Technical Terms and Measurements Guide

Regulatory Limits

• MCL (Maximum Contaminant Level) - Legal limit set by EPA

• MCLG (Maximum Contaminant Level Goal) - Health-based goal (often zero)

• AL (Action Level) - Concentration that triggers treatment requirements

• TT (Treatment Technique) - Required process instead of specific limit

• SMCL (Secondary Maximum Contaminant Level) - Non-health based guidelines

  
  

Testing and Reporting Terms

• ND (Not Detected) - Below detection limits

• BDL (Below Detection Limit) - Same as ND

• NTU (Nephelometric Turbidity Units) - Measures cloudiness

• MPN (Most Probable Number) - Statistical method for bacteria counting

  
  

Units of Measurement Explained

Concentration Units (Smallest to Largest)

• ppt (parts per trillion) = ng/L (nanograms per liter)

  • Like 1 second in 32,000 years

• ppb (parts per billion) = μg/L (micrograms per liter)

  • Like 1 second in 32 years

• ppm (parts per million) = mg/L (milligrams per liter)

  • Like 1 second in 11.5 days

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Radioactivity Units

• pCi/L (picocuries per liter) - Measures radioactive decay

• Bq/L (becquerels per liter) - Alternative radioactivity unit

Biological Units

• CFU (Colony Forming Units) - Viable bacteria count

• Oocysts/L - Parasite cyst count

• % Positive - Percentage of samples with detection

Statistical Terms Explained

Common Statistical Measures

• Maximum - Highest level detected during the year

• Minimum - Lowest level detected

• Range - Shows minimum to maximum (e.g., "0.5 - 2.3 ppm")

• Average/Mean - Sum of all results divided by number of samples

• Running Annual Average - Moving average updated throughout the year

Special Statistical Terms

• 90th Percentile - 90% of all samples were below this level

  • Used for lead and copper monitoring

• LRAA (Locational Running Annual Average) - For disinfection byproducts

• Geometric Mean - Special average used for bacteria

Understanding Detection and Reporting

Detection Limits

• MDL (Method Detection Limit) - Lowest amount instrument can detect

• MRL (Minimum Reporting Level) - Lowest amount utility must report

• PQL (Practical Quantification Limit) - Lowest reliable measurement

Reporting Conventions

• <0.5 - Detected but below 0.5 units

• 0.5 - Exactly 0.5 units detected

• ND or BDL - Nothing detected above detection limits

• Range: ND-2.1 - Some samples had no detection, highest was 2.1

Violation Types Explained

Health-Based Violations

• MCL Violation - Exceeded legal health limit

• MRDL Violation - Too much disinfectant

• Treatment Technique Violation - Didn't follow required treatment

• Coliform Rule Violation - Problems with bacteria monitoring

Monitoring Violations

• Monitoring/Reporting Violation - Missed required testing

• Public Notification Violation - Failed to notify customers properly

How Limits Are Set

Health-Based Standards (MCLs)

1. MCLG determined - Health goal (often zero for carcinogens)

2. Technology assessed - What treatment can achieve

3. Cost-benefit analysis - Balance health protection with feasibility

4. MCL set - Legal limit as close to MCLG as feasible

   
   

Secondary Standards (SMCLs)

• Based on taste, odor, color, or other aesthetic factors

• Not health-based but affect water acceptability

• Examples: iron, manganese, chloride, sulfate

Reading Data Tables

Typical Table Format

| Contaminant | Units | MCLG | MCL | Level Found | Range | Violation | Source |

How to Interpret Each Column

• Contaminant - What was tested

• Units - How it's measured (ppm, ppb, etc.)

• MCLG - Health goal

• MCL - Legal limit

• Level Found - Actual result (may be average, maximum, or 90th percentile)

• Range - Lowest to highest individual results

• Violation - Yes/No if MCL was exceeded

• Source - Where contamination typically comes from

Special Monitoring Situations

Lead and Copper Rule

• Uses Action Level (AL) instead of MCL

• Based on 90th percentile of home samples

• Lead AL = 15 ppb, Copper AL = 1.3 ppm

• If exceeded, utility must improve corrosion control

Disinfection Byproducts

• TTHMs (Total Trihalomethanes) - MCL = 80 ppb

• HAA5 (Haloacetic Acids) - MCL = 60 ppb

• Based on running annual averages

• Higher in summer, lower in winter

Radionuclides

• Gross Alpha - Screening test for radioactivity

• Radium 226+228 - Specific radium isotopes

• Radon - May be regulated in future

• Natural occurrence varies by geology

Common Measurement Conversions

Quick Reference

• 1 ppm = 1,000 ppb = 1,000,000 ppt

• 1 mg/L = 1 ppm

• 1 μg/L = 1 ppb

• 1 ng/L = 1 ppt

Practical Examples

• Fluoride: Typically 0.7-1.2 ppm (mg/L)

• Lead: Action Level is 15 ppb (μg/L)

• PFAS: Often measured in ppt (ng/L)

• Chlorine: Usually 0.2-4.0 ppm (mg/L)

Red Flag Terms to Watch For

Immediate Attention Needed

• "Exceeded MCL"

• "Action Level Exceeded"

• "Treatment Technique Violation"

• "E. coli detected"

• "Boil Water Notice"

Monitor Closely

• "Approaching MCL"

• "Increasing trend"

• "Customer complaints"

• "System modifications planned"

Getting Help with Technical Terms

When You Need Clarification

• Contact your water utility directly

• Ask for explanation in plain language

• Request historical data for comparison

• Ask about health significance of specific levels

Resources for Definitions

• EPA Safe Drinking Water Hotline: 1-800-426-4791

• Your state drinking water program

• Utility customer service departments

• Water quality professional associations