Understanding Water Quality Data

What EPA SDWIS Tracks

The Safe Drinking Water Information System is the EPA's primary database for public drinking water compliance. Every community water system, from a small rural district serving 25 people to a metropolitan utility serving millions, is required to regularly test its water and report results to the state primacy agency, which forwards data to EPA.

SDWIS records include: contaminant violations (MCL exceedances), treatment technique failures, monitoring and reporting deficiencies, public notification failures, and formal enforcement actions. PlainWater surfaces this data for all 50 states and territories, letting you trace the compliance history of any system.

Maximum Contaminant Levels (MCLs)

The EPA regulates over 90 contaminants in public drinking water under the Safe Drinking Water Act. For each, the agency sets a Maximum Contaminant Level, the highest permissible concentration in water delivered to customers. MCLs are legally enforceable limits, not guidelines.

MCLs are expressed differently by contaminant type. Microbial contaminants like total coliform bacteria use a presence/absence standard. Chemical contaminants like nitrate or arsenic are expressed as milligrams per liter (mg/L) or parts per billion (ppb). Radiological contaminants use picocuries per liter (pCi/L).

Browse contaminant violation data by county or search specific water systems to see which contaminants have triggered violations in your area.

Types of Violations

Not all violations are equal. SDWIS categorizes them into several distinct types:

• MCL violations: A contaminant was detected above its legal limit. These are the most serious health-based violations.
• Treatment Technique violations: The system failed to follow required treatment processes (e.g., inadequate filtration, insufficient disinfection).
• Monitoring & Reporting violations: Required testing was not performed or results were not submitted on time. The water may be fine, but compliance cannot be confirmed.
• Public Notification violations: The system failed to notify customers of a violation within the required timeframe.

PlainWater distinguishes health-based violations from monitoring violations on every system and county page. A high count of monitoring violations warrants scrutiny, a pattern of missing tests can obscure actual water quality problems.

How Water Systems Are Classified

SDWIS classifies public water systems into three main types based on who they serve:

• Community Water Systems (CWS): Serve the same population year-round, cities, towns, water districts, homeowner associations. These face the most comprehensive regulations and are the focus of most water quality concern.
• Non-Transient Non-Community (NTNC): Serve the same people regularly but not their primary residences, schools, factories, office complexes. Subject to most of the same standards as CWS.
• Transient Non-Community (TNC): Serve different people over time, campgrounds, gas stations, restaurants with their own wells. Face fewer regulations since exposure is intermittent.

Common Contaminant Categories

Microbial contaminants

Bacteria, viruses, and parasites such as E. coli, total coliform, Cryptosporidium, and Giardia. These present immediate acute-illness risk and trigger urgent public-notification requirements when MCLs are exceeded.

Inorganic chemicals

Naturally occurring or industrial pollutants, arsenic, nitrate, lead, copper, fluoride. Most have long-term cumulative-exposure standards and require chronic monitoring rather than emergency response.

Disinfection byproducts

Chemicals formed when chlorine reacts with organic material in water, trihalomethanes (TTHM) and haloacetic acids (HAA5). Tradeoff: necessary disinfection produces small but regulated amounts of these compounds.

Radionuclides

Naturally occurring radioactive elements, uranium, radium, gross alpha emitters. Tracked under separate monitoring schedules and most prevalent in groundwater systems in the western United States.

Worked example: reading an arsenic violation

Suppose a quarterly arsenic test reports $0.018 mg/L when the MCL is $0.010 mg/L, that is roughly 80% above the legal limit. A typical compliance schedule sets a 12-month return-to-compliance window with confirmation samples; if a follow-up sample drops to $0.008 mg/L (20% below MCL) the event closes. You would see this pattern in SDWIS as a violation start date, an enforcement action, and a return-to-compliance date.

"A violation history doesn't mean water is unsafe today, it shows where regulators have flagged risk. Always pair the SDWIS record with the most recent annual Consumer Confidence Report."

Reading a Violation Record

When you view a water system's violation history on PlainWater, key fields to understand include:

• Violation start date: When the violation period began (e.g., a monitoring quarter where testing was missed).
• Violation end date / return to compliance: When the system corrected the problem or came back into compliance.
• Contaminant code: The specific regulated substance or rule that was violated.
• Is health based: Whether the violation directly involved a safety standard exceedance.
• Enforcement action: Any formal response by the state or EPA, compliance schedules, fines, notices.

A single resolved monitoring violation from years ago is very different from an ongoing health-based violation with no return-to-compliance date. Look at the full pattern, recency, type, and resolution status all matter.

Frequently Asked Questions