Assessing groundwater flux from underlying fractured bedrock to the overburden aquifer system Fredericton, New Brunswick, Canada.

摘要

Fredericton, the capital city of New Brunswick, Canada, draws its water from wells in a semi-confined river valley alluvial aquifer which produce approximately 25,000 m3/day for a population of 50,535 (Statistics Canada, 2007). The water that is produced by the well field comes from either the Saint John River via riverbank induced infiltration, surface infiltration, the underlying fractured bedrock, or a combination of these sources. In the past, lower water supply demands from a smaller population in Fredericton were met from the aquifer system. As a result, the hydrogeology of the fractured sedimentary bedrock, in terms of contribution to the water supply was not addressed in depth. With the increased water supply demand from a growing population and the potential variation in recharge rates to affect groundwater quantity however, the need to understand and estimate the groundwater contribution from the bedrock aquifer has been recognized. A hydrogeological characterization, and 3D numerical model assessment of the Fredericton area, was completed to: 1) determine how the flux would vary under both natural flow and well field pumping conditions; and 2) determine how sensitivity to variations in recharge, would impact the quantity of the groundwater flux from the fractured bedrock to the overburden aquifer. The steady-state (natural flow) and transient (pumping) model simulations indicated the bedrock fluid flux in the well field area was approximately 1000 m3/d and was not greatly affected by changes to recharge or pumping rates. With changes in precipitation or temperature, a possible result of climate change, the resulting hydraulic head and fluid flux in the overburden was more sensitive than that of the bedrock, which represented a more stable fluid flux because of the lower fractured rock mass permeability.