groundwater

Groundwater is water held underground in the pore spaces or fractures of rock and soil. It resides in the saturated zone below the water table. It is part of the hydrologic cycle and is a major source of fresh water used for drinking, irrigation, and industry. The upper boundary of the groundwater is the water table; above it is the unsaturated zone.

Groundwater forms as water infiltrates from precipitation, rivers, or artificial recharge. Recharge rates depend on geology, soil, vegetation, and climate. The water moves slowly through aquifers, driven by hydraulic gradients, and is stored in porous rocks and layers that transmit water. Some aquifers are confined by an impermeable layer (aquitard); others are unconfined, with the water table exposed to atmospheric pressure.

Humans extract groundwater via wells and pumps, providing a sizable share of domestic, agricultural, and industrial water. Sustainable management requires balancing recharge with withdrawal; overexploitation can cause declining water tables and subsidence.

Groundwater can be contaminated by nitrate from fertilizer, pesticides, heavy metals, industrial solvents, and naturally occurring substances such as arsenic or radon. In coastal areas, pumping can cause saltwater intrusion. Poor land-use practices and waste disposal threaten aquifers.

Protection and management include monitoring groundwater levels and quality, protecting recharge areas, applying managed aquifer recharge, and regulatory frameworks. Advances in modeling help assess availability and guide policy.