An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit. The word was coined by William Whewell at the request of the scientist Michael Faraday from two Greek words: elektron, meaning amber, hodos, a way; the electrophore, invented by Johan Wilcke, was an early version of an electrode used to study static electricity. An electrode in an electrochemical cell is referred to as either a cathode; the anode is now defined as the electrode at which electrons leave the cell and oxidation occurs, the cathode as the electrode at which electrons enter the cell and reduction occurs. Each electrode may become either the anode or the cathode depending on the direction of current through the cell. A bipolar electrode is an electrode that functions as the anode of one cell and the cathode of another cell. A primary cell is a special type of electrochemical cell in which the reaction cannot be reversed, the identities of the anode and cathode are therefore fixed; the anode is always the negative electrode.
The cell can be discharged but not recharged. A secondary cell, for example a rechargeable battery, is a cell in which the chemical reactions are reversible; when the cell is being charged, the anode becomes the positive and the cathode the negative electrode. This is the case in an electrolytic cell; when the cell is being discharged, it behaves like a primary cell, with the anode as the negative and the cathode as the positive electrode. In a vacuum tube or a semiconductor having polarity the anode is the positive electrode and the cathode the negative; the electrons exit the device through the anode. Many devices have other electrodes to control operation, e.g. base, control grid. In a three-electrode cell, a counter electrode called an auxiliary electrode, is used only to make a connection to the electrolyte so that a current can be applied to the working electrode; the counter electrode is made of an inert material, such as a noble metal or graphite, to keep it from dissolving. In arc welding, an electrode is used to conduct current through a workpiece to fuse two pieces together.
Depending upon the process, the electrode is either consumable, in the case of gas metal arc welding or shielded metal arc welding, or non-consumable, such as in gas tungsten arc welding. For a direct current system, the weld rod or stick may be a cathode for a filling type weld or an anode for other welding processes. For an alternating current arc welder, the welding electrode would not be considered an anode or cathode. For electrical systems which use alternating current, the electrodes are the connections from the circuitry to the object to be acted upon by the electric current but are not designated anode or cathode because the direction of flow of the electrons changes periodically many times per second. Electrodes are used to provide current through nonmetal objects to alter them in numerous ways and to measure conductivity for numerous purposes. Examples include: Electrodes for fuel cells Electrodes for medical purposes, such as EEG, ECG, ECT, defibrillator Electrodes for electrophysiology techniques in biomedical research Electrodes for execution by the electric chair Electrodes for electroplating Electrodes for arc welding Electrodes for cathodic protection Electrodes for grounding Electrodes for chemical analysis using electrochemical methods Inert electrodes for electrolysis Membrane electrode assembly Electrodes for Taser electroshock weapon Chemically modified electrodes are electrodes that have their surfaces chemically modified to change the electrode's physical, electrochemical, optical and transportive properties.
These electrodes are used for advanced purposes in research and investigation
Cotton candy is a form of spun sugar. The confection is sugar, with small amounts of either flavoring or food coloring being added. Cotton candy is made by spinning it out through minute holes, it resolidifies in minutely thin strands of "sugar glass".. The final cotton candy contains air, with a typical serving weighing around 1 ounce or 28 grams, it is served at fairs, circuses and Japanese festivals, sold on a stick or in a plastic bag. Similar light halva confections include the Indian sohan papdi and pootharekulu, the Persian pashmak, the Turkish pişmaniye, although the latter is made with flour and water in addition to sugar. Tatar cuisine has similar flour-honey sweet sawdust talqysh-kalava. Similar sweets include Chinese dragon's-beard candy and Korean honey skein kkul-tarae. Several places claim the origin of cotton candy, with some sources tracing it to a form of spun sugar found in Europe in the 19th century. At that time, spun sugar was an expensive, labor-intensive endeavor and was not available to the average person.
Others suggest versions of spun sugar originated in Italy as early as the 15th century. Machine-spun cotton candy was invented in 1897 by dentist William Morrison and confectioner John C. Wharton, first introduced to a wide audience at the 1904 World's Fair as "Fairy Floss" with great success, selling 68,655 boxes at 25¢ per box. Joseph Lascaux, a dentist from New Orleans, invented a similar cotton candy machine in 1921. In fact, the Lascaux patent named the sweet confection “cotton candy” and the "fairy floss" name faded away, although it retains this name in Australia. In the 1970s, an automatic cotton candy machine was packaged it; this made it easier to produce and available to sell at carnivals and stores in the 1970s and on. Tootsie Roll Industries of Canada Ltd. the world's largest cotton-candy manufacturer, makes a bagged, fruit-flavored version called Fluffy Stuff. The United States declared National Cotton Candy Day to be on December 7. Typical machines used to make cotton candy include a spinning head enclosing a small "sugar reserve" bowl into which a charge of granulated, colored sugar is poured.
Heaters near the rim of the head melt the sugar, squeezed out through tiny holes by centrifugal force. Colored sugar packaged specially for the process is milled with melting characteristics and a crystal size optimized for the head and heated holes; the molten sugar solidifies in the air and is caught in a larger bowl which surrounds the spinning head. Left to operate for a period, the cotton-like product builds up on the inside walls of the larger bowl, at which point machine operators twirl a stick or cone around the rim of the large catching bowl, gathering the sugar strands into portions which are served on stick or cone, or in plastic bags; as the sugar reserve bowl empties, the operator recharges it with more feedstock. The product is sensitive to humidity, in humid summer locales, the process can be messy and sticky. Modern innovations in cotton-candy equipment include vending machines which automatically produce single servings of the product, developed in Taiwan, lighted or glowing sticks.
The source material for candy mesh is both colored and flavored. When spun, cotton candy is white because it is made from sugar, but adding dye or coloring transforms the color. Cotton candy was just white. In the US, what's known as'floss sugar' is available in a wide variety of flavors, but two flavor-blend colors predominate –'blue raspberry' and'pink vanilla', both formulated by the Gold Medal brand. Cotton candy comes out purple, when mixed, making a significant favorite at fairs. Cotton candy machines were notoriously unreliable until Gold Medal's invention of a sprung base in 1949, since they have manufactured nearly all commercial cotton-candy machines and much of the floss sugar in the US. Once spun, cotton candy is only marketed by color. Absent a clear name other than'blue', the distinctive taste of the blue raspberry flavor mix has gone on to become a compound flavor that some other foods borrow to invoke the nostalgia of cotton candy that people only get to experience on vacation or holidays.
Pink bubble gum went through a similar transition from specific branded product to a generic flavor that transcended the original confection, and'bubble gum flavor' shows up in the same product categories as'cotton candy flavor'. In 1978, the first automated machine was used for the production of cotton candy. Since the creations and innovations of this machine have become greater and greater, they range in sizes from counter-top accessible to carnival size. Modern machines that are made for commercial use can hold up to 3 pounds of sugar and have compartments for storage of extra flavors; the rotating bowl at the top spins at 3,450 revolutions per minute. Dragon's beard candy Candy making The complete Confectioners, Cook And Baker by M. Sanderson Cotton Candy Express. "History of Cotton Candy." Cotton Candy Express. N.p. n.d. Web. Sep 14, 2011. Cotton Candy. N.p. Aug 11, 2010. Web. Sep 14, 2011
Radioactive waste is waste that contains radioactive material. Radioactive waste is a by-product of nuclear power generation and other applications of nuclear fission or nuclear technology, such as research and medicine. Radioactive waste is hazardous to all forms of life and the environment, is regulated by government agencies in order to protect human health and the environment. Radioactivity decays over time, so radioactive waste has to be isolated and confined in appropriate disposal facilities for a sufficient period until it no longer poses a threat; the time radioactive waste must be stored for depends on the type of radioactive isotopes. Current approaches to managing radioactive waste have been segregation and storage for short-lived waste, near-surface disposal for low and some intermediate level waste, deep burial or partitioning / transmutation for the high-level waste. A summary of the amounts of radioactive waste and management approaches for most developed countries are presented and reviewed periodically as part of the International Atomic Energy Agency Joint Convention on the Safety of Spent Fuel Management and on the Safety of Radioactive Waste Management.
Radioactive waste comprises a number of radionuclides: unstable configurations of elements that decay, emitting ionizing radiation which can be harmful to humans and the environment. These isotopes emit different types and levels of radiation, which last for different periods of time; the radioactivity of all radioactive waste weakens with time. All radionuclides contained in the waste have a half-life—the time it takes for half of the atoms to decay into another nuclide—and all radioactive waste decays into non-radioactive elements. Certain radioactive elements will remain hazardous to humans and other creatures for hundreds of thousands of years. Other remain hazardous for millions of years. Thus, these wastes must be shielded for centuries and isolated from the living environment for millennia. Since radioactive decay follows the half-life rule, the rate of decay is inversely proportional to the duration of decay. In other words, the radiation from a long-lived isotope like iodine-129 will be much less intense than that of a short-lived isotope like iodine-131.
The two tables show some of the major radioisotopes, their half-lives, their radiation yield as a proportion of the yield of fission of uranium-235. The energy and the type of the ionizing radiation emitted by a radioactive substance are important factors in determining its threat to humans; the chemical properties of the radioactive element will determine how mobile the substance is and how it is to spread into the environment and contaminate humans. This is further complicated by the fact that many radioisotopes do not decay to a stable state but rather to radioactive decay products within a decay chain before reaching a stable state. Exposure to radioactive waste may cause serious death. In humans, a dose of 1 sievert carries a 5.5% risk of developing cancer, regulatory agencies assume the risk is linearly proportional to dose for low doses. Ionizing radiation causes deletions in chromosomes. If a developing organism such as a fetus is irradiated, it is possible a birth defect may be induced, but it is unlikely this defect will be in a gamete or a gamete-forming cell.
The incidence of radiation-induced mutations in humans is small, as in most mammals, because of natural cellular-repair mechanisms, many just now coming to light. These mechanisms range from DNA, mRNA and protein repair, to internal lysosomic digestion of defective proteins, induced cell suicide—apoptosisDepending on the decay mode and the pharmacokinetics of an element, the threat due to exposure to a given activity of a radioisotope will differ. For instance iodine-131 is a short-lived beta and gamma emitter, but because it concentrates in the thyroid gland, it is more able to cause injury than caesium-137 which, being water soluble, is excreted through urine. In a similar way, the alpha emitting actinides and radium are considered harmful as they tend to have long biological half-lives and their radiation has a high relative biological effectiveness, making it far more damaging to tissues per amount of energy deposited; because of such differences, the rules determining biological injury differ according to the radioisotope, time of exposure and sometimes the nature of the chemical compound which contains the radioisotope.
Radioactive waste comes from a number of sources. In countries with nuclear power plants, nuclear armament, or nuclear fuel treatment plants, the majority of waste originates from the nuclear fuel cycle and nuclear weapons reprocessing. Other sources include medical and industrial wastes, as well as occurring radioactive materials that can be concentrated as a result of the processing or consumption of coal and gas, some minerals, as discussed below. Waste from the front end of the nuclear fuel cycle is alpha-emitting waste from the extraction of uranium, it contains radium and its decay products. Uranium dioxide concentrate from mining is a thousand or so times as radioactive as the granite used in buildings, it is refined from yellowcake converted to uranium hexafluoride gas. As a gas, it undergoes enrichment to increase the U-235 content from 0.7% to about 4.4%. It is turned into a hard ceramic oxide for assembly as reactor fuel elements; the main by-product of enrichment is depleted uranium, principally the U-238 isotope, with a U-235 content of ~0.3%.
It is stored, either as UF6 or as U3O8. Some is used in applications where its extreme
Chemistry is the scientific discipline involved with elements and compounds composed of atoms and ions: their composition, properties and the changes they undergo during a reaction with other substances. In the scope of its subject, chemistry occupies an intermediate position between physics and biology, it is sometimes called the central science because it provides a foundation for understanding both basic and applied scientific disciplines at a fundamental level. For example, chemistry explains aspects of plant chemistry, the formation of igneous rocks, how atmospheric ozone is formed and how environmental pollutants are degraded, the properties of the soil on the moon, how medications work, how to collect DNA evidence at a crime scene. Chemistry addresses topics such as how atoms and molecules interact via chemical bonds to form new chemical compounds. There are four types of chemical bonds: covalent bonds, in which compounds share one or more electron; the word chemistry comes from alchemy, which referred to an earlier set of practices that encompassed elements of chemistry, philosophy, astronomy and medicine.
It is seen as linked to the quest to turn lead or another common starting material into gold, though in ancient times the study encompassed many of the questions of modern chemistry being defined as the study of the composition of waters, growth, disembodying, drawing the spirits from bodies and bonding the spirits within bodies by the early 4th century Greek-Egyptian alchemist Zosimos. An alchemist was called a'chemist' in popular speech, the suffix "-ry" was added to this to describe the art of the chemist as "chemistry"; the modern word alchemy in turn is derived from the Arabic word al-kīmīā. In origin, the term is borrowed from the Greek χημία or χημεία; this may have Egyptian origins since al-kīmīā is derived from the Greek χημία, in turn derived from the word Kemet, the ancient name of Egypt in the Egyptian language. Alternately, al-kīmīā may derive from χημεία, meaning "cast together"; the current model of atomic structure is the quantum mechanical model. Traditional chemistry starts with the study of elementary particles, molecules, metals and other aggregates of matter.
This matter can be studied in isolation or in combination. The interactions and transformations that are studied in chemistry are the result of interactions between atoms, leading to rearrangements of the chemical bonds which hold atoms together; such behaviors are studied in a chemistry laboratory. The chemistry laboratory stereotypically uses various forms of laboratory glassware; however glassware is not central to chemistry, a great deal of experimental chemistry is done without it. A chemical reaction is a transformation of some substances into one or more different substances; the basis of such a chemical transformation is the rearrangement of electrons in the chemical bonds between atoms. It can be symbolically depicted through a chemical equation, which involves atoms as subjects; the number of atoms on the left and the right in the equation for a chemical transformation is equal. The type of chemical reactions a substance may undergo and the energy changes that may accompany it are constrained by certain basic rules, known as chemical laws.
Energy and entropy considerations are invariably important in all chemical studies. Chemical substances are classified in terms of their structure, phase, as well as their chemical compositions, they can be analyzed using the tools of e.g. spectroscopy and chromatography. Scientists engaged in chemical research are known as chemists. Most chemists specialize in one or more sub-disciplines. Several concepts are essential for the study of chemistry; the particles that make up matter have rest mass as well – not all particles have rest mass, such as the photon. Matter can be a mixture of substances; the atom is the basic unit of chemistry. It consists of a dense core called the atomic nucleus surrounded by a space occupied by an electron cloud; the nucleus is made up of positively charged protons and uncharged neutrons, while the electron cloud consists of negatively charged electrons which orbit the nucleus. In a neutral atom, the negatively charged electrons balance out the positive charge of the protons.
The nucleus is dense. The atom is the smallest entity that can be envisaged to retain the chemical properties of the element, such as electronegativity, ionization potential, preferred oxidation state, coordination number, preferred types of bonds to form. A chemical element is a pure substance, composed of a single type of atom, characterized by its particular number of protons in the nuclei of its atoms, known as the atomic number and represented by the symbol Z; the mass number is the sum of the number of neutrons in a nucleus. Although all the nuclei of all atoms belonging to one element will have the same
Ceramic glaze is an impervious layer or coating of a vitreous substance, fused to a ceramic body through firing. Glaze can decorate or waterproof an item. Glazing renders earthenware vessels suitable for holding liquids, sealing the inherent porosity of unglazed biscuit earthenware, it gives a tougher surface. Glaze is used on stoneware and porcelain. In addition to their functionality, glazes can form a variety of surface finishes, including degrees of glossy or matte finish and color. Glazes may enhance the underlying design or texture either unmodified or inscribed, carved or painted. Most pottery produced in recent centuries has been glazed, other than pieces in unglazed biscuit porcelain, terracotta, or some other types. Tiles are always glazed on the surface face, modern architectural terracotta is often glazed. Glazed brick is common. Domestic sanitary ware is invariably glazed, as are many ceramics used in industry, for example ceramic insulators for overhead power lines; the most important groups of traditional glazes, each named after its main ceramic fluxing agent, are: Ash glaze, important in East Asia made from wood or plant ash, which contains potash and lime.
Feldspathic glazes of porcelain. Lead-glazed earthenware, is shiny and transparent after firing, which needs only about 800 °C, it has been used for about 2,000 years around the Mediterranean, in Europe, China. It includes Victorian majolica. Salt-glazed ware European stoneware, it uses ordinary salt. Tin-glazed pottery, which coats the ware with lead glaze made opaque white by the addition of tin. Known in the Ancient Near East and important in Islamic pottery, from which it passed to Europe. Includes faience, maiolica and Delftware. Modern materials technology has invented new vitreous glazes that do not fall into these traditional categories. Glazes need to include a ceramic flux which functions by promoting partial liquefaction in the clay bodies and the other glaze materials. Fluxes lower the high melting point of the glass formers silica, sometimes boron trioxide; these glass formers may be drawn from the clay beneath. Raw materials of ceramic glazes include silica, which will be the main glass former.
Various metal oxides, such as sodium and calcium, act as flux and therefore lower the melting temperature. Alumina derived from clay, stiffens the molten glaze to prevent it from running off the piece. Colorants, such as iron oxide, copper carbonate, or cobalt carbonate, sometimes opacifiers like tin oxide or zirconium oxide, are used to modify the visual appearance of the fired glaze. Glaze may be applied by dry-dusting a dry mixture over the surface of the clay body or by inserting salt or soda into the kiln at high temperatures to create an atmosphere rich in sodium vapor that interacts with the aluminium and silica oxides in the body to form and deposit glass, producing what is known as salt glaze pottery. Most glazes in aqueous suspension of various powdered minerals and metal oxides are applied by dipping pieces directly into the glaze. Other techniques include pouring the glaze over the piece, spraying it onto the piece with an airbrush or similar tool, or applying it directly with a brush or other tool.
To prevent the glazed article from sticking to the kiln during firing, either a small part of the item is left unglazed, or it's supported on small refractory supports such as kiln spurs and Stilts that are removed and discarded after the firing. Small marks left by these spurs are sometimes visible on finished ware. Decoration applied under the glaze on pottery is referred to as underglaze. Underglazes are applied to the surface of the pottery, which can be either raw, "greenware", or "biscuit"-fired. A wet glaze—usually transparent—is applied over the decoration; the pigment fuses with the glaze, appears to be underneath a layer of clear glaze. An example of underglaze decoration is the well-known "blue and white" porcelain famously produced in Germany, the Netherlands and Japan; the striking blue color uses cobalt as cobalt cobalt carbonate. Decoration applied on top of a layer of glaze is referred to as overglaze. Overglaze methods include applying one or more layers or coats of glaze on a piece of pottery or by applying a non-glaze substance such as enamel or metals over the glaze.
Overglaze colors are low-temperature glazes. A piece is fired first, this initial firing being called the glost firing the overglaze decoration is applied, it is fired again. Once the piece is fired and comes out of the kiln, its texture is smoother due to the glaze. Glazing of ceramics developed rather as appropriate materials needed to be discovered, firing technology able to reliably reach the necessary temperatures was needed. Glazed brick goes back to the Elamite Temple at Chogha Zanbil, dated to the 13th century BC; the Iron Pagoda, built in 1049 in Kaifeng, China, of glazed bricks is a well-known example. Lead glazed earthenware was made in China during the Warring States Period, its production increased during the Han Dynasty. High temperature proto-celadon glazed stoneware was made earlier than glazed earthenware, since the Shang Dynasty. During the Kofun period of Japan, Sue ware was decorated with greenish natural ash glazes. From 552 to 794 AD, differently colored glazes were introduced.
The three colored glazes of the Tang Dynasty were used for a period, but were phased out.
In condensed matter physics and materials science, an amorphous or non-crystalline solid is a solid that lacks the long-range order, characteristic of a crystal. In some older books, the term has been used synonymously with glass. Nowadays, "glassy solid" or "amorphous solid" is considered to be the overarching concept, glass the more special case: Glass is an amorphous solid that exhibits a glass transition. Polymers are amorphous. Other types of amorphous solids include gels, thin films, nanostructured materials such as glass doors and windows. Amorphous materials have an internal structure made of interconnected structural blocks; these blocks can be similar to the basic structural units found in the corresponding crystalline phase of the same compound. Whether a material is liquid or solid depends on the connectivity between its elementary building blocks so that solids are characterized by a high degree of connectivity whereas structural blocks in fluids have lower connectivity. In pharmaceutical industry, the amorphous drugs were shown to have higher bioavailability than their crystalline counterparts due to the high solubility of amorphous phase.
Moreover, certain compounds can undergo precipitation in their amorphous form in vivo, they can decrease each other's bioavailability if administered together. Amorphous materials have some shortrange order at the atomic length scale due to the nature of chemical bonding. Furthermore, in small crystals a large fraction of the atoms are the crystal; the most advanced structural characterization techniques, such as x-ray diffraction and transmission electron microscopy, have difficulty in distinguishing between amorphous and crystalline structures on these length scales. Amorphous phases are important constituents of thin films, which are solid layers of a few nanometres to some tens of micrometres thickness deposited upon a substrate. So-called structure zone models were developed to describe the micro structure and ceramics of thin films as a function of the homologous temperature Th, the ratio of deposition temperature over melting temperature. According to these models, a necessary condition for the occurrence of amorphous phases is that Th has to be smaller than 0.3, the deposition temperature must be below 30% of the melting temperature.
For higher values, the surface diffusion of deposited atomic species would allow for the formation of crystallites with long range atomic order. Regarding their applications, amorphous metallic layers played an important role in the discussion of a suspected superconductivity in amorphous metals. Today, optical coatings made from TiO2, SiO2, Ta2O5 etc. and combinations of them in most cases consist of amorphous phases of these compounds. Much research is carried out into thin amorphous films as a gas separating membrane layer; the technologically most important thin amorphous film is represented by few nm thin SiO2 layers serving as isolator above the conducting channel of a metal-oxide semiconductor field-effect transistor. Hydrogenated amorphous silicon, a-Si:H in short, is of technical significance for thin-film solar cells. In case of a-Si:H the missing long-range order between silicon atoms is induced by the presence by hydrogen in the percent range; the occurrence of amorphous phases turned out as a phenomenon of particular interest for studying thin-film growth.
Remarkably, the growth of polycrystalline films is used and preceded by an initial amorphous layer, the thickness of which may amount to only a few nm. The most investigated example is represented by thin multicrystalline silicon films, where such as the unoriented molecule. An initial amorphous layer was observed in many studies. Wedge-shaped polycrystals were identified by transmission electron microscopy to grow out of the amorphous phase only after the latter has exceeded a certain thickness, the precise value of which depends on deposition temperature, background pressure and various other process parameters; the phenomenon has been interpreted in the framework of Ostwald's rule of stages that predicts the formation of phases to proceed with increasing condensation time towards increasing stability. Experimental studies of the phenomenon require a defined state of the substrate surface and its contaminant density etc. upon which the thin film is deposited. R. Zallen; the Physics of Amorphous Solids.
Wiley Interscience. S. R. Elliot; the Physics of Amorphous Materials. Longman. N. Cusack; the Physics of Structurally Disordered Matter: An Introduction. IOP Publishing. N. H. March. A. Street. P. Tosi, eds.. Amorphous Solids and the Liquid State. Springer. D. A. Adler. B. Schwartz. C. Steele, eds.. Physical Properties of Amorphous Materials. Springer. A. Inoue. Amorphous and Nanocrystalline Materials. Springer. Journal of non-crystalline solids
Groundwater is the water present beneath Earth's surface in soil pore spaces and in the fractures of rock formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water; the depth at which soil pore spaces or fractures and voids in rock become saturated with water is called the water table. Groundwater is recharged from and flows to the surface naturally. Groundwater is often withdrawn for agricultural and industrial use by constructing and operating extraction wells; the study of the distribution and movement of groundwater is hydrogeology called groundwater hydrology. Groundwater is thought of as water flowing through shallow aquifers, but, in the technical sense, it can contain soil moisture, immobile water in low permeability bedrock, deep geothermal or oil formation water. Groundwater is hypothesized to provide lubrication that can influence the movement of faults, it is that much of Earth's subsurface contains some water, which may be mixed with other fluids in some instances.
Groundwater may not be confined only to Earth. The formation of some of the landforms observed on Mars may have been influenced by groundwater. There is evidence that liquid water may exist in the subsurface of Jupiter's moon Europa. Groundwater is cheaper, more convenient and less vulnerable to pollution than surface water. Therefore, it is used for public water supplies. For example, groundwater provides the largest source of usable water storage in the United States, California annually withdraws the largest amount of groundwater of all the states. Underground reservoirs contain far more water than the capacity of all surface reservoirs and lakes in the US, including the Great Lakes. Many municipal water supplies are derived from groundwater. Polluted groundwater is less visible and more difficult to clean up than pollution in rivers and lakes. Groundwater pollution most results from improper disposal of wastes on land. Major sources include industrial and household chemicals and garbage landfills, excessive fertilizers and pesticides used in agriculture, industrial waste lagoons and process wastewater from mines, industrial fracking, oil field brine pits, leaking underground oil storage tanks and pipelines, sewage sludge and septic systems.
An aquifer is a layer of porous substrate that transmits groundwater. When water can flow directly between the surface and the saturated zone of an aquifer, the aquifer is unconfined; the deeper parts of unconfined aquifers are more saturated since gravity causes water to flow downward. The upper level of this saturated layer of an unconfined aquifer is called the water table or phreatic surface. Below the water table, where in general all pore spaces are saturated with water, is the phreatic zone. Substrate with low porosity that permits limited transmission of groundwater is known as an aquitard. An aquiclude is a substrate with porosity, so low it is impermeable to groundwater. A confined aquifer is an aquifer, overlain by a impermeable layer of rock or substrate such as an aquiclude or aquitard. If a confined aquifer follows a downward grade from its recharge zone, groundwater can become pressurized as it flows; this can create artesian wells that flow without the need of a pump and rise to a higher elevation than the static water table at the above, aquifer.
The characteristics of aquifers vary with the geology and structure of the substrate and topography in which they occur. In general, the more productive aquifers occur in sedimentary geologic formations. By comparison and fractured crystalline rocks yield smaller quantities of groundwater in many environments. Unconsolidated to poorly cemented alluvial materials that have accumulated as valley-filling sediments in major river valleys and geologically subsiding structural basins are included among the most productive sources of groundwater; the high specific heat capacity of water and the insulating effect of soil and rock can mitigate the effects of climate and maintain groundwater at a steady temperature. In some places where groundwater temperatures are maintained by this effect at about 10 °C, groundwater can be used for controlling the temperature inside structures at the surface. For example, during hot weather cool groundwater can be pumped through radiators in a home and returned to the ground in another well.
During cold seasons, because it is warm, the water can be used in the same way as a source of heat for heat pumps, much more efficient than using air. The volume of groundwater in an aquifer can be estimated by measuring water levels in local wells and by examining geologic records from well-drilling to determine the extent and thickness of water-bearing sediments and rocks. Before an investment is made in production wells, test wells may be drilled to measure the depths at which water is encountered and collect samples of soils and water for laboratory analyses. Pumping tests can be performed in test wells to determine flow characteristics of the aquifer. Groundwater makes up about twenty percent of the world's fresh water supply, about 0.61% of the entire world's water, including oceans and permanent ice. Global groundwater storage is equal to the total amount of freshwater stored in the snow and ice pack, including the north and south poles; this makes it an important resource that can act as a natural storage that can buffer against shortages of surface water, as in during times of drought.
Groundwater is replenished b