subsequentes mostraram que houve um rápido fluxo das águas subterrâneas no aquífero e também uma rápida troca entre a superfície do solo e o aquífero. O perfilamento de condutividade elétrica (CE) durante um teste de bombeamento de três dias mostrou que a maior parte do fluxo ocorreu através de plano de fraturas espaçadas a cerca de 10 m, que houveram substanciais contrastes na CE nos principais fluxos de fraturas, e que houveram rápidas mudanças ao longo do tempo. Amostras de coliformes totais revelaram uma contaminação transiente das águas subterrâneas, particularmente depois de chuvas pesadas e permaneceu por alguns dias. Estas características podem ser entendidas em termos da natureza de porosidade dual do aquífero. A maior parte do armazenamento está na matriz, porém isto pode ser considerado estático a curto prazo. Grande parte do transporte é através da rede de fraturas, que tem acelerado fluxo das águas subterrâneas (∼ 100 m/dia) e rápida transmissão de pulsos de pressão devido à alta difusão hidráulica. A rápida recarga pode ocorrer através da sobrecarga fina e /ou fraturada e nos locais de fonte/nascente onde o fluxo é revertido devido ao bombeamento durante episódios de inundações superficiais. Estas características facilitaram o ingresso de bactérias derivadas da superfície no aquífero, e seu rápido transporte dentro dos poços de bombeamento. A presença de bactérias é comum em aquíferos carbonaticos, e este fato pode ser explicado pela forte conexão e grande abertura nas redes de fraturas nestes aquíferos de porosidade dual, mesmo que muitos, como em Walkerton, carecem de formações cársticas.

Notes

Acknowledgments

Thanks go to Brad Simpson for help with fieldwork. This work was partially funded by Concerned Walkerton Citizens, by the Walkerton Inquiry, and by Western University.

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Copyright information

© Springer-Verlag Berlin Heidelberg 2017

Authors and Affiliations

  • Stephen R. H. Worthington
    • 1
    Email author
  • C. Christopher Smart
    • 2
  1. 1.Worthington GroundwaterDundasCanada
  2. 2.Department of GeographyWestern UniversityLondonCanada

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