Lecture 18 Ground Water (1)Aquifer and Aquitard

• Definitions

• Confined and unconfined aquifer

• Geological structure

General Comments• Largest reservoir of unfrozen fresh water in the world (97%)

• 40% within 1 km from the surface

• Equivalent of 19.6 m water sheet globally, or 67.2 m on land only

• Spatially variable

• Almost all from precipitation

Ground Water

Definition: Water stored in the saturated zone beneath the soil surface

Figure 5.1, WR

Terms to RememberWater table: the depth at which soil is saturated. It can be variable with time and topography

Soil zone: unsaturated zone of the top soil layer

Zone of saturation: completely saturated zone below the water table

Capillary fringe: a thin layer immediately above the water table and affected by the capillary rise

Intermediate zone: a subsurface zone between the bottom of the soil zone and the top of the capillary fringe.

Geological Structure

Geological structure of a location determines the distribution of underground water. Sedimentary deposits, such as sandstones and limestones, formed from the erosion and deposition of other rocks have many pores which can store water. Igneous and metamorphic rocks formed under conditions of high temperatures and pressures have few pores and have low water-bearing capacity (Figure 5.1 WR)

Terms to RememberAquifers: layers of rock or unconsolidated deposits that contain sufficient saturated water.Aquitards: layers of less permeable formations that transmit water more slowly than the adjacent aquifersUnconfined aquifers: the upper boundary is water tableConfined aquifers: the upper boundary is aquitardPerched aquifer: a special case of unconfined aquifer where the underlying permeable or impermeable bed is not continuous over a very large area and is situated at some height above the main aquifer of the location Potentiometric surface: the imaginary surface to which water rises in wells tapping confined aquifersPiezometer: a device for measuring the elevation of the potentiometric surface. The probe is placed inside the aquifer.Artesian: confined aquifer or a well that penetrates a confined aquifer, showing a continuous out flow of water