5 Groundwater
Learn how groundwater is stored and moves, how aquifers are replenished and affected by pumping, and how groundwater exchanges water with rivers.
in the hydrologic cycle
is stored below Earth's surface in openings in soil, sediment, and rock. It is part of the hydrologic cycle: water infiltrates the ground, moves through subsurface materials, and eventually returns to the surface through springs, rivers, lakes, wetlands, or wells. Most does not move through underground rivers; it travels through connected pores and fractures.
Aquifers and storage
An is saturated rock or sediment that can store and transmit enough water to supply wells or springs. Sand and gravel, fractured rock, and some porous limestone can form aquifers. A transmits water poorly and may restrict flow between aquifers.
describes the proportion of a material's volume made up of openings. describes how readily water can move through the material, especially how well those openings connect. A material can have many pores yet transmit water slowly if the pores are poorly connected.
types and water levels
In an , the forms the upper boundary of the saturated zone and can rise or fall as and discharge change. In a , water is trapped beneath a and is under pressure. When a well penetrates a , water rises to the 's . It reaches the land surface only if that pressure level is above the ground.
storage is not simply the total volume of water in an 's openings. Some water is held tightly to grains and is not readily drained, while some can be released as water levels or pressure fall. Unconfined aquifers generally release water as gravity drains some pore spaces. Confined aquifers release water mainly as pressure changes slightly compress the water and materials.
movement and
moves from places with higher —the water's energy level, commonly related to elevation and pressure—toward places with lower head. Gravity and pressure drive this movement. Its rate depends on the hydraulic gradient and the 's , or ease of transmitting water. summarizes this relationship: flow increases when the gradient is steeper or the material is more conductive.
movement is usually much slower than river flow, but rates vary greatly. Water may travel short, shallow paths to a nearby stream, or take deeper, longer routes that last decades or far longer. Local geology and the distance between and discharge areas shape these paths.
How reaches aquifers
is water that reaches the saturated zone and adds to storage. Commonly, rain or snowmelt infiltrates the ground, moves through the unsaturated zone, and percolates down to the . Some water is instead taken up by plants, evaporates, or runs off into streams. can also come from water leaking from rivers, lakes, and wetlands, as well as from human activities such as irrigation and managed .
varies with climate, soil, geology, vegetation, and land use. A permeable sandy area may allow more water to infiltrate than a paved or clay-rich area, but surface infiltration is not automatically : some infiltrated water can remain in the soil or return to the atmosphere before reaching the saturated zone. may occur unevenly, in pulses after storms or snowmelt, and replenishment of deep aquifers can be slow.
Wells and pumping
A well is a hole drilled or dug into an to observe or withdraw . In a water-table well, the resting water level indicates the local . In a well tapping a , the level to which water rises reflects pressure in that , not necessarily the 's depth. If confined pressure lifts water above the ground, the well is a .
Pumping lowers water level or pressure near a well, creating a , a roughly cone-shaped zone of drawdown. Water initially comes from nearby storage. Over time, pumping can also reduce natural discharge to a stream or draw stream water into the .
If withdrawals persistently exceed replenishment and captured inflows, levels can decline, and wells may become less productive or go dry.
and rivers
and rivers are connected in many landscapes. A receives flowing through its bed or banks into the channel. This contribution, especially important between rainstorms, is called . A sends water from its channel into the ground, where it recharges an . A river may gain along one reach and lose along another, or switch between the two as water levels and seasons change.
The direction of exchange depends on the relative water levels, or hydraulic heads, in the river and nearby . If head is higher, water tends to flow toward the river; if river head is higher, water tends to seep into the . Pumping near a river can lower head, reducing flow to the river or inducing river water to seep into the . These exchanges move dissolved substances between rivers and aquifers, linking their water quality as well as their water supply.
and surface water are therefore connected parts of one hydrologic system: aquifers store and transmit water, adds water to the saturated zone, wells withdraw it, and rivers may gain , lose water to aquifers, or do both in different places and at different times.