Groundwater Explained: Aquifers, Water Tables, and Underground Water Systems

How Water Gets Underground

Groundwater originates as precipitation that infiltrates the soil surface and percolates downward through the unsaturated zone (also called the vadose zone), where pore spaces contain both air and water. Below a certain depth, all pore spaces are completely filled with water. The boundary between the unsaturated zone and this saturated zone is the water table. The position of the water table fluctuates with the seasons, rising after periods of heavy rainfall or snowmelt (recharge) and falling during dry periods or when water is withdrawn by wells (discharge). In humid regions, the water table may be near the surface or even at the surface in wetlands and river valleys. In arid regions, it may lie tens or hundreds of meters below the surface.

The rate at which water infiltrates and percolates depends on the properties of the surface materials. Sandy and gravelly soils allow rapid infiltration, while clay-rich soils and impermeable surfaces (pavement, rooftops) restrict it. Urban development dramatically reduces infiltration by covering the ground with impervious surfaces, increasing stormwater runoff and reducing groundwater recharge. This reduction in recharge can lower water tables, decrease stream baseflow, and increase flooding downstream.

Aquifers and Aquitards

An aquifer is a body of rock or sediment that is sufficiently porous and permeable to store and transmit useful quantities of groundwater. The two most important properties of an aquifer are porosity (the percentage of the rock volume that consists of open spaces) and permeability (the ability of those spaces to transmit water). High porosity means the rock can store a lot of water, but if the pores are very small and poorly connected (as in clay), the water cannot move through them easily. Sandstone, gravel, and fractured limestone are common aquifer materials because they combine adequate porosity with good permeability.

An aquitard (or confining layer) is a layer of low-permeability material, such as clay or unfractured shale, that restricts groundwater flow. Aquitards separate aquifers and control the vertical movement of water between them. Unconfined aquifers are directly connected to the surface through the unsaturated zone, and their upper boundary is the water table. Confined aquifers are sandwiched between aquitards, and the water in them is under pressure greater than atmospheric. When a well penetrates a confined aquifer, water rises above the top of the aquifer and may flow to the surface without pumping if the pressure is sufficient. Such wells are called artesian wells, named after the Artois region of France where they were first widely used.

The Ogallala Aquifer (High Plains Aquifer) in the central United States is one of the largest and most economically important aquifer systems in the world, underlying about 450,000 square kilometers across eight states. It provides water for approximately 30 percent of the groundwater used for irrigation in the United States, supporting a major agricultural region. However, decades of pumping have drawn water from the Ogallala far faster than natural recharge can replace it, lowering the water table by tens of meters in some areas and raising serious concerns about the long-term sustainability of agricultural production in the region.

Groundwater Flow

Groundwater moves through aquifers from areas of high hydraulic head (essentially, high water pressure or elevation) to areas of low hydraulic head. The rate of flow is governed by Darcy law, which states that the discharge through a porous medium is proportional to the hydraulic gradient (the change in hydraulic head per unit distance) and the hydraulic conductivity (a measure of how easily water moves through the material). In most aquifers, groundwater moves very slowly, typically centimeters to meters per day, though velocities can be much higher in fractured rock or karst (dissolved limestone) systems where water flows through open fractures and caves.

Groundwater eventually discharges to the surface at springs, seeps, rivers, lakes, wetlands, and the ocean. In many river systems, groundwater baseflow sustains streamflow during dry periods, keeping rivers from going dry between storms. The interaction between groundwater and surface water is continuous and bidirectional: gaining streams receive groundwater discharge, while losing streams recharge groundwater by leaking water through their beds. Understanding these connections is essential for water resource management because pumping groundwater near a river can reduce the stream flow, and surface water contamination can migrate into aquifers.

Groundwater Quality and Contamination

Natural groundwater chemistry is determined by the minerals in the rock through which it flows. Water passing through limestone dissolves calcium and magnesium carbonate, producing hard water that can leave scale deposits in pipes and appliances. Water in contact with evaporite deposits may become excessively saline. Volcanic aquifers may naturally contain elevated concentrations of arsenic, fluoride, or other elements that pose health risks at high concentrations. In many parts of South and Southeast Asia, naturally occurring arsenic in groundwater is a major public health concern affecting tens of millions of people.

Human contamination of groundwater includes agricultural chemicals (nitrate fertilizers and pesticides that leach through soil into aquifers), industrial solvents and heavy metals (which can migrate through the subsurface for decades), leaking underground storage tanks (a common source of petroleum contamination), landfill leachate, and septic system effluent. Because groundwater moves slowly, contaminant plumes can persist for years or decades, and cleanup is extremely difficult and expensive. Once an aquifer is contaminated, it may remain unusable for generations. Prevention through proper waste management, monitoring, and land use planning is far more effective than remediation after contamination has occurred.

Land Subsidence and Overextraction

When groundwater is pumped from aquifers faster than it is replenished by natural recharge, water tables decline and the aquifer may compact permanently. In aquifers composed of clay-rich layers, the reduction in water pressure allows the clay to compress, causing the ground surface above to subside (sink). Land subsidence from groundwater overextraction has caused billions of dollars in damage worldwide. The San Joaquin Valley in California has subsided by up to 9 meters in some areas due to decades of agricultural pumping. Mexico City, built on the clay sediments of a former lake bed, continues to subside at rates exceeding 30 centimeters per year in some districts as groundwater is extracted from beneath the city. Jakarta, Bangkok, and many other major cities face similar problems. Subsidence is largely irreversible because compacted clay cannot re-expand to its original volume even if water levels are restored.