Water-quality information is commonly grouped into physical, chemical and biological parameters. The groups describe different properties and require different measurement methods. No group—and no short list of field sensors—can establish complete water safety on its own.
Use the classification as a planning tool. Select a parameter because it answers a defined question, then identify the correct method, unit, sampling point and comparison requirement.
Physical water-quality parameters
Physical parameters describe measurable characteristics such as temperature, optical appearance and solids behavior.
| Parameter | What it describes | Important limitation |
|---|---|---|
| Temperature | Thermal condition at the measurement point | Changes interpretation of other parameters but does not determine safety |
| Turbidity | Optical scattering under the instrument method | Not a universal mass concentration of suspended solids |
| Color | Method-defined apparent or true color | Does not identify the responsible compound by itself |
| Suspended solids | Method-defined mass retained under the analytical procedure | Not interchangeable with turbidity without matrix-specific evidence |
| Dissolved solids | Residue/constituents under a defined laboratory method or an estimate | Does not identify ions or quantify all organic matter |
Turbidity can support filtration, sediment and event monitoring. A value does not become “pollution” at one universal NTU level, and clear water can still contain dissolved or microbiological contaminants.
Chemical water-quality parameters
Chemical parameters describe acid-base condition, ionic conductance, gases, nutrients and named analytes.
| Parameter | What it can support | What it cannot prove alone |
|---|---|---|
| pH | Acid-base condition and treatment/process change | Drinking-water safety, alkalinity or contaminant identity |
| Conductivity | Ionic-conductance trends | Which ions or pollutants are present |
| Salinity | Method/scale-specific dissolved-salt context | Complete ionic composition or safety |
| Alkalinity/acidity | Capacity under a titration definition | The same thing as pH |
| Hardness | Mainly calcium/magnesium hardness under the method | All scaling/corrosion behavior or water safety |
| DO | Oxygen condition at the point/time | Total pollution or cause of a low/high value |
| Residual disinfectant | Named disinfectant residual under a validated method | Absence of pathogens or by-products |
| COD/BOD | Method-defined oxygen-demand information | Interchangeable values, pollutant identity, toxicity or biodegradability by themselves |
| Ammonia/ammonium | A stated NH3/NH4+/TAN reporting basis | Toxic NH3 without pH, temperature and basis |
Conductivity usually rises as dissolved ionic content increases, but a high value does not identify contamination and a low value does not establish purity. TDS estimates derived from conductivity depend on the configured factor and water composition.
pH alone does not make water safe to drink. Any pH range should be tied to the water use, jurisdiction, method and authority. Chlorine species and residuals also require method and application context; the presence of a residual does not prove that every contaminant is absent.
Biological water-quality parameters
Biological monitoring can target microorganisms, algal/pigment indicators, plankton, benthic communities or other organisms. The sampling and analysis must match the target.
| Biological target | Typical evidence route | Limitation |
|---|---|---|
| Indicator bacteria | Culture, membrane filtration or validated molecular method | One indicator does not detect every pathogen |
| Viruses/protozoa | Target-specific laboratory method | Common field sensors do not measure them |
| Algal pigments | Optical/fluorescence screening plus suitable confirmation | Pigment response does not identify species or toxin concentration |
| Biological community | Standardized ecological sampling and identification | Results depend on habitat, season and survey design |
Do not apply one E. coli number across drinking water, recreational water and wastewater. The organism, unit, method, duration/use and jurisdiction must be local to the criterion.
How should the three groups be used together?
Physical, chemical and biological evidence can complement one another. For example, turbidity, conductivity and DO can describe different aspects of an event, while laboratory microbiology or targeted chemistry can answer a specific safety question. A correlation between groups does not identify a cause.
For selection and cross-parameter interpretation, use the Multiparameter Monitor guide. For sample planning and field/laboratory workflow, use How to Test Water Quality.
Selection checklist
- Define water source, intended use and decision.
- Select parameters that answer that decision.
- Identify the named method, unit and expected range.
- Choose representative locations, depths and times.
- Plan calibration, blanks/checks, duplicates or reference samples as applicable.
- Compare results only with a requirement that matches the analyte, method, unit and jurisdiction.