TEST METHOD: Field testing of nutrients in waterbodies

Revised 28 November 2006 (addition of ammonium details)

RISK ASSESSMENT

Assess Safety, Health, Quality and Environmental aspects of each specific step.

OVERVIEW

Nitrogen and Phosphorus are tested as indicators of the health of waterbodies, providing early warning to manipulate the system to prevent cyano bacterial and chlorophyte blooms. These instructions are suitable for use in the field when studying waters varying from freshwater to hypersaline water.

The total orthophosphate test measures available phosphate, in the field, using the Lovibond seawater water testing kit. Total orthophosphate is all that is required for routine sample runs in well known waterbodies where one is only looking for changes from background readings. It is suitable for field use as no digestion is necessary. Where waterbodies are unknown or contamination with bound phosphates is a possibility, consider returing a sample to the laborarory for total phosphorus analysis.

Total oxidised nitrogen measures the type of nitrogen (nitrite and nitrate) that is immediately available to plants in brine or freshwater and as such provides a useful indicator of a waterbody’s ability to support a bloom. Ammonium is a common nitrogenous pollutant that is toxic to many species in excess amounts. For fieldwork, measurement of both oxidised nitrogen and ammonium is possible, as neither of these require digestion.

Total nitrogen is the most useful routine laboratory measure as N is very labile and changes rapidly, both within the waterbody and in the removed sample. The full suite of nitrogen testing should be conducted where it is important to understand speciation within the waterbody. Should total nitrogen be required, use an appropriate method selected from the Vega, and analyse in the laboratory. Total nitrogen measures Kjeldahl (organic) nitrogen, oxidised nitrogen and ammonia. Subtracting oxidised nitrogen from total nitrogen provides a measure known as TKN (Total Kjeldahl Nitrogen) which is a sum of the organic N and ammonium components. Test methods for ammonium are also available in the Vega, so it is possible to determine all specific forms of nitrogen in a sample.

In the field the following tests are used:

Total oxidised nitrogen (NO2 plus NO3) is tested, as it has shown correlation to algal overgrowth, and because NO2 is toxic to fish.

Ammonium (NH4) is measured as it is a common pollutant and is toxic to many species in high concentrations.

Total orthophosphate is measured as it has a high correlation with cyanobacterial growth.

TASK SAFETY REQUIREMENTS

Safety boots are mandatory. Use PPE (eg safety glasses) as appropriate.

SPECIFIC JOB STEPS RISK ASSESSMENT RISK CONTROL
Preparation of sample for orthophosphate testing: High salt levels interfere with colour development of the method reagents, so dilution may necessary. If dilution is required, see Table One below.

Otherwise, use sample directly.

Dilution reduces the sensitivity of the method.

 

Contamination may occur between samples

Use a 1:2 dilution for most hypersaline samples (not fresh or seawater). Reserve higher dilutions for testing samples where a 1:2 dilution results in a milky colouration developing.

Rinse all equipment after each sample run. Rinse equipment well with demineralised water between samples. If necessary to keep longer, freeze samples.

Field readings for phosphate: Two 10mL aliquots shall be pipetted into Lovibond cells (in the field) and the reagents added as per the Lovibond Test Method instructions. The Lovibond colorimeter disc displays the phosphate level as mg/L PO4. To convert the reading to mg/L P multiply the disc result by 0.32, then multiply the result by any dilution factor.    
Record the mg/L P on the field sheet for the location, or in a notepad with a location note and date.    

Preparation of oxidised nitrogen sample: High salt levels interferes with flocculation of the zinc reductant used in Lovibond oxidised nitrogen field tests, so dilution or filtration may be necessary. Any zinc remaining in the sample will prevent the azo dye from developing its full colour causing underestimation of the oxidized nitrogen concentration, so it must be removed completely.

Either dilute samples as outlined below on Table One, or use a nitrate-free membrane filter attached to a syringe to separate the reduced sample from the zinc powder.

Nitrogen may be fixed from the air by bacteria within the sample

 

Dilution reduces the sensitivity of the method.

Always fill sample jar to the top if it will be a while before testing.

Keep samples cool, test on the day of collection.

Filter preferentially. Use nitrate-free Supor membranes only, not cellulose nitrate! Dilute only for field readings where no filtration is available. Dilute only as much as needed. See Table Onebelow.

Field readings for oxidised nitrogen: One 20mL aliquot of sample or diluted sample shall be pipetted into the plastic reducing tube supplied by Lovibond and, after reduction and floculation/filtration, one 10mL aliquot shall be pipetted into a Lovibond cell. The test shall be conducted as per the Lovibond Test Method instructions. The Lovibond colorimeter disc displays the oxidised nitrogen concentration as mg/L N. Correct for any dilution factor.    
Record the mg/L N on the field sheet for the location, or in a notepad with a location note and date.    
Preparation of sample for ammonium testing: High salt levels interfere with colour development of the method reagents, so dilution may necessary. Similarly, very high ammonium levels, beyond the range of the test, may interfere with proper colour development. If dilution is required follow the instructions in Table One below.

Otherwise, use sample directly.

Dilution reduces the sensitivity of the method.

 

Contamination may occur between samples

Use a 1:2 dilution for most hypersaline samples (not fresh or seawater). Reserve higher dilutions for testing samples where a 1:2 dilution results in a turquoise colouration developing.

Rinse all equipment after each sample run. Rinse equipment well with demineralised water between samples. If necessary to keep longer, freeze samples.

Field readings for ammonium: A 10mL aliquot shall be pipetted into the provided cell (in the field) and the reagents added as per the test method instructions in the field kit. The kit contains a colour chart that displays the ammonium level as mg/L N.    
Record the mg/L N on the field sheet for the location, or in a notepad with a location note and date.    

Table One - Dilution of saline samples

Typical sample specific gravity

Dilution factor

Sample quantity required

Seawater - 1.060

1:1

Pipette required quantity of undiluted sample directly into Lovibond cell

1.061 - 1.100

1:2

Measure 100mL of sample in measuring cylinder, pour into 200mL measuring flask and make up to the mark with demineralised water. Stopper and invert repeatedly to mix thoroughly. Pipette the required quantity of diluted sample across to Lovibond cell.

1.100 - Maiden

1:4

Measure 50mL of sample in measuring cylinder, pour into 200mL measuring flask and make up to the mark with demineralised water. Stopper and invert repeatedly to mix thoroughly. Pipette the required quantity of diluted sample across to Lovibond cell.

These dilutions are indicative only. Observe the reaction to determine whether the method is working as it is supposed to. Milkiness or unusual colours suggest further dilution is required. If this occurs, start the test again with a freshly diluted sample.

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