7 Hydrogeology, Environmental and Engineering Geology (Posters)

MASS FLUX OF NITROGEN IN SHALLOW GROUNDWATER OF AN AGRICULTURAL WATERSHED IN CENTRAL ILLINOIS

Program: Technical Sessions
Time: 1:00 PM – 5:00 PM

Authors

Abstract

Nitrogen is a common contaminant in Illinois’ surface water and groundwater. With the emergence of hypoxia as a national concern, the fate and transport of nitrogen in the environment is getting renewed interest. To address this concern, the fate and transport of nitrogen in an agricultural watershed is under investigation by a multidisciplinary team. This investigation includes field sampling and laboratory studies to estimate the flux of nitrogen in shallow groundwater and to the stream that drains the watershed. Because many researchers consider nitrogen loss via denitrification to be among the most uncertain of all estimates in the nitrogen mass balance, denitrification rates will be quantified by microbial and isotopic methods.

The Big Ditch watershed covers 40 square miles in northern Champaign County. The predominant land use is agriculture. The major crops are corn, soybeans, and seed corn. Twenty monitoring wells were installed to monitor groundwater levels and geochemistry. Streamflow was monitored at the outlet. Rain gages were installed to monitor precipitation and tile flow was selectively monitored.

To date, groundwater samples from most wells have less than 1 mg total N/L. Organic N accounts for 10 to 30% of total N in these samples. Ammonium is the principal N species in samples from deep wells. However, groundwater from 3 wells has consistently shown total N concentrations of 3 to10 mg N/L. More than 95% of this nitrogen is nitrate. These data support the hypothesis that denitrification is a significant process in the subsurface of this watershed. Denitrification activity and the abundance of denitrifying bacteria are shown to vary with depth. Although the numbers of bacteria are lowest in the middle depths, no correlation between the rate of denitrification and depth has been found. Lab results indicate that the denitrification rate increases with depth if carbon is abundant but remains constant if carbon is limited.