Geology Water

Porosity is the amount of pore space in a rock or a sediment, usually expressed as a percentage of total rock volume. You can calculate it using the formula:

% porosity = total volume of pore space/total volume of rock x 100

Permeability is the ability to transmit fluids such as water, oil or gas and can be expressed as a rate of flow. You can calculate permeability using this formula:

Permeability = distance water has traveled/time taken

Factors affecting porosity:

• The degree of sorting. A well sorted rock has a high porosity, a poorly sorted rock has a low porosity because the finer grains fill in the gaps between the coarser ones.

• The amount of diagenesis the rock has undergone. A loose unconsolidated rock has a much higher porosity than a rock that has undergone compaction and cementation.

• Grain shape. Rocks containing rounded grains have a higher porosity than rocks containing angular grains that fit together.

• The packing of the grains (how they fit together)

Factors affecting permeability:

A rock that has a high effective porosity with good interconnections between the pore spaces will also have a high permeability. Although grain size is not a factor in porosity, it is important in determining permeability. Coarse grained rocks have a higher permeability than fine grained rocks because there is less resistance to flow around coarse grains.

Secondary permeability results from the presence of fractures, such as joints and faults.

Groundwater and the water table

Most groundwater originates from rainwater thats has infiltrated into soil and then percolated downwards through the pore space of rocks to reach the water table.

The water table is the level at which water sits within the ground. It separates the unsaturated rock above from the saturated rock.

Hydrostatic pressure and hydraulic gradient

Hydrostatic pressure results from the weight of the overlying column of water and increases with depth. The height of overlying column of water is known as the hydrostatic head.

Pressure differences cause groundwater to flow. Water always flows down the hydraulic gradient from areas of high pressure to areas of low pressure. The rate at which the water flows is proportional to the drop in height of the water table. You can calculate the hydraulic gradient using this formula:

Hydraulic Gradient = difference in hydrostatic pressure or hydrostatic head/ distance between two points

Aquifers are bodies of rock with a high porosity so that large quantities of water can be stored within their pore spaces. They also have a high permeability so water can enter, flow through, and be extracted from the aquifer with ease.

Aquifers with a recharge zone on the surface are replenished by rain water and can  provide a constant supply of water provided the rate of recharge equals the rate of extraction.

Types of aquifers

• An unconfined aquifer is open to the atmosphere, under atmospheric pressure, and is recharged by rain water from directly above. Water will need to be pumped to the surface from a well or borehole sunk into an unconfined aquifer.

• A confined aquifer is overlain by impermeable rocks and the groundwater held within it is under hydrostatic pressure. Groundwater can only be replenished in a confined aquifer if it has recharge zones that are open to the atmosphere.

Artesian basins and artesian wells

Large, synclinal, confined aquifers are termed artesian basins. If a borehole is sunk into an artesian basin, the water may flow up to the surface. We call this an artesian well. Once the hydrostatic pressure falls, the water has to be pumped to the surface.

Groundwater is abstracted by sinking a borehole and pumping the water to the surface. As water is pumped out of the ground from the a well, the level of the water table falls around the well, leading to a cone of depression.

Problems caused by groundwater abstraction

• Lowering of the water table. Shallow wells become dry and have to be sunk deeper. This is a particular problem if wells are situated too close together and their cones of depression overlap. This leads to lowering of the water table.

• Subsidence at the surface resulting from the removal of water from the pore spaces of rocks. Rocks overlying the aquifer collapse downwards creating depressions at the surface which can be several metres in diameter. Subsidence results in compaction of the aquifer and a permanently reduced water storage capacity.

• Saltwater encroachment in coastal areas. Where aquifers occur at coastlines, the less dense fresh groundwater forms a lense floating on top of more dense seawater. Over pumping disturbs the freshwater-saltwater interface and allows seawater to enter the aquifer - making it undrinkable.

Threats to groundwater supply

• Overpumping - if too much groundwater is extracted then there may not be enough left to provide a reliable public water supply.

• Pollution - groundwater is vulnerable to contamination from a variety of sources and once polluted is difficult to clean. Unconfined aquifers are at more of a risk of pollution than confined aquifers.

Lithological springs are the result of of changes in rock type. They occur where the porous and permeable rock overlies impermeable rock. The water table will intersect the land surface at the junction between the two rock types. There will be a spring line along the base of the permeable rock.

Springs resulting from lithology also occur where impermeable igneous intrusions, such as dykes, cut through porous and permeable sedimentary rocks. Springs occur where the contact between the two rock types outcrops at the surface.

Springs at faults are produced if they have moved porous and permeable rock into contact with impermeable rock. A spring line will occur where the fault plane intersects the land surface.

Faults can result in the formation of pressurised springs if they intersect confined aquifers. Water under hydrostatic pressure from the confined aquifer will rise up the fault plane and flow out onto the surface.

Springs at unconformities can also result in the formation of springs. If porous and permeable rock lies unconformably on top of the impermeable older rock, the water table will intersect the land surface at the junction between the two rock types. A spring line will occur where the plane of the unconformity intersects the land surface.

Advantages of surface water supply

• Easy to abstract water from rivers, lakes or reservoirs by direct pumping

• Water can be treated after use and put back into river

• Dams and reservoirs can be used for hydroelectric power generation

• Reservoirs can be used for recreation and other purposes

Disadvantages of surface water supply

• Water will need treatment

• Seasonal, as the volume of water in rivers varies and water loss occurs through evaporation

• Requires construction of expensive and environmentally damaging dams

• Requires flooding of land

• Reservoirs will eventually silt up

• Construction of dams may trigger earthquakes