Recalibrating the human carbon footprint from wastewater

New evidence shows that “fossil carbon” derived from pharmaceuticals, food, personal care products, and more could be much more prevalent in the environment than previously thought and could skew current models of the global carbon cycle. These findings, published in ES&T (DOI 10.1021/es9004043) by David R. Griffith of the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution and his colleagues at Yale University, are derived from carbon isotope dating of dissolved organic carbon (DOC). The researchers find that such petroleum waste products make up one-quarter of the DOC that gets dumped into an ecosystem from wastewater effluent.

Such a large number is new for modelers concerned with the carbon cycle, and it’s twice the amount reported in the National Research Council’s 2003 Oil in the Sea assessment. Some estimates assumed that carbon isotope signatures of wastewater would be closer to modern ratios, such as that of modern-day vegetation. But because the petroleum-derived carbon found in these household products is so old, its carbon-14 isotope fingerprint skews estimates of how long total carbon has resided in an ecosystem. For example, combining the age of fossil carbon with total carbon in the Hudson River basin makes that river system’s carbon signature appear to be about 440 years older than it would be without the wastewater input, Griffith and his colleagues calculate.

Carbon component measurements represent averages, says Mary Ann Moran, an ocean microbiologist at the University of Georgia Athens; many different components that come from various proportions of soil, leaves, or other sources could lead to the same average. The identification of fossil carbon in wastewater “threw a wrench” into older interpretations of the average, Moran adds. “It’s saying you can get very old carbon from a source we never thought of before, but that is very ubiquitous.” That discrepancy could possibly throw off models of global carbon cycles, particularly in the oceans, she says, though that remains to be tested.

Griffith and his colleagues estimate that the total petroleum-derived DOC from U.S. treated wastewaters amounts to about 100 million kilograms per year. They base their calculation on two assumptions: that most wastewater treatment plants in the U.S. use the same biologically activated sludge treatment and that the influent source is mainly from residential customers using similar products. However, they acknowledge that industrial effluent from a rubber tire manufacturer, for example, could have a much heavier petroleum carbon fingerprint.

Their measurements are based on northeastern rivers, for now. “What may be [an] interesting [factor] when we expand this out to the U.S. or the rest of the world is changes in human diet,” Griffith says. For example, in an area where the dietary staple is rice rather than corn, the carbon isotope ratio might be different.

more from EST