So what is groundwater

Water stored underground: vital and vulnerable

So what is groundwater?

Rainwater percolates into the earth. Soil and rock are like a giant sponge, full of holes - typically tiny pores and cracks just millimetres in size. Below the water table, these holes are full of water. This is groundwater. Groundwater slowly travels through connected pores and cracks just centimetres to metres per year.

Water stored underground in cracks and pores

larger image [JPEG, 233.6 kb, 1000 X 876, notice]

Protecting the balance

Groundwater storage is like a bank account. The balance falls when withdrawals exceed deposits. Nature makes deposits through rainfall, and withdrawals through leakage of groundwater to streams and the ocean. Our wells represent further withdrawals. If total withdrawals exceed deposits, we deplete our groundwater storage. Do we know if we are draining our account?

Water table ups and downs through the seasons
The amount of water stored underground changes through the seasons. As winter and spring rains infiltrate the ground, stored groundwater increases and the water table rises. When the rains stop, the water table falls as groundwater leaks into streams and the ocean. Well pumping also removes water and lowers the water table. Excessive pumping of groundwater can result in long-term depletion of groundwater storage.

larger image [JPEG, 136.5 kb, 1000 X 535, notice]

Underground lakes and rivers?

Not on Bowen Island. Large underground streams and lakes only occur in limestone cave systems. Limestone is unique as it dissolves in water, allowing caves to form. Bowen Island's granitic and volcanic rocks do not dissolve in water and so lack cave systems.

Aquifers yield water via wells

larger image [JPEG, 216.5 kb, 1000 X 727, notice]

Tapping into water stored underground

Any body of rock or sediment that yields useful amounts of water is an aquifer. Bowen Island has two types of aquifer: fractured rock, and sand and gravel layers. The amount of water stored in fractured rock is typically limited, and these aquifers can run low during the summer drought. Sand and gravel can store more water and these aquifers are less likely to dry up in the summer. Shallow- dug wells can dry up as the water table falls during the summer.

Groundwater flows from upland recharge areas to valley discharge areas

larger image [JPEG, 178.2 kb, 1000 X 567, notice]

Ensuring our aquifers replenish

Most recharging of aquifers occurs in forested uplands and valley slopes, but land clearing, road building, and ditching reduce water infiltration by creating impermeable surfaces and diverting water into ditches and streams. Infiltration ponds along ditches can increase the return of water into the groundwater system.

Excessive pumping can reduce flow in streams

Okay

larger image [JPEG, 157.2 kb, 900 X 747, notice]

Oops! I dried up the stream
Groundwater springs feed streams year-round. They are the only source of stream water during the dry season. A pumped well draws down the nearby water table. Excessive pumping for an extended period of time can lower the water table over a broad area. This can divert groundwater from streams and even cause streams to dry up. Nothing damages a stream like taking away its water!

Overpumping

larger image [JPEG, 161.6 kb, 900 X 753, notice]

Are we depleting our groundwater?
To determine whether we are overpumping our island aquifers, we need a series of groundwater observation wells on Bowen Island. These are unused wells where water table levels can be regularly checked to determine long-term trends. Some groundwater monitoring has started on Bowen Island, but more observation wells are needed.