Well Water Interpreter: Methodology

How our well water lab report interpreter works, what standards it uses, and what it does not claim.

What Standards Are Used

The interpreter compares user-entered lab values against three categories of EPA standards:

  • Maximum Contaminant Levels (MCLs): Legally enforceable standards for public water systems under the Safe Drinking Water Act (40 CFR 141). Used for nitrate, arsenic, fluoride, uranium, and PFAS.
  • Action Levels (Lead and Copper Rule): Lead and copper are regulated under the Lead and Copper Rule (40 CFR 141 Subpart I), not through standard MCLs. The current lead action level is 15 ppb (dropping to 10 ppb in November 2027 under LCRI). The copper action level is 1.3 mg/L.
  • Secondary Maximum Contaminant Levels (SMCLs): Non-enforceable guidelines for aesthetic quality (40 CFR 143). Used for iron, manganese, pH, TDS, sulfate, and chloride.
  • Health Advisories: Non-regulatory guidance from the EPA for contaminants without MCLs or where aesthetic thresholds differ from health thresholds. Used for manganese (0.3 mg/L lifetime HA) and sulfate (500 mg/L).

The MCL / Private Well Distinction

EPA Maximum Contaminant Levels are enforceable standards for public water systems under the Safe Drinking Water Act. They do not legally apply to private wells. However, MCLs are widely used as reference benchmarks for evaluating private well water quality by state health agencies, university extension programs, and certified testing laboratories.

When this tool says a value "exceeds the EPA reference standard," it means the value is above the MCL that would apply to a regulated public water system. It does not mean the well owner is in violation of any regulation. Private well owners are responsible for their own water quality decisions.

How Interpretation Works

For each analyte the user enters, the tool:

  1. Converts the value to canonical units if needed (for example, grains per gallon to mg/L for hardness, or mg/L as NO3 to mg/L as N for nitrate).
  2. Compares the canonical value against threshold ranges defined in our standards data.
  3. Assigns a status: within limits, needs attention, nuisance/aesthetic, or informational.
  4. Returns the interpretation text, reference standard citation, and recommended action steps from our standards database.

All interpretation text comes from a curated standards database, not from dynamic generation. Thresholds and citations are sourced from published federal regulations and EPA guidance documents. Some standards are recent or in transition (PFAS MCL rule finalized April 2024; lead action level changing from 15 ppb to 10 ppb in November 2027). We monitor regulatory changes and update the tool accordingly.

What the Tool Does Not Claim

  • It does not determine whether your water is "safe" or "unsafe." Safety is a complex determination that depends on exposure duration, individual health factors, and the interaction between multiple contaminants.
  • It does not replace professional water testing or consultation. A certified water testing professional can evaluate your specific well construction, geology, and surrounding land use.
  • It does not account for contaminants not entered. A passing result on tested analytes does not mean the water is free of all contaminants.
  • It does not account for well construction, aquifer geology, or seasonal variation. A single test is a snapshot, not a comprehensive assessment.

Data Sources and Citations

  • National Primary Drinking Water Regulations: 40 CFR Part 141
  • National Secondary Drinking Water Regulations: 40 CFR Part 143
  • Lead and Copper Rule: 40 CFR 141 Subpart I (1991); Lead and Copper Rule Improvements (LCRI, effective 2027)
  • PFAS National Primary Drinking Water Regulation: Final Rule, April 2024
  • Revised Total Coliform Rule: 40 CFR 141 Subpart Y (2013)
  • Radionuclides Rule: 40 CFR 141.66 (2003)
  • EPA Drinking Water Health Advisories for manganese (2004) and sulfate

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