Abstract
The transport and fate of nitrate (NO3−) to in the top 15 cm of a streambed has been well-documented, but an understanding of greater depths is limited. This work examines the transport and fate of nitrate (NO3−) at depths of 30 cm, 60 cm, 90 cm, and 150 cm below the stream-streambed interface. Concentrations of nitrate as nitrogen (NO3-N) and chloride (Cl−) were measured in the waters from the streambed, the stream water, and the groundwater. Mixing models predicted values of ΔNO3-N, the difference between measured NO3-N and theoretical NO3-N. At a 30-cm depth, the mean ΔNO3-N value was −0.25 mg/L, indicating a deficit of NO3-N and the removal of NO3-N from the system. At deeper levels, the values of ΔNO3-N began to approach zero, reaching a mean value of −0.07 mg/L at 150 cm. The reduction of NO3-N does not appear to be controlled by vegetation, as it was not correlated to either temperature or visible light. Larger negative ΔNO3-N values (more removal) occur when stream NO3-N concentrations are higher and organic matter is present.
Highlights
Fertile soils within the upper Mississippi River Basin experience intensive agricultural practices that utilize nitrogen (N) fertilizers to enhance yield
From January to the end of May, the hydraulic gradient decreased as stream stage rose at a greater rate than the groundwater (Figure 4a)
At baseflow, when the stream stage is lower and the stream velocity is slower, downwelling stream water will compete against upwelling groundwater
Summary
Fertile soils within the upper Mississippi River Basin experience intensive agricultural practices that utilize nitrogen (N) fertilizers to enhance yield. Crop yield accounts for 50% of the added N; the excess nitrogen remains within or is exported from the system [1]. 25% of the NO3 -N in the stream system will remain mobile, eventually discharging into the Gulf of Mexico [6,7]. Since 1950, the NO3 -N load discharged into the Gulf of Mexico has tripled [8,9], with the Illinois River identified as the second leading contributor of NO3 − to the Mississippi River [10], accounting for 19% of the NO3 -N load delivered to the Gulf of Mexico by the Mississippi River [11,12,13]
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