How to Read the Data

Every number on this site is derived from raw regulatory test results submitted to Ontario's Drinking Water Quality and Enforcement dataset, plus a smaller set of aesthetic parameters sourced directly from individual municipalities. Nothing here is a live feed or a predicted value — it's a plain-language summary of results that were already reported.

What the status labels mean

Within limit / below Ontario standard — the measured result (or running annual average, for THMs and HAAs) is below Ontario's regulatory limit for that parameter in the latest reporting period.

Exceedance — at least one sample, or the running annual average, was above Ontario's regulatory limit. An exceedance requires the water system to investigate and take corrective action; it does not by itself mean a boil-water advisory is currently in effect.

Insufficient data / possible exceedance — there isn't enough sampled history to calculate a full running annual average, or the only available figure is a conservative upper bound rather than a precise average.

Non-detect results

A "non-detect" result means a substance was not found above the lab's detection limit — it does not mean the substance is proven to be completely absent. Non-detect samples count toward total sample counts shown on each parameter card, but are excluded from detected-results averages, since treating a detection limit as if it were a measured value would overstate what was actually found.

Running annual averages (THMs and HAAs)

Trihalomethanes and haloacetic acids are compliance-checked against a running annual average of quarterly results, not a simple average of every sample collected in a year — a system sampled more often than quarterly would otherwise implicitly weight whichever quarter happened to be sampled more. Where a system has fewer than four sampled quarters, or only non-detect quarters, we show a conservative upper-bound figure instead of a precise average — this methodology is auditable row-by-row in the RAA methodology CSV.

Municipal system vs. plumbing samples

Some parameters, notably lead, are sampled both at the municipal water system and separately at building-side plumbing. Lead usually enters drinking water from old plumbing or service lines rather than the municipal supply itself, so the two are always shown as separate figures rather than blended together.

Historical data, not a live advisory

This site reports on Ontario's fiscal-year regulatory reporting periods (April 1 – March 31), which are published after the fact. It is not a real-time water quality feed. If you need current advisory information for your community, check your municipality's own water utility page or Ontario's official sources.

See Data sources for exactly where each figure comes from.