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Hydrogeology Journal

, Volume 20, Issue 1, pp 45–60 | Cite as

Geophysical and geochemical characterization of the groundwater system and the role of Chatham Fault in groundwater movement at the Coles Hill uranium deposit, Virginia, USA

  • John P. Gannon
  • Thomas J. Burbey
  • Robert J. Bodnar
  • Joseph Aylor
Paper

Abstract

The largest undeveloped uranium deposit in the United States, at Coles Hill, is located in the Piedmont region of Pittsylvania County, south-central Virginia, and is hosted in crystalline rocks that are adjacent to and immediately west of Chatham Fault, which separates these crystalline rocks from the metasedimentary rocks of the Danville Triassic Basin (in the east). Groundwater at the site flows through a complex network of interconnected fractures controlled by the geology and structural setting. The role of Chatham Fault in near-surface (<≈200 m) groundwater flow is examined using electrical resistivity profiling, borehole logging, a pumping test, groundwater age dating and water chemistry to determine if the fault represents a permeability barrier or conduit for groundwater flow. The volumetric flow per unit width flowing eastward across the fault is estimated at 0.069–0.17 m2/day. Geochemical data indicate that groundwater in the granitic crystalline rocks represents a mixture of modern and old water, while the Triassic basin contains a possible deeper and older source of water. In regions with shallow water tables, mine dewatering during operation presents significant mining costs. The study’s results yield important information concerning the effect that Chatham Fault would have on groundwater flow during Coles Hill mining operations.

Keywords

Fractured rocks Tracer tests Electrical resistivity Aquifer testing USA 

Caractérisation géophysique et géochimique du système hydrologique et rôle de la Faille Chatham dans l’écoulement de nappe, gisement d’uranium de Coles Hill, Virginie, USA

Résumé

Le plus grand gisement d’uranium non exploité aux Etats unis, à Coles Hill, est localisé dans la région de Piémont de Pittsylvania County, centre Sud de Virginie, et encaissé dans des roches cristallines immédiatement adjacentes à l’Ouest de la Faille Chatham, qui sépare ces roches cristallines des roches sédimentaires du bassin triasique de Danville (à l’Est). Sur le site, la nappe s’écoule à travers un réseau complexe de fractures interconnectées contrôlées par la géologie structurale. Le rôle de la faille Chatham dans l’écoulement de sub surface (<≈200 m) est examiné en utilisant profil de résistivité électrique, diagraphie, un test de pompage, datation de l’eau de nappe et chimie de l’eau, pour déterminer si la faille représente une barrière hydraulique ou un conduit pour l’écoulement souterrain. Le débit par unité de surface s’écoulant en direction de l’Est d’un côté à l’autre de la faille est estimé à 0.069–0.17 m2/jour. Des données géochimiques indiquent que l’eau de nappe dans la roche granitique représente un mélange d’eau récente et ancienne, tandis que le bassin triasique contient une eau pouvant être plus profonde et plus ancienne. Dans les régions à surface de nappe libre peu profonde, l’exhaure pendant l’extraction représente des coûts d’exploitation considérables. Les résultats de l’étude fournissent des informations importantes concernant l’incidence que la Faille Chatham aurait sur le flux souterrain pendant l’exploitation minière de Coles Hill.

Caracterización geofísica y geoquímica del sistema de agua subterránea y rol de la Falla de Chatham en el movimiento del agua subterránea en el depósito de uranio de Coles Hill, Virginia, EEUU

Resumen

El mayor depósito de uranio sin explotar en los Estados Unidos, en el Coles Hill, está situado en la región del piedemonte del Condado de Pittsylvania, centro sur de Virginia, y está alojado en rocas cristalinas localizadas inmediatamente al oeste y adyacentes a la falla de Chatham, que separa estas rocas cristalinas de las rocas metasedimentarias de la cuenca Triásica de Danville (en el este). El agua subterránea en el sitio fluye a través de una compleja red de fracturas interconectadas controladas por la configuración geológica y estructural. Se examina el rol de la falla de Chatham en el flujo subterráneo cercano a la superficie (<≈200 m) usando perfilajes de resistividad eléctrica, registros de perforaciones, un ensayo de bombeo, datación de la edad del agua subterránea y la química del agua para determinar si la falla representa una barrera permeable o un conducto para el flujo del agua subterránea. El flujo volumétrico por unidad de ancho que fluye hacia el este a través de la falla está estimado en 0.069–0.17 m2/día. Los datos geoquímicos indican que el agua subterránea en las rocas graníticas cristalinas representa una mezcla de agua moderna y antigua, mientras que la cuenca Triásica contiene una posible fuente de agua más profunda y más antigua. En las regiones con el nivel freático somero, el drenaje de la mina durante las operaciones presenta costos mineros significativos. Los resultados del estudio brindan información importante concerniente al efecto que la Falla Chatham tendría en el flujo de agua subterránea durante las operaciones mineras en Coles Hill.

地下水系统的地球物理和地球化学特征以及Chatham断裂对地下水运动的影响---以美国弗吉尼亚州Coles山铀矿为例

摘要

美国Coles山的最大的未开采铀矿位于弗吉尼亚州中南部Pittsylvania市的山前地区,发育于结晶岩内,且邻近Chatham断裂且位于其西侧,该断裂将Danville三叠系盆地(东部)的变质沉积岩和结晶岩分开了。该区地下水流流经一个较复杂的受地质和构造控制的相互交汇的断裂系统。通过电阻剖面法、钻孔测井、抽水试验、地下水定年以及水化学研究了Chatham断裂对近地表(<≈200 m)地下水流的影响,用于确定断层是导水的还是阻水的。单位宽度穿过断裂向东的水流体积流量大约是0.069–0.17 m2/天。地球化学数据表明花岗岩结晶岩里的地下水是老水和现代水的混合体,而三叠纪盆地里含有一些可能更深和更老的水源。地下水位较浅的地区,施工过程中产生的矿坑排水占据了相当的采矿成本。本次研究成果给出了Coles山铀矿开采过程中Chatham断裂对地下水流影响的重要信息。

Caracterização geofísica e geoquímica do sistema hidrogeológico e o papel da Falha Chatham no escoamento da água subterrânea no jazigo de urânio Coles Hill, Virgínia, EUA

Resumo

O maior jazigo de urânio não explorado dos Estados Unidos, em Coles Hill, está localizado na região de Piedmont em Pittsylvania County, centro-sul da Virgínia, e jaz em rochas cristalinas que são adjacentes e imediatamente do lado oeste da Falha Chatham, a qual separa essas rochas cristalinas das rochas metassedimentares da bacia triássica de Danville (a leste). As águas subterrâneas fluem no local através de uma complexa rede de fracturas interconectadas, sob o controlo de elementos geológicos e estruturais. O papel da Falha Chatham no fluxo da água subterrânea superficial (<≈ 200 m) é avaliado através de perfis de resistividade eléctrica, diagrafias de furos, um ensaio de bombeamento, datação da idade das águas subterrâneas e hidroquímica de modo a determinar se a falha representa uma barreira ou um eixo de escoamento. O fluxo volumétrico por unidade de comprimento que flui para leste através da falha estima-se em 0.069–0.17 m2/dia. Dados geoquímicos indicam que as águas nas rochas graníticas cristalinas representam uma mistura de água moderna e antiga, enquanto que a bacia do Triássico contém uma água de origem possivelmente mais profunda e antiga. Em regiões com níveis freáticos mais superficiais, a drenagem durante a mineração reveste-se de custos significativos. Os resultados deste estudo forneceram informações importantes sobre o efeito que a Falha Chatham terá no fluxo de águas subterrâneas durante as operações de mineração de Coles Hill.

Notes

Acknowledgements

The authors would like to thank Virginia Uranium, Inc. for their financial support during the course of this investigation, and for providing unpublished data related to the Coles Hill deposit. Mr. Walter Coles, Sr, is thanked for permission to work on his property. Special thanks is extended to David L. Nelms and Amy E. Jensen of the US Geological Survey for collecting the chemistry, CFC, SF6, helium and tritium data used in this study. Special thanks are also extended to Bill Henika, who provided indispensable geologic consultation, as well as all those who helped in the many hours of fieldwork required for this study. Finally, we thank Bill Seaton and the two anonymous reviewers for their extremely valuable comments, which greatly enhanced the quality of this manuscript.

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Copyright information

© Springer-Verlag 2011

Authors and Affiliations

  • John P. Gannon
    • 1
  • Thomas J. Burbey
    • 1
  • Robert J. Bodnar
    • 1
  • Joseph Aylor
    • 2
  1. 1.GeosciencesVirginia TechBlacksburgUSA
  2. 2.Virginia Uranium, Inc.ChathamUSA

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