GroundwaterBeneath the Island soil is a vast supply of water that has soaked down into the ground. We call it "groundwater". It fills and moves slowly through the tiny spaces around soil particles and within cracks in the sandstone bedrock. It feeds our streams and rivers, supplies our wells, and supplies all of our water needs across the Island.
Groundwater is not like an underground pond or river. Rather, it soaks into the tiny spaces in the soil and bedrock until it becomes saturated, just like a sponge. We can sink wells down into it and take enough water from it to supply a home or even a city. This saturated soil and bedrock sponge is called an "aquifer". At shallow depths, the tiny spaces in the soil, known as pore spaces, still have some air in them - they are unsaturated. The boundary between the zone of unsaturated soil and that of the saturated aquifer beneath it, is called the "water table".
In some cases, the situation is reversed: the soil filled with water is sitting above unsaturated soil. When this happens, it is usually because there is a layer of poorly draining material, such as clay, between the two zones. Water takes a long time to seep through the clay and so the soil above the clay becomes saturated. Once the water passes through the clay, however, it moves more quickly, leaving an unsaturated zone immediately below the clay. The top zone in this case is known as a "perched" water table. These make less reliable sources of water than the permanent water table because they can dry up during periods of dry weather.
Like surface water, groundwater is always moving. It is pulled down through the aquifer by the force of gravity. However, groundwater moves very slowly. Rather than flowing through large open channels, as surface water does, groundwater is forced through very small, often poorly connected pore spaces and fractures in the bedrock. The more permeable the aquifer and the steeper the slope of the water table, the faster the groundwater flows. But it's still very slow, sometimes moving only a few metres per year.
The movement of water through an aquifer is called a "groundwater flow system". On PEI, the boundaries of groundwater flow systems are virtually the same as for the watersheds on the surface. (This is not always the case elsewhere in the world. In some places, a single large groundwater flow system lies underneath several surface watersheds. And in other places a single watershed contains several separate groundwater flow systems.)
Groundwater Recharge and Water Table
Groundwater comes originally from the surface, from rain and snow that has filtered down through the porous soil. Every year, about one third of our rainfall finds its way into underground aquifers. As it soaks down into the soil, it is said to "recharge" the aquifer. The greatest amount of recharge happens in the spring when there is lots of water from melting snow as well as normal precipitation to soak into the ground. Also, there is not much loss of water through evaporation or transpiration because temperatures are low and there is as yet little plant growth. This is why the water table is highest in the spring.
During the summer months, higher temperatures and vigorous plant growth result in more evaporation and transpiration of water into the air. This leaves less water available to soak into the ground. So from spring until fall, the water table gradually lowers.
In the fall, plant growth slows and temperatures drop. As a result, it is not unusual on PEI to have a second, smaller, recharge event in the fall. It lasts until the ground freezes or precipitation falls as snow. Water table levels then gradually become lower until the spring.
The extent of rise and fall of the water table varies from place to place and from year to year depending on weather conditions and topography. In general, though, the greatest fluctuations happen in the higher portions of a watershed, where the water table can fluctuate by five metres or more. Closer to sea level, the water table may rise or fall by a metre or less in the course of a year.
Although the levels of PEI's water tables vary from place to place and from season to season, there is always plenty of groundwater on PEI. In fact, we use only about two per cent of the total recharge to our groundwater system. In some areas with heavy industrial or municipal water demands, withdrawals may be as high as 50 per cent of recharge. As a general rule, this is the maximum level of withdrawal that is approved, so that enough groundwater is left to discharge into our streams and maintain a healthy level of flow.