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Hydrogeology Journal

, Volume 24, Issue 2, pp 439–457 | Cite as

The puzzle of high heads beneath the West Cumbrian coast, UK: a possible solution

  • John H. BlackEmail author
  • John A. Barker
Paper

Abstract

A region of high heads within the Borrowdale Volcanic Group (BVG; a fractured crystalline rock) beneath the coastal plain of West Cumbria, England (UK), is identified as a possible relic left over by the Late Devensian ice sheet. It was found during investigations in the 1990s. Contemporary modelling work failed to produce a satisfactory explanation of the high heads compatible with the ‘cold recharge’ isotopic signature of the groundwater. This study has reassessed the original hydraulic testing results. By plotting density-adjusted heads versus their depth below the water table in the immediate vicinity of the borehole in which they were measured, a depth profile resembling a ‘wave’ was revealed with a peak value located at 1,100 m depth. The possibility that this wave represents relic heads from the last major ice sheet has been assessed using one-dimensional mathematical analysis based on a poroelastic approach. It is found that a wet-based ice sheet above the West Cumbrian coast was probably thick enough and sufficiently long-lasting to leave such relic heads providing that the hydraulic diffusivity of the BVG is in the order of 10−6 m s−1. Initial assessment 20 years ago of the long-interval slug tests suggested that such low values are not likely. More recent interpretation argues for such low values of hydraulic diffusivity. It is concluded that ice sheet recharge is the most likely cause of the raised heads, that the BVG contains significant patches of very low conductivity rock, and that long-interval single-hole tests should be avoided in fractured crystalline rock.

Keywords

Crystalline rocks Repository host rock Paleohydrology Conceptual models UK 

Le casse-tête des hautes charges hydrauliques sous la côte ouest du Comté de Cumbria, Royaume-Uni: une solution possible

Résumé

Une région à charges hydrauliques élevées au sein du groupe volcanique de Borrowdale (BVG; une roche cristalline fracturée) sous la plaine côtière de l’ouest du comté de Cumbria, en Angleterre (Royaume-Uni) est identifiée comme une relique possible laissé par le couvert de glace du Dévensien tardif. Il a été constaté cours d’investigations dans les années 1990. Le travail contemporain de modélisation a échoué à produire une explication satisfaisante des charges hydrauliques compatibles avec la signature isotopique de la ‘recharge froide’ des eaux souterraines. Cette étude a réévalué les résultats des tests hydrauliques. En traçant les charges hydrauliques corrigées de l’effet de densité en fonction de la profondeur sous l’eau dans le voisinage immédiat du forage dans lequel elles ont été mesurées, un profil de profondeur qui ressemble à une « vague » a été révélé avec une valeur d’un pic localisé à une profondeur de 1,100 m. La possibilité que cette vague représente la charge hydraulique, relique de la dernière calotte glaciaire majeure a été évaluée en utilisant l’analyse mathématique unidimensionnelle basée sur une approche poroélastique. Il a été trouvé qu’une couche de glace à base humide recouvrant la côté occidentale du Comté de Cumbria était probablement assez épaisse et d’une durée suffisante pour laisser ces reliques de charges hydrauliques indiquant que la diffusivité hydraulique de la BVG est de l’ordre de 10−6 m s−1. L’évaluation initiale datant d’il y a 20 ans des slug tests à long intervalle suggérait que de telles faibles valeurs n’étaient pas plausibles. L’interprétation plus récente plaide en faveur de valeurs faibles de diffusivité hydraulique. Il en est conclu que la recharge de la calotte glaciaire est la cause la plus probable des charges hydrauliques élevées, du fait que le BVG contienne des éléments significatifs de roche de très faible conductivité hydraulique, et que des tests en forage de long intervalle devraient être évités dans une roche cristalline fracturée.

El enigma de altas cargas hidráulicas debajo de la costa occidental de Cumbria, Reino Unido: una posible solución

Resumen

Se identifica una región de las altas cargas hidráulicas dentro del Grupo Volcánico Borrowdale (BVG; una roca cristalina fracturada) debajo de la llanura costera de West Cumbria, Inglaterra (Reino Unido), como un posible relicto dejado por una capa de hielo del Devensiano Tardío. Se encontró durante las investigaciones en la década de 1990. El trabajo de modelado contemporáneo no logró producir una explicación satisfactoria de las altas cargas hidráulicas compatibles con la firma isotópica de la "recarga fría" del agua subterránea. Este estudio ha reevaluado los resultados de las pruebas hidráulicas originales. Al representar las cargas hidráulicas ajustadas por densidad en función de la profundidad por debajo del agua en las inmediaciones del pozo de sondeo en el que ellos fueron medidos, se reveló un perfil de profundidad que se asemeja a una ‘onda’ con un valor del pico situado en 1,100 m de profundidad. La posibilidad de que esta onda represente cargas hidráulicas relictos a partir de la última capa de hielo importante se ha evaluada utilizando análisis matemático unidimensional basado en un enfoque poroelástico. Se encontró que una capa de hielo de base húmeda por encima de la costa oeste de Cumbria fue probablemente lo suficientemente gruesa y duradera para dejar tales relictos de carga hidráulica si se supone que la difusividad hidráulica de la BVG es del orden de 10−6 m s−1. La evaluación inicial de hace 20 años con prolongados ensayos slug sugirió que tales valores bajos no eran probables. La interpretación más reciente presenta argumentos favorables para tales valores tan bajos de difusividad hidráulica. Se concluye que la recarga de la capa de hielo es la causa más probable de las elevadas cargas hidráulicas, que el BVG contiene parches importantes de roca de muy baja conductividad, y que las pruebas prolongadas en pozos simples se deben evitar en la roca cristalina fracturada.

英国坎伯兰西海岸之下的高水头之谜:一个合理的答案

摘要

英国西坎伯兰沿海平原之下Borrowdale火山组之下高水头区被认为可能是末次冰期冰盖的遗留物。20世纪90年代调查中发现,同时期的模拟工作不能得到高水头与地下水“寒冷补给”同位素印记相匹配的满意解释。这项研究再次评价了原始水力测试结果。通过绘制密度调整的水头对测量的钻孔旁边水下深度,揭示了类似“波浪”的深度剖面,峰值位于1100米的深度。基于两相介质方法采用一维数学分析评价了这个波浪代表最后主要冰盖遗迹水头的可能性。发现,西坎伯兰海岸之上基于潮湿的冰盖可能足够厚和有足够的持续时间,留下这样的遗迹水头,Borrowdale火山组水力扩散系数大约为10−6 m s−1。长间隔的微水试验20年前的初步评价表明,如此低的值是不可能的。最近的解译支持水力扩散系数如此的低值。结论就是,冰盖补给是水头增加最可能的原因,Borrowdale火山组包含着导水率非常低岩石的碎片,在断裂结晶岩中应该避免长间隔的单孔试验。

O enigma das altas cargas hidráulicas abaixo da costa Oeste da Cúmbria, Reino Unido: uma possível solução

Resumo

A região das altas cargas hidráulicas entre o Grupo Vulcânico Borrowdale (GVB; uma rocha cristalina fraturada) abaixo da planície costeira do Oeste da Cúmbria, Inglaterra (RU), é identificada como uma possível relíquia que sobrou da camada de gelo do Devoniano superior. Isso foi descoberto durante as investigações na década de 1990. Estudos de modelagem contemporâneos falharam em produzir uma explicação satisfatória para as altas cargas hidráulicas que fossem compatíveis com a assinatura isotópica das águas subterrâneas da “recarga de gelo”. Este estudo reavaliou os resultados do teste hidráulico original. Através da representação gráfica da densidade-ajustada das cargas contra a sua profundidade abaixo da água nas mediações vizinhas dos poços no qual eles foram medidos, um perfil de profundidade que se assemelha a uma “onda” foi revelado com um valor de pico localizado a uma profundidade de 1,100 m. A possibilidade de que essa onda represente cargas reminiscentes da última principal camada de gelo, tem sido avaliada com o uso de uma analise matemática unidimensional baseada em uma abordagem poroelástica. Foi descoberto que uma camada de gelo de base úmida (wet-based) acima da costa Oeste da Cúmbria foi provavelmente, suficiente espessa e suficientemente persistente para deixar tais cargas hidráulicas reminiscentes contando que a difusividade hidráulica do GVB seja na ordem do 10−6 m s−1. Avaliações inicias de 20 anos atrás de testes de permeabilidade (slug tests) de longo intervalo sugerem que valores tão baixos não são comuns. Recentes interpretações defendem tais valores baixos de difusividade hidráulica. É concluído que a recarga da camada de gelo é a mais provável causa do aumento das cargas hidráulicas, que o GVB contem significantes trechos de uma rocha de baixa condutividade, e que o teste de furo único com longo intervalo (single-hole) deveriam ser evitados numa rocha cristalina fraturada.

Notes

Acknowledgements

The authors would like to thank the anonymous reviewer who directed our analysis in a more appropriate direction. We would like to acknowledge the expertise of the test team managed by Gerry O’Hara, Paul Yerby, Mark Brightman and Bob Jeffrey who produced all of the EPM results except borehole 3.

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Copyright information

© Springer-Verlag Berlin Heidelberg 2015

Authors and Affiliations

  1. 1.In Site Hydro LtdEast BridgfordUK
  2. 2.University of SouthamptonSouthamptonUK

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