Seasonal Variations in Nitrate Flux, Transport, and Sources in the Upper Illinois River Basin

Excess N contributes to numerous environmental issues such as eutrophication, hypoxia, and loss of habitat and biodiversity. This study was conducted to study the sources, inventory, and transport of nitrate in the Upper Illinois River Basin (UIRB)—one of the basins with the highest N delivery amount and rate to the Gulf of Mexico. We combined the two most powerful approaches for the study of N: isotopic/chemical measurements and hydrological modeling were integrated on basin and subbasin levels to gain full understanding of N behavior in the UIRB, in order to assist people to make scientific and efficient management plans for N control and reduction. River samples were collected on the Upper Illinois River and its major tributaries from 2004 to 2008, in order to gain useful information on the basin nutrient transport and mixing processes. The measurements of nitrate concentration and isotopic values of water samples provided us with insights into the major sources of nitrate and its seasonal and temporal variations, which were indicators for studying land use impact on nutrient transport, and in-stream processes within the basin. Our study demonstrated that isotopic composition of nitrate is controlled by land use patterns, weather, and location. The influence of wastewater effluents, tributary inputs, and agricultural land on nitrate concentration and isotopic values were well documented. Potential increase in N fertilizer use is expected for expanding production of corn and cellulosic materials. SWAT (Soil and Water Assessment Tool) simulations of various fertilizer application scenarios were conducted to study the direct impact of changes in fertilization on N export and crop output respectively at basin and subbasin levels. SWAT also yielded outputs that assist us to understand in-stream denitrification processes, basin nitrate export mechanisms, and control of hydrological conditions on denitrification and N transport at basin and subbasin levels. The SWAT model results are consistent with results of the geochemical study of UIRB nitrate. SWAT simulations also add missing pieces to the understanding of environmental N behavior within the UIRB. This study demonstrates that there are huge potential benefits in modeling and geochemical studies that will allow us to enhance our knowledge of management of N and N sources and sinks, which are required to achieve the extremely challenging nitrogen reduction goal.