Aquifers and artesian basins

Define aquifers

Permeable or porous water-bearing rocks such as chalk

Where is groundwater extracted for public supply?

From aquifers by wells and boreholes

What is a major contribution to the baseflow?

Emerging in springs and seepages, groundwater rivers and makes a major contribution to the base flow

What is the water table?

Within an aquifers the upper surface of saturation

What happens to the water table seasonally?

It fluctuates seasonally and is also affected by periods of exceptional rainfall, drought and abstraction

When does recharge happen?

When the water table goes back up

Define artesian pressure

Hydrostatic pressure exerted on groundwater that when tapped will rise to the surface under its own pressure.

What is an artesian aquifer?

• When sedimentary rocks form a syncline, an aquifer confined between impermeable rock layers may contain groundwater which is under artesian pressure.

• If groundwater is tapped by a well or borehole, water will flow to the surface under its own pressure.

What is the potentiometric surface determined by?

The height of the water table in areas of recharge on the edges of the basin

Give an example of an artesian aquifer?

• London is in the centre of a synclinal structure which forms ab artesian basin. Groundwater in the chalk aquifer is trapped between impermeable London clay and Gault day.

• Rainwater enters the chalk aquifers where it outcrops on the edge of the basin. Thus under natural conditions the wells and boreholes in the London area are under artesian pressure.

• Groundwater calk is an important source of water from the capital. However, fall in the water table.

• In central London it fell by nearly 90m. In the past 50 years, declining demands from industries and populations the water table reached 3m/year in the 90s. Now there are licences to control this.