Recent U.S. Geological Survey research has found that natural biochemical processes in water moving back and forth between a stream and its underlying sediment were significant in removing nitrate from streams in the Illinois River basin, one of the world’s most intensively farmed regions.
The USGS study in a nitrogen-polluted stream found that the flow of streamwater through a very thin zone of sediment enhances chemical reactions that decrease nitrate delivery to coastal areas where nitrogen fuels formation of hypoxic "dead zones."
Beneath all streams and rivers is a shallow layer of sediment that is permeated by water exchange across the sediment surface. This boundary between the world of earth and water in streams is referred to by scientists as the "hyporheic" zone, from Greek words meaning "under the flow." The hyporheic zone can be thought of as the stream's "skin," since it serves vital functions such as the removal of dissolved and particulate contaminants being transported by the stream.
Previous research has established under laboratory conditions that hyporheic flow should be critical to sparking reactions that improve stream water quality, but field studies have generally been unable to reveal the contribution of hyporheic flow to decreasing the flow of contaminants to sensitive downstream waters.
This field study determined that a very thin skin, a mere four centimeters (1.6 in.) of sediment, was effective in removing nitrate from streams of the Illinois River basin during late summer. The crucial investigative approach was labeling in-stream nitrate with an isotopic tracer that could be followed at very fine scales in the sediment and simultaneously tracked for kilometers downstream.
The study scientists found that hyporheic flow increased nitrate removal by renewing the supply of dissolved organic carbon and nitrate to specialized bacteria in the sediment that performed denitrification, a reaction that converts dissolved nitrate to gaseous nitrogen and so removes nitrate permanently from flowing water.
The top four centimeters of sediment had the greatest abundance of denitrifying bacteria, in addition to the highest levels of hyporheic flow. Sediment properties in this thin layer were also conducive to the formation of oxygen-free micro zones that are required for the reaction to take place.
The study was published in the October 2013 edition of Water Resources Research. The findings were presented December 11 at the fall meeting of the American Geophysical Union.
source: U.S. Geological Survey
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