A physical hydrogeological and hydrochemical study of groundwater/surface water interaction in a granitic terrain in eastern Newfoundland

Schillereff, Herbert Scott (1992) A physical hydrogeological and hydrochemical study of groundwater/surface water interaction in a granitic terrain in eastern Newfoundland. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

This thesis presents a hydrogeological and hydrochemical investigation of groundwater and surface water interaction in a fractured granite terrain, within the Seal Cove River valley in eastern Newfoundland. The overall approach for this work was to first determine the structural geologic framework of the study are and surrounding region, then use this framework as a basis for interpreting the physical, hydrochemical and isotopic responses of groundwater and surface water to hydrologic stress, and for numerical simulations of groundwater flow in the study area. -- The Seal cove River valley study area (referred to as the SCRV) is 81.7 ha. in size and includes a 1.44 km reach of a branch of the Seal Cove River, with a 300 m x 50 m beaver pond midway along this reach. Lithologically homogeneous granitic rocks occur in glaciated outcrops on over 30% of a hillside above the study reach and sporadically on the valley floor. Overburden consists of thin glacial drift and peat deposits. Geophysical surveys indicate that the buried granite surface is essentially flat, dipping on average 0.5 degrees toward the north-northwest. -- The structural geologic framework was compiled from air photos, outcrop mapping and scanline fracture surveys across the pluton, and core logging in the SCRV. The Holyrood pluton intrudes multiply-deformed volcanic rocks of the Harbour Main Group and lies within the Conception Bay Anticlinorium, bounded to the east and west by the Topsail and Duffs Faults. Within the pluton, fracture lineaments and the regional mesoscopic fracture system both show preferred northerly, northeasterly and southeasterly strikes. Trace length and fracture frequency patterns show little variation in western parts of the pluton (including the SCRV area). There are no obvious variations in structural trends on a macroscopic scale in the vicinity of the SCRV. -- In the SCRV, three subvertical mesoscopic fracture sets are identified based on cluster analysis of scanline data. These set correspond with orientations of subsurface fractures and macroscopic fracture in the SCRV and with the regional fracture system. A fourth set comprises subhorizontal sheeting joints. Mesoscopic fracture trace lengths and spacings tend to vary smoothly across the SCRV without significant variation within or between subvertical fracture sets. -- The major boundary faults are interpreted to predate the Holyrood pluton, acting as feeders for the sill-like intrusion. Siluro-Devonian reactivation of these faults is interpreted to have generated the tectonic fracture system in the pluton (and the SCRV). A kinematic deformation model for the pluton is presented in with the regional tectonic fracture system formed as Riedel and secondary P-shear fractures during a single phase of progressive, left-lateral bulk simple shear. This model is the only detailed analysis of deformation in the pluton to date. -- The structural framework implies that 1) groundwater flow around the SCRV occurs primarily in a shallow or intermediate flow systems; 2) near-surface mesoscopic fractures and subvertical macroscopic fault and fracture zones are the likely principal conduits for shallow groundwater flow in the SCRV; 3) fracture-controlled hydraulic properties of the granite (e.g. hydraulic conductivity) can be assumed to be uniform around the SCRV (at scales of 10’s to 100’s of meters); and 4) the present SCRV fracture data set is inadequate for sophisticated determination of directional permeability in the granite (e.g. by permeability tensor formulation or fracture network analysis), requiring a simpler approach in formulation input parameters for numerical models of groundwater flow in the SCRV. Use of the more sophisticated methods mentioned above would require a more complete and unbiased characterization of fracture geometry than is available here, i.e. a statistical description of the geometry of subhorizontal fractures (Set 4), and unbiased estimates of fracture spacing for all sets. [Please see thesis for remainder of abstract.]

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