Standard measurement of water's organic pollution may be highly flawed

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A popular method for determining water pollution may be wildly inaccurate. (AP Photo/Gerry Broome)

A technique used worldwide to determine organic carbon pollution in natural waters has been revealed to overestimate concentration levels by up to 90%, meaning some environmental policies may be based on considerably incorrect data.

Published April 14 in Science Advances, a study pinpointed what its authors say is a vital issue with the routine method called chemical oxygen demand, or COD. It incorporated statistics from varying latitudes, including in China, Canada and the U.S., spanning from farmlands to marine waters and salt lakes. 

The metric in question is meant to inform policymakers regarding organic pollution levels, because high amounts are known to deplete water's oxygen content. But if results aren't supplemented by other tests for organic pollution, the novel study suggests, it can mislead legislators who craft environmental laws

For instance, the U.S. Clean Water Act calls for the use of four different metrics, one of which was endorsed and built upon by the researchers behind the new study.

According to the paper, however, "many" countries solely use chemical oxygen demand to make environmental assessments and control water quality. China and Japan, for instance, use decreasing chemical oxygen demand as a reference point for evaluating the performance of their environmental policies.  

"Organic pollution affects water quality because the decomposition of organic matter … consumes oxygen dissolved in natural waters, which can lead to low oxygen concentration harmful to aquatic animals," said study author Louis Legendre, a France-based professor emeritus at Sorbonne University and a researcher at Villefranche Oceanography Laboratory. 

High levels of organic pollution can also lead to dead zones, waters that lack oxygen-respiring organisms, and eventually give rise to ocean acidification. They might even contribute to climate change, Legendre says.

Because such pollution can be rapidly — and relatively easily — detected by chemical oxygen demand, the method at the center of the new study remains very popular. 

It essentially works by finding the amount of dissolved organic carbon, or DOC, in natural waters by placing a powerful oxidizing agent in a water sample and then seeing how much of it becomes oxidized. In an indirect way, it is considered to accurately index overall carbon pollution, which collectively refers to organic pollution.

However, along with co-author Nianzhi Jiao, a professor at Xiamen University, in China, Legendre explained to The Academic Times that perhaps policymakers shouldn't be so quick to follow the numbers that chemical oxygen demand gives them. 

That's because as a stand-alone metric, the method doesn't address the fact that dissolved organic carbon has two categories: labile and refractory. Differentiating between them is crucial for obtaining an accurate reflection of organic pollution, the authors say.

"In general, most of the DOC produced by food webs is used within a few days or weeks by microorganisms, mostly heterotrophic bacteria and archaea — this part of the DOC is called labile, or LDOC," Legendre explained. As microbial organisms interact with dissolved organic carbon, it starts to degrade and contribute to pollution. 

Meanwhile, microorganisms are less interested in the refractory category.

"RDOC is not used rapidly by microorganisms and, in some cases, remains in aquatic environments during hundreds and even thousands of years," Legendre said. "Hence, the COD method overestimates organic pollution in natural waters when RDOC is abundant."

The alarming conclusion that chemical oxygen demand yields a potential inaccuracy rate of up to 90% is based on the researchers' calculations that refractory dissolved organic carbon can account for that much of the total dissolved organic carbon content in natural waters.

The study calls on one severe example of a country using only chemical oxygen demand to determine natural water organic pollution levels.

"Lake Biwa was notorious for its organic pollution in the 1970s," the study states, noting that over the years, the Japanese lake saw lower amounts of organic pollution. 

However, based on chemical oxygen demand, it appeared that dissolved organic carbon levels went up instead of down. As it turned out, the study says, the technique was picking up on harmless refractory dissolved organic carbon — the type that doesn't add to pollution.

A way around the apparently overgeneralized method, the authors explained, is to supplement it with a second type, called biochemical oxygen demand, which can distinguish between the two types of dissolved organic carbon. 

The team also proposes a new way to measure biochemical oxygen demand, arguing that the traditional way tends to require conducting tedious and delicate chemical reactions on many samples of water. The alternative solution, which uses a kind of oxygen sensor called an oxygen optode, minimizes the wearisome nature of the test. It consolidates the samples into one — also improving reliability — and, as an added benefit, avoids using pollutive chemicals.

"We hope that the scientific community and all water management agencies will use BOD," in either the original or newly proposed way, Legendre said. "We also hope that this will result in an overall better management of organic pollution in natural waters. In other words, improvements in scientific knowledge could lead to improved policies."

The paper, "Correcting a major error in assessing organic carbon pollution in natural waters," published April 14 in Science Advances, was authored by Nianzhi Jiao, Zongqing Lv, Ruanhong Cai, Xilin Xiao, Jianning Wang, Rui Wang, Xingyu Huang, Jia Sun, Rui Zhang, Yao Zhang, Kai Tang and Qiang Zheng, Xiamen University; Jihua Liu, Fanglue Jiao, John Batt, Douglas Wallace and Helmuth Thomas, Joint Laboratory for Ocean Research and Education at Dalhousie University; Bethanie Edwards, University of California-Berkeley; Yongqin Liu and Bixi Guo, Institute of Tibetan Plateau Research; Paul Hill, Julie LaRoche, Hugh MacIntyre and Marlon Lewis, Dalhousie University; Farooq Azam, University of California, San Diego; Wei-Jun Cai, University of Delaware; Chen He and Quan Shi, China University of Petroleum-Beijing; Gerhard J. Herndl, University of Vienna; David Hutchins, University of Southern California; Luca Polimene, Plymouth Marine Laboratory; Carol Robinson, University of East Anglia; Curtis A. Suttle, University of British Columbia; and Louis Legendre, Sorbonne University. 

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