Information AboutAquifers |
| CATEGORIES ABOUT AQUIFER | |
| hydraulic engineering | |
| hydrology | |
| aquifers | |
| SHOPPER'S DELIGHT | |
|
MISCONCEPTION ABOUT AQUIFERS AND GROUNDWATER A common Misconception is that Groundwater exists in ''underground rivers'' (e.g. Cave s where water flows freely underground). This is only sometimes true in Eroded Limestone areas known as Karst Topography which make up only a small percentage of Earth's area. More usual is that the Pore Spaces of rocks in the subsurface are simply saturated with water — like a kitchen sponge, which can be Pump ed out and used for agricultural, industrial or municipal uses. The Beach is an example of what most aquifers are like. If you dig a hole into the Sand at the beach you will find very wet or saturated sand at a shallow depth. This hole is a crude Well , the beach sand is an aquifer, and the level which the water rises to in this hole represents the Water Table . AQUIFER CLASSIFICATION This diagram indicates typical flow directions in a Cross-section al view of a simple confined/unconfined aquifer system (two aquifers with one Aquitard between them, surrounded by the bedrock aquiclude) which is in contact with a Stream (typical in Humid regions). The Water Table and Unsaturated Zone are also illustrated. Saturated versus unsaturated zone'' (e.g., aquifers, aquitards, etc.) and the ''unsaturated zone'' (also called the Vadose Zone ). Saturated means the pressure head of the water is greater than Atmospheric Pressure (it has a gauge pressure > 0). The definition of the Water Table is surface where the 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 Suction . The 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 Sand y soils with larger pores the head will be less than in Clay ey soils with very small pores. The normal capillary rise in a clayey soil is less than six feet but can range between 5 and 100 feet. {Link without Title} 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.
Aquifers versus aquitards Aquifers are typically saturated regions of the subsurface which produce an economically feasible quantity of water to a Well or Spring (e.g., Sand and Gravel or fractured Bedrock often make good aquifer materials). '''Aquitards''' (sometimes, if completely impermeable, called an '''aquiclude''' or '''aquifuge''') are saturated regions, which due to lower Hydraulic Conductivity , do not yield a sustainable amount of water in an economic fashion (e.g., Clay , Silt or fresh bedrock often form aquitards). ''Economically feasible'' is a relative term; for example, an aquifer that is quite adequate for local domestic use, as in a rural area, might be considered an inadequate ''aquitard'' for industrial, Mining , or 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, due to 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.
Confined versus unconfined 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 Water Table or phreatic surface. Typically (but not always) the shallowest aquifer at a given location is unconfined, meaning it does not have a confining layer (an aquitard or aquiclude) between it and the surface. Unconfined aquifers usually receive recharge water directly from the surface, from 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. A "perched aquifer" occurs when the porous, water-bearing segment of rock is located on top of a layer of non-porous rock. If the distinction between confined and unconfined is not clear geologically (it is not known if a clear confining layer exists, or the geology is more complex, e.g., a fractured bedrock aquifer), the value of storativity returned from an Aquifer Test can be used to determine it (although aquifer tests in unconfined aquifers should be interpreted differently than confined ones). Confined aquifers have very low Storativity values (much less than 0.01, and as little as 10-5), 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 ).
HUMAN DEPENDENCE ON GROUNDWATER Most land areas on Earth have some form of aquifer underlying them, sometimes at significant depths. Fresh water aquifers, especially those with limited recharge by meteoric water, can be over-exploited and, depending on the local Hydrogeology , may draw in non-potable water or Salt water ( Saltwater Intrusion ) from hydraulically connected aquifers or surface water bodies. This can be a serious problem especially in coastal areas and other areas where aquifer pumping is excessive. Aquifers are critically important in Human habitation and Agriculture . Deep aquifers in arid areas have long been water sources for Irrigation (see Ogallala below). Many Village s and even large cities draw their water supply from wells in aquifers. Some aquifers are "riparian aquifers". These are related to River s, fluvial deposits, or unconsolidated deposits along river corridors, and are usually rapidly replenished by infiltration of surface water. Some municipal well fields are specifically designed to take advantage of induced infiltration of surface (usually river) water, leaving them potentially vulnerable to Water Quality Problems in the surface water body ( Chemical spills, Petroleum spills, and Bacteriological problems). Aquifers that provide sustainable fresh groundwater to urban areas and for Agricultural Irrigation are typically close to the ground surface (within a couple of hundred meters) and have some recharge by fresh water. This recharge is typically from rivers or meteoric water (precipitation) that percolate into the aquifer through overlying unsaturated materials. SUBSIDENCE In unconsolidated aquifers, groundwater is produced from pore spaces between particles of gravel, sand, and silt. If the aquifer is confined by low-permeability layers, the reduced water pressure in the sand and gravel causes slow drainage of water from the adjoining confining layers. If these confining layers are composed of compressible silt or clay, the loss of water to the aquifer reduces the water pressure in the confining layer, causing it to compress due to the weight of overlying geologic materials. In severe cases, this compression can be observed on the ground surface as subsidence. Unfortunately, much of the subsidence due to groundwater extraction is permanent (elastic rebound is small). Thus the subsidence is not only permanent, but the compressed aquifer has a permanently-reduced capacity to hold water. EXAMPLES An example of a significant and sustainable carbonate aquifer is the Edwards Aquifer {Link without Title} in central Texas . This carbonate aquifer has historically been providing high-quality water for nearly 2 million people and, even today, is completely full because of tremendous recharge from a number of area streams, rivers and Lake s. The primary risk to this resource is human development over the recharge areas. One of the largest aquifers in the world is the Guarani Aquifer , with 1.2 million km&2 of area, from central Brazil to northern Argentina. Aquifer depletion is a global problem, and is especially critical in northern Africa ; see the Great Manmade River project of Libya for an example. However, new methods of groundwater management such as artificial recharge and injection of surface waters during seasonal wet periods has extended the life of many freshwater aquifers, especially in the United States. The Ogallala Aquifer of the central United States is one of the world's great aquifers, but in places it is being rapidly depleted for growing municipal use, and continuing agricultural use. This huge aquifer, which underlies portions of eight states, contain primarily Fossil Water from the time of the last Glaciation . Annual recharge, in the more arid portions of the aquifer, is estimated to total only about ten percent of annual withdrawals. The and reducing potential user conflicts. The Great Artesian Basin is one of the largest groundwater aquifers in the world. It plays a large part in water supplies for remote parts of South Australia. For more aquifers, see '' List Of Aquifers ''. EXTERNAL LINKS |