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Hydrogeology

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Brittany
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« on: May 11, 2007, 12:46:24 am »

Hydrogeology (hydro- meaning water, and -geology meaning the study of the Earth) is the part of hydrology that deals with the distribution and movement of groundwater in the soil and rocks of the Earth's crust, (commonly in aquifers). The term geohydrology is often used interchangeably. Some make the minor distinction between a hydrologist or engineer applying themselves to geology (geohydrology), and a geologist applying themselves to hydrology (hydrogeology).

Hydrogeology, as stated above, is a branch of the earth sciences dealing with the flow of water through aquifers and other shallow porous media (typically less than 450 m or 1,500 ft below the land surface.) The very shallow flow of water in the subsurface (the upper 3 m or 10 ft) is pertinent to the fields of soil science, agriculture and civil engineering, as well as to hydrogeology. The general flow of fluids (water, hydrocarbons, geothermal fluids, etc.) in deeper formations is also a concern of geologists, geophysicists and petroleum geologists. Groundwater is a slow-moving, viscous fluid (with a Reynolds number less than unity); many of the empirically derived laws of groundwater flow can be alternately derived in fluid mechanics from the special case of Stokes flow (viscosity and pressure terms, but no inertial term).

The mathematical relationships used to describe the flow of water through porous media are the diffusion and Laplace equations, which have applications in many diverse fields. Steady groundwater flow (Laplace equation) has been simulated using electrical, elastic and heat conduction analogies. Transient groundwater flow is analogous to the diffusion of heat in a solid, therefore some solutions to hydrological problems have been adapted from heat transfer literature.

Traditionally, the movement of groundwater has been studied separately from surface water, climatology, and even the chemical and microbiological aspects of hydrogeology (the processes are uncoupled). As the field of hydrogeology matures, the strong interactions between groundwater, surface water, water chemistry, soil moisture and even climate are becoming more clear.
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Brittany
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« Reply #1 on: May 11, 2007, 12:48:29 am »

Water table



The water table or phreatic surface is the surface where the water pressure is equal to atmospheric pressure.

A large amount of water within a body of sand or rock below the water table is called an aquifer, and the ability of rocks to store such groundwater is dependent on their porosity and permeability.

The practice of drilling wells to extract groundwater is dependent on understanding the water table. Because wells must reach the water table, its depth determines the minimum depth of a viable well, and thus the feasibility of drilling it.
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« Reply #2 on: May 11, 2007, 12:49:37 am »

Form

The form of a water table may change and vary due to seasonal changes or topography. In undeveloped regions, or areas with high amounts of precipitation, the water table roughly follows the contour of the overlying land surface, and rises and falls with rainy or dry weather. Springs and oases occur when the water table reaches the surface. Springs commonly form on hillsides, where the earth's slanting surface may "intersect" with the water table. Other, unseen springs are found under rivers and lakes, and account for the sometimes surprisingly well-preserved water levels which occur in times of mild drought.


Surface topography

Within an aquifer, the water table is rarely horizontal, but reflects the surface relief due to the effect of gravity.[citation needed] In hilly regions, the variation in gradient give rise to rivers, springs or oases when the water table intersects the surface.


Perched water tables

A perched water table (or perched aquifer) is an aquifer that occurs above the main water table. This occurs when there is an impermeable layer of rock (aquiclude) above the main aquifer but below the surface. Water percolating down to the main aquifer gets trapped above this second impermeable rock layer. If a perched aquifer's flow intersects the Earth's surface, at a valley wall for example, the water is discharged as a [[spring (hydrosphere)|spring].

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« Reply #3 on: May 11, 2007, 12:51:19 am »

Fluctuations



Seasonal fluctuations in the water table. During the dry season, river beds may dry up.

Seasonal fluctuations

In some regions (Britain for example), winter precipitation is often higher than summer precipitation. The groundwater storage is not recharged by precipitation in summer, consequently, the water table is lowered in the April-October period yearly. This disparity between the level of the winter and summer water table is known as the zone of intermittent saturation, wherein the water table will fluctuate in response to climatic conditions.

Long term fluctuations

Fossil water is groundwater that has remained in an aquifer for millennia, and occurs mainly in deserts. Fossil water is non-renewable by present day rainfall due to their depth below the surface, and any extraction ('mining') causes a permanent change in the water table in such regions.
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« Reply #4 on: May 11, 2007, 12:52:38 am »

Aquifer

An aquifer is an underground layer of water-bearing permeable rock or unconsolidated materials (gravel, sand, silt, or clay) from which groundwater can be usefully extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology.

Shallow aquifers

Aquifers can occur at various depths. Those closer to the surface are not only more likely to be exploited for water supply and irrigation, but are also more likely to be topped up by the local rainfall. Many desert areas have limestone hills or mountains within them or close to them which can be exploited as groundwater resources. Parts of the Atlas Mountains in North Africa the Lebanon and Anti-Lebanon ranges of Syria, Israel and Lebanon, the Djebel Akhdar in Oman and parts of the Sierra Nevada and neighbouring ranges in South West USA have shallow aquifers which are exploited for their water supplies. Over exploitation can lead to the exceeding of the practical sustained yield, i.e. more water is taken out than can be replenished. Along the coastlines of certain countries, such as Libya and Israel, population growth has led to over-population which has caused the lowering of water table and the subsequent contamination of the groundwater with saltwater
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« Reply #5 on: May 11, 2007, 12:53:31 am »



Typical aquifer cross-section
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« Reply #6 on: May 11, 2007, 12:54:36 am »

Classification
 
This diagram indicates typical flow directions in a cross-sectional view of a simple confined/unconfined aquifer system (two aquifers with one aquitard, also known as an confining or impermeable layer, between them, surrounded by the bedrock aquiclude) which is in contact with a gaining stream (typical in humid regions). The water table and unsaturated zone are also illustrated.


Saturated versus unsaturated

Groundwater can be found at nearly every point in the earth's shallow subsurface, to some degree; although aquifers do not necessarily contain fresh water. The earth's crust can be divided into two regions: the 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 aeration), where there are still pockets of air with some water that can be replaced by water.

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 sandy soils with larger pores the head will be less than in clay 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). [1]

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.

http://en.wikipedia.org/wiki/Aquifer
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« Reply #7 on: May 11, 2007, 12:56:38 am »

Cold seep



Tubeworms, soft corals and chemosynthetic mussels at a seep located 3,000 metres down on the Florida Escarpment. Eelpouts, a Galatheid crab and an alvinocarid shrimp feed on mussels damaged during a sampling exercise.
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« Reply #8 on: May 11, 2007, 12:58:22 am »

A cold seep (sometimes called a cold vent) is an area of the ocean floor where hydrogen sulfide, methane and other hydrocarbon-rich fluid seepage occurs. Cold seeps are distinct from hydrothermal vents: the former's emissions are of the same temperature as the surrounding seawater, whereas the latter's emissions are super-heated. Cold seeps constitute a biome supporting several endemic species.

Entire communities of light independent organisms - known as extremophiles - develop in and around cold seeps, most relying on a symbiotic relationship with chemoautotrophic bacteria. These prokaryotes, both Archaea and Eubacteria, process sulfides and methane through chemosynthesis into chemical energy. Higher organisms, namely vesicomyid clams and siboglinid tube worms use this energy to power their own life processes, and in exchange provide both safety and a reliable source of food for the bacteria. Other bacteria form mats, blanketing sizable areas in the process.

 


Beggiatoal bacterial mat at a seep on Blake Ridge, off South Carolina. The red dots are range-finding laser beams.

Unlike hydrothermal vents, which are volatile and ephemeral environments, cold seeps emit at a slow and dependable rate. Likely owing to the differing temperatures and stability, cold seep organisms are much longer-lived than those inhabiting hydrothermal vents. Indeed, recent research has revealed that the seep tubeworm Lamellibrachia luymesi may be the longest living noncolonial invertebrate known, with a minimum lifespan of between 170 and 250 years.

Cold seeps were first discovered in 1984 by Dr. Charles Paull in the Gulf of Mexico at a depth of 3,200 metres. Since then, seeps have been discovered in other parts of the world's oceans, including the Monterey Canyon just off Monterey Bay, California, the Sea of Japan, off the Pacific coast of Costa Rica, in the Atlantic off of Africa, in waters off the coast of Alaska, and under an ice shelf in Antarctica [1]. The deepest seep community known is found in the Japan trench at a depth of 7326 m.

http://en.wikipedia.org/wiki/Cold_seep

Cold seeps develop unique topography over time, where reactions between methane and seawater create carbonate rock formations and reefs. These reactions may also be dependent on bacterial activity. Ikaite, a hydrous calcium carbonate, can be associated with oxidizing methane at cold seeps.

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