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Introduction to environmental analysis - Reeve R.

Reeve R. Introduction to environmental analysis - Wiley publishing , 2002. - 312 p.
ISBN 0-471-49295-7
Download (direct link): introductiontoenvironmental2002.pdf
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DQ 3.5
What regular variations would you expect in concentrations of the following:
• Dissolved oxygen;
• Nitrate?
Answer
Oxygen is produced by photosynthesis in daytime but is consumed by respiration or by oxidation of organic material continuously. There will be a continuous but slow replenishment from the atmosphere. A drop in oxygen concentration during the night would be expected.
Variation of nitrate would be more complex. This is a nutrient which is necessary for growth and so if there were no additional inputs it would decrease in the spring growing season and increase in winter; however, if a farmer put an excessive amount of nitrate-containing fertilizer on a
Water Analysis - Major Constituents
43
neighbouring field, there would be a sudden increase in any river into which the field drained.
You should recall Figure 2.9 which gives an example of seasonal nitrate concentration changes.
(iii) Decide on the total number of samples you are taking, remembering that each location should be sampled in duplicate. Although it is good practice to start by taking as many samples as you feel necessary for complete monitoring, you do also have to take into consideration the time required for the analyses. It is very common to severely underestimate the time involved in the laboratory analyses.
A further consideration if there is to be any statistical treatment of results is that there are sufficient samples for the treatment to be significant.
(iv) Decide on the location of the sampling and the sampling apparatus. If you are to take samples regularly from one location, the first consideration must be ease of access. Remember that the weather may not always be perfect. Surface water sampling requires little sophistication in sampling apparatus (often directly into a sample bottle or a bucket) but the surface may not be the best location for sampling. It may not provide the most representative sample. There also is the possibility of contamination by surface pollutants. Surface contamination can be largely overcome by inserting the bottle upside down in the water and inverting to fill it from just below the surface. Ideally, however, the river should be sampled further underneath the surface, in its main flow and at similar depths for each sample. A simple sub-surface sampler would be a weighted, stoppered bottle on an attachment line. The stopper is removed at the required depth by a cable. More complex designs, such as the Van Dorn sampler shown in Figure 3.3, are open cylinders with valves at each end and produce less disturbance to the river on sampling. The sampler is sealed by using a weight (messenger) dropped down the attachment line to activate the valve mechanism.
If you are monitoring the effect of a discharge into a river, samples should be taken far enough downstream for the discharge to be completely mixed (Figure 3.4). Samples taken further upstream would be unrepresentative as the analysis would depend on how much the discharge had mixed with the river.
(v) Decide on the sample volume to be taken to the laboratory and the sample-storage containers. The latter are usually made of glass or polyethylene. However, these materials (and those in the container top) are not as inert as you might think. Polyethylene containers may leach organic compounds into the sample, while glass bottles can leach inorganic species (sodium, silica and other components of the glass). How much you fill the container is also important. If you are analysing volatile material or dissolved gases, the container must always be full. For other components, it is beneficial
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Introduction to Environmental Analysis
Figure 3.3 Schematic of a Van Dorn water sampler.
Water Analysis - Major Constituents
45
Figure 3.4 Sampling to monitor the affect of a discharge.
not to fill the container completely as the contents can then be more easily mixed before analysis. Try attempting to mix the contents of a completely full container!
At this stage, it would also be worthwhile to check the equipment used at all stages of the sampling procedure to ensure that nothing will introduce contamination - the sampler itself, funnels and any tubing used. If you are sampling from a motorized boat without care, the boat itself could introduce contamination into the water and may disturb the sediment.
(vi) Decide on the method of storage of samples. Standard methods are available for most components to minimize analyte loss. The method varies according to the physical and chemical properties of the species. For example:
Nitrate - store at 4°C to lower biological degradation.
Pesticides - store in the dark to avoid photochemical decomposition.
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Introduction to Environmental Analysis
Metal ions - acidify the sample to prevent adsorption of metal ions on to the sides of the container.
Phenols - add sodium hydroxide to lower the volatility.
For some analyses (e.g. biochemical oxygen demand - see below), no preservation is possible and the analysis should be performed as soon as possible after sampling, keeping the sample cool during transportation to the laboratory.
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