Underground water supplies

We access water by extracting it from surface water bodies such as streams, lakes, and reservoirs. However, in some regions there is little surface water. This is the case in arid regions, where precipitation levels are low and temperatures are high, causing surface water to evaporate. Here, an alternative source of water is groundwater. Groundwater is held in bedrock below the water table, known as the saturated zone. Some types of bedrock, called aquifers, are porous (they contain spaces within them, such as cracks and small holes or pore spaces) and/or permeable (the spaces are joined up, or connected, allowing water to flow through).

It is estimated that groundwater sources provide water for almost 50% of the global urban population. As global population grows further, and the climate becomes warmer and drier, pressure on water supplies increases. However, overuse of these supplies can have a range of adverse consequences.

Groundwater

Water enters aquifers and groundwater stores by infiltrating into the soil during or after a rainfall event. The water percolates through the soil to the bedrock. This sub-surface water can move horizontally, due to gravity, but can also move laterally, from wetter to drier areas.

Groundwater can be stored below the surface for a long time, but it can also flow away from the bedrock. It gradually travels into surface water bodies such as streams, lakes, or oceans, it can re-emerge at the surface as a natural spring, and it can be lost via evaporation, particularly where water tables are close to the surface. Water can also be abstracted from groundwater reserves by people, through the digging of boreholes and wells.

Over-abstraction

The amount of water entering and leaving groundwater reserves is finely balanced. In some regions, humans extract groundwater more quickly than it can recharge. This over-abstraction is driven by population growth, and agricultural and industrial expansion. Coupled with rising temperatures and falling precipitation levels, this causes groundwater reserves to become depleted and the water table to fall, which can have several environmental, financial and societal consequences.

If the water table drops below the level of existing wells used for water abstraction, these will dry up as groundwater becomes inaccessible. Wells will then need to be deepened, or new wells dug. A lower water table also means that pumping costs increase, to bring the water up the well to the surface.

A reduction in the amount of groundwater means that less water is available to flow into rivers, and lakes. Groundwater often maintains base flow - this is where river flow comes from between precipitation events. If groundwater contributions to rivers are decreased, flows will reduce, which may cause some rivers to become ephemeral or intermittent, drying up completely at some times of the year.

The drying out of rivers and soils can have consequences for the ecosystems in these environments. Where streams dry up, fish are left stranded, and die. A reduction in the amount of groundwater available to seep to the surface also causes drying of wetlands and soils. This influences the species which can grow there, leading to a reduction in vegetation and fauna biodiversity.

Impact on the Carbon cycle

The drying of soils and vegetation leads them to become more susceptible to wildfires. Wildfires damage soils and vegetation and can cause large amounts of gaseous carbon to be released to the atmosphere through the process of combustion.

The drying of wetlands can also impact carbon cycling, as reductions in the water table in these environments increase carbon emissions from respiration. This is because pore spaces in the soil are filled with air as opposed to water, increasing the amount of aerobic respiration from organisms which need oxygen to function.

Salinisation

Where the lowering of water tables causes groundwater to stop flowing into surface water bodies, the reverse may occur, with water travelling from wet areas to drier ones. In coastal regions, this may cause seawater to flow into aquifers, known as saltwater intrusion. This saline water is not suitable to drink. Sodium, magnesium, and calcium can accumulate in soils as water evaporates and these soluble salts precipitate at the surface. This leads to a reduction in fertility, further harming ecosystem functioning and crop yields.

Subsidence

In some areas where groundwater has been abstracted, land subsidence can occur. Water in soils and bedrock can provide support for the overlying ground surface, and any buildings or infrastructure on it. When water is removed through abstraction, the weight of the overlying infrastructure can cause soils and rocks to collapse and compact, causing a fall in ground level, which causes damage to properties and risks life.

Water insecurity

If groundwater and other water supplies become depleted such that the available water becomes very limited, communities may begin to suffer from water insecurity. This includes a decline in food production as less water is available for irrigation, limited industrial capacity, increased risk of disease if only dirty drinking water is available and, in some areas, a disruption to education as girls are required to collect safe water instead of attending school. These issues particularly affect developing countries and hinder further development.

Conclusion

The importance of protecting groundwater reserves is becoming increasingly recognised. In the UK, for example, most groundwater usage requires a licence, with checks to ensure that the abstraction does not cause environmental degradation. In order to manage groundwater effectively, both the supply from groundwater sources and the demand for water must be managed effectively.