Redwood Terrace Mutual, CA
Exceeded the action level in the past
Latest 90th-percentile lead: 4 ppb (monitoring period 2026-2026). The federal action level is 15 ppb today and drops to 10 ppb in Nov 2027; EPA's health goal is 0. The latest result is under the action level, but this system has gone over 15 ppb before. Lead comes from pipes, not the source water, so a home with a lead service line can still be exposed. Test your own tap.
Who runs the water here
Redwood Terrace Mutual serves about 56 people through 17 service connections, which makes it a very small system under EPA's size classes. It is privately owned and draws surface water, in San Mateo County, California. EPA system ID: CA4100510; phone on file: 650-799-5366. Ask them for your address in their service line inventory - every system had to publish one by Oct 2024.
Lead results over time
That is down 10 ppb from 14 ppb in the period ending 2025. The highest 90th-percentile result EPA holds for Redwood Terrace Mutual is 61 ppb across 12 monitoring periods, 2 of them over the 15 ppb action level.
| Period ending | 90th-percentile lead |
|---|---|
| 2026 | 4 ppb |
| 2025 | 14 ppb over 10 |
| 2025 | 7 ppb |
| 2024 | 0 (not detected) |
| 2023 | 0 (not detected) |
| 2023 | 0 (not detected) |
| 2022 | 61 ppb over 15 |
| 2021 | 15 ppb over 10 |
| 2020 | 7.5 ppb |
| 2019 | 28 ppb over 15 |
| 2017 | 1.9 ppb |
| 2016 | 0 (not detected) |
Against other California systems
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. Redwood Terrace Mutual's 4 ppb sits at the 89th percentile in California and the 87th nationally (higher means more lead).
Other water systems in San Mateo County
35 other systems share San Mateo County; 6 of them have a higher latest lead result than Redwood Terrace Mutual. 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-S San Francisco | 62,609 | 4.1 ppb |
| 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 |
What to do in your house
Test your own tap with a certified lab first-draw sample; La Honda, 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 Redwood Terrace Mutual whether it covers that cost before hiring anyone - many utilities now do. Its latest copper 90th percentile is 1560 ppb (copper action level 1,300 ppb).
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. What a 90th-percentile result does and does not tell you: how to read a lead result. Why an old exceedance still matters: over the limit before, under it now.