Hydrogeology Groundwater Water Resources Environment

Glossary of Hydrogeological Terms (Derived from a range of sources, including Environment Agency R&D Publication 20, 1999)

Absorption: The incorporation of a chemical within a solid or liquid.

Adsorption: The attachment of a chemical to the surface of a solid or liquid.

Advection: Mass transport caused by the bulk movement of flowing groundwater.

Aquifer: A permeable geological stratum or formation that is capable of both storing and transmitting significant volumes of water, typically through pores, intergranular pore-space and/or transmissive fissures and fractures.

Attenuation: Reduction in contaminant concentration through physical, chemical and biological processes as it passes through a medium.

Biodegradation: The breakdown of a substance or chemical by living organisms, commonly bacteria.

Compliance point: Location agreed by the regulator where the remedial target concentration must be achieved.

Conservative pollutants: Pollutants that can move readily through the aquifer with negligible reaction with the rock matrix and that are unaffected by biodegradation. (chloride can be seen as a conservative pollutant).

Controlled waters (as defined by Water Resources Act 1991, Part III, Section 104): All rivers, streams, springs, canals, lakes, groundwaters, estuaries and coastal waters to three nautical miles from the shore.

Dense non-aqueous phase liquid (DNAPL): A liquid immiscible with water that has a density greater than water and so sinks in water.

Diffusion: Migration of substances by natural movement of their particles.

Dilution: Reduction in concentration brought about by the addition of water.

Dispersion: Irregular spreading of solutes due to aquifer heterogeneities at pore-grain scale (mechanical dispersion) or at field scale (macroscopic dispersion)

Effective rainfall: The amount of rain available for recharge to the aquifer after evapotranspiration (length units).

Eluate:A solution resulting from the mixing of soil and water in order to remove sorbed substances.

[Chemical] Equilibrium: No net transfer between two phases.

Free phase contamination: Product (e.g. gasoline, diesel) that is present in its original state and at a high saturation. May also include coal tars.

Groundwater: The mass of water in the ground below the water table (saturated zone) occupying the total pore space in the rock and moving slowly down the hydraulic gradient where permeability allows.

Henry’s law constant: Coefficient that represents the equilibrium partitioning factor between a solute in the water and vapour phases.

Hydraulic conductivity: A coefficient of proportionality describing the rate at which water can move through a permeable medium. The density and kinematic viscosity of the water must be considered in determining hydraulic conductivity.

Hydraulic gradient: The change in total head with a change in distance in a given direction. The direction is that which yields a maximum rate of decrease in head.

Hydraulic head: The sum of the elevation head, the pressure head and the velocity head at a given point in the aquifer.

Intergranular: The space between the grains of a rock or soil.

Light non-aqueous phase liquid (LNAPL): A liquid immiscible with water that has a density less than water and so floats on water.

Non-aqueous phase liquid (NAPL): Liquids that are immiscible with water.

Non-polar molecule: A molecule not susceptible to permanent charge, usually without ionisable groups attached.

Partition coefficient: In a heterogeneous system of two or more phases in equilibrium , the ratio of activities (or less accurately the concentrations) of the same molecular species in the phases is a constant at constant temperature. This constant is termed the partition coefficient.

Partitioning: The process by which a contaminant, released originally in one phase (e.g. adsorbed to soil grains), becomes distributed between other phases (i.e. vapour and dissolved phase).

Pathway: A route along which a particle of water, substance or contaminant moves through the environment.

Perched water: This is a layer of saturated soil formed above the main water table due to a layer of low permeability material intercepting water moving downwards through the unsaturated zone.

Permeability: Measure of the ability to transmit water. Defined as the volume of water passing through 1m² of aquifer under unit hydraulic gradient; units m³/m²d or m/d.

Polar molecule: A charged molecule that is affected by changes in pH.

Pollution [as defined by Environmental Protection Act 1990]: Pollution of the environment due to the release (into any environmental medium) from any process of substances which are capable of causing harm to man or any other living organism supported by the environment.

Pollution [of groundwater, as defined in Groundwater Directive]: The discharge by man, directly or indirectly, of substances or energy into groundwater, the results of which are such as to endanger human health or water supplies, harm living resources and the aquatic ecosystem or interfere with other legitimate uses of water.

Pore water: Any free water (that is, not adsorbed within the matrix of a soil or rock and incapable of participating in contaminant movement) contained within the primary pore space or within fissures in either the unsaturated or saturated zone.

Porosity: The ratio of the volume of void spaces in a rock or sediment to the total volume of the rock or sediment.

Receptor: An entity/organism or controlled water that is being or could be harmed by a potential pollutant.

Recharge: The amount of water that reaches a water source such as an aquifer, which is calculated as rainfall less run-off, evapotranspiration and soil storage.

Remedial target: The goal of remedial activity set at the compliance point, in the form of a desired concentration in the soil or groundwater.

Retardation: A measure of the reduction in solute velocity relative to the velocity of the advecting groundwater caused by processes such as adsorption.

Saturated zone: The zone in which the voids of the rock or soil are filled with water at a pressure greater than atmospheric. The water table is the top of the saturated zone in an unconfined aquifer.

Sentinel borehole: A monitoring borehole up-gradient of a receptor.

Source Protection Zone (SPZ): An area designated around a groundwater source, the maximum extent of which is the catchment area for the source and within which there are limits to the processes and activities that can occur within that area.

Sorption: Absorption and adsorption considered jointly.

Surrogate borehole: A borehole located between the source and the receptor, at which a target concentration may be defined.

Target concentration: Derived chemical concentration at a regulatory agreed compliance point.

Total soil concentration: The total concentration of contaminants within the soil matrix, whether it be absorbed, adsorbed or in free phase.

Unsaturated zone [also called zone of aeration and vadose zone]: The zone between the land surface and the water table. It includes the root zone, intermediate zone and capillary fringe. The pore spaces contain water at less than the atmospheric pressure, as well as air and other gases. Saturated bodies, such as perched groundwater, may exist in the unsaturated zone.
Volatilisation The process by which the pure liquid phase turns to the gaseous phase, or boils.