This article throws light upon the top three sampling methods for flowing water. The methods are: 1. Isokinetic, Depth-Integrated Sampling 2. Single Vertical at Centroid-of-Flow (VCF) Method 3. Non-Isokinetic Sampling Method.
Sampling Method # 1. Isokinetic, Depth-Integrated Sampling:
Collection of isokinetic, depth-integrated samples involves using either an equal-width-increment (EWI) or equal-discharge-increment (EDI) sampling method.
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The EWI and EDI methods are used to divide a selected cross section of a stream into increments having a specified width. The term vertical refers to that location within the increment at which the sampler is lowered and raised through the water column.
i. EWI verticals are located at the midpoint of each width increment.
ii. EDI verticals are located at the centroid.
When using an isokinetic sampler there should be no change in velocity (speed and direction) as the sample enters the intake (fig. 2).
For isokinetic sampling, the mean velocity of the vertical that is sampled must exceed the minimum-velocity requirement of an isokinetic sample. The minimum velocity requirements are 1.5 ft/s for a bottle sampler, 2 ft/s for a 1- or 3-liter bag sampler, or 3 ft/s for a 6-liter bag sampler.
(a) Equal-width-increment (EWI) method:
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For the EWI sampling method, the stream cross section is divided into a number of equal-width increments (fig. 3). Samples are collected by lowering and raising a sampler through the water column at the center of each increment. (This sampling location is referred to as the vertical).
Collection of sample for EWI:
i. For organic compounds:
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Select equipment with fluorocarbon polymer, glass or metal components if components will directly contact samples to be analyzed for organic compounds. Do not use plastics other than fluorocarbon polymers.
ii. For inorganic constituents:
Select equipment with components made of fluorocarbon polymer or other relatively inert and uncoloured plastics or glass if components will directly contact samples to be analyzed for inorganic constituents. Do not use metal or rubber components for trace-element sampling.
(b) Equal-discharge-increment (EDI) method:
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The objective of the EDI method is to collect a discharge-weighted sample that represents the entire flow passing through the cross section by obtaining a series of samples, each representing equal volumes of stream discharge.
The EDI method requires that flow in the cross section be divided into increments of equal discharge. Equal-volume, depth-integrated samples are collected at the centroid of each of the equal-discharge increments along the cross section. Centroid is defined as that point in the increment at which discharge is equal on both sides of the point.
Collection of sample for EDI:
i. For organic compounds:
Select equipment with fluorocarbon polymer, glass or metal components if components will directly contact samples to be analyzed for organic compounds.
ii. For inorganic constituents:
Select equipment with components made of fluorocarbon polymer or other relatively inert and uncolored plastics or glass if components will directly contact samples to be analyzed for inorganic constituents. Do not use metal or rubber components for trace-element sampling.
Sampling Method # 2. Single Vertical at Centroid-of-Flow (VCF) Method:
Samples may be collected at a single vertical at the centroid of stream flow if the section is known to be well mixed laterally and vertically with respect to concentrations of target analytes.
The VCF method for collecting water samples is identical to the EDI method except that there is one centroid of flow for the stream cross section and therefore only one vertical is sampled.
Sample Method # 3. Non-Isokinetic Sampling Method:
These samples have important uses for unattended stream sampling and for sampling to determine constituent occurrence and distribution, but they have limited value for collecting samples used to calculate, constituents discharge.
Three nonisokinetic sampling methods most commonly used are the dip (weighted-bottle), discrete and pump methods.
(a) Dip sampling method:
Dip sampling involves either (1) dipping a narrow-mouthed bottle into a water body, or (2) using either the DH-81 or D-95 sampler as described below. To use a DH-81 or D-95 sampler in a stream with velocity less than 1.5 ft/s, remove the nozzle from the sampler and precede to sampler the entire vertical. This is especially useful at a deep-water site.
(i) To collect a dip sample in water that is to shallow to submerge an isokinetic, depth-integrating sampler, wade to where the sample(s) will be collected and immerse a hand-held, narrow-mouth bottle at the centroid of flow or at multiple locations along a cross section.
(ii) To collect a dip sample where water is too deep to wade and velocity is too great for use of an isokinetic sampler: lower a weighted-bottle sampler at the centroid of flow or at multiple locations along a cross section.
(b) Discrete sampling method:
Discrete or point sampling involves either lowering a sampler to a specified depth and collecting a sample by first opening, then closing the sampler, or (2) using a single-stage sampler, which fills when stream stage rises to a predetermined height.
(c) Pump-sampling method:
Pump sampling involves either suction-lift or submersible pump systems designed to collect water-quality samples. Pump systems can be portable or can be permanently installed and automated for sampling.
Collection of sample for testing:
i. For organic compounds:
Select equipment with fluorocarbon polymer, glass, or metal components if components will directly contact samples to be analyzed for organic compounds. Do not use other plastics expect fluorocarbon polymers.
ii. Inorganic constituents:
Select equipment with components made of fluorocarbon polymer or other relatively inert and uncolored plastics or glass if components will directly contact samples to be analyzed for inorganic constituents. Do not use metal or rubber components for trace-element sampling.
iii. Sedimentation:
The samples to be analyzed for sediment concentration and (or) particles-size distribution should be collected using a separate set of clean sample bottles.
Volatile organic compound:
Samples for the analysis of volatile organic compounds (VOCs) are collected as a single-vertical point sample in a flowing stream. The VOC sampler should be deployed where the stream velocity represents the average flow, which typically is near mid-channel in the cross section.
When collecting samples for VOC analyses, special care must be taken to avoid contamination from any oily film and debris floating on the stream surface. VOC samples are collected directly into laboratory-supplied prebaked 40-mL amber-glass vials.
To collect a VOC sample without the sampler:
Wearing gloves, submerge a capped VOC vial into the stream section, remove the cap underwater, let the vial fill to overflowing, and then firmly recap the vial underwater. Repeat this for each of the VOC vials.