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From a distance, it might seem there’s little mystery involved with nutrient pollution in waterways that flow through the intensively farmed landscapes of the American Midwest. Farmers put fertilizer on fields, and some of it ends up in streams when it rains.
But how does fertilizer “end up” in streams?
To answer that question you might want a detective. Or better still, a scientist who operates like one.
Enter Conor Neal, with his newly minted Master’s degree in Geology from the U of I. Over the past year and a half, he has been working with Department of Geology Professor Alison Anders to figure out how one key nutrient, phosphorous, is transported from the surrounding landscape into the lower reach of Wildcat Slough, a tributary that feeds into the Sangamon River just south of Fisher.
How does nutrient migration involve geology, you ask? It’s in the fine sediment. As Neal and Anders explained it to me, some nutrients, such as nitrogen, dissolve easily in water and are thus carried in solution. Others, including phosphorous, prefer to attach to particles of soil. Fine sediment—defined by geologists as silt and clay—is the most important of these, since fine sediment particles offer so much surface area. In short, says Neal: “Fine sediment carries phosphorous. Find the source of fine sediment in a stream, and you find the source of phosphorous.”
The fieldwork for Neal’s project was pretty straightforward. Using satellite images he identified the six types of land cover in the watershed, 99 percent of which (literally) is devoted to row-crop agriculture. The five much smaller types included forested uplands, forested floodplain, pasture, channel banks and grass.
Then he took samples of fine sediment from those areas. This involved simply digging up a sandwich bag’s worth of soil with a garden trowel. These samples were to serve as a reference for samples of sediment taken from Wildcat Slough during spring floods (since that’s when the water carries significant amounts of suspended sediment).Finally, he collected suspended sediment from Wildcat Slough during heavy rains between March and June of last year. He did so using a simple device called a sediment trap, which is a four-foot-long piece of PVC pipe fitted with a cone-shaped nose that’s anchored to a fencepost in the stream. As flowing water enters the trap through the nose its velocity decreases dramatically, which causes whatever fine sediment it carries to drop out before it exits through a tube at the tail.
[Photo by Katelyn Zatwarnicki shows Neal installing the sediment trap in Wildcat Slough.]
Like detectives, scientists often have a pretty good idea of what they’re going to find when they begin an investigation. In the case of Neal’s study, he and Anders both anticipated the fine sediment suspended in the lower reach of Wildcat Slough during spring floods would be traceable to the agricultural fields in the watershed—after all, that’s where the bare soil is.
But Neal’s analysis showed something very different; less than 5 percent of the fine sediment collected in the stream was traceable to farm fields. A much greater portion, roughly half, came directly from the channel banks, with the rest contributed by forested and grassy areas. Why that’s so offers a new mystery to be investigated.
What these results suggest, says Anders, is that the soil conservation practices employed by farmers in that part of the watershed are doing exactly what they’re supposed to do, keeping the soil on the fields and out of the stream. In doing so they’re also preventing phosphorous pollution.
When we spoke, Conor Neal emphasized to me his great debt of gratitude to the families who allowed him onto their land to do field work; without their cooperation such a study could not have been done.
Neal’s project is a precursor to a much larger, five-year effort involving UI researchers from multiple disciplines. They will be collaborating with partners from other universities and conservation agencies to understand water, nutrient and sediment transport in the Upper Sangamon River Basin.
The ultimate success of this project, called the “Intensively Managed Landscapes Critical Zone Observatory,” will also depend on partnerships with local landowners, farm operators, land managers and urban planners. I’ll return it in this column as it develops, but in the meantime more information is available at http://criticalzone.org/iml/.
