Hydrogeological investigation of Quaternary and late Cretaceous bedrock aquifers, Comox Coalfield, Vancouver Island, British Columbia, Canada

Date

2009-04-30T21:22:15Z

Authors

Fisher, Gypsy C.

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Abstract

This study involved a regional hydrogeological assessment of the Comox Coalfield on Vancouver Island, British Columbia. Two site-specific geological and hydrogeological investigations were conducted. The first involved generating a 2.5 dimensional hydrostratigraphic model of part of the Quadra Sand Comox-Merville Aquifer using lithology information from 196 drilled domestic-use groundwater wells. Well logs were standardized with respect to lithologic and hydraulic characteristics. Contact surfaces were created for identifiable hydrostratigraphic units employing an iterative geostatistical interpolation process that incorporated contact points from well logs and interpreted points based on the regional hydrogeology. Modeled hydrostratigraphic surfaces were compared to logged contacts and to exposures at Willemar and Lazo bluffs at Comox. Six lithostratigraphic units were identified in the coastal exposures. Hydraulic conductivity values, estimated from grain size data using the Hazen method, for the lowermost 4 units were: 2.3 x 10-3 cm/s, 9.1 x 10-6 cm/s, 9.4 x 10-3 cm/s, and 4.7 x 10-6 cm/s, respectively. The hydrostratigraphic model was verified using statistical variance analysis, field reconnaissance data, and the identification of a separate surficial aquifer within the study area. The model identified all units mapped in the field and two units below sea level, inferred to be the Cowichan Head Formation. The Comox Bluff model successfully predicted, within 2 m vertically, subsurface hydrostratigraphic boundaries 80% of the time. The second component of the study included a hydrogeological investigation of stacked Quaternary and Late Cretaceous bedrock aquifers at Oyster River. This investigation incorporated drilling logs, borehole geophysics, aqueous geochemistry, pumping and recovery test data, and hydrostratigraphic interpretation of surficial exposures. The potential for hydraulic communication between the Late Cretaceous Nanaimo Group fractured sedimentary bedrock and the overlying unconsolidated Quaternary aquifers was examined. Two adjacent groundwater observation wells were drilled; one completed in bedrock (146.9 m) and one in the surficial sediments (7.3 m). The deeper well penetrated the Trent River and Comox Formations of the Nanaimo Group. A water-bearing fracture zone approximately 3 m wide was encountered at 135 metres below ground surface, coincident with the Comox Y and Y Lower coal seams. Dissolved methane gas was detected in the bedrock aquifer, with an initial concentration of 2,123 mg/L. Schoeller diagrams reveal that the gas in bedrock is coal related. A pumping and recovery test in the deep well suggests that there is unlikely any hydraulic communication between the bedrock and surficial aquifers encountered at Oyster River. This assessment is based on infrequent water level measurements in the shallow well, which did not consistently draw down during pumping of the deeper well. However, the pumping rate was not sustainable for this test and it could not be held constant. Fracture transmissivity and hydraulic conductivity for the bedrock aquifer were estimated using the Theis Recovery method at 7.06 x 10-7 m2/s and 2.29 x 10-7 m/s, respectively. The hydrogeological research conducted at Comox and Oyster River highlights the effectiveness of a multidisciplinary approach for subsurface investigations. This study contributes site level data upon which regional inferences can be built for the Comox Coalfield.

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Keywords

hydrogeology, Quaternary geology, Late Cretaceous Nanaimo Group, fractured flow, coalbed gas, geostatistical modeling, Quadra Sand

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