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Methods of Sample Collection

Methods of sample collection used by the NASQAN and National Monitoring Network programs conform to the USGS National Field Manual for the Collection of Water-Quality Data

To the greatest extent possible, isokinetic, depth integrated sampling techniques that provide samples representative of stream conditions required by the NASQAN program. As of 2002, the US DH-2, US DH-95, US D-95, and US D-96 isokinetic samplers have been approved by the Interagency Technical Committee of the Subcommittee on Sedimentation, and are used for water-quality and suspended-sediment sample collection (fig. 1). The D-99 sampler became available in 2005. The report "A Guide to the Proper Selection and Use of Federally Approved Sediment and Water-Quality Samplers" discusses stream conditions suitable for the use of each sampler. Prior to 2002, the now superseded USGS Frame Bag, D-77, or D-77 bag samplers were typically used (see section 2.1 of version 1 of USGS Techniques of Water-Resources Investigations, Book 9, Chapter A2 for details on these samplers.

Figure 1. Isokinetic depth-integrating samplers: (A) US DH-81, (B) US DH-95, (C) US D-95, (D) US D-96, and (E) US D-99 (from Lane, S.L., Flanagan, Sarah, and Wilde, F.D., March 2003, Selection of equipment for water sampling, version 2: U.S. Geological Survey Techniques of Water-Resources Investigations, Book 9, Chapter A2, p. 25)

Isokinetic depth-integrated samplers accumulate a representative water sample continuously and isokinetically, that is, stream water approaching and entering the sampler intake does not change in velocity, from a vertical section of a stream while transiting the vertical at a uniform rate (fig. 2).

Figure 2. Relation between intake velocity and sediment concentration for isokinetic and nonisokinetic collection of water samples that contain particulates greater than 0.062 millimeters (from U.S. Geological Survey, 2006, Collection of water samples, version 2: U.S. Geological Survey Techniques of Water-Resources Investigations, Book 9, Chapter A4, p. 39).

Collection of isokinetic, depth-integrated samples is done using either an equal-width-increment (EWI) or equal-discharge-increment (EDI) sampling method. The methods typically result in a composite sample that represents the streamflow-weighted concentrations of the stream cross section being sampled. The following material excerpted from Chapter A2 of the USGS Field Manual (Selection of equipment for water sampling) describes these methods.

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. EWI verticals are located at the midpoint of each width increment. EDI verticals are located at the centroid, a point within each increment at which stream discharge is equal on either side of the vertical. If properly implemented, EDI and EWI methods should yield identical results.

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. The combination of the same constant transit rate used to sample at each vertical and the isokinetic property of the sampler results in a discharge-weighted sample that is proportional to total streamflow.

Figure 3. Equal-width-increment method for collection of water samples (from U.S. Geological Survey, 2006, Collection of water samples, version 2: U.S. Geological Survey Techniques of Water-Resources Investigations, Book 9, Chapter A4, p. 42).

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 (fig 4.). The EDI method requires that flow in the cross section be divided into increments of equal discharge.

Figure 4. Equal-width-increment method for collection of water samples (from U.S. Geological Survey, 2006, Collection of water samples, version 2: U.S. Geological Survey Techniques of Water-Resources Investigations, Book 9, Chapter A4, p. 51).

Currents in the large rivers sampled by the NASQAN program require the use of heavy samplers (the D-96 sampler weighs 132 pounds and the D-99 weighs 275 pounds) to keep the sampler vertical in the current. Due to this weight, it is necessary to suspend the sampler from a crane assembly (fig 5.). Because of the large size of the rivers sampled by the NASQAN program, it is necessary to collect most samples by boat (fig. 6). See chapter 5 of U.S. Geological Survey Water Supply Paper 2175 for a more complete description of cranes and associated equipment used to suspend water sampling equipment. A more complete description of water quality sampling by boat is available at http://mo.water.usgs.gov/wtrqual/sampling_boat.htm.

Figure 5. D-77 water quality sampler suspended from crane used to collect water samples from non-wadable rivers from a bridge. The crane is on wheels so that it can be easily moved between sampling verticals along the bridge. (Photo from USGS Water Quality Image Library.)

Figure 6. D-96 water quality sampler suspended from boat mounted crane used to collect water samples from non-wadable rivers by boat. (Photo from NASQAN Yukon River Basin Studies Web site.)