Hydrogeologic aspects of dewatering at Welland

Realignment of the Welland Canal between Port Robinson and Port Colborne and the building of underpass structures necessitated both temporary and permanent depressurizing of an artesian aquifer. The aquifer is a thin zone of fractured dolomite found immediately beneath approximately 60 to 100 ft (18 to 30 m) of poorly-permeable glacial till and lacustrine sediments. Since most of the private wells in the Welland area obtain water from the bedrock aquifer, extensive well interference problems were anticipated. Problems in connection with the dewatering and depressurizing included design of deep-well pumping systems for excavation work, prediction and monitoring of drawdown effects, and the determination of responsibility for interference in specific areas since several dewatering systems with overlapping cones-of-depression would be operating simultaneously. To overcome a problem of insufficient hydrologic data prior to the commencement of dewatering, a contour map of the original piezometric surface was constructed from drillers' records. This map showed a normal groundwater flow system where the movement of groundwater was from local upland recharge areas to local discharge areas along the Welland River and the existing canal. A contour map of the drawdown was drawn on the assumption that deviations from the reconstructed original piezometric surface were caused by dewatering. Analysis of the cone-of-depression along with analyses of pre-engineering pumping tests provided values for aquifer coefficients required for solution of the unsteady flow equations. Because the coefficients used were average values for a relatively small area extrapolated over a broad area, the predicted drawdown showed considerable variation in reliability. The work did provide, however, a check on the numerical method (Frind 1970) which takes into account local variations.