Modelling the Oro Moraine multi-aquifer system: Role of geology, numerical model, parameter estimation and uncertainty.

摘要

A steady-state groundwater model of the Oro Moraine aquifer system, near Barrie (Ontario), is developed. The model is used to identify controls on baseflow to the Minesing Swamp, a 7000 hectares wetland of international significance. Lithologic descriptions contained in the extensive database of well records are used to develop a hydrostratigraphic model of the predominantly glacial aquifer system and to distribute hydraulic conductivity using kriging. A pseudo-unsaturated model is developed and coupled with a recharge spreading layer to simulate the complex recharge mechanism for the aquifer system and to provide a simulation of both surface and subsurface fluxes to streams. A hypothetical flow problem illustrates that the model is able accurately predict recharge fluxes for heterogeneous aquifers. The Oro Moraine model is calibrated to water level data and stream gauge measurements using an inverse algorithm that is developed based on existing theory. The model is able to predict observed runoff and baseflow quantities to streams that are represented by Dirichlet boundaries. The adjoint method is used to identify recharge areas for streams and wetlands. It is found that the nearby Snow Valley upland is most important for groundwater discharge to the Minesing Swamp and that the Oro Moraine is the most critical recharge area for baseflow to the Matheson and Willow Creeks that flow into the swamp. Hypothetical urbanization impact calculations suggest that low to medium density residential development in the uplands will result in a modest decrease in baseflow to the swamp. This baseflow in turn only makes up a relatively small fraction of the total inflow to the swamp. However, groundwater provides a relatively steady inflow of water over larger portions of the wetland which may be important for nutrient cycling and for maintaining soil moisture conditions. Subtle changes in baseflow may therefore have a large unforeseen impact on the ecology of the swamp. The urbanization impact calculations further indicate that a loss of recharge in the uplands may dramatically impact baseflow to the headwaters of local streams. The model thus provides valuable insight into crucial characteristics of the aquifer system. An application of stochastic methods to quantify model uncertainty is hampered by the complexity of the aquifer system.