New USGS Field Method is a Breakthrough for Contaminant Analysis in Water Samples

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TACOMA, Wash. — The U.S. Geological Survey recently published a new techniques and methods report highlighting a portable continuous-flow centrifuge which aims to save time and money on contaminant analysis of particles suspended in water samples.

In the past, contaminant analysis required the collection of large volume samples and separation of the water from the suspended sediment to obtain enough contaminant for reliable chemical analysis in the laboratory. Separation of suspended sediment from water in a laboratory is time consuming and costly. It requires shipping large amounts of water to a laboratory for separation or shipment of a large industrial centrifuge to the field. Consequently, few studies or monitoring programs have measured the chemical quality of suspended sediment because of the difficulty in consistently obtaining samples for laboratory analysis.

 “The new field technique developed by the USGS overcomes these difficulties by utilizing a small portable continuous-flow centrifuge for the separation of suspended sediment from large volumes of water in the field,” said Kathy Conn, USGS Water Quality Specialist and lead author of the report. “This results in a small solid sample that can be easily shipped to the laboratory for chemical analysis.”

 The report describes project scoping considerations such as determining the time required to obtain sufficient sediment mass, deployment of equipment and system layout options and results from various field and laboratory quality-control experiments. The testing confirmed the applicability of the technique for the determination of many inorganic and organic chemicals sorbed on suspended sediment, including trace elements, pesticides, polycyclic aromatic hydrocarbons and polychlorinated biphenyls. The particle-size distribution of the captured sediment changes to a more fine-grained composition during centrifugation. This shift in grain size requires users of the technique to account for this change when extrapolating chemical concentrations on the centrifuged sediment sample to the environmental sample.

Toxic chemicals were measured on river suspended sediment during centrifuge field testing even when chemical concentrations were not detected in corresponding water samples. This suggests that, because many chemicals prefer to sorb to sediment rather than remain in water, occurrence and concentrations of toxic chemicals in water bodies may be underestimated if the suspended sediment-bound fraction is not measured.

Data on suspended sediment-bound chemical concentrations produced using these protocols will support management decisions, such as chemical source-control efforts or in-stream restoration activities. When coupled with streamflow data, measurements of the mass of suspended sediment and the concentration of a contaminant in the sediment can be used to provide the amount of contaminant transported by the stream over a given time period. These data will improve estimates of the amounts and concentrations of trace-level contaminants being discharged by the stream to downstream freshwater and coastal marine ecosystems, and will aid in assessing the importance and impacts of suspended sediment-bound chemicals to human health and aquatic life.

The report, “Continuous-flow Centrifugation to Collect Suspended Sediment for Chemical Analysis” is published and available online as U.S. Geological Survey Techniques and Methods Report 1-D6.