New research clarifies the capacity of rivers to filter pollutants

Like a human body's circulatory system that moves blood and carries nutrients, Earth’s river networks are similar conduits.

One of a river's important functions is removing pollution that ends up in its waters -- from roads, lawns, septic systems, sewage treatment plants and other sources -- before those waters reach sensitive downstream ecosystems like estuaries and oceans.

New research published in Nature Communications finds that watershed size plays a major role in a river network's ability to do that work. The findings further the understanding of which estuaries and coastal areas will be impacted by human development in their watersheds.

Using a model that integrates what is known about how streams and rivers function, the scientists found that when the size of a watershed -- the area of land that drains into an aquatic system -- increases, the rate at which rivers filter pollution doesn't increase at a linear rate. Instead, it increases faster, thanks to the larger rivers that commonly go hand-in-hand with larger watersheds.

"It’s not well-known what controls how much pollutant filtration river networks can do," says Wilfred Wollheim of the University of New Hampshire, lead author of the paper. The research was funded by the U.S. National Science Foundation, and includes work at the NSF Plum Island Ecosystems and Arctic Long-Term Ecological Research sites.

"This study, which analyzes previously existing data, demonstrates the value of investments in long-term data archiving for the study of scale-dependent processes," says Francisco (Paco) Moore, a program director in NSF's Division of Environmental Biology, which supported the research along with NSF’s Division of Ocean Sciences. "Our understanding of ecological processes will continue to increase as we reexamine existing data in a deeper context."

Wollheim and his colleagues describe what they uncovered about watershed size and river function as superlinear scaling, and say it occurs because larger rivers contribute disproportionately to the pollution-filtering function of entire networks of aquatic ecosystems, including lakes, streams, rivers and wetlands.

To keep as much pollution as possible out of estuaries and oceans, says Wollheim, it may be more important to manage land use and mitigate nonpoint source pollution -- such as runoff carrying fertilizers, herbicides, insecticides and toxic chemicals -- in smaller watersheds, which are less able to filter pollutants, than larger watersheds.

It’s also important, the researchers found, to mitigate nonpoint pollution in parts of the watershed that are closer to an estuary or coastal area, where the system will have less chance to filter pollutants before they reach those critical areas.