The Role of the Tyrrell Sea Aquitard and Bioherms in the Hydrogeology of the James Bay Lowlands Under Dewatering Conditions

Abstract

The Hudson-James Bay Lowlands (HJBL) are composed of fens and bogs frequently intersected by rivers and punctured by bioherms (ancient reef mounds) rising from bedrock aquifer. The peatlands exist partly due to minimal vertical losses limited by the low hydraulic conductivity (K) of the Tyrrell Sea sediments and small vertical gradients between the surface and bedrock aquifers. The recent development of an open-pit mine; DeBeers Victor Diamond Mine within the HJBL requires dewatering of the regional limestone aquifer to a depth of 200 mbgs. Two major components in assessing the potential impacts of dewatering on the hydrogeological regime and the sensitive peatland ecosystem are: the characteristics and behaviour of the Tyrrell Sea aquitard, and bioherms as potential recharge enhancing features. The Victor Tyrrell Sea (VTS) deposits are clayey silt with low LL, low PI, and no smectite clay minerals. The clay fraction consists of quartz, illite, chlinochlore, and usually calcite. The deposits are normally consolidated with Cc values of 0.08-0.155 and void ratios of 0.52-0.77. The VTS deposits are grey with pockets of black graphite and frequent shells. The K rages from 6.6x10-9 to 4.7x10-8 m/s. GEO-SLOPE™ finite element modeling software was used to investigate the sensitivity of surface drainage and consolidation behaviour in regard to the range of measured VTS parameters. The behaviours were also compared to classic clay aquitard types such as Lake Agassiz and Lake Champlain. Unit thickness, K, and the K modifier function have the greatest impact on the rate and magnitude of consolidation, and vertical drainage of the surface. The K of the underlying till is extremely important because they can be a buffer between the dewatered bedrock aquifer and the VTS. The characteristics of the sediments and the flow regime surrounding three outcropping bioherms which increase vertical drainage are investigated and a conceptual model of flow is proposed. The sediments in profile around bioherms have different levels of stratification but always contain silt and sand units. The hydraulic conditions at eight nested piezometers and the stratigraphy as defined by a series of auger hole profiles was considered to develop a conceptual model. The flow regime presented is of radial flow towards the bioherms enhanced by high K sediments between the peat and bedrock becoming vertical flow when the high K sediments contact vertical channels in the rock.