Một “tầng ngậm nước” là một lớp nước dưới đất ở trong đá thấm nước hoặc các chất xốp (sỏi, cát, bùn, hoặc đất sét) từ đó nước ngầm có thể được hút lên qua giếng nước. Ngành khoa học nghiên cứu về dòng chảy trong các tầng ngậm nước và mô tả các đặc điểm của các tầng ngậm nước được gọi là địa chất thủy văn.

== Phân loại ==
nhỏ|phải|200px|Mặt cắt ngang điển hình của tầng ngậm nước

Biểu đồ này cho biết các hướng dòng chảy điển hình trong một mặt cắt ngang của một hệ thống tầng ngậm nước có áp/không áp đơn giản (hai tầng ngậm nước với một tầng cách nước giữa chúng, bao quanh bởi nền đá không thấm nước). Hệ thống này liên hệ với một dòng suối (điển hình trong các vùng ẩm ướt). Đường mặt nước và miền chưa bão hòa cũng được minh họa.

=== Bão hòa vs. Chưa bão hòa===

saturated zone'' or ''phreatic zone'' (e.g., aquifers, aquitards, etc.), where all available spaces are filled with water, and the ''unsaturated zone'' (also called the vadose zone), where there are still pockets of air that can be replaced by water. 

'''Saturated''' means the pressure head of the water is greater than pressure head is equal to atmospheric pressure (where gauge pressure = 0). '''Unsaturated''' conditions occur above the water table where the pressure head is negative (absolute pressure can never be negative, but gauge pressure can) and the water which incompletely fills the pores of the aquifer material is under water content in the unsaturated zone is held in place by surface adhesive forces and it rises above the water table (the zero gauge pressure isobar) by capillary action to saturate a small zone above the phreatic surface (the capillary fringe) at less than atmospheric pressure. This is termed tension saturation and is not the same as saturation on a water content basis. Water content in a capillary fringe decreases with increasing distance from the phreatic surface. The capillary head depends on soil pore size. In sandy soils with larger pores the head will be less than in clayey soils with very small pores. The normal capillary rise in a clayey soil is less than 1.80 m (six feet) but can range between 0.3 and 10 m (1 and 30 feet). [http://www.ces.ncsu.edu/plymouth/programs/vepras.html] 

The capillary rise of water in a small diameter tube is this same physical process. The water table is the level to which water will rise in a large diameter pipe (e.g. a well) which goes down into the aquifer and is open to the atmosphere.

<br>''Xem: chứa nước và độ ẩm của đất''

=== Tầng ngậm nước vs. tầng cách nước===
'''Aquifers''' are typically saturated regions of the subsurface which produce an economically feasible quantity of water to a well or spring (e.g., rock with low urban water supply.

In non-mountainous areas (or near rivers in mountainous areas), the main aquifers are typically unconsolidated alluvium. They are typically composed of mostly horizontal layers of materials deposited by water processes (rivers and streams), which in cross-section (looking at a two-dimensional slice of the aquifer) appear to be layers of alternating coarse and fine materials.  Coarse materials, because of the high energy needed to move them, tend to be found nearer the source (mountain fronts or rivers), while the fine-grained material will make it farther from the source (to the flatter parts of the basin or overbank areas - sometimes called the pressure area).  Since there are less fine-grained deposits near the source, this is a place where aquifers are often unconfined (sometimes called the forebay area), or in hydraulic communication with the land surface.

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=== Có áp vs. Không áp ===
There are two end members in the spectrum of types of aquifers; ''confined'' and ''unconfined'' (with semi-confined being in between).  '''Unconfined''' aquifers are sometimes also called ''water table'' or ''phreatic'' aquifers, because their upper boundary is the precipitation or from a body of surface water (e.g., a river, stream, or lake) which is in hydraulic connection with it. '''Confined''' aquifers have the water table above their upper boundary (an aquitard or aquiclude), and are typically found below unconfined aquifers. The term "perched" refers to ground water accumulates above a low-permeability unit.  This term is generally used to refer to a small local area of ground water that occurs at an elevation higher than a regionally-extensive aquifer.  The difference between perched and unconfined aquifers is their size (perched is smaller).  

If the distinction between confined and unconfined is not clear geologically (i.e., if it is not known if a clear confining layer exists, or if the geology is more complex, e.g., a fractured bedrock aquifer), the value of storativity returned from an storativity values (much less than 0.01, and as little as 10<sup>-5</sup>), which means that the aquifer is storing water using the mechanisms of aquifer matrix expansion and the compressibility of water, which typically are both quite small quantities.  Unconfined aquifers have storativities (typically then called specific yield) greater than 0.01 (1% of bulk volume); they release water from storage by the mechanism of actually draining the pores of the aquifer, releasing relatively large amounts of water (up to the drainable porosity of the aquifer material, or the minimum volumetric water content).

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== Quan niệm sai==
A common eroded pore spaces of rocks in the subsurface are simply saturated with water &mdash; like a kitchen sponge &mdash; which can be pumped out and used for agricultural, industrial or municipal uses.  

The well, the wet sand represents an aquifer, and the level to which the water rises in this hole represents the water table.

Un '''aquifère''' est une couche de terrain ou une roche, suffisamment poreuse (qui peut stocker de l'eau) et perméable (où l'eau circule librement), pour contenir une nappe d'eau souterraine. Une nappe d'eau souterraine est un réservoir naturel d'eau douce susceptible d'être exploitée.

La nappe qu'il contient est susceptible d'alimenter des ouvrages de production d'puits, puits.

On distingue les '''aquifères poreux''' des '''aquifères lamellés"

*Dans les '''aquifères poreux''', l'eau est contenue dans les pores ouverts de la roche et peut y circuler librement (graviers, grès, scories volcaniques, etc.). La perméabilité est matricielle.

*Dans les '''aquifères fissurés''', l'eau est contenue et circule dans les failles, fissures ou diaclases de la roche (calcaires , granites, coulées volcaniques etc.). La perméabilité est fissurale.

*Les '''aquifères karstiques''' sont des systèmes complexes particuliers associant une zone superficielle plus ou moins fissurée et insaturée (en eau) servant de zone d'infiltration, et une zone inférieure fissurée, présentant également des conduits, grottes etc. Cette zone est saturée en dessous d'un certain niveau et l'eau circule avec de grandes vitesses comparativement aux systèmes poreux.

Attention : Les argiles sont des roches qui peuvent contenir une grande quantité d'eau du fait de leur porosité élevée mais qui ne la laissent pas circuler librement (faible perméabilité ou imperméabilité).

==Exemples d'aquifères importants==
===Afrique===
*L'aquifère du Bassin de Nubie (Afrique du Nord)
===Amérique du Nord===
*L'Aquifère de Guelph-Amabel (Ontario - Canada)
*L'Aquifère Ogallala (États-Unis)

===Amérique du Sud===
*L'Aquifère Guarani (Amérique du Sud)
===Antarctique===
===Asie===
L'Aquifère de la Montagne (Israël, Cisjordanie)
===Océanie===
===Europe===
*Nappe de la Plaine d'Alsace
*Nappe de la craie (Bassin Parisien - France)
*Nappe des Calcaires Carbonifères (Belgique)
*Nappe du Bassin aquitain (France)

== Porosité ==
La quantité d'eau que peut contenir un aquifère est fonction de sa composition. 

on note n= volume des vides /volume total