Introduction

In urban centres, the primary source of freshwater supply comes from the Ottawa River and major tributaries, whereas in the rural areas, freshwater is obtained from bedrock and overburden aquifers. The abundance and quality of groundwater varies significantly throughout the National Capital Area because of important differences in the composition, texture and structure bedrock and surficial formations, major variations in the surface and bedrock topography, and in the thickness of unconsolidated Quaternary deposits. The following information provides background data for a regional hydrogeological synthesis, and variations may occur at the local level.

Climatic factors

Because there are no significant seasonal variations in the amount of precipitation throughout the year, recharge of groundwater is mainly controlled by seasonal temperatures. Recharge occurs in the spring, beginning with the melting of the snow, and gradually decreases during May and June. Potential evapotranspiration exceeds the rainfall during approximately four months of the year, corresponding to the warmer season. Groundwater levels generally reach their lowest levels at the end of August. During autumn, there is some recharge before deep frost penetration occurs. Seasonal variations in groundwater levels can affect the supply of water for domestic use in shallow wells, but generally have no effect in deeper wells drilled into bedrock.

Aquifers and aquitards

Aquifers are water saturated zones in unconsolidated surficial materials or bedrock that can transmit significant quantities of water to wells and springs. Groundwater comes from rain and melted snow that infiltrates the topsoil and percolates downwards until it reaches a zone which is already saturated with water, known as water table. In general, aquifers can vary in area from less than an hectare to thousands of square kilometres, and may be centimetres to hundreds of metres thick. In complex stratigraphy, such as in the National Capital Area, several aquifers can be found in one location at different depths. Groundwater usually flows slowly and is controlled by the permeability of the material. The flow can be from less than a centimetre a century in nearly impermeable layers to several metres a day in very permeable sediments or fractured bedrock. Groundwater generally flows from high areas towards low areas such as rivers and lakes, but the direction of flow does not necessarily correspond to the flow direction of surface water, as the flow is influenced not only by the surface topography but also by the topography of aquifers and aquitards.