Groundwater is water which is stored underground - not surprisingly. It can be confined, which means that a deposit of water is surrounded by nonpermeable rock, typically in aquifers or unconfined, in which case it is surrounded by permeable rock, gravel, soil, and other materials. Around 20% of the world's freshwater is groundwater, and groundwater makes up a significant portion of the potable water consumed worldwide, with up to 50% of some populations relying on groundwater.
Precipitation - rain and snow - falls onto the surface of the earth. Some of it remains on the surface, flowing back to the oceans via trickles and streams, lakes and rivers. Some of it is absorbed into the earth, into the unconfined material and confined aquifers. Water movement underground is much slower than on the surface, and once water is in aquifers, its motion is often measured in hundreds of years.
In the Cowichan Watershed, a heavy November rainfall in the mountains above Lake Cowichan can be spilling into the ocean at Cowichan Bay in hours. But some of that rainfall seeps into the ground, where it may remain for many generations.
In between, there is constant and complex interactions between the surface water bodies, unconfined grounwater, and water in aquifers. In all the watersheds in British Columbia, there is very little mapping or data about this complex interplay, or even, in many watersheds, about the hydrological features underground.
Further complicating the hydrological context, are human impacts on groundwater. We log, pave, dam, dump, pump, and drill on, over, into and out of our watersheds. Sometimes, to our own great peril.
Groundwater, Aquifers and the Water Act in the Cowichan Valley is a good introduction to the topic in the Cowichan Watershed.
Groundwater Information Resources
Water Protection and Sustainability
BC Ministry of Environment
Groundwater, Aquifers and the Water Act in the Cowichan Valley
Pat Lapcevic and Mike Wei, Nov 2010
MOE Groundwater Report
R.P. Richards, 1986