USGS-OWQ: NFM 4.1.1.A EWI: Collection of stream samples using the equal-width-increment (EWI) method
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Official Method Name
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Collection of samples using the equal-width-increment (EWI) method |
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Current Revision
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Media
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WATER |
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Instrumentation
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Isokinetic Sampler (see Method Summary for more information) |
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Method Subcategory
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Sampling/Preparation |
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Method Source
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Citation
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Brief Method Summary
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The EWI method requires that a stream cross section be divided into a pre-determined number of increments of equal width. Depth-integrated, discharge-weighted samples are collected through the vertical profile (?vertical?) at the center of each increment, using the same and constant transit rate at each vertical, resulting in a discharge-weighted sample that is proportional to total streamflow. The samples collected at the verticals of each equal-width increment are combined as a composite, usually in a churn or cone splitter. The method requires use of appropriate isokinetic sampling devices and can be applied at wadeable and unwadeable stream cross sections. Consult the source citation for detailed instructions in determining the number of equal-width increments, location of the vertical, determination of transit rate, and use of isokinetic equipment. Consult http://pubs.water.usgs.gov/twri9A/, Chapter A2, section 2.1.1.A for selection and use of isokinetic samplers. |
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Scope and Application
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EWI method can be used only in streams with flowing water and requires use of an isokinetic, depth-integrating sampling device. For valid application of the method, streamflow velocity must not be less than the velocity for which the sampling device is rated: for example, the flow velocity must equal or exceed 1.5 ft/s when an isokinetic bottle sampler is used, 2 ft/s when a 1- or 3-liter isokinetic bag sampler is used, and 3 ft/s when a 6-liter isokinetic bag sampler is used. For descriptions and limitations of isokinetic samplers, refer to section 2.1.1.A of the following report: http://pubs.water.usgs.gov/twri9A/. Isokinetic, depth-integrating methods are designed to produce a discharge-weighted sample that is a composite of samples collected at multiple equally spaced increments along a stream cross section, and that is proportional to total streamflow. Each unit of stream discharge is equally represented in the sample. Rule of thumb for selecting the EWI method: the stream cross-section has relatively uniform depth and velocity. |
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Applicable Concentration Range
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Depends on target analyte(s), analytical method, and sampling method and equipment capabilities, generally within tenths of ug/L to thousands of mg/L. |
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Interferences
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Take care not to disturb the streambed by bumping the sampler on it; bed material may enter the nozzle, resulting in erroneous data. Do not overfill the sampler container; overfilling results in a sample that is not isokinetic and that could be enriched with heavy particulates. Do not underfill the sampler container; underfilling will result in a sample that is not isokinetically collected because the maximum transit rate has been exceeded. |
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Quality Control Requirements
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Follow the appropriate quality control procedures for the sample analyte (organic, inorganic, or microbiological), including selection and preparation of non-contaminating equipment, clean and/or sterile handling methods, and collection/analysis of quality-control samples. |
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Sample Handling
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Implement recommended methods to prevent contact of sample with foreign substances (including wet, dry, and gaseous atmospheric inputs) that could compromise the integrity of the sample and subsequent analysis. |
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Maximum Holding Time
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Samples should be appropriately processed, treated, and submitted for analysis as soon after collection as possible. Some samples require analysis within 24 hours; consult the analyzing laboratory for holding-time requirements. |
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Relative Cost
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Unknown |
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Sample Preparation Methods
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See http://pubs.water.usgs.gov/twri9A/, |
