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Water Analysis

The following methods and protocols are all related to the chemical and physical properties of water.

Unless stated, the methods are currently being used in the Laboratory and have Risk Assessments associated with them. If you are planning on carrying out any work in the lab you must read and sign the Risk Assessment first. Please contact the Laboratory Supervisor for details.

Water Sampling
  1. Obtain pH, temperature and conductivity readings in triplicate.
  2. Perform an alkalinity titration on 100 ml of water using bromo-cresol green methyl red indicator. This is titrated to a pinky-grey/green end point. This should be performed 3 times per site. (Note - this method is for Total Alkalinity)
  3. Collect 100 ml of unfiltered water TP analysis.
  4. Filter 100 ml water with GF/F filters for filterable reactive phosphorus and nitrate nitrogen.
  5. Filter 100 ml water each with GF/F filters for cation analysis. The latter should be acidified to pH 2.0 with Aristar-grade concentrated nitric acid.
  6. Rinse the filter apparatus well and then filter 100 ml water with cellulose nitrate filters (0.45 µm pore size) for silica analysis.
  7. Rinse again and prepare a series of field blanks with deionised water. There should be 100 ml unfiltered, 200 ml filtered with GF/F, a sterilin of GF/F filtered and acidified, and a sterilin of cellulose nitrate filtered water.

All collection vessels should be acid-washed prior to sampling.

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Total Alkalinity

The total alkalinity of a water body refers to its ability to neutralise a strong acid, ie. its buffering capacity. Although the alkalinity may in theory be caused by any weak acid anion it is usually only carbonate, or more strictly bicarbonate, alkalinity that is important in freshwaters (Wetzel and Likens 1991).

The measurement of total alkalinity is achieved by titrating a known volume of a strong acid (eg. 1.6N H2SO4 against the water sample until all the carbonate has been used. This equivalence endpoint can be identified using a Bromocresol-green Methyl-red indicator. The end point of pH 4.5 is recommended for waters containing elevated levels of phosphate.

In the field, a 100ml sample of water is collected in a well-rinsed measuring cylinder and transferred to a 250 ml conical flask. To this 6-8 drops of Bromocresol-green Methyl-red indicator are added turning the sample blue-green. The titration can be performed using a Hach digital titrator (model 16900-01) with 1.6N H2SO4 as the titrant until the sample turns to light violet grey (pH 4.5). The total alkalinity is then read directly from the digital display and expressed as mg/L CaCO3. This should be repeated three times to give a mean value.

The titration can also be carried out in the laboratory using a standard burette and a standardised acid solution.

The endpoint of the titration can also be determined potentiometrically - this method is recommended for people with impaired colour vision.


References

  • American Public Health Association (1989) Standard Methods for the Examination of Water and Wastewater (17th ed.). American Public Health Association, Washington DC. 1550pp.
  • Wetzel, R. G. & Likens, G. E. (1991) Limnological Analysis. (2nd ed.) Springer-Verlag, Berlin. 391pp.

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Total Phosphorus

Total Phosphorus Digestion

Adapted from the American Public Health Association (1989) Standard Methods for the Examination of Water and Wastewater (17th ed.). American Public Health Association, Washington DC.

This method is for the determination of total P i.e. dissolved and particulate. The method, therefore, uses unfiltered water. Also applicable for total “dissolved” P if using filtered samples.


Reagents

  • Potassium persulphate (K2S2O8)
  • Standard phosphate solution (50 mg l-1)
  • Sulphuric acid (300 ml conc H2SO4 diluted to 1000 ml with DDW)
  • 4 M Sodium hydroxide solution (Dissolve 160 g NaOH in 1000 ml DDW)
  • Aqueous phenolphthalein indicator

A set of standards must be treated simultaneously. Set up using the following dilutions of the standard solutions.

Take 1.00 ml of standard phosphate solution (50 mg l-1) and make up to 100 ml in a volumetric flask to produce a P stock sol’n of 500 µg l-1

(a) 10 ml of DDW in reaction vessel   (0 µg l-1 PO4-P)
(b) 9.5 ml DDW, 0.5 ml P stock(25 µg l-1 PO4-P)
(c) 9.0 ml DDW, 1.0 ml P stock (50 µg l-1 PO4-P)
(d) 8.0 ml DDW, 2.0 ml P stock (100 µg l-1 PO4-P)
(e) 6.0 ml DDW, 4.0 ml P stock     (200 µg l-1 PO4-P)

Method

  1. Prepare oxidising reagent (must be fresh).
  2. Dissolve 10 g K2S2O4 in 100 ml DDW in a beaker, using gentle heating to <40°C.
  3. Pipette 10 ml of sample into each microwave reaction vessel
  4. Add 2 ml of oxidising reagent to each standard and all samples
  5. Add 0.2ml of the H2SOto each standard and all samples
  6. Cap vessels, ensuring the white caps are clean and dry - this is extremely important in the control vessel to prevent overheating.
  7. Place in turntable, ensuring all vessels are within a protective sleeve and vessels are distributed evenly in the carousel. Place in microwave.
StageMax. Power% PowerTimePressureTempHold
11600w10015:00-17010:00

8. Program a new method using “ramp to temperature” And assuming using 24 vessels 

9. When the digestion run is completed allow samples to cool to room temperature before uncapping.

10. Transfer 8 ml of each of the digested standards and samples to centrifuge tubes and add one drop of aqueous phenolphthalein indicator.

11. Add a know volume of 4M NaOH solution to achieve a faint pink colour – record volume added (expect to add 2-4 ml) 

12. Top tubes up with DDW so all samples are the same final volume

13. Add 1 ml of composite solution (see SRP method), cap and invert to mix, and leave for 1-2 hrs (<8hrs) for colour to develop and read absorbance at 885 nm using the DDW standard as a zero.

14. Digestion vessels and lids should be thoroughly rinsed with DDW prior to re-use or soaked in an acid bath overnight, rinsed and dried for the next user if not being re-used.


Important

Check the microwave digester unit in the first few minutes to ensure that temperature increases – the Mars microwave tends to over-read at low temperatures so don’t be surprised if the samples are already 60C when they go in. It is recommended that replicates of samples are done where possible and that a DDW blank is placed with each run. Standards MUST be used with every separate digestion to account for any variation between digestions.

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Soluble Reactive Phosphorus

Use filtered water samples (Whatman GF/F glass-fibre filters preferred). The method is also suitable for TP samples following digestion.


Reagents

  1. Sulphuric acid: add 70ml conc H2SO4 to 450 ml DDW
  2. Ammonium molybdate: dissolve 1.5 g of (NH4)6Mo7O24.4H2O in 50ml DDW (do not use if more than 6 weeks old)
  3. Ascorbic acid: dissolve 2.7 g of ascorbic acid in 50 ml DDW [make up fresh (very gentle warming may be required)]
  4. Potassium antimonyl tartrate: dissolve 0.34 g in 250ml DDW [make up fresh (gentle heating may be required)]
  5. Standard phosphate solution (50 mg/l): dissolve 0.2195 g of oven-dried KH2PO4 in DDW and make up to 1000 ml (in the Lab fridge)

Prepare the mixed reagent in a fixed ratio as follows - reagent (a) 5 : (b) 2 : (c) 2 : (d) 1, mixed in that order - Make up fresh. 1 ml required per sample, i.e.:

  • For 100 ml mixed reagent: 50 ml: 20 ml: 20 ml: 10 ml
  • For 50 ml mixed reagent: 25 ml: 10 ml: 10 ml: 5 ml
  • For 20 ml mixed reagent: 10 ml: 4 ml: 4 ml: 2 ml

Where the concentrations of PO4-P are greater than 10 µg/l, samples and standards can be prepared using lower volumes. Prepare 10 ml volumes of the samples and the standards in disposable centrifuge tubes and add 1 ml of the composite solution; cap and invert the tube several times to mix. This method is particularly suitable when larger numbers of samples are being analysed. The laboratory centrifuge tubes are sterile and P free.

In practice, this method has been found to provide accurate and repeatable results on samples with PO4-P concentration in the range of 5.0-1000 µg/l. It is however recommended that samples with very high PO4-P concentrations are diluted with DDW to below 200 µg/l PO4-P.


Procedure

Take 1.00 ml of standard phosphate solution (50 mg/l) and make up to 100 ml to produce a working P stock solution of 500 µg/1 as P. A set of standards must be treated simultaneously to the water samples. Set up the standards using the following dilutions of the 500 µg/1P stock solution.

  1. 10.00 ml DDW (0 µg/L PO4-P). This is the reaction blank
  2. 9.50 ml DDW: add 0.50 ml P stock (25 µg/l PO4-P)
  3. 9.00 ml DDW: add 1.00 ml P stock (50 µg/l PO4-P)
  4. 8.00 ml DDW: add 2.00 ml P stock (100 µg/l PO4-P)
  5. 7.00 ml DDW: add 3.00 ml P stock (150 µg/l PO4-P)
  6. 6.00 ml DDW: add 4.00 ml P stock (200 µg/l PO4-P)
  7. Place 10.0 ml of the filtered water samples into 15 ml centrifuge tubes

  8. Add 1.00 ml of the mixed reagent to each tube (including the standards), cap tightly and invert to mix.

  9. After 0.5-12 hours measure the absorbance in a quartz cuvette at 885 nm against the reagent blank (0 µg/L PO4-P).

Where the concentrations of PO4-P are very low (e.g. <5.0 µg/l), larger sample volumes should be used (200 ml) and an additional step is required whereby after the composite solution is added the blue complex is extracted into an organic solvent (e.g. isobutanol) in a separating funnel.

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pH Measurement

The measurement of pH is performed electrometrically using a pH meter with either a general-purpose combination electrode or a low ionic combination electrode, depending upon the ionic concentration of the sample. In low ionic strength solutions, a standard pH electrode will give a sluggish response and poor reproducibility. However, a low ionic electrode must not be used in solutions of high ionic strength (above 50uS) as this will result in damage to the electrode.

The electrode should never be allowed to dry out and therefore must be returned to a beaker or bottle of either pH4 buffer or standard electrode filling solution between measurements. In the absence of either of these solutions, distilled water can be used, but the electrode should not be stored for any great length of time in this.

The principle of the probe requires the glass electrode to adsorb a layer of the sample onto its surface; the resultant potential difference being a function of the hydrogen ion (H+) concentration in the sample and the electrolyte contained within the electrode (Wetzel and Likens, 1991).

Prior to measurement, the pH meter is calibrated using a freshly made buffer solution (pH7) and the slope of the electrode is adjusted against a pH4 buffer. Temperature compensation is adjusted manually according to the ambient sample temperature. The electrode is thoroughly rinsed with distilled water before each measurement.

Water samples should be collected in a clean glass beaker, well flushed with the sample, and the electrode allowed to stand for several minutes without agitation before the pH value is determined.

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Dissolved Silca

Silica is an important nutrient for diatoms. Below levels of approximately 0.5mg/L most diatoms lose the ability to reproduce effectively. This method is suitable for concentrations below 100mg/L.


Collection

Water samples for silica analysis should be collected in clean polythene bottles and filtered on-site with non-glass filters (e.g. cellulose nitrate 0.5µm pore size). The filtrate is relatively stable but should be analysed within 28 days. Samples can be frozen but this converts the silica to an unreactive form; it is, therefore, necessary to allow thawed samples to stand at room temperature for one or two hours before analysis. It is preferable to keep samples refrigerated.


Principal

In solution, silica exists as either silicic acid (H4SiO4) or silicate (SiO32-). This reacts with acidic ammonium molybdate to form a yellow silico-molybdate complex, which on reduction with sodium sulphite forms a molybdate blue colour. This is then compared to standards of known concentration using a spectrophotometer at 700nm in a 1cm cell.


Reagents

  1. Hydrochloric acid, 0.25N: mix 22ml of concentrated HCl (sp. gr. 1.18) with water and make up to 1 litre
  2. Ammonium molybdate, 5%: dissolve 5.2g (NH4)6Mo7O24.4H2O in water, dilute to 100ml
  3. Disodium EDTA, 1%: dissolve 1g of disodium EDTA in water and dilute to 100ml
  4. Sodium sulphite, 17%: dissolve 42.5g of sodium sulphite in water and dilute to 250ml (i.e. 170g in 1 litre)
  5. Silica Standard: stabilise approximately 30g of Na2SiO3.5H2O by placing it in a desiccator for 3 hours. Dissolve 17.65g of the stable compound in water and dilute to 1 litre. Pipette 10ml of this solution into a volumetric flask and make up to 1 litre with deionised water.
  6. 1.00ml = 0.050mg SiO2

General Procedures

All samples (including blanks, but not standards) should be analysed in duplicate.

  1. Pipette 10ml of each sample into 50-100ml Erlenmeyer flasks.
  2. Pipette 10ml of deionised water into a flask to serve as a reagent blank.
  3. Make up a series of standards from the stock solution. Make up each standard to 10ml with deionised water:
  4. 0.5ml = 0.025mg/10ml = 2.5mg/L
  5. 1.0ml = 0.05mg/10ml = 5.0mg/L
  6. 2.0ml = 0.10mg/10ml = 10.0mg/L
  7. 3.0ml = 0.15mg/10ml = 15.0mg/L
  8. 4.0ml = 0.20mg/10ml = 20.0mg/L
  9. 8.0ml = 0.40mg/10ml = 40.0mg/L
  10. Add 5ml of reagent a. to each flask; swirl.
  11. Add 5ml of reagent b. to each flask; swirl.
  12. Add 5ml of reagent c. to each flask; swirl.
  13. 5 minutes after the addition of the molybdate (b) add 10ml of reagent d.; mix and allow to stand for approximately 30 min. The colour is stable for several hours after this time. (NB. Do not use rubber stoppers to close the flasks)
  14. Using a wavelength of 700nm and a 1cm cell measure the absorbance of the samples and standards against a deionised water blank.
  15. Silica concentration can then be determined from a plot of the absorbancies of the standards against their known concentrations.

If refrigerated, all samples should be analysed within 28 days of collection.

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Ammonium Ions

Ammonium ions react with phenol and hypochlorite in an alkaline solution to form indophenol blue: the reaction is catalysed by nitroprusside. The resulting absorbance is proportional to the concentration of ammonium ions present and is measured spectrophotometrically at 640nm.


Reagents

  1. Phenol solution: dissolve 20g phenol in 200ml of 95% ethanol
  2. Sodium Nitroprusside solution: dissolve 1g of sodium nitroprusside in 200ml of water [store solution in the dark]
  3. Alkaline solution: dissolve 100g tri-sodium citrate and 5g of Sodium hydroxide in 500ml of water
  4. Sodium Hypochlorite solution: take 600ml commercial sodium hypochlorite and dilute to 1 litre
  5. Ammonium Sulphate standard solution: dissolve 0.9433g of ammonium sulphate in water and make up to 1 litre (1ml = 200ugNH4-N)

Procedure

Remember to set up your calibration curve at the same time as processing your samples.

  1. Place 50ml of filtered water sample into a 100ml conical flask. Dilute samples likely to contain more than 400ug/L (effluent, anaerobic waters, water from ochreous dykes) appropriately - usually 1 in 5.
  2. Make up an oxidising solution by mixing 4 parts solution C with 1 part solution D.
  3. Add 2ml of solution A to each sample and mix.
  4. Add 2ml of solution B to each sample and mix.
  5. Add 5ml of oxidising solution to each sample and mix.
  6. Store the samples in the dark. After 1.5-12 hours, measure the absorbance in a cm cell at 640nm against the standard blank.

Standards

Prepare a calibration curve using the following dilutions of the ammonia standard (E). Take 1ml of solution E and dilute to 500ml. This is the working standard (X) (400 ug/L).

  • 400 ug/L - 50ml X
  • 280 ug/L - 35ml X made up to 50ml
  • 200 ug/L - 25ml X made up to 50ml
  • 120 ug/L - 15ml X made up to 50ml
  • 80 ug/L - 10ml X made up to 50ml
  • 0 ug/L - 50ml of water

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Chlorophyll a Concentration

Determination of algal biomas using phytosynthetic pigments

Samples of sediment and attached algae are always contaminated by detritus and other organisms but even phytoplankton are usually intimately mixed with detritus and bacteria. It is often necessary to determine the size of the crop of algae but the contamination makes measures of organic and carbon content or dry weight meaningless. However, although each phylum differs in the precise suite of pigments its members contain, all algae have the green chlorophyll a and a collection of carotenoids, which are orange-yellow in colour. The ratio of chlorophyll a to organic content of the cells varies with the physiological state of the cells and the ration of carotenoid to chlorophyll a can give a useful index of, for example, nitrogen deficiency.


Determination of pigment content of phytoplankton

Filter a known volume (250ml–1litre) of well-shaken water through a glass-fibre filter under vacuum. Suck the filter dry and transfer it to a mortar. Add a pinch of sand, 1ml of MgCO3 suspension (5%) and about 1ml of acetone from the wash bottle. Grind to a smooth paste then wash the paste carefully into a measuring cylinder or graduated centrifuge tube and make up to 10ml with acetone. Place tubes in a chilled cold box for 30mins to allow pigments to be extracted into the acetone and the solids to settle out, then transfer the supernatant to a 1cm spectrophotometer cell with a pasteur pipette. Against an acetone, blank determine the absorbance at 750nm, 663nm, 480nm, 430nm, and 410nm in the spectrophotometer. The value at 750 corrects for any fine colloidal matter and should be subtracted from each of the other values.


Calculate the chlorophyll a concentration as follows:

Chlorophyll a Concentration
where...

 

  • A = absorbance at 663
  • v = volume (ml) of 90% acetone used for extraction (in this case 10ml)
  • V = volume of water filtered (in litres)
  • d = path length (cm) of the spectrophotometer cell (in this case 1cm)

The value 11.0 is derived from the specific absorbance (absorbance per mole) of chlorophyll a. The units calculated are mg chlorophyll a per litre of water. Typical values for freshwater lakes range 1 - 300mg/L.


Calculate the carotenoid concentration as follows:

Chlorophyll a Concentration 2
 

 

The unit here is the m Specific Pigment Unit (mSPU), which is roughly equivalent to 1mg per litre. A mixture of substances is being measured, hence the precise measure of weight cannot be used.

Now calculate the ratios A480: A663 and A430: A410. The following table will help you to interpret these.

Chlorophyll a Concentration 3

Phytoplankton Primary Productivity

There are two main ways of determining the rate of photosynthesis (primary productivity) of a sample of algae. One involves the uptake of radioactive CO2, and the other involves the measurement of the rate of evolution of oxygen which is documented here. The basic titration for measurement of dissolved oxygen has a variety of uses in projects (e.g. microbial or animal respiration experiments) as well as measurements of photosynthesis and is given in detail elsewhere.

Fill two clear glass-stoppered bottles (25ml) and one opaque bottle with the algal suspension. The bottles should be filled to the brim and air bubbles should not be trapped when the stoppers are inserted. Place one clear bottle and the opaque bottle on a light rack or back in the lake and record the time. To the remaining clear bottle add Winkler reagents and determine the initial oxygen concentration. After several hours remove the other bottles, record the time and determine their oxygen concentration.

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Nitrate-Nitrogen

Collection

Collect 100ml of water (filtered using Whatman GF/F) in an acid washed polythene bottle and refrigerate.


Cadmium Reduction

This procedure is for soluble nitrate using filtered water samples (GF/F). Samples should be diluted to an approximate concentration of 500µg/L NO3-N. (test with nitrate strips).


Principal

Nitrate is reduced to nitrite on exposure to cadmium chips. The nitrite is then combined with reagents to form a red/pink azo dye which can be measured spectrophotometrically.


Reagents

For Cadmium reduction only:

  • Spongy cadmium: cadmium granules can be purchased as 0.3-1.5 mm chips. Otherwise, place zinc rods in a 20% w/v solution of cadmium sulphate overnight. Scrape off the cadmium and divide it into small particles with a spatula. The cadmium is then washed with 2% HCl followed by thorough rinsing in distilled water. The cadmium is then stored under water in a well-sealed vessel. Re-wash with acid 10 min. before use and then with distilled water immediately before use [WARNING - Cadmium is highly toxic]
  • Ammonium chloride: dissolve 2.6g in 100ml of water.
  • Borax (Disodium tetraborate) Na2B4O7.10H2O: dissolve 2.1g in 100ml of water.
  • Sulphanilamide: dissolve 1.00g in 100ml of 10% v/v HCl (i.e. 10ml concentrate HCl added to 100ml of water).
  • N-1-naphthylethylene diamide dihydrochloride: dissolve 1.00g in 1 litre of water
  • Hydrochloric acid, 2% v/v: add 2ml concentrate HCl to 100ml of water.
  • Standard nitrate solution: dissolve 7.218g of potassium nitrate, KNO3 in water and make up to 1 litre: 1.0ml = 1.00mg NO3-N. 
    • This solution can be preserved with 2ml of chloroform and will keep for 6 months. For the working solution dilute 1.0ml of the stock solution to 1000ml with deionised water: 1.0ml = 1.0mg/L NO3-N


General procedure

The calibration curve should be prepared simultaneously with the samples.


Calibration

From the working stock solution prepare a range of dilutions, make up each standard to 10ml with deionised water.

  • 10ml of deionised water as a laboratory blank
  • 0.50ml = in 10ml H2O = 0.05mg/L
  • 1.00ml = in 10ml H2O = 0.10mg/L
  • 2.00ml = in 10ml H2O = 0.20mg/L
  • 3.00ml = in 10ml H2O = 0.30mg/L
  • 4.00ml = in 10ml H2O = 0.40mg/L
  • 5.00ml = in 10ml H2O = 0.50mg/L
  • 7.50ml = in 10ml H2O = 0.75mg/L
  • 10.0ml = in 10ml H2O = 1.00mg/L

Samples

  1. Pipette 10ml of each sample (and the above calibration dilutions + blanks) into 30ml sterilin universal containers.
  2. To each sample add 3.0ml of ammonium chloride solution (b).
  3. To each sample add 1.0ml of borax solution (c).
  4. To each sample add 0.5-0.6g of spongy cadmium (a). N.B. extreme care should be taken at all times when handling cadmium.
  5. Screw on the caps and shake on a mechanical shaker for 20 minutes (timing is critical).
  6. Transfer 1.4ml of each sample to a 10ml test tube and add 0.2ml of sulphanilamide reagent (d) and swirl to mix.
  7. After 4-6 minutes add 0.2ml of N-1-naphthylethylene diamide dihydrochloride solution (e) and swirl to mix.
  8. Make up to 10ml with deionised water.
  9. After 10 -120 min. measure the absorbance on a spectrophotometer at 543nm in a 1cm cell using the deionised water sample as a blank.
  10. From the calibration samples, plot absorbance against concentration and determine, from the nearest fit line, the concentration (0-1mg/L) per unit absorbance.
  11. For the water samples multiply the absorbance by the concentration per unit absorbance (determined in step 10) to give nitrate concentration. If the sample was diluted scale up by dilution factor.

Cadmium Recovery

Due to the toxicity of cadmium great care should be taken to retain all traces of the metal. Any solutions which have come in contact with cadmium should be kept for safe disposal. The spongy cadmium used can be recovered by filtration and then washed with 2% (v/v) HCl, followed by thorough rinsing in distilled water. Storage should be in a clearly labelled sealed vessel.

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SCP's from Rainwater and freshwater samples
Based on Rose, N.L. et al. (2001)

 

Concentrations of SCPs in water samples can be determined very simply, as long as a sufficient volume of sample is used initially. In rain samples, this can be as low as 2 - 3 litres. In surface waters, concentrations are far lower and volumes of 40 - 50 litres should consider a minimum. As these volumes have to be filtered, this raises its own problems, as lake water filters quickly become blocked in all but the most oligotrophic of sites when attempting to filter such large volumes. However, this can be overcome by collecting the required volume in a suitable container and allowing this to settle, covered and undisturbed, overnight. The next day, the upper water can then be siphoned off and the bottom few litres collected and filtered.

GF/C filters should be used of as small a diameter as possible. Use a single filter if at all possible. Note the volume of the sample that has been filtered (or in the case of using the 'settling' procedure the volume allowed to settle).

  1. Once the sample has been filtered, tightly roll or fold the filter and place it in a 12 ml polypropylene tube.

  2. Add 3ml of concentrated HF to dissolve the filter.

  3. Top-up the tube with distilled water and centrifuge at 1500 rpm for five minutes.

  4. If the filter was small, then the suspension may be clear and after further washes, with distilled water, the cover-slips and slides can be made up at this point and counted as described in the standard sediment SCP procedure. If not, then a white precipitate may have formed in the HF step. This can be removed by washing with concentrated HCl as follows.

  5. Add 3ml of concentrated HCl to the tube. Allow a few moments for the precipitate to dissolve and then top-up with distilled water and centrifuge at 1500 rpm for 5 minutes. If the white precipitate persists then repeat this procedure until it has gone.

  6. If the solution is now clear, proceed to make up the cover-slips and slides as described in the standard SCP sediment procedure. However, if there is considerable organic material in the sample this can be removed by adding 3 ml of concentrated nitric acid to each tube and heating it in a water bath at 80 °C for 2 hours.

  7. Once the precipitate is gone, and any organic matter removed, then the cover-slips and slides can be made-up as described under the standard sediment SCP procedure and the SCPs counted.


Calculation of concentrations and fluxes

Here, the percentage of the suspension evaporated onto the coverslip (E) is used to calculate the total number of SCPs on the filter and then divided by the volume ('V'; in litres) of the original sample to get a concentration of SCPs per litre, i.e:

SCP concentration = (100/E) l-1 / V

If this is rainfall then the SCP flux (number of SCPs m-2) for the period covered by the rain sample can be calculated as:

SCP flux = (100/E) * R / VFILT

where R is rainfall in metres for the period covered by the sample and VFILT is the volume of rain filtered (in m3).

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