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Counties → California

San Mateo County, California

SDWIS compliance plate

Sheet CA081 · San Mateo County 52 sys · 1,318 viol · #81/3254

VIOL-SEVERE · HEALTH-LOW · RANK-TOP · PHOTO-FINISH · PFAS-LOW · SRC-SW · SYS-MID · BOOK-FULL

  • VIOL-SEVERE 96.2% flagged
  • HEALTH-LOW 18.4% health
  • RANK-TOP #81/3254
  • PHOTO-FINISH ±0.0 pp
  • PFAS-LOW 5.3% detect
  • SRC-SW SW lead 69%
  • SYS-MID 52 systems
  • BOOK-FULL 7/7 fields

Nearest flagged-rate peer: Calaveras County (+ 0 pp flagged-rate)

According to EPA SDWIS county population served used on /counties, San Mateo County ranks #81 of 3,254 counties by people served, and #13 of 58 counties in California. See methodology · corpus placement.

San Mateo County standing, mid-flagged corpus third

Flagged-system rate 96.2% is in the corpus middle third (≤ 100.0% flagged) across 2,973 counties with ≥3 SDWIS systems . Health-based share 18.4% is health-based share in the middle third (≤ 24.5%) . UCMR5 detect rate 5.3% is UCMR5 detect rate in the tested-pool middle third (≤ 33.3%) . Population served ranks #81 of 3,254 · #13 of 58 in California.

Flagged-rate neighbourhood around San Mateo County

Lowest/highest poles plus in-state counties nearest San Mateo County on % of SDWIS systems with ≥1 violation. Full ladder: /counties.

0% 20% 40% 60% 80% 100% Alpine · lowest Monterey · near Calaveras · near Sutter · highest 70% 95.7% 95.9% 96.2% 96.2% 96.9% 97.1%

Source: EPA SDWIS (county aggregates) As of 2026-05-15

Water Systems

System Name Population Violations PFAS
City of Daly City 104,901 12 Clean
California Water Service - San Mateo 102,348 7 Clean
City of Redwood City 92,819 8 Clean
California Water Service-S San Francisco 64,146 8 Clean
California Water Service - Bear Gulch 61,431 9 Clean
City of San Bruno 41,327 2 Clean
North Coast County Water Dist 38,546 5 Clean
Estero Municipal Improvement District 36,556 7 Clean
California Water Service - San Carlos 35,344 8 Clean
City of Burlingame 31,451 9 Clean
Mid-Peninsula Water District 30,609 6 Clean
City of East Palo Alto 29,519 23 Clean
City of Millbrae 22,795 0 Clean
Menlo Park Municipal Water 22,362 3 Clean
Coastside County Water District 18,940 33 Clean
Westborough Water District 13,486 4 Clean
Town of Hillsborough 11,766 5 Clean
Montara Water and Sanitary District 6,078 11 Clean
City of Brisbane 3,402 17 -
Palo Alto Park Mutual Water Company 2,900 9 -
O'Connor Tract Co-Operative Water Co. 2,789 12 -
San Mateo County Memorial Park 1,790 26 -
Guadalupe Valley Mid 1,472 8 Detected
Cuesta la Honda Guild Inc 915 74 -
Csp - Ano Nuevo State Park 709 2 -
Costanoa Campground 583 19 -
County Service Area 11 450 47 -
Camp Jones Gulch 357 8 -
County Service Area 7 300 84 -
Camp Loma Mar 208 69 -
Optimists Volunteers for Youth 207 4 -
Pescadero High School 192 14 -
Csp - Portola State Park 181 32 -
Butano Canyon Mutual 150 49 -
Redwood Glen Camp & Conference Ctr 114 25 -
Harley Farms Goat Dairy 113 0 -
Butano State Park - Csp 95 10 -
Portola Improvement Association MWC 90 53 -
Pigeon Point Hostel 67 7 -
Loma Mar Mutual 65 67 -
Bianchi Flowers Inc 65 9 -
Redwood Terrace Mutual 56 137 -
Marchi Central Farm 50 24 -
Skylawn Memorial Park (Skylawn Inc) 50 131 -
Martins Beach Water System 49 13 -
Oku Incorporated 35 94 -
Uc Elkus Ranch 34 30 -
La Honda Center 33 17 -
Gazos Creek Beach House 33 3 -
San Gregorio Company 32 27 -

Compare beyond California

Mid-flagged peers for San Mateo County

Cross-state counties nearest San Mateo County on flagged-system share (96.2%) and cumulative violation count (1,318). In-state neighbours stay on the compliance plate above.

Similar flagged-system rate

Nearest cross-state counties by % of SDWIS systems with at least one violation on record.

4 peers

Similar violation load

Nearest cross-state counties by cumulative SDWIS violation count (all systems in the county).

4 peers

Read our methodology · Check your water system - how this data is sourced, computed, and verified.

Data sourced from EPA SDWIS and UCMR5 PFAS monitoring data. See our methodology for details. Retrieved and formatted by PlainWater

Every figure on PlainWater is rendered directly from EPA SDWIS drinking-water data, no number is typed in by an editor. This page draws directly on EPA SDWIS data, no figure is typed in by an editor. See our editorial standards & corrections policy, the methodology behind these numbers, or report a data error.