California Water Service-S San Francisco, CA
Lead detected, under the action levels
Latest 90th-percentile lead: 4.1 ppb (monitoring period 2023-2025). The federal action level is 15 ppb today and drops to 10 ppb in Nov 2027; EPA's health goal is 0. Lead was measured in sampled homes but the 90th percentile is under both action levels. EPA's health goal for lead is zero, so test your own tap if your home was built before 1986 or anyone in it is pregnant or a young child.
California Water Service-S San Francisco at a glance
California Water Service-S San Francisco serves about 62,609 people through 16,457 service connections, which makes it a large system under EPA's size classes. It is privately owned and draws surface water, in San Mateo County, California. EPA system ID: CA4110009; phone on file: 408-367-8593. They can tell you what your service line is made of and whether they pay to replace it.
Every lead result on file
Against the period ending 2022 (3.2 ppb) the latest result is about the same. The highest 90th-percentile result EPA holds for California Water Service-S San Francisco is 4.1 ppb across 4 monitoring periods, none of them over 15 ppb.
| Period ending | 90th-percentile lead |
|---|---|
| 2025 | 4.1 ppb |
| 2022 | 3.2 ppb |
| 2019 | 0 (not detected) |
| 2018 | 0 (not detected) |
Where this result ranks
Of the 2,884 California systems with a lead result, the median latest reading is 0 ppb; 30 are over 15 ppb right now and 215 have gone over at some point. California Water Service-S San Francisco's 4.1 ppb sits at the 90th percentile in California and the 88th nationally (higher means more lead).
Other water systems in San Mateo County
35 other systems share San Mateo County; 5 of them have a higher latest lead result than California Water Service-S San Francisco. If you are unsure which one supplies your house, the name on your water bill decides it.
| System | People | Latest lead |
|---|---|---|
| California Water Service - San Mateo | 103,536 | 4.4 ppb |
| City Of Daly City | 102,593 | 0 (not detected) |
| City Of Redwood City | 90,043 | 0 (not detected) |
| California Water Service - Bear Gulch | 60,382 | 4 ppb |
| City Of San Bruno | 42,631 | 0 (not detected) |
| North Coast County Water Dist | 38,546 | 0 (not detected) |
| Estero Municipal Improvement District | 36,556 | 0 (not detected) |
| California Water Service - San Carlos | 34,801 | 5.8 ppb |
| City Of Burlingame | 32,453 | 6.3 ppb |
| Mid-Peninsula Water District | 31,059 | 0 (not detected) |
Next steps for San Jose, CA
Test your own tap with a certified lab first-draw sample; San Jose, CA may offer free or low-cost kits through the utility. If your own line is lead, replacing the private side runs about $1,200-$12,300 (EPA average about $4,700). Ask California Water Service-S San Francisco whether it covers that cost before hiring anyone - many utilities now do.
These figures describe homes the utility sampled, not your tap. Only your service line record or a test of your own water answers the question for your house. The lead result guide covers how utilities pick the homes they sample. Systems that went over once often dip back under; here is what that means.