Laboratory and in situ analysis of samples

8. LABORATORY AND IN-SITU ANALYSIS OF SAMPLES

Sample analysis usually consists of determining the following parameters: sediment concentration, sediment composition, sediment density, and chemical analysis.

Samples for chemical or bioassay analysis should be immediately chilled and stored at 4 oC after collection.

8.1 Sediment concentration

The two most commonly used methods are evaporation and filtration. The filtration method may be somewhat faster for samples of small concentrations. However, large concentrations tend to clog the filter material (silt concentrations > 100 mg/l).

8.2 Bed material composition

Preparation of samples prior to analysis is of the utmost importance if accurate and reproducible results are to be obtained. Samples containing clay minerals or organic material are very liable to cracking on drying and care should always be taken to avoid samples drying out prior to analysis. However, when samples may have dried out naturally when collected on a mudflat or a riverbank, then the aggregates should be broken down (hydrogen peroxide treatment).

Organic materials ranging from macroscopic plant and coal to microscopic colloidal humus does affect average specific weight and greatly affects the particle size and/or fall velocity, if present in sufficient quantities. Quantitative determination of organic material, usually is recommended if the sample consists of 10% or more of organic material. Complete removal of organic material is necessary for all samples to be analyzed for particle size or fall velocity when other than native water is used because the organic material may bind together the sediment particles.

Samples having a size range from pebbles or cobbles down to fine sands will require hand separation of the largest particles. If possible, the size-distribution of the large particles (cobbles) should be determined in-situ by manual measurement of the nominal diameter or by means of photographic methods.

8.3 Suspended sediment composition

The physical analysis of suspended sediment samples should be focussed on the determination of the particle fall velocity distribution because this latter parameter is of essential importance in sedimentation studies. Therefore, the sedimentation methods (settling tests) must be preferred above the other methods such as sieving or the Coulter counter. These latter two methods may be used to check the results of the sedimentation tests.