San Diego, CA vs Herndon, VA

When comparing tap water quality between San Diego, CA and Herndon, VA, Herndon currently holds a noticeable 11-point lead in average WaterQ score, with a score of 97 out of 100 versus 86 for San Diego. San Diego, CA carries a B grade while Herndon, VA carries an A. A WaterQ score reflects an aggregate of EPA-reported violations, monitoring data, and compliance history across all public water systems serving each city, so a higher score generally indicates fewer recent violations and a stronger compliance record, though it does not guarantee that every tap in the city tests identically.

San Diego, CA is the larger of the two, with 362,084 more residents than Herndon. San Diego is served by 6 public water systems, compared with 2 in Herndon — a difference of 4 systems. A larger number of systems often reflects a more fragmented distribution network spanning multiple utilities, suburbs, or rural service areas, while a single large utility serving a big population can benefit from economies of scale in treatment and monitoring, but may also face more complex, aging infrastructure to maintain.

For residents of either San Diego or Herndon, the most useful next step is to look up the specific water system that serves your address, since city-wide averages combine results from every system in the area. Visit the San Diego or Herndon city pages below for a breakdown of local systems, recent violations, and contaminant-specific data, or browse WaterQ's contaminants directory to learn what each measured substance means for health.