Water quality / Ottawa
A weekly lab result cannot keep up with a combined sewer overflow
Near the New Edinburgh boathouse, Ottawa already knows when pipes discharge. What it still lacks is fast, public water testing at the moment contamination risk actually changes.
The Ottawa River at the NCC River House — the old New Edinburgh Club boathouse — is one of the city’s best urban swim sites. It is also a few hundred metres from combined sewer overflow (CSO) outfalls that still discharge to the Ottawa River after heavy rain, including Hemlock and Sandridge.1
Ottawa has spent hundreds of millions of dollars to reduce those overflows. It still tests recreational water mostly the 19th-century way: collect a bottle, culture bacteria overnight, post a number the next day. That method is scientifically valid. It is also too slow for the events that matter.
The core problem
Culture-based E. coli tests take 18–24 hours. Combined sewer overflows happen in hours. After the 1 July 2026 storm, the City reported 218,820 m³ of combined overflow to the Ottawa River. Beach sampling that week was not increased to match the event.23
What Ottawa already measures well
The City publishes CSO dates, causes and volumes. Federal Wastewater Systems Effluent Regulations files include overflow-point coordinates and reported volumes. Ottawa Riverkeeper and the NCC post River House dock results on Swim Guide. Carleton researchers filled a Rideau River gap in summer 2025 with weekly E. coli samples, including extra samples after reported CSOs.456
That is not nothing. It is also not a system that can tell a paddler at Stanley Park, or a swimmer at River House, whether yesterday’s cloudburst made this afternoon different.
Reported CSO volume to Ottawa waterways
Figure 1. City of Ottawa historical CSO volumes during the April 15–November 15 reporting window. The Combined Sewage Storage Tunnel (in service late 2020) cut frequency and volume, but large events still occur. Source: City of Ottawa CSO pages.2
Why weekly culture tests miss the spike
Ontario’s recreational standard is a geometric mean of five samples at or below 200 E. coli CFU/100 mL, with a common single-sample caution at 400. Health Canada’s current freshwater beach action value is 235 CFU/100 mL per sample.78
In 2025 Ottawa Public Health moved supervised-beach sampling from daily to weekly. OPH argues — correctly — that a lab result is already yesterday’s water, that rivers flush, and that weekly testing meets the Ontario Public Health Standards. Its Board of Health analysis found weekly tests predicted the next week’s pass/fail status almost as well as daily tests overall (about 78.5% vs 80.3%).9
That average hides the days people actually worry about. Ottawa Riverkeeper’s review of 2014–2024 daily data found large day-to-day swings. After the Canada Day 2026 rain, OPH did not add event sampling; Riverkeeper’s extra samples at Petrie and Mooney’s Bay were still above the threshold 48 hours later.310
A weekly Wednesday sample is a trend tool. It is a poor nowcast. OPH itself now tells the public to use rainfall, clarity and wildlife as real-time cues — which is an admission that the published E. coli number is not operational intelligence.
Time to an answer, by method
Figure 2. Approximate time from sample to a usable E. coli or proxy result. Culture remains the legal reference method in Ontario beach programs. Faster tools exist and are already used for operational decisions elsewhere.111213
The Rideau–Rockcliffe gap is specific
Hemlock CSO and Sandridge Tank Overflow discharge to the Ottawa River north of Sir George-Étienne Cartier Parkway. Clegg discharges to the Rideau at Clegg Street. River House swimming is on the Ottawa River, in a high-flow reach that usually tests well — NCC/Riverkeeper data have shown strong pass rates and relatively low average E. coli compared with stagnant beaches.114
Good average quality is not the same as coverage during overflows. There is still no public, paired dataset that says: Sandridge discharged X m³ at 16:40; here is an E. coli or enzymatic result at the docks at 18:00, 22:00 and the next morning. City CSO telemetry and recreational sampling live in different dashboards.
Who tests what near this shoreline
| Program | What you get | What you do not get |
|---|---|---|
| City CSO map / overflow log | Which outfall, rain vs equipment, volume | Bacteria or chemistry in the river |
| NCC + Ottawa Riverkeeper at River House | Seasonal E. coli, usually weekly; extra after some storms | Hourly data; guaranteed event sampling at every CSO |
| Ottawa Public Health beaches | Weekly geometric mean at five supervised beaches | River House, Rideau parks, CSO outfalls |
| Carleton Rideau study (2025) | Weekly park sites plus some CSO-day grabs | A standing automated network |
| ECCC / City / RVCA river chemistry | Long-term nutrients, metals, index scores | Same-day swim advice |
Better testing does not mean more bottles on Wednesday
The useful upgrade is event-driven and faster, not infinite daily culture at every beach. A practical stack already exists:
- Use the sensors Ottawa already paid for. CSO regulators, the Combined Sewage Storage Tunnel, and rain gauges know when a discharge starts. That signal should automatically trigger sampling at River House, Hemlock/Sandridge, Clegg, and the lower Rideau parks.
- Add a rapid microbial method for operations. Enzymatic analyzers such as ColiMinder can report β-glucuronidase activity on the order of 30–90 minutes. Handheld systems such as Fluidion ALERT can quantify E. coli in about two hours. qPCR is faster than overnight culture and is already discussed in Ottawa health documents as a future provincial option.11129
- Run continuous proxies between lab days. Turbidity, conductivity, temperature and flow will not replace E. coli, but they flag the plume. University of Waterloo researchers have also prototyped a low-cost on-site E. coli sensor with results in minutes.13
- Publish one feed. Overflow on/off, rainfall, latest rapid result, and latest culture result should sit on the same public page as Swim Guide and the City CSO map.
A simple operating picture after a storm
Figure 3. Illustrative timeline, not a specific 2026 event. Culture posted the next afternoon can miss the window when people actually enter the water. Automated proxies plus a 2-hour rapid test would close that gap while culture remains the confirmation method.
A pilot that would settle the argument
OPH and Riverkeeper are not describing different rivers. They are describing different jobs. Weekly culture is fine for seasonal trend. It is the wrong instrument for a CSO night beside a public dock.
A one-season pilot at River House and the lower Rideau would be cheap next to the CSST:
- keep weekly five-sample culture as the reference;
- install one continuous multiparameter sonde;
- tie City overflow alarms to same-day rapid E. coli or enzymatic runs at 0, 6, 24 and 48 hours;
- release the data the same day, with the usual “not a legal beach posting” caveat.
If the rapid series stays low after Hemlock and Sandridge events, that is useful reassurance for a high-flow Ottawa River site. If it spikes and stays high, weekly Wednesday sampling is not protecting the people who swim on Thursday. Either result is better than inferring safety from a volume number and a week-old geometric mean.
Bottom line
Ottawa already knows the pipes. It already knows the river is often swimmable. What it does not yet do is measure the water on the same clock as the overflow. Automated triggers and faster tests are how you stop asking the public to guess.
References
- City of Ottawa. Combined sewer overflows, station locations. Hemlock CSO Outfall and Sandridge Tank Overflow discharge to the Ottawa River north of Promenade Sir George-Étienne Cartier. ottawa.ca CSO map.
- City of Ottawa. Overflow activity and historical CSO volumes, including 218,820 m³ on 1 July 2026. ottawa.ca CSO overview.
- Ottawa Riverkeeper. “Closing the Gaps in Recreational Water Testing,” 25 July 2026. Notes no extra OPH testing after the 1 July rainfall and elevated results 48 hours later at sampled beaches. ottawariverkeeper.ca.
- Environment and Climate Change Canada. Wastewater Systems Effluent Regulations reported data (identification and CSO point files). open.canada.ca.
- Swim Guide / Ottawa Riverkeeper. River House Swimming Docks community monitoring. theswimguide.org/beach/11046. NCC dock status: ncc-ccn.gc.ca/places/docks.
- Banu Örmeci, Carleton University. Summer 2025 Rideau River E. coli testing, including CSO-day samples. carleton.ca/ormeci.
- Ottawa Public Health. Beach water quality monitoring: weekly five-sample geometric mean; Ontario standard 200 E. coli/100 mL. ottawapublichealth.ca.
- Health Canada. Guidelines for Canadian Recreational Water Quality — fecal indicators. Freshwater E. coli beach action value 235 CFU/100 mL. canada.ca.
- Ottawa Public Health / Board of Health materials, 2025–26: shift from daily to weekly testing; lab turnaround 18–24 hours; weekly vs daily predictive comparison. See also iPolitics, 16 July 2026. ipolitics.ca.
- Ottawa Riverkeeper. Evaluating a change from daily to weekly testing at City beaches, September 2025. PDF.
- Khanafer, Z. Polytechnique Montréal thesis on ColiMinder enzymatic near-real-time E. coli monitoring (~30 minutes). publications.polymtl.ca.
- Fluidion. ALERT One field E. coli quantification in as little as two hours. fluidion.com.
- University of Waterloo. Portable microwave/E. coli sensor with on-site results in minutes (2025). design-engineering.com.
- Laura Reinsborough. Comparison of Ottawa River E. coli at NCC River House with other urban rivers, Ottawa Citizen, 24 July 2024. ottawacitizen.com.







