ABSTRACT: The Annapolis Valley located in western Nova Scotia, Canada, represents the southern-southeastern extension of the early Mesozoic Fundy Rift Basin. Its northern ridge reaches 220 m above the valley floor and is formed by a Jurassic basalt cap that overlays fractured sedimentary rocks. Geophysical logging and pumping tests conducted in a well drilled into this basalt show that water is supplied by only a few high-angle fractures that preferentially strike parallel to the valley axis. In addition, elastic properties of the principal bedrock units as determined from full-waveform sonic logs reveal a contrast in Poisson?s ratios, õ, between the basalt and the underlying sandstone. Interpretation of these field results indicates that the hydrologic properties of the basalt may be directly influenced and enhanced by a lack of buttressing that produces extensional stresses perpendicular to the ridge. In the case of the Annapolis Valley, this gravitationally induced stress state is exacerbated by lateral spreading due to contrasts in õ between superposed formations and a regional compressional stress component that is aligned with the valley axis. A finite-element model of the valley-ridge configuration is developed that incorporates these stress conditions and that considers their potential consequences on the hydrologic characteristics of the basalt.
INTRODUCTION A comprehensive, multidisciplinary study has recently been undertaken to evaluate the hydrogeologic characteristics of the Annapolis- Cornwallis Valley in western Nova Scotia, Canada [1] (Figure 1). This northeast-southwest trending feature, heretofore referred to simply as Annapolis Valley, encompasses approximately 2100 km2 and is bounded by the North and South Mountains (Figure 2a,b). Groundwater is a major source of water supply for a variety of uses in this region, including domestic, agricultural, and industrial. Approximately 90 percent of residents rely on groundwater from private or municipal wells.
As part of this project, geophysical logs and other downhole measurements were obtained in 12 water wells scattered throughout the study area from Annapolis Royal (west) to Wolfville (east), but strategically located such that they penetrated the major geologic formations (Figure 2b). This paper focuses on field data obtained in one particular well drilled through the basalt cap that covers North Mountain and located just north of the town of Fig. 1. Location of study area in western Nova Scotia. Berwick (Figure 2b). At this site, results indicate that stresses associated with valley shape, configuration, and composition may combine to directly influence the hydrologic properties of the basalt aquifer and potentially affect groundwater availability among local users. Information derived from the analysis of geophysical logs is incorporated into a finite-element model to simulate the effects of stress and structure on fracture systems and groundwater flow.