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What are sediment basins good for?

Sampling and testing of sediment basins is usually left until the last minute. However, if these technical activities are not carried out to a high level of competency, the whole process can be quickly undermined. 
{{image2-a:r-w:200}}SAMPLING
There is a series of Australian Standards AS5667 that cover design and implementation of plans for sampling of different water bodies. Part four of this standard covers sampling from lakes, both natural and man-made. This standard outlines the equipment to be used and the best points to take samples from. 
When sampling for quality control, the sample should be taken from near the outlet point or near the inlet of a major water source. For sediment basins, this means near the pump intake or spillway. It is also important to take the sample so water from the centre of the body is sampled, not the surface layer and not the sediment lying on the bottom.
TESTING: PH, TSS, TURBIDITY
The most common tests required on sediment basins are pH, total suspended solids (TSS) and/or turbidity. pH is a measure of the hydrogen ion concentration in the water and gives an overall indication of the health of a water system, particularly when it comes to changes over time. 
Limits of between 6.5 and eight or 8.5 are typically applied, although sometimes changes to background level are a better indication, particularly where environments are naturally acidic (low pH, such as in many forested creeks) or basic (high pH, such as in limestone regions). It is tested most accurately using an electrode that needs to be calibrated daily but indicator papers are available for field use if less accuracy and technical competence is required.
{{image3-a:l-w:200}}TSS is not a field test, although VGT has adapted standard laboratory equipment to be used in a back of the truck situation. The water sample is filtered using a vacuum pump through a glass fibre paper and the amount of sediment caught is dried in an oven and weighed. A laboratory will take a minimum of one to two hours to perform the test, although they usually have a standard time of about a week. It takes a very small amount of sediment, only 0.05g in one litre, for the dam to fail a typical 50mg/L criteria. This means sampling technique is critical.
Turbidity is a measure of the amount of light that passes through a sample. It is most accurately tested in the field using a meter that electronically measures the difference in light. The meters should be calibrated at least every 12 months, and checked every time they are used. Turbidity tubes are a low-tech alternative: water is poured into a tube until the lines on the bottom are no longer visible. This test can be a bit subjective but is suitable for screening basins for further treatment or testing. 
Turbidity is a different test to TSS and although correlations can be made, they usually show very low correlation co-efficients. Better to use a laboratory that will give the fast testing time you require than risk a turbidity test giving an incorrect TSS answer.
VGT regularly runs training seminars on sampling water and dust deposition gauges, as well as the basics of field testing and 
result interpretation. ?
Source: VGT Pty Ltd

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