Water Quality Glossary

Key terms and definitions to help you understand your water quality report.

67 terms

1

1,4-Dioxane

A synthetic industrial chemical used as a solvent stabilizer and a contaminant in many cosmetic products. The EPA health advisory is 0.35 µg/L (one of the strictest non-MCL benchmarks). Linked to liver and kidney effects. Tested under UCMR 3. See full 1,4-Dioxane profile →

9

90th percentile

A statistic used in the Lead and Copper Rule: the value below which 90% of sampled homes fall. If a system's 90th-percentile lead exceeds 15 µg/L, the EPA action level is triggered.

A

Action Level (AL)

A concentration of a contaminant in drinking water that triggers treatment or other requirements a water system must follow. For example, the action level for lead is 15 parts per billion (ppb).

Arsenic

A naturally occurring element found in rocks and soil that can dissolve into groundwater. The EPA MCL for arsenic in drinking water is 10 parts per billion (ppb). Long-term exposure is linked to cancer, cardiovascular disease, and diabetes. See full Arsenic profile →

B

Boil Water Advisory

A public notice issued by a water utility or health authority advising consumers to boil tap water before drinking it, usually due to detected contamination or a loss of system pressure.

C

CCR (Consumer Confidence Report)

An annual water quality report that every public water system must deliver to its customers by July 1 each year. It lists detected contaminants, their levels, and whether the system met all EPA standards.

Chloramine

A disinfectant formed by combining chlorine with ammonia. Over 20% of US water systems use chloramine as a secondary disinfectant because it lasts longer in pipes than chlorine. The EPA maximum residual disinfectant level (MRDL) is 4 mg/L. See full Chloramine profile →

Chromium-6 (Hexavalent Chromium)

A toxic form of chromium that can enter drinking water from both natural sources and industrial contamination. There is no federal MCL specific to chromium-6; the EPA regulates total chromium at 100 ppb. Research links ingested chromium-6 to stomach cancer. See full Chromium-6 profile →

Coliform

A group of bacteria used as indicators of water quality. While most coliforms are harmless, their presence can signal that disease-causing organisms may also be in the water supply.

Copper

A metal that enters drinking water primarily through corrosion of copper plumbing and fixtures. The EPA action level for copper is 1.3 mg/L (1,300 ppb), measured at the 90th percentile under the Lead and Copper Rule. High levels can cause gastrointestinal illness. See full Copper profile →

Compliance

A water system is in compliance when it meets all applicable drinking water regulations, including contaminant limits, monitoring schedules, and public notification requirements.

Contaminant

Any physical, chemical, biological, or radiological substance found in water. The EPA regulates over 90 contaminants with enforceable standards under the Safe Drinking Water Act.

Chlorate

A disinfection byproduct that can form when chlorine dioxide or hypochlorite is used to treat drinking water. Chlorate is unregulated federally; the EPA lifetime health advisory is 210 µg/L. High exposure may impair thyroid function. Tested under UCMR 3 (2013-2015). See full Chlorate profile →

Cobalt

A transition metal that can leach into water from natural mineral deposits and industrial processes. Cobalt is unregulated in drinking water and has no federal MCL or health advisory; EPA UCMR 3 testing detected it in <3% of US public water systems. See full Cobalt profile →

CDC Community Water Fluoridation

A CDC dataset tracking the percentage of each state's population served by water systems that adjust fluoride levels for dental health. Most states publish 2020 data at the county level. WaterVerge surfaces this on state and county pages.

D

DBP (Disinfection Byproduct)

Chemical compounds formed when disinfectants like chlorine react with natural organic matter in water. Common DBPs include trihalomethanes (THMs) and haloacetic acids (HAA5). See full DBP profile →

Domestic Well

A private well that serves a single household, exempt from federal Safe Drinking Water Act regulation. Roughly 13% of US residents drink from domestic wells. The USGS Powell Domestic-Well Database compiles ~110K samples nationally.

E

EPA (Environmental Protection Agency)

The federal agency responsible for setting and enforcing national drinking water standards under the Safe Drinking Water Act. The EPA delegates primary enforcement to states in most cases.

Exceedance

When the measured level of a contaminant in drinking water exceeds the Maximum Contaminant Level (MCL) or other applicable standard. An exceedance may or may not result in a formal violation.

ECHO (Enforcement and Compliance History Online)

An EPA portal that provides public access to enforcement, compliance, and inspection data for regulated facilities. WaterVerge sources its bulk SDWIS download from ECHO every quarter.

F

Filtration

A water treatment process that removes particles, microorganisms, and other impurities by passing water through granular media, membranes, or other filtering materials.

G

Groundwater

Water found beneath the earth's surface in aquifers and underground formations. About one-third of US public water systems rely on groundwater as their primary source.

H

HAA5 (Haloacetic Acids)

A group of five disinfection byproducts formed when chlorine reacts with organic matter in water. The EPA MCL for HAA5 is 60 parts per billion (ppb). See full HAA5 profile →

Health Advisory

Non-enforceable guidance from the EPA providing information on contaminants that may cause health effects at certain concentrations. Health advisories exist for contaminants not yet regulated under the SDWA.

Hazard Index (HI)

A unitless ratio used to evaluate combined exposure to multiple PFAS compounds. Calculated as the sum of (measured concentration ÷ MCL) across all detected PFAS analytes. A Hazard Index > 1 indicates the system exceeds federal PFAS limits.

HAA6Br (Brominated Haloacetic Acids)

A subset of bromine-containing haloacetic acids — a category of disinfection byproducts. Unregulated federally; tested under UCMR 4 to inform future regulation.

HAA9 (Total Haloacetic Acids 9)

The full set of nine haloacetic acid disinfection byproducts. Includes HAA5 plus four additional bromine-containing species. Tested under UCMR 4 as a candidate for future regulation.

I

Industrial proximity penalty

A v3+ component of the contaminant subscore that deducts up to -3 points based on the count of EPA Superfund NPL sites and the lbs of TRI surface-water releases within 10 miles of a city centroid. Computed via haversine distance.

L

Lead and Copper Rule (LCR)

An EPA regulation that requires water systems to monitor lead and copper levels at consumer taps. If the 90th percentile exceeds the action level (15 ppb for lead, 1.3 ppm for copper), the system must take corrective action. See full Lead and Copper Rule profile →

Lithium

An alkali metal that occurs naturally in groundwater. Lithium has no federal MCL or health advisory in drinking water. UCMR 5 (2023-2025) is the first nationwide lithium monitoring round; concentrations vary widely by region with the highest readings in the western US. See full Lithium profile →

M

Maximum Contaminant Level (MCL)

The highest level of a contaminant allowed in drinking water, set as close to the MCLG as feasible using best available treatment technology. MCLs are legally enforceable standards.

MCL Goal (MCLG)

The level of a contaminant in drinking water below which there is no known or expected risk to health. MCLGs are non-enforceable public health goals. For carcinogens, the MCLG is typically set at zero.

Monitoring Violation

A violation issued when a water system fails to conduct required testing or fails to report results on schedule. Monitoring violations do not necessarily indicate contaminated water but represent a gap in oversight.

Molybdenum

A trace metal essential to human health in small amounts but potentially harmful at high doses. The EPA lifetime health advisory for molybdenum in drinking water is 40 µg/L. Tested under UCMR 3. See full Molybdenum profile →

Manganese

A naturally occurring metal essential in trace amounts. The EPA secondary MCL is 50 µg/L (aesthetic — staining and taste); the WHO lifetime health-based guideline is 80 µg/L. High chronic exposure has been linked to neurological effects. Tested under UCMR 4. See full Manganese profile →

N

Nitrate

A nitrogen compound that enters drinking water primarily from agricultural fertilizer runoff, septic systems, and natural deposits. The EPA MCL for nitrate is 10 mg/L. High nitrate levels are especially dangerous for infants, causing methemoglobinemia (blue baby syndrome). See full Nitrate profile →

NDMA (N-Nitrosodimethylamine)

A semi-volatile organic compound classified as a probable human carcinogen. NDMA can form as a disinfection byproduct of chloramination. There is no federal MCL; California sets a 10 ng/L Public Health Goal — the most protective US benchmark. Tested under UCMR 2 (2008-2010). See full NDMA profile →

NPL (National Priorities List)

The EPA's list of the most contaminated hazardous waste sites in the United States, scheduled for long-term cleanup under Superfund. WaterVerge tracks 1,379 active NPL sites and flags any within 10 miles of a city centroid.

P

PFAS (Per- and Polyfluoroalkyl Substances)

A large class of synthetic "forever chemicals" that do not break down easily in the environment. PFAS have been linked to cancer, immune system effects, and other health problems. The EPA finalized enforceable limits for several PFAS in 2024. See full PFAS profile →

PFOA (Perfluorooctanoic Acid)

A specific PFAS compound formerly used in manufacturing nonstick coatings and other products. The EPA set a maximum contaminant level of 4 parts per trillion (ppt) for PFOA in 2024. See full PFOA profile →

PFOS (Perfluorooctane Sulfonic Acid)

A specific PFAS compound historically used in firefighting foams, stain repellents, and other industrial applications. The EPA set a maximum contaminant level of 4 parts per trillion (ppt) for PFOS in 2024. See full PFOS profile →

Public Water System (PWS)

A system that provides water for human consumption to at least 25 people or 15 service connections for at least 60 days per year. There are over 148,000 public water systems in the United States.

Perchlorate

A salt used in rocket propellants, road flares, and explosives. Perchlorate disrupts thyroid function by interfering with iodide uptake. There is no federal MCL; California regulates at 6 µg/L and Massachusetts at 2 µg/L. Tested under UCMR 1 (2001-2005). See full Perchlorate profile →

Per-analyte cap (scoring v4)

A v4 methodology rule that limits any single contaminant to a maximum -3 point deduction from the contaminant subscore. Prevents one outlier reading from dominating a city's overall grade.

PWSID (Public Water System ID)

A unique identifier assigned to each public water system, formatted as a state code prefix plus a 7-digit number (e.g. TX1234567). Used to join violation, sample, and demographic data.

R

Recency multiplier (scoring v4)

A v4 methodology rule that scales each UCMR round's penalties by age: UCMR 1 = 0.20×, UCMR 2 = 0.30×, UCMR 3 = 0.60×, UCMR 4 = 0.80×, UCMR 5 = 1.00×. Reflects that older monitoring describes historical conditions, not current treatment quality.

S

SDWA (Safe Drinking Water Act)

The primary federal law governing drinking water quality in the United States, originally passed in 1974. It authorizes the EPA to set national standards for drinking water contaminants.

SDWIS (Safe Drinking Water Information System)

The EPA's federal database that tracks information about public water systems, including violations, enforcement actions, and system characteristics. WaterVerge sources much of its data from SDWIS.

Surface Water

Water from lakes, rivers, reservoirs, and streams used as a drinking water source. Surface water systems are generally required to provide filtration and disinfection treatment.

Strontium

A naturally occurring alkaline earth metal that can leach from rocks and soils. The EPA Health Reference Level is 1,500 µg/L. Strontium is chemically similar to calcium and can affect bone development at high exposures. Tested under UCMR 3. See full Strontium profile →

Superfund

The federal program created in 1980 to clean up hazardous waste sites that pose risks to human health and the environment. Superfund sites can contaminate nearby groundwater and surface water sources for decades.

Scoring version

The version of the WaterVerge methodology used to compute a city's grade. Stored per city in the city_profiles.scoring_version column. v2 = Phases 1-3 baseline; v3 = added Phase 4 analytes + Superfund/TRI proximity; v4 = added per-analyte cap + UCMR recency discount.

SNC (Significant Non-Complier)

An SDWIS designation for water systems with the most serious or persistent violations. SNC violations carry a 2× multiplier in the WaterVerge violation subscore.

T

THM (Trihalomethane)

A group of four disinfection byproducts (chloroform, bromoform, bromodichloromethane, and dibromochloromethane) formed during water chlorination. The EPA MCL for total THMs is 80 ppb. See full THM profile →

Treatment Technique

A required process or technology that a water system must use to reduce a contaminant to acceptable levels. Treatment techniques are used instead of MCLs when measuring a contaminant is not practical.

TRI (Toxic Release Inventory)

An EPA database tracking annual releases of toxic chemicals to air, water, and land by industrial facilities. WaterVerge surfaces TRI lbs released to surface water within 10 miles of each city. The 2023 TRI covers ~21,800 facilities.

U

UCMR (Unregulated Contaminant Monitoring Rule)

An EPA program that requires water systems to monitor for contaminants not currently regulated under the SDWA. UCMR data helps the EPA decide whether new regulations are needed. The most recent cycle, UCMR 5, focused heavily on PFAS.

USGS Powell Domestic-Well Database

A USGS data release compiling ~110,000 arsenic and manganese samples from US domestic (private) wells. Used by WaterVerge to surface state-level "% of wells exceeding the EPA arsenic MCL" stats. Note: this is private-well data; community water systems are graded separately.

UCMR 1

The first Unregulated Contaminant Monitoring Rule round (2001-2005). Tested 26 contaminants including perchlorate, MTBE, and chemical pesticides. WaterVerge applies a 0.20× recency multiplier to UCMR 1 data because it predates many treatment improvements.

UCMR 2

The second UCMR round (2008-2010). Tested 25 contaminants including N-nitrosodimethylamine (NDMA) and other nitrosamines. WaterVerge applies a 0.30× recency multiplier.

UCMR 3

The third UCMR round (2013-2015). Tested 30 contaminants including hexavalent chromium, vanadium, chlorate, molybdenum, cobalt, strontium, and 1,4-dioxane. Recency multiplier: 0.60×.

UCMR 4

The fourth UCMR round (2018-2020). Tested 30 contaminants including HAA5 disinfection byproducts, manganese, and pesticides. Recency multiplier: 0.80×.

UCMR 5

The fifth and most recent UCMR round (2023-2025). Tested 29 PFAS compounds plus lithium. WaterVerge uses UCMR 5 data at full weight (1.0× multiplier) since it represents the most current monitoring.

V

Violation

A formal determination that a water system has failed to meet a drinking water regulation, such as exceeding an MCL, missing a monitoring deadline, or failing to notify the public of a problem.

Vanadium

A transition metal that occurs naturally in some rock and soil. The EPA short-term health advisory is 21 µg/L. Vanadium is unregulated in US drinking water. Tested under UCMR 3 (2013-2015). See full Vanadium profile →

W

Water System

An infrastructure network that collects, treats, stores, and distributes drinking water to consumers. Systems are classified as community, non-transient non-community, or transient non-community based on the population they serve.

WQP (Water Quality Portal)

A joint EPA + USGS national repository for water-quality monitoring samples. WaterVerge ingests state-level summaries (median, P90, max) for analytes like nitrate and arsenic.