The purpose of this Glossary is to provide a list of terms used in this document and commonly used by hydrogeologists, as well as some specific terms used in groundwater contamination assessments and Drinking Water Source Protections. These definitions are adapted from Gill and Lund (1984), EPA (1991) and the Drinking Water Source Protection Rule (R309-113).
Adit: Horizontal or nearly horizontal passage from the surface from which a mine is entered.
Alluvium: A general term for clay, silt, sand, gravel or similar unconsolidated material deposited during comparatively recent geologic time by a stream or other body of running water.
Analytical model: A model that provides approximate or exact solutions to simplified mathematical forms of the differential equations for water movement and solute transport. Analytical models can generally be solved using calculators or computers.
Anisotropy: The condition of having different properties in different directions. The condition under which one or more of the hydraulic properties of an aquifer vary according to the direction of flow.
Anticline: A fold in rock strata that is convex upward.
Aquifer test: A test to determine hydrologic properties of an aquifer, involving the withdrawal of measured quantities of water from, or addition of water to, a well and the measurement of resulting changes in head in the aquifer both during and after the period of discharge or addition. Same as pump test.
Aquifer/Aquifer System: A formation, group of formations, or part of a formation that contains sufficient saturated permeable material to yield sufficient, economical quantities of water to wells, springs, and drain tunnels.
Aquitard: The less-permeable beds in a stratigraphic sequence that tend to restrict or impede groundwater flow relative to the more permeable beds that serve as aquifers.
Area of influence: Area surrounding a pumping or recharging well within which the water table or potentiometric surface has been changed due to the well's pumping or recharge.
Artesian Conditions: In a confined aquifer, when the water level in a well rises above the top of the aquifer.
Attenuation: The process of diminishing contaminant concentrations in groundwater, due to filtration, biodegradation, dilution, sorption, volatilization, and other processes.
Collection area: The area surrounding a groundwater source which is underlain by collection pipes, tile, tunnels, infiltration boxes, or other groundwater collection devices.
Colluvium: Loose, heterogeneous, incoherent mass of soil material and/or rock fragments deposited chiefly by mass-wasting.
Conduit Flow: Groundwater movement down-gradient along fractures, faults, joints, bedding planes and solution openings resulting in "flashiness" -- a rapid response to recharge, turbulence, and highly variable chemistry, temperature and flow rates. Darcy's law is not operative under conduit flow conditions. (See Section II for further discussion.)
Cone of depression: A depression in the groundwater table or potentiometric surface that has the shape of an inverted cone and develops around a well from which water is being withdrawn. Its trace (perimeter) on the land surface defines the zone of influence of a well. Also called pumping cone and cone of drawdown.
Confined aquifer: The following criteria are met in order to verify and maintain an upward hydraulic gradient in the producing aquifer: an effective confining layer must exist between the ground surface and the producing aquifer. This confining layer must have a lower hydraulic conductivity than the producing aquifer; and the potentiometric surface of the producing aquifer must remain higher in elevation than the potentiometric surface of the overlying aquifer. If there is no overlying aquifer, then the potentiometric surface of the producing aquifer must remain higher in elevation than the upper surface of the overlying confining layer. These criteria must be maintained during periods of maximum and long-term pumping and seasonal groundwater fluctuations. Not all confined aquifers in nature have an upward hydraulic gradient; however, for the purposes of R309-113, an upward hydraulic gradient must be maintained.
Contact: The surface where two different kinds of rock come together.
Contaminant: An undesirable substance not normally present, or an unusually high concentration of a naturally occurring substance, in water, soil, or other environmental medium.
Contamination: The degradation of natural water quality as a result of man's activities.
Controls: The codes, ordinances, rules, and regulations currently in effect to regulate a potential contamination source.
Criteria: The conceptual standards that form the basis for WHPA delineation to include distance, groundwater time of travel, aquifer boundaries, and groundwater divides.
DDW: Division of Drinking Water.
Designated person: The person appointed by a PWS to ensure that the requirements of R309-113 are met.
Diffuse Flow: Groundwater movement down-gradient through interconnected fractures, faults, joints and bedding planes, resulting in less turbulent flow, less dramatic response to recharge events, and more uniform chemistry than observed in conduit flow. Groundwater movement is similar to that in porous flow environments, and may be described using Darcy's law. Groundwater movement through a fractured-rock aquifer may be considered to be diffuse flow if: (1) the fractures are closely spaced, (2) the fractures are evenly sized, evenly distributed, and randomly oriented, and (3) the area of consideration is large relative to the spacing of the fractures. Criteria such as pumping test responses, configuration of the water table, water chemistry variations, distribution of hydraulic conductivity, and the ratio of the fracture scale to the problem scale may be used to determine which type of flow regime exists. (See Section II for further discussion.)
Dike: Tabular igneous intrusion that cuts across planar bedding or foliation of the surrounding rock.
Dispersion: The spreading and mixing of chemical constituents in groundwater caused by diffusion and mixing due to microscopic variations in velocities within and between pores.
Drawdown: The vertical distance groundwater elevation is lowered, or the amount head is reduced, due to the removal of groundwater. Also the decline in potentiometric surface caused by the withdrawal of water from a hydrogeologic unit. The distance between the static water level and the surface of the cone of depression. A lowering of the water table of an unconfined aquifer or the potentiometric surface of a confined aquifer caused by pumping of groundwater from wells.
Existing groundwater source of drinking water: A public supply groundwater source for which plans and specifications are submitted to DDW on or before the effective date of the DWSP Rule.
Fissure: A fracture or crack in a rock along which there is a distinct separation.
Flow line: The general path that a particle of water follows under laminar flow conditions. Line indicating the direction followed by groundwater toward points of discharge. Flow lines generally are considered perpendicular to equipotential lines.
Flow model: A computer model that calculates a hydraulic head field for the study area using numerical methods to arrive at an approximate solution to the differential equation of groundwater flow.
Flow path: The path a water molecule or solute follows in the subsurface.
Flow System/Hydraulic Boundary: A hydrologic feature that prevents the flow of groundwater. Examples include groundwater divides or low permeability material that impedes groundwater flow.
Flowing Artesian: When the water level in a well rises above and flows at the ground surface.
Footwall: The lower side of a horizontal or inclined rock body or fault. If the fault has dip-slip translational movement along a normal fault, the footwall block is upthrown; the footwall block is downthrown along a reverse fault.
Fracture: A general term for any break in a rock, which includes cracks, joints, and faults.
Fractured (Rock) Hydrogeologic Setting: Groundwater movement down-gradient through any break in a rock (fracture). Fractures include cracks, joints, and faults caused by mechanical failure of the rock due to stress. Groundwater moving through fractures may behave as if it is in either porous (i.e. diffuse) flow or conduit flow.
Groundwater barrier: Rock or artificial material with a relatively low permeability that occurs (or is placed) below ground surface, where it impedes the movement of groundwater and thus may cause a pronounced difference in the heads on opposite sides of the barrier.
Groundwater basin: General term used to define a groundwater flow system that has defined boundaries and may include more than one aquifer. The basin includes both the surface area and the permeable materials beneath it. A rather vague designation pertaining to a groundwater reservoir that is more or less separate from neighboring groundwater reservoirs. A groundwater basin could be separated from adjacent basins by geologic boundaries or by hydrologic boundaries.
Groundwater divide: Ridge in the water table, or potentiometric surface, from which groundwater moves away at right angles in both directions. Line of highest hydraulic head in the water table or potentiometric surface.
Groundwater mound: Raised area in a water table or other potentiometric surface, aerated by groundwater recharge.
Groundwater source: Any well, spring, tunnel, adit, or other underground opening from or through which groundwater flows or is pumped from subsurface water bearing formations.
Hanging wall: The upper side of a horizontal or inclined rock body or fault. The hanging wall is downthrown along a normal fault with dip-slip movement; the hanging wall is upthrown along a reverse-slip fault.
Head, total: Height of the column of water at a given point in a groundwater system above a datum plane such as mean sea level. The sum of the elevation head (distance of a point above datum), the pressure head (the height of a column of liquid that can be supported by static pressure at the point), and the velocity head (the height to which the liquid can be raised by its kinetic energy).
Heterogeneity: Characteristic of a medium in which material properties vary from point to point.
Homogeneity: Characteristic of a medium in which material properties are identical throughout.
Hydraulic Conductivity (K): A coefficient of proportionality describing the rate at which water can move through a permeable medium. It is a function of the porous medium and the fluid.
Hydraulic Gradient (i): Slope of a water table or potentiometric surface. More specifically, change in head per unit of distance in a given direction, generally the direction of the maximum rate of decrease in head. The difference in hydraulic head divided by the distance along the flowpath.
Hydrogeologic methods: The techniques used to translate selected criteria and criteria thresholds into mappable delineation boundaries. These methods include, but are not limited to, arbitrary fixed radii, analytical calculations and models, hydrogeologic mapping, and numerical flow models.
Hydrogeologic unit: Any soil or rock unit or zone that because of its hydraulic properties has a distinct influence on the storage or movement of groundwater.
Impermeable: Characteristic of geologic materials that limit their ability to transmit significant quantities of water under the head differences normally found in the subsurface environment.
Interference: The result of two or more pumping wells, the drawdown cones of which intercept. At a given location, the total well interference is the sum of the drawdowns due to each individual well. The condition occurring when the area of influence of a water well comes into contact with or overlaps that of a neighboring well, as when two wells are pumping from the same aquifer or are located near each other.
Isotropy: The condition in which the properties of interest (generally hydraulic properties of the aquifer) are the same in all directions.
Karst Hydrogeologic Settings: Groundwater movement down-gradient through a network of permeable conduits which evolved as a consequence of dissolution of the host rocks (host rock must be soluble, e.g., limestone). Two major end-member types of groundwater movement occur in karst aquifers -- conduit flow and diffuse flow, as defined above.
Leakage: The vertical flow of groundwater; commonly used in the context of vertical groundwater flow through confining strata.
Limestone: A bedded sedimentary deposit consisting chiefly of calcium carbonate.
Management area: The area outside of zone one and within a two-mile radius where the Optional Two-mile Radius Delineation Procedure has been used to identify a protection area.
Management strategies: Zoning and non-zoning controls which include, but are not limited to, the following: zoning and subdivision ordinances, site plan reviews, design and operating standards, source prohibitions, purchase of property and development rights, public education programs, groundwater monitoring, household hazardous waste collection programs, water conservation programs, memoranda of understanding, written contracts and agreements, and so forth.
Maximum contaminant level (MCL): Maximum permissible level of a contaminant in water that is delivered to the users of a PWS. Maximum containment level is defined more explicitly in Safe Drinking Water Act (SDWA) regulations (40 CFR Section 141.2).
New groundwater source of drinking water: A public supply groundwater source of drinking water for which plans and specifications are submitted to DDW after the effective date of the DWSP Rule.
Nonpoint source: Any conveyance not meeting the definition of point source.
Normal fault: A fault, with an angle usually between 45-90 degrees, at which the hanging wall (upper block) has moved downward relative to the footwall (lower block).
Observation well: A well drilled in a selected location for the purpose of observing parameters such as water levels or water chemistry changes.
Permeability: Capacity of a rock or soil material to transmit a fluid.
Piezometric surface: See potentiometric surface.
Point source: Any discernible, confined, and discrete conveyance, including but not limited to any pipe, ditch, channel, tunnel, conduit, well, discrete fissure, container, rolling stock, animal feeding operation with more than ten animal units, landfill, or vessel or other floating craft, from which pollutants are or may be discharged. This term does not include return flows from irrigated agriculture.
Porosity: The ratio of the volume of void spaces in a rock or sediment to the total volume of the rock or sediment.
Porous Flow: Groundwater movement down-gradient through the pore space of aquifer host rocks, such as uncemented or poorly-cemented sandstones. Darcy's law is operative in porous flow environments. Groundwater movement through a fractured-rock aquifer may be considered to resemble porous flow if (1) the fractures are closely spaced (2) the fractures are evenly sized, distributed, and spatially oriented, and (3) the area of consideration is large relative to the spacing of the fractures.
Potable water: Suitable for human consumption as drinking water.
Potential contamination source: Any facility or site which employs an activity or procedure which may potentially contaminate ground water. A pollution source is also a potential contamination source.
Potentiometric Surface: A surface that represents the level to which water will rise in tightly cased wells. If the head varies significantly with depth in the aquifer, then there may be more than one potentiometric surface. The water table is a particular potentiometric surface for an unconfined aquifer.
Public Water System (PWS): See PWS.
Pump Test: A test to determine hydrologic properties of an aquifer, involving the withdrawal of measured quantities of water from, or additional of water to, a well and the measurement of resulting changes in head in the aquifer both during and after the period of discharge or addition.
PWS (EPA): Public Water System. Any water system providing water for human consumption for an average of at least 25 persons per day (or 15 or more service connections) and in use for at least 60 days each year. Further defined as follows:
Non-Transient, Non-Community Water System: A Public Water System that is not a community water system and that regularly serves at least 25 of the same persons over 6 months per year.
Radial flow: The flow of water in an aquifer toward a well.
Recharge area: Area in which water reaches the groundwater reservoir by surface infiltration. An area in which there is a downward component of hydraulic head in the aquifer.
Reverse fault: Fault with a dip greater than 45 degrees at which the hanging wall (upper block) appears to have moved upward relative to the footwall (lower block).
Sandstone: A cemented or otherwise compacted detrital sediment composed predominantly of quartz sand grains.
Shale: A laminated sediment in which the constituent particles are composed of clay. Same as mudstone, except mudstone may be composed of a percentage of silt and may or may not be laminated.
Stagnation point: A place in a groundwater flow field at which the groundwater is not moving.
Thrust fault: Fault with a dip of 45 degrees or less in which the hanging wall (upper block) appears to have moved upward relative to the footwall (lower block).
Time of travel (TOT): The time required for a particle of water to move in the saturated zone from a specific point to a groundwater source of drinking water.
Unconfined Aquifer: Any aquifer that does not meet the definition of a confined aquifer. An aquifer over which there is no confining strata and the water table forms the upper boundary.
Well field: An area containing two or more wells supplying a public water supply system.
Wellhead Protection Area (WHPA): The surface and subsurface area surrounding a water well or wellfield, supplying a Public Water System, through which contaminants are reasonably likely to move toward and reach such water well or well field.
Wellhead: The physical structure, facility, or device at the land surface from or through which groundwater flows or is pumped from subsurface, water-bearing formations.
Zone of Contribution (ZOC): The area surrounding a pumping well, spring, or tunnel that encompasses all areas and features that supply groundwater to the well spring, or tunnel.
Zone of Influence (ZOI): The area of influence, or cone of depression formed when pumping a well or well field.