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Related Pages

	Isokinetic & Nonisokinetic Sampling (35KB)
	Equal-Width -Increment (EWI) (33KB)
	Equal -Discharge -Increment (EDI) (31KB)

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Flowing streamwater is preferably collected using isokinetic depth and width-integrating sampling methods. Isokinetic depth-integrating methods are designed to produce a discharge-weighted sample, that is, each unit of discharge is equally represented in the sample.

Collection of isokinetic, depth-integrated samples involves using either an equal-width-increment (EWI) or equal-discharge-increment (EDI) sampling method. EWI or EDI esually results in a composite sample that represents mean discharge weighted concentrations of the stream cross-section.

Isokinetic samplers are usually used to obtain a discharge-weighted sample across a section of flowing water. An isokinetic sampler collects water so that there is no change in speed and direction as the sample enters the intake.

The number of verticals to be sampled in order to get a discharge-weighted sample at a site is related primarily to data objectives (the accuracy needed) and how well-mixed or homogeneous the stream is with respect to the physical, chemical, and biological characteristics (variation) of the cross-section.

In-Situ Measurement of Physical Properties
In-situ measurement of physical properties provides the most accurate assessment of these characteristics for a given water body because these properties immediately begin to change once a sample of water is removed from the water body. Physical properties most commonly measured include:

• Water Temperature
• Specific Conductance
• Dissolved Oxygen
• pH
• Turbidity
• Oxidation-Reduction Potential

Single- or multi-parameter instruments may be used to make in-situ measurement of physical properties. Many of these instruments also include data-logging and unattending-monitoring capabilities. Routine maintenance and careful calibration to known standards are required for accurate measurements.

Chemical Compatibility of Equipment and the Water Sample
Attention must be given to the materials used to construct the equipment because these can directly affect sample chemistry. Equipment designed for water-quality work often is constucted of a combination of materials, the most inert being for components that will contact the sample. Materials used in equipment can include plastics, glass, and metals. Chemical reactivity varies widely within the same group, depending on the chemical composition, the physical configuration, and the manufacturing process.

Field personnel must also use techniques to minimize potential contamination, implement quality-assurance procedures, and quantify potential effects by using quality-control sample analysis.