Detecting water pollution - a sentinel sensor on conductivity and pH
The hardest thing about catching pollution is that it comes without warning - and a sample taken once a month will almost never hit that one moment when something flowed into the river. The people who protect watercourses, water intakes or valuable natural stretches know this well, and so they reach for a different mode of work: a sentinel sensor that measures non-stop and never sleeps.
Continuous measurement of conductivity and pH turns a single point on a river into an early-warning system. Some inflows and events - sewage, salty run-off, acidic and alkaline discharges - leave a trace in these parameters before turbidity or smell become visible. Not all of them: oils, fuels and many organic compounds barely change conductivity, and a small tributary into a large river is lost in the noise. The absence of a signal does not mean the absence of pollution - choose the set of sensors to match the hazards you actually have upstream of the site. For someone protecting an intake or a valuable stretch the difference is simple: instead of guessing after the fact, they see the event while it is happening.
Why conductivity and pH in particular
These are sensitive indicators, fast and cheap to measure continuously:
- Electrolytic conductivity rises with the load of dissolved ions - a sudden spike is a classic signal of an inflow (sewage, salinity, run-off from roads/fields). After rain, conductivity usually falls, because rainwater dilutes the ions; the exception is often the first flush from paved surfaces, and in winter road salt.
- pH reacts to many chemical discharges - a sharp deviation is a sign that something unusual is happening.
Neither of them will tell you what exactly flowed in. When an event changes the ionic composition or the pH, both will show that something happened and when - and that is often enough to head into the field at the right moment.
How to set up sentinel monitoring
- Place the sensor at a strategic point - below a potential source, above an intake or a valuable stretch.
- Run continuous logging / telemetry so you can see the data live.
- Establish a baseline - the typical range of conductivity and pH for that water under various conditions.
- Define thresholds as deviations from the baseline, not “textbook” values - every water has its own norm.
- See to servicing and calibration, because a sentinel has to be trustworthy.
A threshold that catches the event
The whole value lies in the reaction time. A breach of the threshold (e.g. a sudden rise in conductivity by a set amount) is treated as a call: check, take a sample now, establish the source, notify whoever needs it. An event caught red-handed gives evidence and a chance to intervene; noticed a week later - guesswork at best.
| What you see in the data | Conductivity | pH | A hypothesis to check |
|---|---|---|---|
| A jump with no rain, outside working hours | sudden, sharp rise | usually unchanged or slightly down | often a sewage discharge or a failure |
| A change together with rain, building and fading slowly | usually a fall (dilution); a rise at the first flush from roads and paved yards, or with road salt | usually stable | catchment run-off or a combined sewer overflow |
| A winter rise correlated with road de-icing | clear rise, lasting for days | stable | road salt (chlorides) |
| A pH jump with no change in conductivity, during the day | stable | rises | intense photosynthesis (not a pollution event) |
| A sharp fall or rise in pH at any time of day | usually changes too | sharp deviation | chemical discharge |
These are patterns, not a diagnosis. Confirm each with a sample and with context: time of day, weather, what lies upstream - rain does not settle the source, because combined sewer overflows discharge sewage precisely during rainfall. The most common mistake is to take a daytime rise in pH at constant conductivity for an event - it is usually photosynthesis, a normal state, not an alarm.
The most common mistakes
- “Textbook” thresholds instead of a local baseline - false alarms or missed events.
- No calibration/servicing - a sentinel you cannot trust is worse than none.
- Poor placement - a sensor out of reach of the inflow will detect nothing.
- Reacting after the fact - without an immediate warning, the window to establish the source is lost.
How often
By definition continuously - this is not a campaign but constant supervision. Two things are needed: an uninterrupted series (the baseline) and a signal at the moment of deviation.
Sources and further reading
The basics of measuring conductivity and pH are described in standards (PN-EN 27888:1999, which adopts ISO 7888:1985, and PN-EN ISO 10523:2012, which adopts ISO 10523:2008), and the principles of water monitoring - in GIOŚ materials. A sentinel sensor is well complemented by periodic sampling for confirmation in the laboratory.
📊 See it live: Global Rivers & Lakes Watch - a public dashboard from three stations: the Maumee River (Ohio, USA), the river Main in Frankfurt and the LéXPLORE research platform on Lake Geneva. Temperature, oxygen, pH, conductivity, turbidity and chlorophyll-a are fetched automatically from USGS, WSV Pegelonline and Eawag Datalakes; plus correlation tiles and a station map. No login.
How to make it easier
A sentinel works when the data flows in live and when a deviation speaks up - as fast as readings arrive. In LimnoLog it looks like this:
- connectors / telemetry - sensor readings reach the station on the connector’s schedule (how often depends on your plan) or right after your logger sends them to the webhook address; alongside, you can pull in data from public APIs (rainfall from IMGW-PIB), because changes in conductivity often go hand in hand with rain;
- threshold alerts - you set guidelines around your own baseline and switch the alert on for the indicator. When a received conductivity or pH reading breaches them, project admins and the members you choose get an e-mail (and a push, if their notifications are on); a second signal follows when the value is back to normal. How soon depends on how often the sensor or connector delivers readings. It is a signal to head into the field and take a sample before the trace disappears;
- the full series and the deviations are on a chart, ready to be shared via a link (e.g. for cooperating services) and exported as documentation of the event.
And since the same system covers, alongside conductivity and pH, turbidity, oxygen, level and any other indicator - your sentinel sees the water from many sides at once.
Early users get free access for now and keep the core app free for good (add-ons on Founder programme terms). If you protect your watercourse or intake - take a look and try it.
See it in the LimnoLog app
Explore LimnoLog features