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APPEND 8 A Glossary alluvial Pertaining to processes or materials associated with transportation or deposition by running water. aguiclude A formation that may contain ground water but is incapable of transmitting it in significant quantities. Aquicludes often form confining layers for aquifers. aquifer A sufficiently permeable formation storing, transmitting, and yielding ground water in useable quantities. aquitard A layer of low permeability that may contain and transmit water from one aquifer to another but cannot itself serve as an aquifer. An aquitard is a leaky, semiconfining layer. argillaceous Containing appreciable clay. artesian aquifer See confined aquifer. bulk density The weight of a dry porous medium per unit volume of the material in its natural conditions (including pores and voids). calcareous Containing sufficient free carbonate to effervesce visibly when treated with cold 0.1 M HC1. carbonaceous Pertaining to, or rich in, carbon. elastic Consisting of fragments of rocks or of organic structures that have been moved individually from their places of origin. -113
-114- coastal plain Any plain that has its margin on the shore of a large body of water, particularly the sea. colluvium A general term applied to deposits on a slope or at the foot of a sloppier cliff that were moved there chiefly by gravity. confined aquifer An aquifer with an upper boundary that is a confining layer and having a pressure potential at this boundary in excess of atmospheric. The potentiometric surface of a confined aquifer is above the top of the aquifer. Also known as an artesian aquifer. confining layer A layer of material having a permeability lower than that of the associated aquifer. If the permeability is essentially zero, the confining layer is impermeable and may be an aquifuge or an aquiclude. If the permeability is small relative to that of the adjoining aquifer, the layer is said to be leaky and is called an aquitard. Darcy's equation A formula stating that the flow rate of water through a porous medium is proportional to the hydraulic gradient. The factor of proportionality is the hydraulic conductivity. fluvial Of, or pertaining to, rivers; produced by river action. , fracture A crack in a rock formation usually formed by stresses imposed on or released from the formation. Fractures may be interconnected or isolated. fragipan A natural subsurface soil horizon with high bulk density and/or high mechanical strength relative to the soil horizons above, seemingly cemented when dry, but when moist showing a moderate to weak brittleness. ground water Subsurface water under a pressure greater than atmospheric. hysteretic function The difference between water content and water pressure relations in unsaturated porous media due to antecedent conditions, i.e., wetting or drying.
- 115 - hydraulic conductivity The factor of proportionality in Darcy's equation relating flow velocity to hydraulic gradient having units of length per unit of time. A property of the porous medium and the water content of the medium. , hydraulic head The energy per unit weight of water made up of the sum of the pressure potential (head), velocity potential (head), and elevation potential (head). The velocity head is often negligible and taken as zero for subsurface flow. Also called water potential. illuvial (clay) Clay that has moved into a soil horizon from the soil horizons above. intrinsic permeability A quantitative measure of water-transmitting ability of a porous medium that is related to the size and interconnectedness of the void openings. lenticular Shaped approximately like a double convex lens. Ethology The physical character of a rock, generally as determined megascopically or with the aid of a low-power magnifier. loess A sediment, commonly nonstratified and commonly unconsolidated, composed dominantly of silt-size particles, ordinarily with accessory clay and sand, deposited primarily by the wind. perched aquifer A localized unconfined aquifer formed above a relatively impermeable layer. May be seasonal due to recharge patterns and leakage through and flow around the restricting layer. permeability A description of the ease with which a fluid may move through a porous medium; abbreviation of intrinsic permeability. phreatic surface Same as water table. piezometric surface The surface defined by a pressure potential and position. For an unconfined aquifer it is equal to the elevation of the water table. For a confined aquifer it is equal to the elevation to which water would rise in a well penetrating and open to the aquifer. porosity A measure of the total void space present in a volume of formation. The percentage of any volume of material occupied by voids.
-116- Voids may be filled with water or air. If all of the voids are filled with water, saturation exists. Porosity consists of primary and secondary porosity. potential gradient or hydraulic gradient The change in hydraulic head per unit distance. primary porosity Porosity composed of voids between individual particles such as grains of sand and gravel or clay or silt particles. residuum Unconsolidated and partly weathered mineral materials accumulated by disintegration of consolidated rock in place. saline (soil) A soil containing sufficient soluble salt to adversely affect the growth of most crop plants. saturation The condition that exists when all voids are filled with water. secondary porosity Porosity due to fractures, solution channels, root channels, and animal burrows. sodic (soils A nonsaline soil containing sufficient exchangeable sodium to adversely affect crop production and soil structure under most soil and plant type conditions. solution channels Channels formed within soluble rocks (such as limestones by the action of water and chemicals in dissolving rock material. specific capacity The Yield of a well per unit crop or the water ~eve' In the we'' (volume per unit time per unit lengthy. ~ ~ , specific retention The volume of water retained per unit volume of material when the material is allowed to drain due to gravity. Similar to field capacity in an agricultural setting. Specific retention plus specific yield equals porosity. specific Yield The volume of water drained per material is Specific yield _ the same for an unconfined aquifer. Specific yield plus specific retention equals porosity. unit volume of material when the allowed to drain due to gravity and the storage coefficient are
-117- storage coefficient The volume of water released per unit area of aquifer and per unit drop in head. till (glacial) Nonsorted, nonstratified sediment carried or deposited by a glacier. unconfined aquifer An aquifer with a water table as an upper surface. Also known as a water table aqua: :er. vadose zone The zone between the surface and the aquifer containing water at pressures less than atmospheric. It includes the capillary fringe. water table The surface defining the location where the pressure potential is atmospheric for an unconfined aquifer. Equivalent to the phreatic surface. The water table is the top of an unconfined aquifer .