When it comes to surface water pollution, people tend to think big. The focus is often on metropolitan sewer districts, large dairy operations, or sediment build-up behind giant dams. It turns out that small-scale dams used on farms to create things like livestock watering holes and irrigation ditches play just as important a role in our water quality.
Steve Powers, formerly a CFL PhD student in Emily Stanley’s lab and now a post doc for the University of Notre Dame’s Environmental Change Initiative, recently published a study that says we should give more respect to the humble dams dotting America’s farmlands. In fact the sum total of the water bodies created by small dams have a global surface area comparable to that of all large reservoirs added together.
When sediment carrying fertilizer from croplands or manure from livestock drains into these water bodies, it settles out behind the dam and is prevented from heading further downstream. That’s a great service for anyone living in the watershed. In Madison, for example, our robust summer algal blooms can be traced to the nutrient phosphorous that is commonly used in fertilizers and runs off of farms, especially during late winter and early spring rain events.
But the pollution control these dams provide is endangered, says Powers. Small dams can be more than 100 years old and “were usually constructed at low cost and have more primitive engineering than larger dams,” he says. “Because of this, small dams are not only more likely to fail, but also are more likely to be removed intentionally by stakeholders, both of which promote transport of sediment and pollution.” What’s more, the waters behind small dams can fill in with sediment in only a decade or two, so they serve as a sink for nutrient pollution for a relatively short period.
Before all of these dams crumble and let nutrients take a more direct path downstream, it’s crucial that we understand what it will mean for our water quality, Powers says.
“Currently, water quality benefits of small dams are almost entirely absent from conversations about dam and water resource management, and that needs to change,” he says. “Dams are not a silver bullet to our water pollution problems, but they could be managed to help limit the damage. Small dams have been providing a collective benefit to water quality for decades. Unfortunately, this won’t last forever, so we need to get a better handle on what those dams are doing for downstream water quality, how they are changing, and this research is part of all that.”
Powers was lead author on the study “Retention and transport of nutrients in a mature agricultural impoundment,” published in the Journal of Geophysical Research-Biogeosciences. The article will soon be highlighted as a “Research Spotlight” for the American Geophysical Union’s newsletter, Eos.
For more on nutrient pollution and strategies to prevent run off, visit the U.S. EPA’s “Water: Pollution Prevention and Control” website.