Pacifica State Beach at Linda Mar consistently ranks among the
least healthy beaches on the California
coast, and the contamination comes from San Pedro Creek. This page shows every water test since
2000 — San Mateo County
Environmental Health's weekly monitoring and the
Linda Mar Water Quality
Coalition's four creek sites — alongside the conditions that shape how bad the pollution gets: rainfall,
estimated creek flow, ground wetness, and temperature. The pollution has a source. These conditions raise and
lower its levels. This page is updated as new results come in. The county's beach record here reaches back to 2000; its creek testing begins in September 2015; the coalition's four-site program begins in July 2024. The goal of this work is to find the source of
the pollution, eliminate it, and make the beach safe to use. Use the filters to choose what is displayed.
The views below the graph show the patterns in the data and explain them.
How to use the graph below
The graph opens with every raw sample: the coalition’s four creek sites (weekly since July 2024 — the density jump that summer is the coalition’s program beginning), the county’s creek station (weekly since 2015), and the county’s beach record (weekly since 2000). The sites average and Surfrider’s creek-mouth series are in the Samples row.
Red dots are samples over the health standard (dashed line); orange dots passed. Every bacteria count on this page is in MPN per 100 mL — the most probable number of bacteria colonies in a 100-milliliter water sample, the standard measure used by all three labs. A ✕ beside a number marks it over its health standard. With Color by source on, dots keep their source’s color instead — judge by the dashed lines. Each gridline on the bacteria scale is ten times the one below it.
The two organisms counted — E. coli and enterococcus — are fecal indicator bacteria: mostly harmless themselves, they are counted because finding them means fecal contamination is in the water, and fecal contamination is what carries the germs that do make people sick. The ocean is full of microbes that pose no threat to people; these two are measured precisely because they should not be there.
Estimated flow, ground wetness, and water temperature are calculated from rainfall and air temperature records, not measured — no instruments in the valley measure them directly. Methods describes how each estimate was checked.
Zoom with a two-finger pinch or Ctrl/⌘ + scroll. Turn layers on and off in the filters.
Samples
Bacteria
Conditions
Annotations
—
Filled circle = enterococcus, ring = E. coli (set by the Bacteria filter); size = concentration (log scale); numbers = that day’s readings, in MPN per 100 mL. Red = over the limit, orange = under. Dashed = a selected source not sampled that week; each site appears only from when its program began.
Hover the timeline for any day’s full record. Click to pin; the arrows step from sample to sample.
Guided views of the evidence
Patterns that may explain what happens in the creek. Selecting a view
sets the graph above, explains what to look for, and offers a short tour of real examples from the record.
Views
Ask the data a question
Years of testing point to a handful of things that actually move the creek — warmth, rain and its
aftermath, the winter’s stored water, when and where the sample was taken, and, at the beach, the tide. Each
question below puts one of them to the record: days are split into three groups, from low to high, and the
typical result of each group is compared. Bars that climb mean the factor matters; bars that look alike mean
it doesn’t — and the questions that show nothing are reported just as plainly. Each card names the data it
uses and explains what the pattern suggests.
Question
Only look at
Compare
How to read it: each bar is the typical result (geometric mean) for that group of days.
The short T-shaped marks crossing each bar's top are the uncertainty range — a small T means the
estimate is precise. n = number of water tests.
The full record
Every test on the graph, row by row: the coalition’s four sites, the county’s creek and
beach records, and Surfrider’s creek-mouth series. Click a column to sort; the source for each tab is named
beneath the table.
Click a column to sort. “Spill ≤7d” marks samples within a week after a reported surface-water spill.
ERA5 weather-model records via
Open-Meteo (free API; watershed point 37.585, −122.47).
Tides
NOAA CO-OPS harmonic predictions, Princeton/Half Moon Bay station 9414131 — the open-coast station nearest Linda Mar. (The SF Golden Gate gauge runs 30–65 minutes behind the coast; earlier versions of this page used it.)
County weekly grab samples via CEDEN — creek: E. coli 2015→, enterococcus on some dates from spring 2026;
beach: enterococcus 2000→ and E. coli 2001→. Coalition weekly
samples at four creek sites (both organisms, 2024→), plus county LM5 taken the same mornings, from
lindamarwaterquality.org. Replicates collapsed to the maximum. Values per 100 mL. The sites average shown
is a calculated value: the geometric mean of whichever of the four sites were sampled that week (at least two required).
Why these two organisms
E. coli and enterococcus are the standard fecal indicator bacteria: strains that live in the guts
of warm-blooded animals and are usually harmless on their own. Finding them in a creek or surf zone
means fecal material is present — and with it, potentially, the viruses, bacteria, and parasites that
actually cause illness. Sea water teems with microbes that pose no threat; these two are singled out
because they do not belong there. The standards below come from epidemiological studies linking
indicator counts to swimmer illness rates.
Standards
Freshwater E. coli 320; freshwater enterococcus 110; ocean enterococcus 104; ocean E. coli 320
(single-sample values). The map applies the standard matching each site and organism. These are health-based
state standards, not choices made for this page: the freshwater values are California’s statewide
Bacteria Provisions
(State Water Board, 2018), adopted from EPA’s
2012
Recreational Water Quality Criteria, which swimmer-illness studies calibrated to a ceiling of about 32
gastrointestinal illnesses per 1,000 swimmers. The ocean enterococcus 104 is California’s beach standard
(AB 411, 1997), the same line the county posts advisories on. Each single-sample value is the companion to a
stricter primary test — a rolling geometric mean (freshwater E. coli 100, drawn with the Trend lines;
ocean enterococcus 35).
Measurement noise
Two labs sampling the same reach the same morning agree on pass/fail 80% of the time (74 paired
days; log–log r = 0.56). Single grabs carry ~±half-log noise; read patterns, not points.
Rain
ERA5 reanalysis daily precipitation at the watershed. No gauge exists in the valley.
Estimated flow (inferred from rain)
Ft = 0.90·Ft−1 + ½Pt + ½Pt−1. Checked against five
comparable gauged coastal creeks: r = 0.69–0.73 (log–log daily). Like ground wetness, the scale is a
unitless index — relative shape, not gauged flow; compare levels, not absolute numbers.
The rain shadow (in the filters and the Rainstorms view) fades at the empirically fitted risk rate —
excess exceedance halves every ~2.2 days — while flow halves every ~6.6 days. The gap is a finding:
risk clears ~3× faster than the water recedes (first flush carries the load).
Ground wetness (inferred from rain)
An index of how much past storm water the ground is still holding, built with a standard hydrology
technique — an antecedent wetness index. Each day it keeps 97% of the previous day’s value and adds a
share of the previous day’s estimated flow, so storms fill it and it drains about 3% a day, taking
months to empty. The scale has no units, because nothing in the valley measures groundwater (no wells,
no gauge): 0 means fully drained, and the wettest winters on record reach about 300. Only comparisons
carry meaning — twice the number means twice the stored water. Its rises and falls were checked against
dry-season baseflow (= groundwater discharge) at the five analog creeks: r = 0.65–0.74.
Formula: Gt = 0.97·Gt−1 + 0.10·Ft−1.
Water temperature (inferred from air)
No creek in the region has measured water temperature since 1979 — but San Gregorio Creek did,
daily, 1965–1979. Fitting that record against the same weather-model air temperatures used here:
water ≈ −1.6 + 1.05 × (7-day mean air), r = 0.85, typical error ±2°C over 3,932 paired days.
That transfer, applied to Pacifica's air record, is the water-temperature presumption shown — labeled
inferred, like every estimate on this page.
Rain vs normal (season running total)
Water-year cumulative rain vs the 1998–2026 day-of-season average. Summers after wet winters run
~3× summers after dry ones in rain-free weeks.
Sewer spills
State CIWQS reports, City of Pacifica system (WDID 2SSO10100); surface-water-reaching spills only.
Reported spills do not explain the chronic dry-weather signal; they are plotted so that can be checked.
What this page does not claim
Correlation is evidence for hypotheses, not proof of mechanism. Each view states what it predicts;
the project findings carry falsifiable tests, including a dated prediction for summer 2027 recorded
before this El Niño winter.