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 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 considered 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 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 in a 12 ml polypropylene tube.
2. Add 3ml of conc HF to dissolve the filter.
3. Top-up the tube with distilled water and centrifuge at 1500 rpm for 5 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 conc. HCl as follows.
5. Add 3ml of conc. 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 conc nitric acid to each tube and heating 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).