Mass Balance of Nitrogen and Phosphorus in an Agricultural Watershed:

The Shallow Groundwater Component

Edward Mehnert, Illinois State Geological Survey

Mass balances for nitrogen (N) and phosphorus (P) were conducted in a tile-drained, agricultural watershed by a multidisciplinary team, which was sponsored by the Illinois Council on Food and Agricultural Research (C-FAR). While cooperating researchers studied N and P fluxes for crops, soils, surface water, and atmospheric deposition, our efforts focused on the flux of N and P through shallow groundwater. This mass balance was conducted in the Big Ditch watershed, a 38 sq. mile watershed in northern Champaign County. The shallow geologic materials consist of thin loess overlying clayey glacial diamictons interbedded with sand and silt. Monitoring wells were installed to monitor the water table at 11 sites and to monitor the deeper sand-and-gravel layers at 9 of these sites. An analytical, steady state, groundwater flow model was developed and calibrated using water levels and stream flow data.

Groundwater samples were collected from the 20 monitoring wells on 28 occasions from May 2000 through June 2003. Overall, N concentrations in the groundwater tended to be low. Only 6 of the 20 wells (5 shallow & 1 deep) had total N concentrations that consistently exceeded 1 mg/L. Nitrate comprised almost 100% of the N in the two wells with the highest total N concentrations (median total N of 8 and 18 mg/L). In the other 18 wells, organic N generally comprised 10 to 40% of the total N. Nitrate was rarely detected in the deep wells, while ammonia was rarely detected in the shallow wells. Phosphorus was rarely detected in groundwater samples. Thus, the watershed soils appear to be a sink for P, binding it and preventing its transport into shallow groundwater.

Denitrification in the subsurface appears to be a significant N sink. Stable isotope ratios of O and N in nitrate from shallow groundwater samples revealed enrichments in the heavy isotopes. These enrichments suggested that denitrification in shallow groundwater was generally greater than 50% complete, but varied from 0% to 98% complete across the watershed. Denitrifying bacteria varied in abundance by a factor of 10 in water bearing layers throughout the watershed, but were considered sufficiently abundant to denitrify a significant amount of N. Denitrification rates were dependent on the availability of biodegradable organic carbon and averaged 0.82 mg N/kg-d with an amendment of 0.2 mM carbon as acetate (2.4 mg/L as C).

To estimate the dynamics of nitrogen in the shallow groundwater of the watershed, we adopted a simple approach—N denitrified equaled the product of the change in N over the shallow groundwater flow path and the volume of water moving through that path. We assumed that high N concentrations entering shallow groundwater were reduced to zero before groundwater discharged to the stream. The volume of water through the flow path was determined using a steady-state groundwater model, constrained by stream flow (0.9 to 10 cfs). Using this approach, the mass of N denitrified varied from 0.3 to 25.5% of the N applied, with the best estimates being 2.3%, 1.2%, and 15.1% of N applied for water years 2000, 2001, and 2002, respectively. These estimates varied with annual precipitation and should be considered minimums because this approach does not account for transient flow.

Dr. Edward Mehnert (May-nert) is a Senior Geohydrologist with the Hydrogeology Section, Illinois State Geology Survey. He has worked at the ISGS since 1985 and is currently an Associate Editor for Ground Water.

He holds degrees in Civil Engineering from Oklahoma State University (B.S., 1982), University of Notre Dame (M.S., 1984), and the University of Illlinois at Urbana-Champaign (Ph.D./Hydrosystems, 1997).

Dr. Mehnert’s research focus is the fate and transport of chemicals in groundwater and the characterization of the physical and chemical properties of aquifers.