Unusual water indicator values - what they can mean
When a reading clearly departs from what you usually see in a given water, it is worth knowing what may lie behind it. Below are the most common causes of deviations in key indicators - these are pointers for further checking, not diagnoses: always confirm them with context and a follow-up measurement.
High electrolytic conductivity
Most often it means a rise in the load of dissolved ions: an inflow of sewage, run-off from roads (winter salinity), run-off from fields, geogenic salinity. A sudden spike can be a signal of an event (see detecting pollution).
Low dissolved oxygen
Typical causes: high temperature (warm water holds less oxygen), intensive decomposition of organic matter, night-time respiration, eutrophication, stagnation near the bottom of a stratified lake. Dangerous especially towards morning (a risk of an oxygen crash).
High nitrogen and phosphorus
They indicate an inflow of nutrients - run-off from fields (fertilisers), sewage, catchment development. This is the main engine of eutrophication and of blooms.
Sudden turbidity
Usually erosion and run-off after rainfall, stirred-up sediment or a bloom (see turbidity). It is worth setting it against the rainfall.
Extreme pH
Very high pH can be the result of intensive photosynthesis (during the day, with a bloom); very low - the influence of geology (humic waters) or a discharge. Always read pH with the time of day.
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.
In practice
Deviations are easiest to catch against a baseline and with guidelines. In LimnoLog a threshold on an indicator triggers a notification, and the chart shows the deviation in the context of the series - making it easier to tell a real event from ordinary variability.
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