USGS Scientific Investigations Report 2010–5008 Use of Continuous Monitors and Autosamplers to Predict Unmeasured Water-Quality Constituents in Tributaries of the Tualatin River, Oregon

Table 2. U.S. Geological Survey guidelines for placement of autosampler intake.

[From G.D. Glysson, U.S. Geological Survey, written commun., 2009. Abbreviations: mm, millimeter]

1. Select a stable cross section of reasonably uniform depth and width to maximize the stability of the relation between concentration at a point and the mean concentration in the cross section. This guideline is of primary importance in the decision to use an automatic sampler in a given situation; if a reasonably stable relation between the sample-point concentration and mean cross-section concentration cannot be attained by the following outlined steps, the sampler should not be installed and an alternate location should be considered. If banks are unstable and the sampler has to be installed in that location, install the intake on the cutting side of the channel so that the intake will not be buried.

2. Consider only the part of the vertical that could be sampled using a standard U.S Geological Survey depth- or point-integrating sampler, excluding the unsampled zone, because data collected with a depth- or point-integrating sampler will be used to calibrate the pumping sampler. (See Edwards and Glysson, 1999, fig. 1.)

3. Determine, if possible, the depth of the point of mean concentration in each vertical for each size class of particles finer than 0.250 mm, from a series of carefully collected point-integrated samples.

4. Determine, if possible, the mean depth of occurrence of the mean concentration in each vertical for all particles finer than 0.250 mm.

5. Use the mean depth of occurrence of the mean concentration in the cross section as a reference depth for placement of the intake.

6. Adjust the depth location of the intake to avoid interference by dune migration or contamination by bed material.

7. Adjust the depth location of the intake to ensure submergence at all times.

8. Locate the intake laterally in the streamflow at a distance far enough from the bank to eliminate any possible bank effects.

9. Place the intake in a zone of high velocity and turbulence to improve distribution by mixing, reduce possible deposition on or near the intake, and provide for rapid removal of any particles disturbed during the purge cycle. Avoid placing the intake in an eddy, as it will probably not be representative of the water in the cross section.

10. Consideration must be given to placing the intake and tubing in a place so that they will be protected during high streamflows.