Unusual water indicator values - what they can mean
Does a reading clearly depart from what you usually see in this water? Below are the most common causes of such deviations for six indicators. These are pointers on what to check, not diagnoses - confirm them with context and a follow-up measurement.
| Reading | When to treat it as unusual | Most likely causes | What to check first |
|---|---|---|---|
| High conductivity | a lowland river in a limestone or farmed catchment is usually 300-800 µS/cm; a jump of > 50% against the baseline | sewage, road runoff (de-icing salt), runoff from fields, geogenic salinity | whether the jump is sudden (an event) or seasonal (winter salt); after rain expect rather a fall (dilution) - a rise during rainfall points to the first flush from roads or to a storm overflow |
| Low oxygen | < 4 mg/l - warning; < 2 mg/l - alarm | warm water, decomposition, night-time respiration, stagnation near the bottom | the time of day of the measurement and the temperature |
| High nitrogen / phosphorus | a rise against the station’s own baseline | fertilisers, sewage, development in the catchment | the weather of recent days (runoff after rain) |
| Sudden turbidity | an increase by an order of magnitude in NTU | erosion and runoff after heavy rain, disturbed sediment, a bloom | rainfall in the last 24-48 h |
| Extreme pH | outside 6.5-8.5; during blooms even > 9 | photosynthesis (high), humic waters and geology (low), a discharge | the time of day and the presence of a bloom |
| Temperature out of line | > 2 °C above neighbouring stations | cooling-water discharge, a shallow sunlit spot | whether the sensor was lying in a warmed-up shoreline zone |
The threshold values in the table are indicative - they serve to catch a reading worth checking, not to assess compliance with regulations. Every station has its own baseline and that is the best reference point.
Two mechanisms that explain most deviations
The daily rhythm. Oxygen and pH change through the day through the same process: photosynthesis consumes CO₂ during the day (pH rises) and produces oxygen, while night-time respiration does the opposite. That is why oxygen measured at dawn and in the afternoon are two different numbers for the same water - and why the risk of an oxygen crash is greatest in the early morning. Without the time of measurement, these two indicators are simply uninterpretable.
Rainfall. After rain many indicators change at once. Turbidity, phosphorus and often nitrogen rise, because water from the catchment carries suspended matter and nutrients. Conductivity usually falls - rainwater dilutes the ions - although the first flush from roads and paved yards, and in winter road salt, can raise it briefly. Comparing with rainfall helps form a hypothesis about the source, but does not settle it: a jump without rain points to a discharge, yet discharges also happen in the rain - combined sewer overflows discharge sewage precisely during rainfall. Diffuse runoff usually builds up and fades slowly, an overflow discharge tends to be sharper and appear at the start of the rain - these are hints, not a diagnostic key. Check what lies upstream (outlets, sewers, roads, field ditches) and take a sample during the event. Persistently elevated nutrients are no longer an event but eutrophication.
The rule above all rules
A single deviation is a hypothesis, not a verdict. It is only confirmed by the next measurement, a link with other indicators and context (weather, season, events) - see How to interpret results.
Before you treat a reading as an event in the water, rule out a measurement error: an uncalibrated probe, a dirty sensor, a measurement at a different time of day or a different depth can produce an “anomaly” that never existed in the river.
Sources
- Oxygen bands and their rationale: dissolved oxygen - measurement and interpretation (the full table of bands with the US EPA criteria and the solubility table).
- Reference values for your region: results published by national environmental monitoring services; meteorological data for checking rainfall: national hydrometeorological services.
- Classification thresholds (when you need a formal assessment rather than a signal): national regulations on the classification of surface water status.
In practice
A deviation shows best against a baseline and guidelines. In LimnoLog you switch on an alert on an indicator, and when a saved reading crosses the guideline you get a notification (e-mail, and a push if you have it enabled) - how soon depends on how often readings arrive. The chart shows the deviation in the context of the series, which makes it easier to tell an event from ordinary variability.
Calculate it yourself
- Concentration unit converter mg/l ↔ µg/l ↔ mmol/l ↔ ppm and nitrogen and phosphorus forms (NO₃ ↔ NO₃-N, PO₄ ↔ PO₄-P).
See it in the LimnoLog app
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