Groundwater, aquifers and hydrogeology: the storage and movement of water in rocks; porosity and permeability; aquifers, aquicludes and the water table; artesian conditions; Darcy's law for groundwater flow; and the practical issues of abstraction, recharge, over-abstraction and groundwater pollution.

What this dot point is asking

Part of the Earth materials and natural resources concept. Eduqas wants you to explain the storage and movement of water in rocks (porosity and permeability), to define aquifers, aquicludes, the water table and artesian conditions, to apply Darcy's law to calculate groundwater flow, and to discuss abstraction, recharge, over-abstraction and pollution. The same porosity and permeability ideas as the petroleum system apply here to water.

The answer

Storing and moving water: porosity and permeability

Water is held in the pore spaces of rocks and moves through the connected pores:

• Porosity is the percentage of the rock volume that is pore space; it sets how much water the rock can store. Well-sorted sandstones have high porosity.
• Permeability is the ability to transmit water, which needs the pores to be connected; it sets how readily the water flows. A clay can be porous but impermeable.

A rock needs both to be a useful water source, exactly as a hydrocarbon reservoir does.

Aquifers, aquicludes and the water table

• An aquifer is a porous and permeable rock that stores and transmits useful quantities of water (sandstone, fractured limestone, chalk).
• An aquiclude (aquitard) is an impermeable rock that does not transmit water usefully (clay, unfractured mudstone); it confines or separates aquifers.
• The water table is the upper surface of the saturated zone (below it, all pores are full). Above it is the unsaturated zone. The water table rises with recharge (rain soaking in) and falls with abstraction or drought, and it broadly follows the topography.

Artesian conditions

Where an aquifer is confined between aquicludes and the water enters (recharges) at a higher elevation, the water in the confined aquifer is under pressure. A well drilled into it (an artesian well) will have water that rises above the top of the aquifer, and if the pressure surface is above ground, water flows out without pumping (a flowing artesian well). The classic geometry is an aquifer in a syncline (a basin), recharged on the high rims.

Darcy's law

Groundwater flow is described by Darcy's law, which states that the flow is proportional to the hydraulic gradient:

where $v$ is the flow velocity, $K$ is the hydraulic conductivity (permeability) of the rock, and $i$ is the hydraulic gradient (the drop in head over distance). The volume flow rate is $Q = K \, i \, A$, with $A$ the cross-sectional area. So water flows faster through more permeable rock and down a steeper hydraulic gradient.

Abstraction, recharge and pollution

Groundwater is a vital resource but is easily mismanaged:

• Abstraction (pumping from wells) must not exceed long-term recharge, or the resource is mined.
• Over-abstraction lowers the water table (drying wells and springs), can cause land subsidence as pore pressure falls and sediment compacts, and at the coast allows saline intrusion (denser seawater moving inland into the aquifer).
• Pollution is a serious risk because contaminants (nitrates, industrial chemicals, leachate) move through permeable aquifers and are very slow and costly to clean up; aquifers therefore need protection of their recharge areas.

Examples in context

Example 1. The Chalk aquifer. The Chalk of southern England is a major aquifer, storing water in its porosity and transmitting it through fractures, supplying much of the region; over-abstraction lowers its water table and can dry up its springs.

Example 2. A coastal aquifer and saline intrusion. Heavy pumping near the coast lowers the freshwater head, allowing denser seawater to move inland into the aquifer and contaminate supply wells, a direct consequence of over-abstraction.

Try this

Q1. An aquifer has a hydraulic conductivity of 20 metres per day and a hydraulic gradient of 0.01. Calculate the flow velocity using Darcy's law. [2 marks]

• Cue. $v = K i = 20 \times 0.01 = 0.2$ metres per day.

Q2. State the difference between an aquifer and an aquiclude. [2 marks]

• Cue. An aquifer is porous and permeable and yields useful water; an aquiclude is impermeable and does not transmit water usefully, confining or separating aquifers.

Q3. State two problems caused by over-abstraction of groundwater. [2 marks]

• Cue. Any two of: a falling water table (drying wells and springs); land subsidence as pore pressure falls; saline intrusion of seawater at the coast.