Understanding Groundwater: Resources, Impacts, and Sustainability
Groundwater: A Vital Resource
What is Groundwater?
Groundwater is water held underground in the phreatic zone (saturated zone) beneath the water table. The phreatic zone consists of permanently saturated solid rocks and sediments. Groundwater replenishment can take up to 20,000 years, making it a non-renewable resource.
Importance of Groundwater
Groundwater is a crucial component of the global hydrological cycle and represents the world’s largest freshwater store, holding over 90% of the total supply.
Where is Groundwater Stored?
Groundwater is stored in aquifers and artesian basins.
Aquifers
Aquifers are porous rocks, such as sandstone and limestone, that can effectively store and transmit significant quantities of water. They act as reservoirs, maintaining stream flow during dry periods. Conversely, aquitards are less permeable rock formations that transmit water slowly, hindering its movement.
Artesian Basins
An artesian basin contains an aquifer confined by two aquitards or impermeable rock layers. This confinement creates high hydrostatic pressure. Drilling a bore into an artesian basin allows water to flow to the surface naturally (without pumping) due to this pressure, creating an artesian well. Increased water abstraction can reduce the hydrostatic pressure within the basin.
Groundwater Recharge and Loss
Recharge
Aquifers and artesian basins are recharged naturally and artificially:
- Natural Recharge: Infiltration and percolation after precipitation or snowmelt, and seepage from rivers or adjacent aquifers.
- Artificial Recharge: Leakage from irrigation channels and reservoirs, or intentional pumping of water into aquifers.
Losses
Groundwater loss occurs through:
- Evapotranspiration: Particularly in low-lying areas with a shallow water table.
- Natural Discharge: Spring flow and seepage into surface water bodies.
- Leakage and Outflow: Through aquitards and into adjacent aquifers.
- Artificial Abstraction: Primarily through pumping for human use.
Environmental Impact of Groundwater Abstraction
Falling Water Tables
Excessive pumping exceeding natural recharge rates leads to falling water tables. For example, Australia’s Great Artesian Basin’s upper level has dropped by 120m due to over a century of intensive groundwater use.
Saltwater Intrusion
In coastal regions with depleted water tables, seawater can intrude and contaminate groundwater supplies, leading to salinization. This harms drinking water and agriculture. Over-abstraction in the Gaza Strip exemplifies this, damaging citrus production due to salt intolerance.
Ground Subsidence
Reduced water content in soil due to falling water tables can cause the ground to collapse. Bangkok, Thailand, experiences subsidence of 5-10cm annually due to heavy buildings and excessive groundwater abstraction, increasing flood risks.
River Discharge Reduction
Groundwater abstraction can lower river and wetland flows, impacting ecosystems. The Colorado River in the USA rarely reaches the Gulf of Mexico due to such reductions.
Changes in Groundwater Quality
As water tables fall, the concentration of toxic chemicals in aquifers can increase. Arsenic contamination in Bangladesh’s Ganges Delta exemplifies this, affecting millions and causing health problems.
Case Study: Groundwater Pollution in Bangladesh
Naturally occurring arsenic in groundwater pumped through tube wells has led to increased cancer incidence in Bangladesh. Millions are affected by arsenic-contaminated drinking water. Solutions include concrete butts for rainwater harvesting, filter systems, and addressing the over-reliance on tube wells.
Water Balance in Australia – The Great Artesian Basin
The Great Artesian Basin, the world’s largest artesian basin, stores vast amounts of water. However, unsustainable practices like excessive boreholes reduce pressure and threaten this vital resource.
Conclusion
Understanding the importance of sustainable groundwater management is crucial for preserving this vital resource for future generations. Balancing human needs with environmental concerns is essential to ensure the long-term health of our aquifers and the ecosystems they support.