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Analytical Chemistry

Better Sampling Needed in Wastewater Monitoring

Water Pollutants: Researchers worry improper sewage sample collection will miss fluctuating contaminant levels

by Rajendrani Mukhopadhyay
July 23, 2010

DIRTY JOB
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Credit: Christoph Ort
Researcher Christoph Ort descends into a sewer to monitor how one chemical’s levels fluctuate over time.
PPCPsampling.jpg
Credit: Christoph Ort
Researcher Christoph Ort descends into a sewer to monitor how one chemical’s levels fluctuate over time.

The adage, "If you blink, you'll miss it," applies to wastewater contaminants too. As Australian researchers have now illustrated, improper sampling protocols to detect pharmaceuticals, personal care products, and illicit drugs in sewage can lead researchers to misestimate the chemicals' concentrations or miss the pollutants altogether (Environ Sci Tech, DOI 10.1021/es100778d).

Chemical contaminants appear and disappear over time in wastewater, so whether or not researchers detect them depends on when and how frequently scientists collect samples. Scientists want to quantify the flux of pharmaceuticals, personal care products, and illegal drugs through sewage systems to determine how much we release into the environment and to improve sewage plants treatments. But many scientists lack the instruments that can monitor these contaminants' dynamics with sufficient time resolution, says environmental engineer Christoph Ort of the University of Queensland in Australia.

In a separate, review article in (Environmental Science and Technology, DOI: 10.1021/es100779n), Ort and colleagues examined 87 recent papers that studied 267 different sewer sites, and found that the authors didn't cite or heed internationally-accepted sampling practices. Researchers often collected samples too infrequently and then pooled all their samples together for analysis; in doing so, they lost all information about the dynamics of a compound over time and possibly missed pollutants.

In the present study, Ort and colleagues illustrate the point by tracking a medical-imaging contrast agent at a sewage treatment plant that served 100,000 inhabitants. Based on how frequently doctors use the chemical, the investigators estimated that at most one or two people in that population would excrete the compound into the sewage system during a weekday.

The researchers then monitored the wastewater over a four-hour time period. To ensure that they didn't miss a single toilet flush, they collected 120 samples, each consisting of two minutes of wastewater flow. When they analyzed the samples by mass spectrometry, the contrast agent only showed up in 30. If they had sampled less frequently, the scientists may have missed its presence completely.

Scientists often resort to improper sampling because of expense or logistical limitations, says chemist Bruce Brownawell at Stony Brook University in New York, but this study highlights its pitfalls: "It makes us vigilant about paying attention to the sampling methods."

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