Waterwatch Australia national technical manual

Module 4 - physical and chemical parameters
Waterwatch Australia Steering Committee
Environment Australia, July 2002
ISBN 0 6425 4856 0

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Methods (continued)

Nitrogen

What is it and why does it matter?

Nitrogen (chemical symbol N): an element that is essential for all forms of life

Nitrogen is derived from the atmosphere, where nitrogen gas (N2) is the main constituent. Few living things can use gaseous nitrogen, but most depend on compounds of nitrogen.

The most common nitrogen compounds are ammonia (NH3), nitrate (NO3) and nitrite (NO2). They occur in dissolved, particulate and gaseous forms.

Nitrogen compounds can be found in surface waters and in groundwater. The element nitrogen is recycled continually by plants and animals, and is present in freshwaters at higher concentrations than phosphate. Although both nutrients are required for plant growth, phosphate is considered to be the limiting factor in freshwater. In saltwater ecosystems, however, nitrogen is much less abundant, and it becomes the nutrient that limits algal growth.

What factors affect nitrogen?

As explained above, nitrogen is actually measured by the concentration of nitrate (NO3). The main factors affecting nitrates are:

Suggested methods, equipment and reporting

Nitrates readily dissolve in water and enter rivers more quickly than other nutrients. As a result, nitrates serve as a better indicator than other nutrients of sewage or manure pollution during dry weather.

Waterwatch groups that have chosen to test for nitrate usually use either the cadmium reduction (colour comparator) method or the zinc reduction (colorimeter/ spectrophotometer) method. Both produce a colour reaction that is then measured either by comparison to colours on a colour comparator or by use of a colorimeter.

The cadmium reduction method appears to be more accurate at lower concentrations of nitrate than the zinc reduction method but is far more hazardous because cadmium is very toxic.

Monitoring nitrate is challenging because it can involve measuring very low concentrations - down to 0.01 mg/L as N.

Your first consideration should be the purpose of monitoring for nitrate and the concentrations likely to be found. For example, if your group is monitoring for background changes in nitrate concentrations in a catchment, environmentally significant changes in nitrate may occur but be undetected if they are smaller than the detection limit of your equipment. There is little to be gained if the concentrations of nitrate are persistently less than the detection limit of your equipment, or concentrations are right on the detection limit, where accuracy is often less than for mid-range values.

Colour comparator method

A colour comparator is a low cost, simple piece of equipment with a colour wheel or colour bar. For nitrates the degree of redness of the solution is measured using the colour comparator. The redder the solution, the higher the concentration of nitrates.

Matching the colour of a treated sample to a comparator can be subjective, especially at low concentrations, and can lead to variable results. In addition, people who suffer from colour blindness can find it difficult to read the results.

Colour comparators are useful for identifying the high concentrations (greater than 1 mg/L) that can be expected at heavily polluted sites in a waterbody, e.g. stormwater runoff in urban streams and wastewater treatment outfalls.

The cadmium reduction method requires that the water samples are not turbid. Turbid samples should be filtered (if filters are available to the group). If copper, iron, or other metals are present in concentrations above several mg/L, the reaction with the cadmium will be slowed down and the reaction time will have to be increased.

The reagents used for this method are often pre-packaged for various concentration ranges of nitrate in the water. It is not possible to know the appropriate range for your waterbody, unless other measurements have been made previously. Therefore, make some trial measurements with reagents for one range (perhaps chosen according to the trigger values for the designated uses for your waterbody), and then move to a more suitable range of reagents if necessary, once you get a feel for the values likely for that waterbody.

Your Waterwatch coordinator may have access to local values for nitrate in other waterbodies, and will be able to discuss the appropriate trigger values for nitrate suggested by the revised water quality guidelines (ANZECC/ARMCANZ 2000).

Equipment

The equipment you will need for this method includes:

Procedure

  1. Collect your sample using the standard technique and using dedicated sample bottles. If not testing nitrate in the field, keep the samples on ice and test them in the laboratory as soon as possible (within 24 hours).
  2. Use safety glasses and gloves to minimise risk of harm from the reagents.
  3. Follow the manufacturer's directions for your particular kit to analyse the sample.
  4. Record your reading on the water quality results sheet.
  5. Pour the treated samples into the 'waste chemical bottle' for disposal as a hazardous toxic waste.
  6. Thoroughly rinse all containers with deionised water before testing the next sample.

Extra safety tips for testing nitrate

Always wear plastic disposable gloves and goggles when analysing samples. They protect you from the reagents and samples, and also protect the samples from contamination by the nitrates and phosphorus that are typically present on the hands of smokers.

Be very careful when you are testing nitrate with the cadmium reduction method. Cadmium is present in the reagent powder and precipitates to the bottom of the test tube at the end of the test. Waste from this test should not be poured down the drain but instead should be stored separately in a hazardous waste jar labelled 'Toxic Waste'. This jar must be disposed of as 'special waste'.

Maintenance

Follow normal acid wash procedures between sampling runs (see Acid-wash cleaning method)

Colorimeter/Spectrophotometer method

Colorimeters and spectrophotometers measure the degree of redness of the treated sample electronically. They measure the amount of light transmitted through the solution or absorbed by it. The light used is at a wavelength of 543 nanometres (nm).

The use of one of these instruments reduces the subjectivity of determining colour and is much more accurate than other instruments. However, colorimeters and spectrophotometers require maintenance and regular calibration.

At very high transmittance (above 90%) the presence of moisture, fingerprints or variable positioning of the sample cells can cause a significant change in readings. Some meters require that you prepare and analyse known standard concentrations before testing in order to convert the transmittance readings of your river sample to milligrams per litre. Other meters read percentage transmittance of light through the sample which can then be converted to milligrams per litre as N, using a chart. The most convenient meters directly display the sample concentration as mg/L of N.

For many rivers, lakes and estuaries in near-natural catchments, environmentally significant changes in nitrate concentrations are very low. As a guide, a field colorimeter or spectrophotometer should have a minimum detection concentration of 0.02 mg/L and accuracy of within 20% of the true value. Measuring lower concentrations can be achieved by sending a sample to a commercial laboratory for testing (see Table 4.7).

Equipment

The equipment you will need for this method includes:

Procedure

These instructions are generalised for a variety of colorimeters. Follow the manufacturer's instructions for your model.

  1. Collect your sample as per standard technique using dedicated sample bottles. If not testing nitrate in the field, keep the samples on ice and test them in the laboratory as soon as possible (within 24 hours).
  2. 'Zero' the meter using a blank (the sample without reagents added to it following the manufacturer's directions).
  3. Pour the recommended sample volume into a mixing container and add reagent powder. Swirl to mix. Wait the recommended time (usually a minimum of 10 minutes) before proceeding.
  4. Pour the first water sample into the sample cell test tube. Wipe the tube with a lint-free cloth to be sure it is clean and free of smudges or water droplets. Insert the tube into the sample cell of the colorimeter.
  5. Place the cover over the sample cell. Read the concentration of the sample or percentage transmittance, and convert to mg/L as N on the chart provided. Record your reading on the water quality result sheet. Note: if the sample concentration is below the minimum detection limit of your instrument, e.g. 0.05mg/L, report the result as <0.05mg/L as N.
  6. Pour the treated samples into the liquid waste bottle.
  7. Rinse the sample cell test tube and mixing container three times with deionised water. Avoid touching the lower portion of the sample cell test tube. Wipe with a clean, lint-free cloth. Be sure that the lower part of the sample cell test tube is clean and free of smudges or water droplets. Be sure to use the same sample cell test tube for each sample. If the test tube breaks, use a new one and repeat step 1 to zero the meter.

Maintenance

Follow normal acid-wash procedures between sampling runs.

Calibration

The laboratory procedure for calibrating a colorimeter is:

  1. Prepare six standard solutions that are in the range of the results expected. Generally 0.00 mg/L, 0.04 mg/L, 0.08 mg/L, 0.12 mg/L, 0.16 mg/L and 0.20 mg/L will be suitable concentrations.
  2. Label six 25 mL volumetric flasks, one for each new standard solution.
  3. Pour about 30 mL of a nitrate standard solution containing 1 mg N/L into a 50 mL beaker.
  4. Using Class A volumetric pipettes (pre-rinsed in standard solution) transfer 0-5 mL of the main standard solution from the beaker to the volumetric flasks, see Figure 4.13).
  5. Fill the volumetric flasks to the line. Swirl.
  6. Analyse a portion of each of these new standard solutions in a colorimeter, as described above, and record the results.
  7. Construct a standard curve from your measured concentrations (mg N/L), with measured concentration on the y axis and desired concentration on the x axis. The points should fall on a straight line (see Figure 4.14).
Figure 4.13: Proportion of main standard solution to concentration of new standard solution
Concentration of
new standard solution		   Volume of
main standard solution
0.00 mg/L   0 mL
0.04 mg/L   1 mL
0.08 mg/L   2 mL
0.12 mg/L   3 mL
0.16 mg/L   4 mL
0.20 mg/L   5 mL
where volume of main standard solution needed =
Desired concentration of new standard
x
Final volume (mL )of new standard
Concentration of main standard solution

Figure 4.14: Acceptable and unacceptable calibration curves for nitrate
Figure 4.14: Acceptable and unacceptable calibration curves for nitrate

Data confidence

Colour comparator
Colorimeter

Calibrate the colorimeter with a reagent blank before each use. To check if sample bottles are contaminating a sample, test a field blank. Test a field replicate after every tenth sample. Every six months, measure a mystery solution supplied by your Waterwatch coordinator.

Check the calibration of the colorimeter at least twice yearly with prepared standards as described below. Some colorimeters require calibrating with prepared standards before each use. For an external quality control check on 10% of samples, split every tenth sample, measure one part and send the other part to another laboratory. Alternatively, during field sampling, make every eleventh sample a replicate of every tenth sample. On measurement, the two samples should give the same results.

Colours of the comparator must be free of scratches and contaminants.

Interpreting your results

Nitrate readily dissolves in water and the concentration in streams can increase during wet weather because of input from run-off and rainfall. So it is important for you to consider stream flow rates and recent weather conditions when interpreting nitrogen data. It is a good idea to measure concentrations at one or more near-natural locations near your study site, to determine typical background concentrations for streams in your catchment.

The natural concentration of ammonia or nitrate in surface water is typically low (less than 1 mg/L). Nitrite is commonly less than 10% of the nitrite/nitrate total. In excessive amounts, nitrates can cause significant water quality problems.

It is not possible to define an upper limit for nitrogen concentrations in surface water that will ensure protection of all ecosystems. Many other factors are important: a combination of flow rate, turbidity, temperature, turbulence, phosphorus concentration and other factors determine the effect of a particular nitrogen concentration in a specific ecosystem. In freshwater ecosystems, increases in concentrations of phosphorus, rather than nitrogen, are believed to trigger eutrophication problems.

Your Waterwatch coordinator will be able to discuss the relevant trigger values for your type of waterbody, its location and designated uses, as suggested by the revised water quality guidelines (ANZECC/ARMCANZ 2000).

 
Table 4.7: Quality control measures for nitrate
Relative data quality Equipment method Sensitivity (minimum detection level) Calibration Type of quality control
Low Colour comparator. 1 mg/L None. Colours must be free of scratches and contaminants
Intermediate Colour comparator or colorimeter 0.02 mg/L For comparator test a calibration blank before each sampling run. For colorimeter, use calibration blank prior to each sample test. Calibrate colorimeter with prepared standards at least twice yearly. Colours of comparator must be free of scratches and contaminants. Test field replicate every 10 samples. Test Waterwatch mystery sample every six months.
High Collection and dispatch of sample under ice to professional laboratory Not applicable As per professional lab practice For 10% of samples, use external field replicates or split sample and send to different laboratories.

Safety considerations and waste disposal tips when sampling and measuring nitrate

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