Zion Red Rock

What it means: Antimony is a metalloid found in natural mineral deposits and released through industrial activities including flame retardant manufacturing, metal smelting, and coal combustion. It can leach into water from certain pipe coatings and solders. Background levels in US drinking water are generally low, with violations rare.

Health effects: Long-term exposure to antimony in drinking water is associated with increased blood cholesterol, decreased blood glucose, and potential cardiovascular effects. Animal studies suggest reproductive and developmental toxicity at elevated doses. EPA classifies it as a possible human carcinogen (Group C) based on animal data.

Limit: 0.006 mg/L (6 ppb)

What it means: Arsenic is a naturally occurring metalloid found in bedrock and soil. It dissolves into groundwater from natural deposits and can also come from mining, smelting and past pesticide use. EPA lowered the MCL from 50 ppb to 10 ppb in 2001.

Health effects: Long-term arsenic exposure increases risk of bladder, lung, and skin cancers. It is also associated with cardiovascular disease, type 2 diabetes, and developmental effects in children. Arsenic is classified as a Group A human carcinogen by the EPA.

Limit: 0.010 mg/L (10 ppb), set in 2001 and enforceable since 2006; the previous MCL was 0.050 mg/L (50 ppb)

What it means: Barium is a naturally occurring alkaline earth metal found in mineral deposits, particularly barite and witherite. It enters drinking water through dissolution of natural deposits and, to a lesser extent, from industrial discharge — metal refining, coal combustion, and oil and gas drilling operations. Groundwater drawn from barium-bearing rock can carry detectable levels.

Health effects: Short-term exposure above the MCL can cause increases in blood pressure. Chronic exposure is associated with cardiovascular effects, kidney damage, and gastrointestinal disturbance. The general population is at risk; those with pre-existing hypertension or kidney disease face greater concern.

Limit: 2 mg/L (2,000 ppb)

What it means: Beryllium is a rare alkaline earth metal that enters drinking water through discharge from metal refineries, coal burning, and electrical and aerospace manufacturing industries. Natural erosion of beryllium-containing minerals can also contribute. It is among the less commonly detected regulated metals in US drinking water systems.

Health effects: Chronic oral exposure to beryllium is associated with intestinal lesions and bone damage. EPA classifies beryllium as a probable human carcinogen (Group B2) based on animal evidence and limited human data. Occupational inhalation exposure causes a distinct lung disease (berylliosis), but the oral route primarily targets the GI tract and bone.

Limit: 0.004 mg/L (4 ppb)

What it means: Cadmium is a heavy metal that enters drinking water primarily through corrosion of galvanized pipes and solder, industrial discharge from metal refining and battery manufacturing, and leaching from natural mineral deposits. It is also a byproduct of zinc production. Levels in water are typically low but can accumulate in plumbing infrastructure over time.

Health effects: Cadmium accumulates in the kidneys and can cause kidney damage with long-term exposure. It is classified by the EPA as a probable human carcinogen (Group B1). Bone demineralization and reproductive effects have also been observed at elevated exposure levels.

Limit: 0.005 mg/L (5 ppb)

What it means: Chromium occurs naturally in rocks, soil, and volcanic dust and enters drinking water through natural leaching from mineral deposits and industrial discharge from electroplating, leather tanning, and textile manufacturing. Hexavalent chromium (Cr(VI)) and trivalent chromium (Cr(III)) are the two main forms; both are regulated together under this MCL.

Health effects: Hexavalent chromium is associated with increased cancer risk through ingestion in animal studies and is classified by EPA as a probable human carcinogen (Group B1) for inhalation, with growing evidence for oral carcinogenicity. Chronic exposure to total chromium causes liver and kidney damage, allergic dermatitis, and gastrointestinal irritation. Individuals with pre-existing liver or kidney conditions face greater risk.

Limit: 0.1 mg/L (100 ppb) — total chromium

What it means: Cyanide in drinking water originates primarily from industrial discharges — metal plating, steel manufacturing, and certain mining operations — and from leaching of road salt formulations that contain ferrocyanide compounds. It can also enter from improper disposal of industrial waste. Natural background levels are negligible. It is measured as free cyanide.

Health effects: Acute high-dose cyanide exposure interferes with cellular oxygen use and can cause rapid deterioration of the central nervous system, including headache, dizziness, and in severe cases, loss of consciousness. Chronic exposure at lower levels is associated with thyroid dysfunction and nerve damage. Most drinking water violations pose chronic rather than acute risk.

Limit: 0.2 mg/L (200 ppb)

What it means: Fluoride occurs naturally in groundwater drawn from fluoride-bearing rock, and many public supplies add it at about 0.7 mg/L to reduce tooth decay. The MCL addresses excess natural levels, which are rare in surface water but possible in some groundwater.

Health effects: Above 2 mg/L: dental fluorosis (mottling/pitting of tooth enamel) in children under 8. Above 4 mg/L: skeletal fluorosis (joint pain, increased bone density) after decades of exposure.

Limit: 4 mg/L primary MCL; 2 mg/L secondary MCL (cosmetic)

What it means: Inorganic mercury enters drinking water through erosion of natural deposits, discharge from refineries and factories, and runoff from landfills. Coal combustion deposits mercury in watersheds via atmospheric deposition, making it a diffuse source for surface water systems. The MCL applies to inorganic mercury; methylmercury bioaccumulation in fish is a separate concern governed by FDA.

Health effects: Chronic exposure to inorganic mercury in water primarily damages the kidneys. Neurological effects — tremors, memory loss, cognitive impairment — are more associated with organic mercury (methylmercury) than with the inorganic form regulated in drinking water. Developing fetuses and young children are most vulnerable.

Limit: 0.002 mg/L (2 ppb)

What it means: Nickel is a naturally occurring metal that leaches from rock and soil into groundwater and surface water. Industrial sources include metal plating, battery manufacturing, and mining operations. Nickel can also leach from faucets, fittings, and stainless steel plumbing components, particularly in low-pH water. It was added to the NPDWR framework as part of the Phase II rule.

Health effects: Chronic oral exposure to nickel in drinking water is associated with cardiovascular and kidney effects. Nickel compounds are classified as known human carcinogens (Group A) by IARC for inhalation; the oral carcinogenicity evidence is less established. Individuals with nickel sensitivity may also experience dermatological reactions.

Limit: 0.1 mg/L (100 ppb)

What it means: Nitrate (NO₃) is the most common chemical contaminant in US groundwater, entering from fertilizers, septic systems, and animal waste. It is odorless and tasteless — the only way to know if it's present is to test.

Health effects: Methemoglobinemia in infants under 6 months: gut bacteria convert nitrate to nitrite, which blocks oxygen transport in blood. Pregnant women should also take precautions. Chronic adult exposure at elevated levels may increase cancer risk.

Limit: 10 mg/L as nitrogen (NO₃-N)

What it means: This code tracks the combined total nitrogen from both nitrate and nitrite. While each has its own MCL (nitrate: 10 mg/L, nitrite: 1 mg/L), the combined sum is also limited to 10 mg/L as nitrogen. The combined measurement applies when both forms are present, particularly in partially treated or mixed sources where nitrification may be incomplete.

Health effects: The combined limit protects against methemoglobinemia — the same risk as nitrate alone and nitrite alone. In infants under 6 months, gut bacteria convert these nitrogen compounds to forms that block oxygen transport in blood ('blue baby syndrome'). Pregnant women and individuals with certain enzyme deficiencies are also at elevated risk.

Limit: 10 mg/L as nitrogen (NO₂-N + NO₃-N combined)

What it means: Nitrite (NO₂) enters water from fertilizer runoff, septic systems, and natural mineral deposits. Unlike nitrate, nitrite acts very quickly in the body — it converts hemoglobin to methemoglobin, which cannot carry oxygen. Nitrite can also form in distribution systems through microbial nitrification when chloramine disinfection breaks down.

Health effects: Methemoglobinemia ('blue baby syndrome') in infants under 6 months: reduced oxygen delivery to tissues can be life-threatening. Pregnant women and people with glucose-6-phosphate dehydrogenase (G6PD) deficiency are also at elevated risk. Adults are generally less susceptible due to more effective methemoglobin reduction.

Limit: 1 mg/L as nitrogen (NO₂-N)

What it means: The 2013 RTCR (effective 2016) replaced the original TCR. It triggers automatic E. coli testing whenever total coliform is found. A single E. coli-positive sample is an acute MCL violation. It also requires systems exceeding 5% total coliform-positive samples to conduct a formal assessment and correct any deficiencies, helping prevent contamination before it escalates.

Health effects: Total coliform bacteria indicate that the disinfection barrier has been breached or fecal contamination is entering the system. E. coli specifically indicates fecal contamination and the possible presence of disease-causing pathogens (norovirus, Salmonella, Shigella, Campylobacter). Illness can be severe in elderly, young children, and immunocompromised individuals.

Limit: Zero E. coli; ≤5% of monthly samples total coliform-positive

What it means: Selenium is a naturally occurring metalloid present in certain soils and rock formations. It enters drinking water through weathering of seleniferous soils and rock, agricultural irrigation return flows, and discharge from petroleum refineries and metal smelters. Selenium is an essential trace nutrient at low concentrations but is toxic at levels above the MCL.

Health effects: Chronic exposure above the MCL causes selenosis — characterized by hair and nail loss, peripheral neuropathy, fatigue, and gastrointestinal disturbance. Liver damage can occur with prolonged high intake. The margin between nutritional requirement and toxic dose is narrow, making the MCL an important threshold.

Limit: 0.05 mg/L (50 ppb)

What it means: Thallium is a rare heavy metal that enters drinking water through leaching from ore-processing operations, discharge from electronics and pharmaceutical manufacturing, and natural mineral weathering. It is one of the least common contaminants detected in US public water systems but has a very low MCL due to its high toxicity.

Health effects: Chronic thallium exposure causes hair loss, changes in blood chemistry, kidney damage, intestinal problems, and liver effects. It can also impair the nervous system. EPA has classified thallium as a possible human carcinogen (Group C). The very low MCL reflects its acute toxicity even at small doses.

Limit: 0.002 mg/L (2 ppb)