Water Quality Glossary

Units of Measurement

How contaminant concentrations and radioactivity are expressed on a water quality report.

ppb (parts per billion)

A concentration unit meaning one part of a substance per billion parts of water, equivalent to micrograms per liter (µg/L). One ppb is roughly one drop of water in an Olympic-sized swimming pool.

ppm (parts per million)

A concentration unit meaning one part of a substance per million parts of water, equivalent to milligrams per liter (mg/L). One ppm is one thousand times more concentrated than one ppb.

µg/L (micrograms per liter)

A measurement of contaminant concentration equal to one millionth of a gram per liter of water. In dilute drinking water it is numerically equal to parts per billion (ppb).

mg/L (milligrams per liter)

A measurement of contaminant concentration equal to one thousandth of a gram per liter of water. In dilute drinking water it is numerically equal to parts per million (ppm).

ng/L (nanograms per liter, ppt)

A measurement of concentration equal to one billionth of a gram per liter, also called parts per trillion (ppt). It is used for extremely potent contaminants such as PFAS, which are regulated at these tiny levels.

pCi/L (picocuries per liter)

A unit measuring the radioactivity of a substance in water, used for radionuclides such as radium, uranium, and radon. One picocurie is about 0.037 radioactive decays per second per liter.

Regulatory & Standards Terms

The legal limits and health goals that define whether water is considered safe.

MCL (Maximum Contaminant Level)

The highest concentration of a contaminant that is legally allowed in public drinking water, set and enforced by the U.S. EPA. MCLs are enforceable standards that balance health risk against the cost and feasibility of treatment.

MCLG (Maximum Contaminant Level Goal)

The level of a contaminant in drinking water below which there is no known or expected risk to health, set by the EPA. MCLGs are non-enforceable public health goals, and for cancer-causing contaminants the goal is often zero.

Public Health Goal (PHG)

A health-protective concentration of a contaminant in drinking water set by California's OEHHA, based only on health risk without regard to cost or treatment feasibility. PHGs are often far stricter than federal legal limits.

Action Level (AL)

The concentration of a contaminant, most often lead or copper, that triggers required treatment or other actions when it is exceeded in tap samples. Unlike an MCL, it is measured at the customer's tap rather than at the treatment plant.

Secondary / Aesthetic Standard (SMCL)

A non-enforceable guideline for contaminants that affect the taste, color, odor, or appearance of water rather than health. Examples include iron, manganese, and chloride, which can stain fixtures or cause off-flavors.

Data & Reporting Terms

How we describe patterns in the underlying testing data.

Detection Rate

The share of water samples in which a contaminant was found at or above the reporting limit. A high detection rate means the contaminant is commonly present, though not necessarily at harmful levels.

Exceedance

A test result that is higher than a health guideline or legal limit for a given contaminant. An exceedance flags water that surpasses a benchmark, but a single exceedance is not always an enforcement violation.

Contaminants

The substances most often reported in Connecticut drinking water, and why they matter.

PFAS

A family of thousands of synthetic “forever chemicals” (per- and polyfluoroalkyl substances) that resist breaking down in the environment and the human body. They are linked to cancer, immune effects, and developmental harm, and are regulated at parts-per-trillion levels.

PFOA

A specific PFAS compound (perfluorooctanoic acid) once used to make nonstick and stain-resistant products. It is a likely human carcinogen and is now regulated by the EPA at 4 parts per trillion.

PFOS

A specific PFAS compound (perfluorooctane sulfonic acid) formerly used in stain repellents and firefighting foam. Like PFOA, it accumulates in the body and is regulated by the EPA at 4 parts per trillion.

Lead

A toxic heavy metal that enters drinking water mainly by leaching from lead service lines, solder, and brass fixtures. There is no safe level of lead exposure, and it is especially harmful to the brain development of infants and children.

Copper

A metal that leaches into drinking water from copper pipes and brass fittings, especially when water sits in plumbing. Low levels are an essential nutrient, but high levels can cause stomach upset and, over time, liver or kidney damage.

Arsenic

A naturally occurring semi-metal that dissolves into groundwater from rock and soil and is also released by some industry. Long-term exposure is linked to skin, bladder, and lung cancer, and it is regulated at 10 parts per billion.

Nitrate

A compound from fertilizer, septic systems, and animal waste that is common in agricultural and well water. High levels are especially dangerous to infants, causing “blue baby syndrome” (methemoglobinemia), and it is regulated at 10 mg/L.

Trihalomethanes (TTHM)

A group of four disinfection byproducts, including chloroform, formed when chlorine reacts with natural organic matter in water. Long-term exposure is associated with cancer and reproductive risks, and the total is regulated at 80 parts per billion.

Haloacetic Acids (HAA5)

A group of five disinfection byproducts formed when chlorine reacts with organic matter in water. They are regulated together at 60 parts per billion because of links to cancer from long-term exposure.

Disinfection Byproducts (DBPs)

Chemical compounds formed when disinfectants such as chlorine react with natural organic material in source water. The most common regulated groups are trihalomethanes (TTHM) and haloacetic acids (HAA5).

Chlorite

A disinfection byproduct formed when chlorine dioxide is used to treat drinking water. At high levels it can affect the nervous system and red blood cells, and it is regulated at 1 mg/L.

Radionuclides (Radium / Uranium / Radon)

Radioactive elements that occur naturally in rock and can dissolve into groundwater. Radium and uranium are regulated in drinking water and linked to cancer and, for uranium, kidney damage; radon is a radioactive gas that is more dangerous inhaled than ingested.

Turbidity

A measure of the cloudiness of water caused by suspended particles such as silt, clay, or microbes. High turbidity can shield pathogens from disinfection and is used as an indicator of treatment effectiveness.

Hardness

A measure of the dissolved calcium and magnesium in water. Hard water is not a health risk but causes scale buildup in pipes and appliances and reduces how well soap lathers.

Treatment & Systems

How water is filtered, certified, and delivered to your tap.

Reverse Osmosis (RO)

A water treatment method that forces water through a semipermeable membrane to remove dissolved contaminants such as lead, arsenic, nitrate, and PFAS. It is highly effective but also removes beneficial minerals and wastes some water.

Activated Carbon

A filter media that removes contaminants by adsorption, trapping chemicals on its porous surface as water passes through. It is effective for chlorine, taste and odor, many disinfection byproducts, and some PFAS, but not for most dissolved metals or nitrate.

NSF/ANSI Standard

A set of independent certification standards for water treatment products developed by NSF International and the American National Standards Institute. A device certified to a specific standard, such as NSF/ANSI 53 for lead, has been independently tested to reduce the named contaminants.

Consumer Confidence Report (CCR)

An annual water quality report that community water systems are required to send to their customers by July 1 each year. It lists detected contaminants, their levels, and how they compare to legal limits.

Public Water System (PWS)

A system that provides drinking water to at least 15 service connections or 25 people for at least 60 days a year. Public water systems are regulated under the Safe Drinking Water Act and must test their water regularly.

Private Well

A privately owned groundwater well serving a home or a small number of households. Private wells are not regulated by the EPA, so testing and treatment are the owner's responsibility.

Keep exploring