If the water level in an artesian well stands above the land surface, the well is a flowing artesian well. A particular aquifer at one place may be a confined aquifer but in other places may behave as an un-confined aquifer, when the water level falls below the base of the overlying confining layer. In the igneous and metamorphic rocks, groundwater may occur in confined conditions in joints and fractures. In volcanic rocks, the interflow spaces and the vesicular beds form confined aquifers. Examples of flowing artesian wells are found in the hard rock areas of South India.
A semi-confined aquifer is an aquifer that is situated between confining layers with a lower permeability. The upper (and lower) confining layers are semi-pervious, through which vertical leakage takes place due to head difference. These transmit limited quantities of groundwater. The general direction of flow in a semi-confined aquifer is horizontal. The preferred direction of flow in the confining layers above and below is vertical.
An unconfined aquifer is not overlain by any confining layer, but it has a confining layer at its bottom and is recharged by water seeping down from above in the form of rainfall and snow melt. Thus, an unconfined aquifer is a partly saturated aquifer bounded below by an aquiclude and above by the free water table or phreatic surface. At the free water table, the groundwater is at atmospheric pressure. In general, the water level in a well penetrating an unconfined aquifer does not rise above the water table, except when there is vertical flow. The unconfined aquifer is normally partly saturated with water, and the top of the saturated surface is known as the water table, which is under atmospheric pressure – it is also known as a water table/phreatic aquifer. The water level in wells perpetrating this aquifer indicates the position of the water table in the surrounding aquifer.
A semi-unconfined aquifer (also known as leaky aquifer) is an aquifer which exhibits characters in between semi-confined and unconfined aquifers as the permeability of the fine grained overlying layers is more than in a semiconfined aquifer and the horizontal flow component in it cannot be neglected. This type of aquifer is a completely saturated aquifer that is bounded below by an aquiclude and above by an aquitard.
If the overlying aquitard extends to the land surface, the aquifer may be partly saturated, but if the aquifer is overlain by an unconfined aquifer that is bounded above by the water table, the aquifer will be fully saturated. If there is hydrological equilibrium, the piezometric level in a well tapping a leaky aquifer may coincide with the water table. In areas with upward or downward flow (i.e., in discharge or recharge areas) the piezometric level may rise above or fall below the water table.
Finally, a multi-layered aquifer is a succession of leaky aquifers sandwiched between aquitards. Systems of interbedded permeable and less permeable layers are common in deep sedimentary basins.
See also: Aquiclude, Aquitard, Aquifuge.
Aromatic Hydrocarbons
An aromatic hydrocarbon (sometimes referred to as an arene) is a hydrocarbon of which the molecular structure incorporates one or more planar sets of six carbon atoms that are connected by delocalized electrons numbering the same as if they consisted of alternating single and double covalent. Aromatic hydrocarbons contain carbon and hydrogen atoms only.
Benzene (C6H6) is the least complex aromatic hydrocarbon, and it was the first one named as such. Each carbon atom in the hexagonal cycle has four electrons to share. One goes to the hydrogen atom, and one to each of the two neighboring carbon atoms which leaves one electron to share with one of the two neighboring carbon atoms, thus creating a double bond with one carbon and leaving a single bond with the other, which is why some representations of the benzene molecule portray it as a hexagon with alternating single and double bonds.
After benzene, aromatic hydrocarbons can be polycyclic (also called condensed aromatic systems) (Figure A-3):
Figure A-3 Examples of aromatic hydrocarbons showing the alternating double bonds and single bonds.
Other depictions of the structure portray the hexagon with a circle inside it, to indicate that the six electrons are floating around in delocalized molecular orbitals the size of the ring itself. This represents the equivalent nature of the six carbon–carbon bonds all of bond order 1.5; the equivalency is explained by resonance forms. The electrons are visualized as floating above and below the ring, with the electromagnetic fields they generate acting to keep the ring flat.
The general properties of aromatic hydrocarbons are: (i) they display aromaticity, (ii) the carbon–hydrogen ratio is high, (iii) they burn with a strong sooty yellow flame because of the high carbon–hydrogen ratio, and (iv) they undergo electrophilic substitution reactions and nucleophilic substitution reaction.
See also: Alkenes, Alicyclic Hydrocarbons, Aliphatic Hydrocarbons.
Arsenic and Selenium
Arsenic is a notoriously poisonous metalloid with many allotropic forms, including a yellow (molecular non-metallic) and several black and gray forms (metalloids). Three metalloid forms of arsenic, each with a different crystal structure, are found free in nature. However, it is more commonly found as arsenide and in arsenate compounds, several hundreds of which are known. Arsenic and its compounds are used as pesticides, herbicides, insecticides, and in various alloys.
Selenium a nonmetal, chemically related to sulfur and tellurium, which rarely occurs in its elemental state in nature. Isolated selenium occurs in several different forms, the most stable of which is a dense purplish-gray semimetal (semiconductor) form that is structurally a trigonal polymer chain. It conducts electricity and is used in photocells. Selenium is found in economic quantities in sulfide ores such as pyrite FeS2), partially replacing the sulfur in the ore matrix. Minerals that are selenide or selenate compounds are also known, but all are rare.
Selenium salts are toxic in large amounts, but trace amounts of the element are necessary for cellular function in most, if not all, animals, forming the active center of the enzymes glutathione peroxidase and thioredoxin reductase (which indirectly reduce certain oxidized molecules in animals and some plants) and three known deiodinase enzymes (which convert one thyroid hormone to another).
Arsenic and selenium occur in biomass to the extent of several parts per million and, on combustion of the coal, a varying quantity of these elements are released or retained in the ash, depending largely on the conditions under which the combustion takes place and on the nature of the ash.
Arsenic and selenium are determined by mixing a weighed sample with the Eschka mixture followed by ignition 750°C (1,380°F). The mixture is dissolved in hydrochloric acid, and the gaseous hydride of each element is generated from the appropriate oxidation state and determined by atomic absorption spectrophotometry. The method permits measurement of the total arsenic and selenium content of coal for the purpose of evaluating these elements where they can be of concern, for example, in coal combustion. When coal samples are prepared for analysis in accordance with standard procedure, the arsenic and selenium are quantitatively retained and are representative of the total amounts in the coal
See also: Periodic Table of the Elements.