Aquifer Definitions

Some useful definitions for talking intelligently about aquifers:

Action Level (AL):
The concentration of a contaminant which, if exceeded, triggers a treatment or other requirement which a water system must follow.

Acute Health Effect: An immediate (i.e. within hours or days) effect that may result from exposure to certain drinking water contaminants (e.g., pathogens).

Aquifer: A natural underground layer, often of sand or gravel, that contains water.

Aquitard: A water-saturated sediment or rock whose permeability is so low it cannot transmit any useful amount of water.

Artesian: A condition produced when an aquifer is confined by layers of low-permeability and the water level in a well rises above the level of the local water table; if it rises as far as the land surface, it is a flowing artesian well.

Best Available Technology (BAT): The water treatment(s) that the U.S. Environmental Protection Agency (EPA) certifies to be the most effective for removing a contaminant.

Chronic Health Effect: The possible result of exposure over many years to a drinking water contaminant at levels above its MCL.

Coliform: A group of related bacteria whose presence in drinking water may indicate contamination by disease-causing microorganisms.

Community Water System (CWS): A water system which supplies drinking water to 25 or more of the same people year-round in their residences.

Compliance: The act of meeting all state and federal drinking water regulations.

Confined Aquifer: An aquifer that is sandwiched between two aquitards. Confinement can raise the water pressure in an aquifer sufficiently high to produce an artesian well.

Contaminant: Anything found in water (including microorganisms, minerals, chemicals, radionuclides, etc.) which may be harmful to human health.

Cryptosporidium: A microorganism commonly found in lakes and rivers which is highly resistant to disinfection. Cryptosporidium has caused several large outbreaks of gastrointestinal illness, with symptoms that include diarrhea, nausea, and/or stomach cramps. People with severely weakened immune systems (that is, severely immuno-compromised) are likely to have more severe and more persistent symptoms than healthy individuals.

Department of Environmental Quality (DEQ):
State agency charged with creating and enforcing drinking water standards. Visit their web site at:

Disinfectant: A chemical (commonly chlorine, chloramine, or ozone) or physical process (e.g., ultraviolet light) that kills microorganisms such as bacteria, viruses, and protozoa.

Distribution System: A network of pipes leading from a treatment plant to customers' plumbing systems.

Environmental Protection Agency (EPA)
Federal agency charged with creating and enforcing drinking water standards. Visit their web site at:

Finished Water: Water that has been treated and is ready to be delivered to customers.

Giardia lamblia: A microorganism frequently found in rivers and lakes, which, if not treated properly, may cause diarrhea, fatigue, and cramps after ingestion.

Grain: A particle of a mineral or rock, generally lacking well-developed crystal faces.

Ground Water (groundwater): The water that systems pump and treat from aquifers (natural reservoirs below the earth's surface).

Health Advisory: An EPA document that provides guidance and information on contaminants that can affect human health and that may occur in drinking water, but which EPA does not currently regulate in drinking water.

Hydraulic Head: A combined measure of the elevation and the water pressure at a point in an aquifer which represents the total energy of the water; since ground water moves in the direction of lower hydraulic head (i.e toward lower energy), and hydraulic head is a measure of water pressure, ground water can and often does flow 'uphill'.

Inorganic Contaminants: Mineral-based compounds such as metals, nitrates, and asbestos. These contaminants are naturally-occurring in some water, but can also get into water through farming, chemical manufacturing, and other human activities. EPA has set legal limits on 15 inorganic contaminants.

Maximum Contaminant Level (MCL): The highest level of a contaminant that is allowed in drinking water. MCL’s are set as close to the MCLG’s as feasible using the best available treatment technology. MCLs are established by the Environmental Protection Agency.

Maximum Contaminant Level Goal (MCLG): The level of a contaminant at (and below) which there is no known or expected risk to health. MCLG’s allow for a margin of safety, however, this goal is not always economically or technologically feasible, and is not legally enforceable.

Microorganisms: Tiny living organisms that can be seen only with the aid of a microscope. Some microorganisms can cause acute health problems when consumed in drinking water. Also known as microbes.

Million fibers per liter (MFL)
Measure of the presence of asbestos fibers that are longer than 10 micrometers.

Millirems per year (mrem/yr)
Measure of radiation absorbed by the body.

Monitoring: Testing that water systems must perform to detect and measure contaminants. A water system that does not follow EPA's monitoring methodology or schedule is in violation, and may be subject to legal action.

Nephelometric Turbidity Unit (NTU)
Nephelometric turbidity unit is a measure of the clarity of water. Turbidity in excess of 5 NTU is just noticeable to the average person.

Non-Detects (ND)
Not detected in the water at the testing limits, i.e. laboratory analysis indicates that the constituent is not present.

Non-Transient, Non-Community Water System: A water system which supplies water to 25 or more of the same people at least six months per year in places other than their residences. Some examples are schools, factories, office buildings, and hospitals which have their own water systems.

Organic Contaminants: Naturally occurring or synthetic substances containing mainly carbon, hydrogen, nitrogen, and oxygen. This includes most pesticides and industrial chemicals. To date, EPA has set legal limits on approximately 56 organic contaminants.

Parts per million (ppm), or milligrams per liter (mg/l)
Indicates the amount of a contaminant found in a million parts of water. This corresponds to one penny in $10,000.

Parts per billion (ppb), or micrograms per liter (ug/l)
Indicates the amount of a contaminant found in a billion parts of water. This corresponds to one penny in $10,000,000.

Parts per trillion (ppt), or nanograms per liter (nanograms/l)
One part per trillion corresponds to one minute in 2,000,000 years, or a single penny in $10,000,000,000.

Parts per quadrillion (ppq), or picograms per liter (picograms/l)
One part per trillion corresponds to one minute in 2,000,000,000 years, or a single penny in $10,000,000,000,000.

Pathogen: A disease-causing organism.

Perched aquifer: A type of unconfined aquifer that sits above another unconfined aquifer because water infiltrating from the surface is trapped or 'perched' on a shallow aquitard.

Permeability: The amount of interconnected porosity and the ease with which water can move through it; clay can have double the porosity of gravel but the pore spaces are so small it may be 10,000 times less permeable.

Picocuries per liter (pCi/L)
Measure of radioactivity in water.

Pore space: The spaces within a rock body that are unoccupied by solid material. Pore spaces include spaces between grains, fractures, vesicles, and voids formed by dissolution.

Porosity: The amount of water held per volume of aquifer material; this is expressed as a percentage e.g. 25% means that one cubic foot of aquifer holds 0.25 cubic feet of water. If all the porosity is filled with water, the rock is termed water-saturated.

Primacy State: A State that has the responsibility and authority to administer EPA's drinking water regulations within its borders. The State must have rules at least as stringent as EPA's.

Public Notification: An advisory that EPA requires a water system to distribute to affected consumers when the system has violated MCLs or other regulations. The notice advises consumers what precautions, if any, they should take to protect their health.

Public Water System (PWS): Any water system which provides water to at least 25 people for at least 60 days annually. There are more than 170,000 PWSs providing water from wells, rivers and other sources to about 250 million Americans. The others drink water from private wells. There are differing standards for PWSs of different sizes and types.

Radionuclides: Any man-made or natural element that emits radiation and that may cause cancer after many years of exposure through drinking water.

Raw Water: Water in its natural state, prior to any treatment for drinking.

Sample: The water that is analyzed for the presence of EPA-regulated drinking water contaminants. Depending on the regulation, EPA requires water systems and states to take samples from source water, from water leaving the treatment facility, or from the taps of selected consumers.

Sanitary Survey: An on-site review of the water sources, facilities, equipment, operation, and maintenance of a public water systems for the purpose of evaluating the adequacy of the facilities for producing and distributing safe drinking water.

Saturated: The condition wherein the pore spaces in a solid (such as rock) are completely filled with liquid (such as water).

Saturated zone: The zone in the subsurface in which all pore spaces are filled with water.

Secondary Drinking Water Standards: Non-enforceable federal guidelines regarding cosmetic effects (such as tooth or skin discoloration) or aesthetic effects (such as taste, odor, or color) of drinking water.

Sole Source Aquifer: An aquifer that supplies 50 percent or more of the drinking water of an area.

Source Water: Water in its natural state, prior to any treatment for drinking.

Surface Water: The water that systems pump and treat from sources open to the atmosphere, such as rivers, lakes, and reservoirs.

TCE and PCE: TCE stands for trichloroethylene. PCE stands for perchloroethylene. These chemicals are chlorinated solvents. They are used commercially as industrial degreasers, spot removers, and in dry cleaning. TCE and PCE are not natural chemicals, they are manmade. Exposure typically occurs via drinking contaminated water. TCE and PCE get into drinking water aquifers when they are improperly disposed of and then leach into the ground. TCE and PCE can also be inhaled. They are Volatile Organic Compounds (VOCs) which means that they vaporize into the air. Chronic TCE or PCE exposure can cause damage to the liver, the kidneys and the central nervous system. It may also lead to increased risk of cancer. The MCLs for both TCE and PCE are 5.0ppb (5 grams per billion grams of water).

Toxic Maximum Daily Load (TMDL): This is a legal determination of the total amount of a particular pollutant that can be discharged into a water supply/water source in one day. This sets the limit on total pollution a water body can receive from all possible sources. TMDLs are established by the Environmental Protection Agency as well as the Division of Environmental Quality. TMDLs are used to set the limits on how much of a particular pollutant each generating source (such as a factory) may discharge.

Transient, Non-Community Water System: A water system which provides water in a place such as a gas station or campground where people do not remain for long periods of time. These systems do not have to test or treat their water for contaminants which pose long-term health risks because fewer than 25 people drink the water over a long period. They still must test their water for microbes and several chemicals.

Treatment Technique (TT): A required process intended to reduce the level of a contaminant in drinking water.

Turbidity: The cloudy appearance of water caused by the presence of tiny particles. High levels of turbidity may interfere with proper water treatment and monitoring.

Unconfined aquifer: An aquifer whose upper boundary is the water table. Also known as a water-table aquifer.

Variances & Exemptions (V&E): State or EPA permission not to meet a certain drinking water standard. The water system must prove that: (1) it cannot meet a MCL, even while using the best available treatment method, because of the characteristics of the raw water, and (2) the variance will not create an unreasonable risk to public health. The State or EPA must review, and allow public comment on, a variance every three years. States can also grant variances to water systems that serve small populations and which prove that they are unable to afford the required treatment, an alternative water source, or otherwise comply with the standard.

Violation: A failure to meet any state or federal drinking water regulation.

Vulnerability Assessment: An evaluation of drinking water source quality and its vulnerability to contamination by pathogens and toxic chemicals.

Water table: A surface at atmospheric pressure defined by the level to which water will rise in a well; a water table by definition is the top of an unconfined aquifer.

Watershed: The land area from which water drains into a stream, river, or reservoir.

Wellhead Protection Area (WHPA): The area surrounding a drinking water well or well field which is protected to prevent contamination of the well(s).