Glossary of Environmental Terms

Our environmental glossary will help you stay up-to-date with key environmental terms and jargon relating to environmental compliance, remediation, and sustainability.


Advection – the transport of dissolved constituents with groundwater and is therefore dependent upon the hydraulic conductivity of the subsurface materials and hydraulic gradient in the aquifer

Aerobic biodegradation – the breakdown of organic contaminants by micro-organisms into smaller organic compounds when oxygen is present; carbon dioxide and water are often the final products

Air Toxics – chemicals released into the air that are known or suspected to cause cancer or other serious health problems

Aliphatic Compounds – a group of organic chemical compounds in which the carbon atoms are linked in open chains or ring compounds with double bonds

Anaerobic biodegradation – the breakdown of organic contaminants by micro-organisms when oxygen is not present

Anaerobic Respiration – some anaerobic bacteria use nitrate, sulfate, iron, manganese, or carbon dioxide as their electron acceptors, and break down organic chemicals into smaller compounds, often producing carbon dioxide and methane as final products

Analyte – compound for which an analytical laboratory has been requested to analyze a given sample or set of samples

Anoxic – “without oxygen”; anoxic groundwater contains no dissolved oxygen which is a common situation at hazardous waste sites–biodegradation processes often use up all of the available oxygen

Anthropogenic – caused by humans rather than nature

Aqueous Solubility – ability of a solution to be dissolved in water

Aquiclude – a body of rock that will slowly absorb water but will not transmit the water fast enough to supply a well or spring (from Dictionary of Geological Terms)

Aquifer – pockets of water existing below the water table; underground layer of water-bearing permeable rock or unconsolidated materials (e.g., gravel, sand, silt, clay) capable of a sustainable yield of a significant amount of water to a well or spring

Aquitard – body of impermeable or distinctly less-permeable material stratigraphically adjacent to one of more aquifers which slows but does not prevent the flow of water to or from an adjacent aquifer

Aromatic Compoundshydrocarbons containing one or more rings with double bonds (e.g., benzene)


Background contamination – the concentration of a regulated substance that is present at the site, but is not related to the release of a regulated substance at the site

BTEX – benzene, toluene, ethylbenzene, and xylene are four Volatile Organic Compounds found in gasoline; often found together and are similar in chemical properties; mobile in the subsurface and in the atmosphere, and of concern for underground storage tank remediation sites due to their mobility and toxicity

Best Management Practices (BMPs) – practical and effective measures taken to protect natural resources; has referred to pollution controls in the fields of industrial wastewater control, stormwater management, and wetland management

Biodegradation – breakdown of organic contaminants into smaller compounds by microbial organisms through metabolic or enzymatic processes, with carbon dioxide or methane as the final product; processes vary greatly but are key in the natural attenuation of contaminants of leaking underground storage tank and hazardous waste spill sites


Capillary Fringe – mostly saturated (or completely saturated) zone just above the water table in which groundwater is drawn up from a water table; water content decreases with distance above the water table (pores at the base of the capillary fringe are fully saturated due to tension saturation; if pore size is small and relatively uniform, it is possible that soils can be completely saturated with water for several feet above the water table; if pore size is large, the saturated portion will extend only a few inches above the water table)

CERCLA – Comprehensive Environmental Response, Compensation, and Liability Act of 1980

Co-metabolic/Co-metabolism – when two or more micro-organisms are required for the biodegradation of petroleum hydrocarbons; some compounds that are resistant to standard monocultural biodegradation have proved to be biodegradable with combinations of two, three, or more different species

Confined Aquifer – an aquifer separated from the ground surface or overlying aquifer by an aquiclude or aquitard to the extent that pressure can be created in the lower reaches of the aquifer without affecting either the soil surface or the upper reservoir of water

Constituents of Concern (COCs)contaminants in environmental media that may cause a risk to human health, safety, or the environment that have been identified for further evaluation, such as risk assessment

Contaminantregulated substance released into the environment


Data Quality Objectives (DQOs) – qualitative and quantitative statements specified to ensure that data of known and appropriate quality are obtained; involves a logical step-by-step procedure for determining which of the complex issues affecting a site are the most relevant to planning a site investigation before any data are collected

Dispersion – spread of dissolved constituents predominantly in the direction of groundwater flow, but also in directions other than would be expected due to groundwater movement only (lateral and vertical); causes some attenuation of the concentrations (lower concentrations) as the constituent move down-gradient

Dense Non-Aqueous Phase Liquid (DNAPL) – “pure product” (e.g., dry cleaning fluid) that remains undiluted as the original bulk liquid in the subsurface; more dense than water and will sink when they reach the water table


Effective Solubility – the maximum dissolved-phase concentration when a compound is part of a chemical mixture, calculated from the compound’s mole fraction in the mixture and its pure-phase solubility in water; always less than the chemical’s pure-phase solubility in water

Environmental Media – soil, sediment, surface water, groundwater, bedrock, and air


Fermentation – takes place when the organic chemical acts as an electron acceptor

Flammable Range – range of a concentration of a gas or vapor that will burn or explode if an ignition source is introduced; also known as “Explosive Range”

Free Productregulated substance that is present as a separate phase liquid; liquid not dissolved in water


Gasoline – typically composed of C4 to C12 hydrocarbons, with the majority of the mass between C4 and C10.


Heating Oil – typically used in the operation of heating equipment, boilers, or furnaces; petroleum that is No. 1, No. 2, No. 4-light, No. 4-heavy, No. 5-light, No. 5-heavy, and No. 6 technical grades of fuel oil; other residual fuel oils (including Navy Special Fuel Oil and Bunker C); other fuels when used as substitutes for one of these fuel oils

Henry’s Law Coefficient (or Constant) – the ratio of the vapor-phase concentration of an organic chemical relative to its dissolved-phase concentration in water

Hydraulic Conductivity (of Soils) – the ease with which pores of saturated soil permit water movement; a quantitative measure of a saturated soil’s ability to transmit water when subjected to a hydraulic gradient (from

Hydrogeology – combines knowledge of hydrology and geology to understand how water interacts with geological systems

Hydrology – the study of surface and subsurface water occurrence, distribution, movement, and quality and its relationship with the environment within each phase of the water cycle


In-Situ – in its original place; unmoved; unexcavated; remaining in the subsurface


Light Non-Aqueous Phase Liquid (LNAPL) – “pure product” (i.e., gasoline) that remains undiluted as the original bulk liquid in the subsurface. LNAPLS (such as gasoline, diesel, other fuels, most crude oils, and creosote) are less dense than water; when petroleum is released into the environment, it is typically as a LNAPL

Lower Explosive Limit (LEL) – below the explosive/flammable range; mixture is too lean to burn; also known as “Lower Flammable Limit


Maximum Contaminant Levels (MCLs) – highest concentration levels allowed by law in public water supplies promulgated by the EPA and/or the applicable State environmental regulatory agency

Methanogenesis/Methanogenicfermentation by micro-organisms of simple organic carbon compounds or oxidation of hydrogen under anaerobic conditions with the production of carbon dioxide and methane; methanogenic conditions prevail in many contamination plumes after all other electron acceptors have been used up by other members of the subsurface microbial community (from

Method Detection Limit – takes into account the reagents, sample matrix, and preparation steps applied to a sample in specific analytical methods

Micro-organisms – bacteria, fungi, protozoa, and actinomycetes typically found in soil that assist in the processes of biodegradation and/or natural attenuation

Monitoring Well (MW) – a well constructed exclusively to monitor and/or sample conditions of a water-bearing aquifer, e.g., water pressure, depth, movement, temperature, or quality

Motor Fuel – petroleum or petroleum-based substance that is motor gasoline, aviation gasoline, No. 1 or No. 2 diesel fuel, or any grade of gasohol, and is typically used in the operation of an internal combustion engine


Natural Attenuation – includes a variety of physical, chemical, or biological processes that act without human intervention to reduce the mass, toxicity, mobility, volume, and/or concentration of contaminants in soil or groundwater

Non-Detectable Concentrations – chemicals that are not detected in a sample above a certain limit, usually the quantitation limit for the chemical in the sample

National Pollutant Discharge Elimination System (NPDES) – the National system for the issuance of permits under section 402 of the Federal Clean Water Act including a state or interstate program which has been approved in whole or in part by the EPA


Over-excavation – additional excavation needed after an underground storage tank and the surrounding soil have been removed; confirmation samples are collected from the walls and floor of the excavation pit and sent for laboratory analysis to ensure that the remaining soil does not present a threat to groundwater

Occupational Safety and Health Administration (OSHA) – purpose is to “assure safe and healthful working conditions for working men and women by setting and enforcing standards and by providing training, outreach, education and assistance”signed into law by President Nixon on December 29, 1970 (via

Overfill – a release that occurs when a tank is filled beyond its capacity

Oxygenate – fuel additives (alcohols and ethers) that contain oxygen, which can boost gasoline’s octane quality, enhance combustion, and reduce exhaust emissions


Physico-Chemical Properties – shorthand phrase referring to the physical and chemical properties of a given chemical or compound; properties are needed to evaluate fate and transport of chemical; including, but not limited to, aqueous solubility, vapor pressure, density, Henry’s Law Coefficient, specific gravity, and biodegradability

Polycyclic Aromatic Hydrocarbons (PAHs) – chemical compounds that consist of fused aromatic rings. Some of these compounds have high toxicity values and have been identified as carcinogenic, mutagenic, and/or teratogenic.

Practical Quantitation Limit (PQL) – the lowest limit that can be reliably achieved within specified limits of precision and accuracy under routine laboratory conditions for a specified matrix; based on quantitation, precision and accuracy, normal operation of a laboratory, and the practical need in a compliance-monitoring program to have a sufficient numbers of laboratories available to conduct the analyses


Receptor – human or other living organism with the potential to be exposed to and adversely affected by contaminants because it is present at the source or along the contaminant migration pathway

Regulated Discharge – point or nonpoint source discharge subject to the permit or approval requirements of state and Federal regulations and any diffuse surface or groundwater discharge to surface waters which has the potential to cause an exceedance of the water quality standards

Regulated Substance – an element, compound, mixture, solution, or substance that, when released into the environment, may present substantial danger to public health, welfare, or environment; includes any hazardous substance as defined by CERCLA, petroleum, and any other substance determined by the environmental regulating agency (via PADEP)

Remediation – to clean up, mitigate, correct, abate, minimize, eliminate, control, or prevent a release of a regulated substance into the environment to protect the present or future of public health, safety, welfare, or the environment, including preliminary actions to study or assess the release

Reportable Release – a quantity or unknown quantity of regulated substance released to or posing an immediate threat to surface water, groundwater, bedrock, soil, or sediment; release reporting quantities and requirements are state- and material-specific

Reporting Limit – lowest concentration at which a contaminant is reported

Risk Assessment – process to quantify the risk posed by exposure of a human or ecological receptor to regulated substances; includes baseline risk assessments, development of site-specific standards, and risk assessment of the remedial alternatives


Saturated Zone – subsurface zone in which all  the spaces in the rock or soil are filled with water; the water table is the top of the saturated zone in an unconfined aquifer

Sediment – solid fragmental material that originates from weathering of rocks and is transported or deposited by air, water, or ice; accumulates by other other natural agents such as chemical precipitation from solution or secretion by organisms; forms in layers; includes sand, gravel, silt, mud, till, loess, and alluvium

Smear Zone – area where free product has been transported through soils due to a fluctuating water table; a zone in soil that is either above or below the water table at any given time

Soil – unconsolidated materials above bedrock

Solubility – relative capability of being dissolved

Stakeholders – all parties with a direct or indirect interest in the outcome of any activity

Stratigraphy – a description of the rock or soil strata in the subsurface, particularly the sequence of layer


Total Dissolved Solids (TDS) – amount of inorganic materials such as minerals, salts, metals, cations, or anions dissolved in water


Upper Explosive Limit (UEL) – above the explosive/flammable range; the mixture is to rich to burn; also known as the “Upper Flammable Limit

Unsaturated Zone – area between the ground surface and the aquifer’s water table within which the moisture content is less than the saturation and the pressure is less than atmospheric; soil pore spaces in this zone may contain air or other gases; also known as the “Vadose Zone


Vapor Pressure – equilibrium between gas and solid; force exerted by the gas or vapor released by a liquid or solid substance in a closed container; an indication of a liquid’s evaporation rate

Volatile Organic Compounds (VOCs) – organic chemicals that are easily evaporated at normal room temperature; BTEX chemicals, benzene, ethylene, glycol, formaldehyde


Water Quality Objectives (WQOs) – narrative on numerical criteria designed to define appropriate levels of environmental quality and to control activities that can adversely affect aquatic systems