Water and Water Quality
4. Water Testing and Monitoring
Learning outcomes
- I can identify indicators of water quality.
- I can explain how water samples are tested and analyzed.
- I can interpret simple water-quality data.
- I can describe the importance of water monitoring.
- I can evaluate the health of a water system using evidence.
Water Testing and Monitoring
Water quality describes the physical, chemical, and biological characteristics of water and whether the water is suitable for a particular purpose.
Water that is suitable for one purpose may not be suitable for another. For example, water quality requirements can differ for:
- drinking water
- swimming
- irrigation
- industrial use
- fish and other aquatic organisms
Scientists monitor water because appearance alone cannot tell us whether water is healthy or safe. Clear water can still contain dissolved chemicals or harmful microorganisms.
Water-quality monitoring usually involves:
COLLECT SAMPLE → MEASURE INDICATORS → RECORD DATA → COMPARE RESULTS → IDENTIFY PATTERNS → EVALUATE WATER HEALTH
Indicators of Water Quality
Scientists cannot usually measure "water quality" with one test.
Instead, they measure several indicators.
Important indicators include:
- temperature
- pH
- dissolved oxygen
- turbidity
- conductivity
- nitrate concentration
- phosphate concentration
- microorganisms
- biological indicators
Together, these measurements provide evidence about the condition of a water system.
Physical, Chemical, and Biological Indicators
Water-quality indicators can be grouped into three broad categories.
Physical indicators
These describe observable or measurable physical properties.
Examples include:
- temperature
- turbidity
- color
- odor
- suspended solids
Chemical indicators
These describe substances or chemical conditions in the water.
Examples include:
- pH
- dissolved oxygen
- nitrates
- phosphates
- dissolved salts
- metals
Biological indicators
These involve living organisms.
Examples include:
- bacteria
- algae
- aquatic invertebrates
- fish populations
Using indicators from all three categories provides a more complete picture than relying on only one measurement.
Temperature
Water temperature is an important environmental measurement.
Temperature affects:
- dissolved oxygen
- metabolic rates
- growth
- reproduction
- chemical reaction rates
- species distribution
Different aquatic organisms are adapted to different temperature ranges.
A major temperature change can therefore alter an aquatic ecosystem.
Temperature and Dissolved Oxygen
Temperature influences how much oxygen can remain dissolved in water.
In general:
colder water can hold more dissolved oxygen than warmer water
Therefore, unusually warm water can sometimes create problems for organisms that require high oxygen concentrations.
This is one reason thermal pollution can affect aquatic ecosystems.
pH
pH measures how acidic or alkaline a solution is.
A simplified pH scale runs from:
0 to 14
with:
pH 7 = neutral
pH below 7 = acidic
pH above 7 = alkaline
Aquatic organisms are generally adapted to particular pH ranges.
Large changes in pH can:
- stress organisms
- interfere with reproduction
- affect chemical reactions
- change the availability or toxicity of some substances
Measuring pH
pH can be measured using:
- indicator paper
- indicator solutions
- electronic pH meters or probes
Electronic probes can provide more precise numerical measurements.
For example:
pH = 7.2
provides more information than simply describing the water as "approximately neutral."
Dissolved Oxygen
Dissolved oxygen, often abbreviated DO, is oxygen gas dissolved in water.
Aquatic organisms such as:
- fish
- insects
- crustaceans
- aerobic microorganisms
depend on dissolved oxygen.
DO is therefore one of the most important indicators of aquatic ecosystem health.
It is commonly measured in:
mg/L
What Affects Dissolved Oxygen?
Dissolved oxygen can be influenced by:
- temperature
- water movement
- photosynthesis
- respiration
- decomposition
- pollution
Fast-moving streams often gain oxygen as water mixes with air.
Aquatic plants and algae can add oxygen through photosynthesis during daylight.
However, respiration and decomposition consume oxygen.
Organic Pollution and Oxygen
Suppose sewage or another source adds large quantities of biodegradable organic material to a river.
Microorganisms begin decomposing this material.
Their respiration consumes oxygen.
The sequence can be:
organic pollution increases
↓
decomposer activity increases
↓
oxygen consumption increases
↓
dissolved oxygen decreases
↓
aquatic organisms experience stress
Severe oxygen depletion can cause fish and other organisms to die.
Biological Oxygen Demand
Biochemical oxygen demand, often called BOD, is a measure related to how much oxygen microorganisms use while decomposing biodegradable organic matter.
Generally:
high BOD → high oxygen demand
A high BOD can indicate substantial organic pollution.
If microorganisms consume large amounts of oxygen:
BOD increases → DO may decrease
BOD and DO therefore provide related but different information.
Turbidity
Turbidity describes how cloudy water is because of suspended particles.
Particles may include:
- clay
- soil
- silt
- organic matter
- microorganisms
- algae
High turbidity can reduce the amount of light passing through water.
Why Turbidity Matters
High turbidity can:
- reduce light penetration
- reduce underwater photosynthesis
- interfere with feeding by some organisms
- cover aquatic habitats with sediment
- damage fish gills
- transport pollutants attached to particles
High turbidity may result from:
- erosion
- construction
- agricultural runoff
- storms
- algal growth
A sudden increase can therefore indicate that conditions in the watershed have changed.
Conductivity
Electrical conductivity measures how easily water conducts an electric current.
Pure water conducts electricity poorly.
Water containing dissolved ions conducts electricity more easily.
Therefore:
more dissolved ions → generally higher conductivity
Conductivity can provide information about the amount of dissolved material in water.
It does not identify exactly which substances are present.
What Can Increase Conductivity?
Conductivity can increase because of:
- dissolved salts
- minerals
- wastewater
- agricultural runoff
- industrial discharge
Natural geology can also affect conductivity.
Therefore, high conductivity does not automatically prove pollution.
Scientists need to consider the normal conditions of the particular water system.
Nitrates
Nitrates contain nitrogen and are important nutrients for plant growth.
Small amounts occur naturally.
Elevated nitrate concentrations may result from:
- fertilizers
- animal waste
- sewage
- wastewater
Excess nutrients can contribute to eutrophication.
Phosphates
Phosphates are another important nutrient.
Sources can include:
- fertilizers
- sewage
- animal waste
- soil erosion
- some human activities
Excess phosphorus can stimulate excessive algae and aquatic plant growth.
In many freshwater ecosystems, phosphorus availability strongly influences algal growth.
Nutrients and Eutrophication
Suppose monitoring detects increasing nitrate and phosphate concentrations.
Scientists might predict:
nutrients increase
↓
algal growth increases
↓
more organic material eventually dies
↓
decomposition increases
↓
oxygen consumption increases
↓
dissolved oxygen may decrease
This illustrates why several indicators should be considered together.
A nutrient measurement by itself tells only part of the story.
Microorganisms
Water may contain microorganisms such as:
- bacteria
- viruses
- protozoa
- algae
Many microorganisms are harmless or beneficial.
Others can indicate contamination or cause disease.
Testing every possible pathogen individually would be difficult, so scientists sometimes test for indicator organisms associated with fecal contamination.
Indicator Bacteria
Certain bacteria can indicate that water has been contaminated by fecal material.
Their presence suggests that disease-causing microorganisms may also be present.
Possible sources include:
- sewage
- leaking sanitation systems
- livestock
- wildlife
- stormwater runoff
Microbiological testing is especially important when evaluating water intended for human contact or consumption.
Biological Indicators
Scientists can also examine organisms living in the water.
Some organisms tolerate pollution better than others.
The types and diversity of organisms present can therefore provide evidence about long-term water quality.
Examples include aquatic:
- insect larvae
- snails
- worms
- crustaceans
These organisms are often called macroinvertebrates when they are large enough to see without a microscope and lack a backbone.
Macroinvertebrates as Indicators
Different macroinvertebrates have different tolerances to pollution.
If a stream contains a diverse community including several pollution-sensitive organisms, this can suggest relatively good conditions.
If only a few pollution-tolerant organisms are present, this may indicate environmental stress.
Biological monitoring is useful because organisms experience the water conditions continuously.
A chemical test provides information mainly about conditions at the time the sample was collected.
Chemical Tests vs Biological Monitoring
Chemical measurements and biological monitoring provide different kinds of evidence.
Chemical measurements
Can identify current conditions such as:
- pH
- nitrate concentration
- dissolved oxygen
- conductivity
Biological monitoring
Can reveal how conditions have affected organisms over longer periods.
For example, a pollution event might have occurred several days before sampling.
The pollutant may no longer be present at a high concentration, but changes in the biological community may remain.
The strongest assessments often combine both approaches.
Collecting a Water Sample
Good water-quality data begin with good sampling.
A sample should be collected carefully so that it represents the water being studied.
Scientists may need to consider:
- sampling location
- sampling depth
- time of day
- weather
- recent rainfall
- water flow
- clean equipment
- contamination from the sampler
Poor sampling can produce misleading results even if laboratory testing is accurate.
Avoiding Sample Contamination
Suppose a student collects a river sample in a bottle containing detergent residue.
The detergent could alter:
- chemical measurements
- biological measurements
- conductivity
The sample would no longer accurately represent the river.
Good practice includes:
- using clean containers
- avoiding contact with the inside of sample bottles
- labeling samples clearly
- following consistent procedures
Sample Labels
A useful water-sample label might include:
- location
- date
- time
- sample number
- collector
- relevant environmental conditions
For example:
Site B – downstream of bridge
5 October, 10:30
Sample B3
Good records allow results to be connected to the correct sampling location and conditions.
Field Testing
Some measurements are best taken directly at the sampling site.
Examples include:
- temperature
- pH
- dissolved oxygen
- conductivity
Portable electronic probes can measure several indicators quickly.
Field measurements are useful because some water characteristics can change during storage and transportation.
Laboratory Testing
Other analyses may require laboratory equipment.
Examples include:
- nutrient concentrations
- bacterial testing
- metals
- chemical contaminants
- suspended solids
Laboratory methods may provide greater sensitivity or precision than simple field tests.
Calibration
Scientific instruments need to provide reliable measurements.
Calibration involves checking or adjusting an instrument using known standards.
For example, a pH meter may be calibrated using solutions with known pH values.
Without proper calibration, an instrument could consistently report values that are too high or too low.
This creates a systematic error.
Repeated Measurements
One measurement may be affected by random variation.
Scientists can improve confidence by taking repeated measurements.
For example:
DO measurements:
6.8 mg/L
6.9 mg/L
6.7 mg/L
Mean:
(6.8 + 6.9 + 6.7) / 3 = 6.8 mg/L
Repeated measurements can help identify unusual results and improve reliability.
Monitoring Over Time
A single sample provides a snapshot.
Monitoring means collecting measurements repeatedly over time.
For example, a river could be tested:
- weekly
- monthly
- seasonally
- before and after storms
This can reveal patterns that one sample would miss.
Why Long-Term Monitoring Matters
Imagine nitrate concentrations are measured once:
4.2 mg/L
That value alone provides limited information.
Now imagine measurements over several months:
| Month | Nitrate |
|---|---|
| January | 2.1 mg/L |
| February | 2.3 mg/L |
| March | 2.4 mg/L |
| April | 4.2 mg/L |
| May | 6.8 mg/L |
| June | 7.1 mg/L |
Now a clear upward pattern appears.
Scientists could investigate possible causes such as:
- seasonal fertilizer use
- increased runoff
- sewage contamination
- changes in land use
Monitoring allows scientists to detect trends.
Sampling Upstream and Downstream
One useful investigation compares water quality:
upstream of a possible pollution source
with:
downstream of the source
Suppose a wastewater discharge enters a river.
Scientists could collect samples:
Site A → upstream
Site B → near discharge
Site C → downstream
Differences between the sites can provide evidence about the effect of the discharge.
Example Water-Quality Data
Consider the following simplified results:
| Indicator | Site A | Site B |
|---|---|---|
| Temperature | 18°C | 24°C |
| pH | 7.2 | 7.0 |
| Dissolved oxygen | 8.5 mg/L | 4.2 mg/L |
| Nitrate | 1.2 mg/L | 6.8 mg/L |
| Turbidity | Low | High |
Which site appears healthier?
Site A appears healthier based on these measurements.
Evidence includes:
- higher dissolved oxygen
- lower nitrate
- lower turbidity
- cooler water
Site B shows several indicators of possible environmental stress.
The conclusion should be based on the combined evidence, not just one measurement.
Looking for Relationships
Water-quality indicators can influence one another.
For example:
temperature rises → oxygen solubility decreases
or:
nutrients increase → algal growth may increase → decomposition may increase → dissolved oxygen may decrease
or:
erosion increases → turbidity increases → light penetration decreases
Recognizing these relationships helps scientists interpret data rather than simply recording numbers.
Interpreting Trends
Consider:
| Location | Nitrate | Dissolved Oxygen |
|---|---|---|
| Upstream | 1.0 mg/L | 8.4 mg/L |
| Farm Drain | 7.5 mg/L | 6.2 mg/L |
| Downstream | 5.8 mg/L | 5.1 mg/L |
A reasonable interpretation is that nutrient pollution may be entering near the farm drain.
Evidence includes:
- nitrate rises sharply
- nitrate remains elevated downstream
- dissolved oxygen decreases
However, this evidence does not automatically prove the exact cause.
Additional measurements and repeated sampling would strengthen the conclusion.
Correlation Is Not Always Causation
Suppose nitrate rises while dissolved oxygen decreases.
The two changes may be related, but scientists should not immediately conclude that nitrate directly caused the oxygen decrease.
Other factors could include:
- temperature
- sewage
- water flow
- algal growth
- organic matter
Good scientific reasoning distinguishes between:
evidence of a relationship
and:
proof of a cause
Establishing Baseline Conditions
To recognize unusual changes, scientists need to know what conditions are normally like.
A baseline is a set of measurements representing typical conditions.
For example, if a stream normally has:
pH ≈ 7.3
and monitoring suddenly records:
pH = 4.5
this would be a major change worth investigating.
Without baseline data, it can be difficult to determine whether a measurement is unusual.
Seasonal Changes
Not every change indicates pollution.
Water quality can naturally vary with:
- season
- rainfall
- temperature
- stream flow
- plant growth
- snowmelt
For example, a stream may naturally be:
- colder in winter
- warmer in summer
- more turbid after heavy rain
Scientists therefore compare measurements with appropriate historical and seasonal data.
Rainfall and Water Quality
Heavy rain can cause rapid changes.
Rainfall may increase:
- runoff
- sediment
- nutrients
- bacteria
- urban pollutants
Therefore, a river tested immediately after a storm may have very different conditions from the same river during dry weather.
Recording weather conditions is important when interpreting results.
Evaluating the Health of a Water System
A good evaluation uses several pieces of evidence.
Consider:
- dissolved oxygen
- pH
- temperature
- nutrients
- turbidity
- conductivity
- microorganisms
- biological diversity
- changes over time
Instead of saying:
"The river is polluted."
a stronger scientific statement would be:
"The downstream site shows evidence of reduced water quality because dissolved oxygen is lower, nitrate concentration and turbidity are higher, and pollution-sensitive macroinvertebrates are less abundant than at the upstream site."
This is an evidence-based conclusion.
Designing a Water-Quality Investigation
Suppose students want to investigate whether a stream changes as it passes through an urban area.
They could choose three sampling sites:
Site A — upstream of the city
Site B — within the city
Site C — downstream of the city
At each site they could measure:
- temperature
- pH
- dissolved oxygen
- turbidity
- conductivity
- nitrate
They could also sample macroinvertebrates.
For a fair comparison, they should try to keep procedures consistent.
Variables in a Water-Quality Investigation
For the previous investigation:
Independent variable:
sampling location
Dependent variables:
water-quality measurements
Examples:
- pH
- DO
- nitrate
- turbidity
Important controlled factors could include:
- sampling method
- sampling depth
- equipment
- measurement procedure
- approximate sampling time
Environmental investigations cannot control every variable, but consistent methods improve comparisons.
Sources of Error
Possible errors include:
- contaminated sample bottles
- poorly calibrated probes
- incorrect reagent amounts
- misreading instruments
- sampling different depths
- testing samples after excessive storage
- incorrectly labeling samples
A good evaluation identifies specific sources of error and explains how they could affect the results.
Monitoring Drinking Water
Water monitoring is not limited to natural ecosystems.
Drinking-water systems must also be monitored.
Testing can check for:
- harmful microorganisms
- disinfectant levels
- turbidity
- metals
- chemical contaminants
- pH
Monitoring helps determine whether treatment is working and whether water remains safe as it moves through the distribution system.
Monitoring Wastewater
Wastewater treatment plants also test water.
Samples may be collected:
- before treatment
- during treatment
- after treatment
Comparing these measurements shows how effectively contaminants are being removed.
For example:
Influent BOD = high
Effluent BOD = much lower
This provides evidence that biological treatment has removed biodegradable organic material.
Citizen Science
Water monitoring can also involve:
- schools
- community organizations
- environmental groups
- volunteers
Citizen-science programs may collect information about:
- stream temperature
- pH
- turbidity
- aquatic organisms
- litter
When standardized procedures are used, these projects can contribute useful environmental information while helping communities understand their local water systems.
Common Mistakes
Assuming Clear Water Is Healthy Water
Many contaminants are invisible.
Water quality must be measured.
Using Only One Indicator
A single measurement rarely provides enough evidence to evaluate an entire ecosystem.
Use multiple indicators.
Assuming High Turbidity Always Means Chemical Pollution
Turbidity may result from natural sediment, erosion, storms, or algal growth.
Further evidence is required.
Assuming High Conductivity Automatically Means Pollution
Natural minerals can also increase conductivity.
Compare results with baseline conditions.
Thinking High Nitrate Directly Kills All Fish
Nutrients often affect aquatic systems indirectly through processes such as eutrophication and oxygen depletion.
Confusing DO and BOD
DO = oxygen currently dissolved in water
BOD = oxygen demand associated with microbial decomposition
Generally:
high BOD can contribute to low DO
Ignoring Temperature
Temperature affects both organisms and dissolved oxygen.
It should often be considered when interpreting DO results.
Drawing Conclusions from One Sample
A single sample represents one location at one moment.
Repeated monitoring provides stronger evidence.
Ignoring Natural Variation
Water quality naturally changes with:
- weather
- season
- flow
- geology
Not every difference indicates pollution.
Claiming Causation Without Enough Evidence
A pattern may suggest a possible cause, but additional testing may be needed to demonstrate it.
Key Terms
Water quality — The physical, chemical, and biological characteristics of water in relation to its suitability for a particular use.
Water-quality indicator — A measurable characteristic used to assess water conditions.
Monitoring — Repeated measurement of environmental conditions over time.
pH — A measure of how acidic or alkaline a solution is.
Dissolved oxygen (DO) — Oxygen gas dissolved in water and available to aquatic organisms.
Biochemical oxygen demand (BOD) — A measure related to the oxygen microorganisms use while decomposing biodegradable organic matter.
Turbidity — Cloudiness caused by suspended particles.
Conductivity — A measure of how easily water conducts electricity, influenced by dissolved ions.
Nitrate — A nitrogen-containing nutrient that can contribute to nutrient pollution at elevated concentrations.
Phosphate — A phosphorus-containing nutrient that can contribute to eutrophication.
Indicator organism — An organism whose presence, absence, or abundance provides information about environmental conditions.
Macroinvertebrate — An animal without a backbone that is large enough to see without a microscope; many aquatic species are useful biological indicators.
Calibration — Checking or adjusting an instrument using known standards.
Baseline — Measurements representing normal or typical environmental conditions.
Sample — A portion of water collected for testing.
Reliability — The degree to which repeated measurements produce consistent results.
Systematic error — An error that consistently shifts measurements in the same direction.
Waterborne disease — Disease transmitted through contaminated water.
Key Takeaways
- Water quality cannot be evaluated from appearance alone.
- Scientists use physical, chemical, and biological indicators.
- Important indicators include temperature, pH, dissolved oxygen, turbidity, conductivity, nutrients, microorganisms, and aquatic organisms.
- Dissolved oxygen is particularly important for many aquatic organisms.
- Warmer water generally holds less dissolved oxygen than colder water.
- High BOD can contribute to reduced dissolved oxygen.
- Turbidity measures the cloudiness caused by suspended particles.
- Conductivity provides information about dissolved ions but does not identify individual pollutants.
- Elevated nitrates and phosphates can contribute to eutrophication.
- Microorganisms can provide evidence of sewage or fecal contamination.
- Macroinvertebrates can provide information about longer-term ecosystem conditions.
- Good sampling procedures are essential for reliable data.
- Instruments should be properly calibrated.
- Repeated measurements improve confidence in results.
- Long-term monitoring can reveal trends that individual samples cannot.
- Upstream and downstream comparisons can help identify possible pollution impacts.
- Natural seasonal and weather-related changes must be considered.
- Several indicators should be evaluated together before judging the health of a water system.
- Strong scientific conclusions explicitly connect claims to evidence.
A useful approach is:
MEASURE → COMPARE → LOOK FOR PATTERNS → CONNECT THE EVIDENCE → DRAW A CONCLUSION
Check Your Understanding
Water-Quality Indicators
1. Define water quality.
2. Give three physical indicators of water quality.
3. Give three chemical indicators.
4. Give two biological indicators.
5. Explain why clear water is not necessarily safe or healthy.
Temperature and Oxygen
6. Explain how temperature affects dissolved oxygen.
7. Why is dissolved oxygen important to aquatic ecosystems?
8. Give three processes that can affect dissolved oxygen.
9. Explain how organic pollution can cause dissolved oxygen to decrease.
10. Explain the difference between DO and BOD.
pH and Turbidity
11. What does pH measure?
12. Classify water with pH 5.5 as acidic, neutral, or alkaline.
13. Classify water with pH 8.2.
14. Define turbidity.
15. Give three possible causes of increased turbidity.
16. Explain how high turbidity can affect aquatic plants.
Nutrients and Conductivity
17. What does conductivity indicate about water?
18. Why does high conductivity not automatically prove that water is polluted?
19. Give two possible sources of elevated nitrate.
20. Explain how high nutrient concentrations can eventually contribute to low dissolved oxygen.
Biological Monitoring
21. What is an indicator organism?
22. Explain why macroinvertebrates can be useful indicators of water quality.
23. Why might biological monitoring reveal information that a single chemical test misses?
24. Distinguish between testing for pathogens and testing for indicator organisms.
Sampling and Monitoring
25. Give four pieces of information that should be recorded when collecting a water sample.
26. Explain why clean sample containers are important.
27. What is calibration?
28. Why are repeated measurements useful?
29. Explain the difference between taking one sample and conducting long-term monitoring.
30. Why should rainfall and weather conditions be recorded during water-quality investigations?
Data Interpretation
Use the following results:
| Indicator | Site A | Site B | Site C |
|---|---|---|---|
| Temperature | 17°C | 19°C | 23°C |
| pH | 7.3 | 7.2 | 7.1 |
| DO | 9.0 mg/L | 7.2 mg/L | 4.6 mg/L |
| Nitrate | 1.1 mg/L | 2.4 mg/L | 7.3 mg/L |
| Turbidity | Low | Medium | High |
31. Which site appears to have the best water quality? Support your answer with at least three pieces of evidence.
32. Which site appears most environmentally stressed?
33. Describe the pattern in dissolved oxygen from Site A to Site C.
34. Describe the pattern in nitrate concentration.
35. Suggest one possible relationship between nitrate and dissolved oxygen that scientists might investigate.
Analysis and Evaluation
36. Why would the data above not be enough to prove exactly what caused the changes?
37. Suggest three additional measurements that could strengthen the investigation.
38. A stream has high dissolved oxygen, low turbidity, neutral pH, low nutrient concentrations, and a diverse population of pollution-sensitive macroinvertebrates. Evaluate the likely health of the stream using this evidence.
39. A river normally has a pH of approximately 7.2, but a measurement of 4.8 is recorded downstream from an industrial area. Explain what scientists should do before concluding that industrial pollution caused the change.
40. Design a simple investigation comparing water quality upstream and downstream from a possible pollution source. Identify the measurements you would take and explain how you would use the evidence to evaluate the health of the water system.