Groundwater: porosity and permeability and how they differ between rock types; aquifers, aquitards and the water table; confined and unconfined aquifers; the calculation of porosity from pore and total volumes; the use of a simple form of Darcy's law to relate groundwater discharge to hydraulic conductivity, hydraulic gradient and area; the issues of over-abstraction and contamination.

What this dot point is asking

OCR wants you to define porosity and permeability and how they vary between rock types, to describe aquifers, aquitards and the water table, to distinguish confined and unconfined aquifers, to calculate porosity from pore and total volumes, to use a simple form of Darcy's law for groundwater flow, and to outline over-abstraction and contamination.

The answer

Porosity and permeability

These two properties control how rocks store and transmit water (and hydrocarbons), and OCR rewards keeping them distinct:

• Sandstone: typically high porosity and high permeability (good aquifer or reservoir).
• Clay or shale: high porosity but very low permeability (water is held but cannot flow), so it acts as a barrier.
• Unfractured crystalline rock (for example granite): low porosity and low permeability.

Aquifers, aquitards and the water table

• Aquifer. A rock that can both store and transmit useful amounts of water (porous and permeable, for example a sandstone or fractured limestone).
• Aquitard (or aquiclude). A low-permeability layer (for example clay) that restricts water flow.
• Water table. The upper surface of the saturated zone, below which the pore spaces are full of water. It broadly follows the topography and rises and falls with recharge.

An unconfined aquifer is open to the surface (its top is the water table); a confined aquifer is sandwiched between aquitards, so its water can be under pressure, sometimes flowing to the surface as an artesian well.

Calculating porosity

Darcy's law

Groundwater discharge through an aquifer is given by a simple form of Darcy's law:

where $Q$ is the discharge (volume per time), $K$ is the hydraulic conductivity (a measure of permeability), $i$ is the hydraulic gradient (the slope of the water table or pressure surface) and $A$ is the cross-sectional area. Discharge rises with higher permeability, a steeper gradient, or a larger area.

Over-abstraction and contamination

• Over-abstraction. Pumping water faster than it is recharged lowers the water table, can dry up springs and wells, and can cause subsidence or, near coasts, saltwater intrusion.
• Contamination. Pollutants (from agriculture, industry or landfill) can enter an aquifer and spread with the groundwater flow, and are hard to remove once present.

Examples in context

Example 1. A chalk or sandstone aquifer. Porous, permeable chalk and sandstone store and transmit large volumes of groundwater and are major sources of drinking water, but they are vulnerable to over-abstraction and to contamination from the surface.

Example 2. Saltwater intrusion at the coast. Over-pumping a coastal aquifer lowers the freshwater head and lets denser seawater move inland into the aquifer, spoiling the water supply, a direct consequence of over-abstraction.

Try this

Q1. A rock has a total volume of $250\ \mathrm{cm^3}$ and a pore volume of $50\ \mathrm{cm^3}$. Calculate its porosity. [2 marks]

• Cue. $\frac{50}{250} \times 100 = 20\%$.

Q2. Explain why clay has high porosity but low permeability. [2 marks]

• Cue. Clay has abundant tiny pore spaces (high porosity), but the pores are minute and poorly connected, so water cannot flow through easily (low permeability).

Q3. State the simple form of Darcy's law and define each term. [2 marks]

• Cue. $Q = KiA$, where $Q$ is discharge, $K$ is hydraulic conductivity (permeability), $i$ is the hydraulic gradient and $A$ is the cross-sectional area.