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Isotopic Evidence of Nitrate Sources and Denitrification in the Mississippi River, Illinois

^a Illinois State Geological Survey, Natural Resources Building, 615 E. Peabody Street, Champaign, IL 61820
^b Illinois State Water Survey, 2204 Griffith Drive, Champaign, IL 61820-7495

View larger version (31K): [in a new window]   Fig. 1. Sampling locations. The light gray area on the U.S. map shows the extent of the Mississippi River watershed.

View larger version (25K): [in a new window]   Fig. 2. Ranges of 18O and 15N values for potential NO3– sources and values measured in Mississippi River and tile drain samples. The domain of soil organic matter is shaded to show the range of greatest (darkest) to lowest (lightest) frequency. The domain of soil organic matter partially overlaps that of reduced N fertilizer, manure, and sewage with respect to 15N. The 18O of NO3– from these reduced N sources was calculated as described in the discussion. The large arrow represents a denitrification vector; as denitrification proceeds, the 15N and 18O values of the remaining NO3– progressively increase in the direction of the vector. The size of circles represents NO3–N concentration; shaded circles indicate tile drain samples. In all cases, analytical errors are smaller than the circle size. Synthetic NO3– Fertilizer refers to synthetic fertilizer applied as NO3– (e.g., the NO3– in NH4NO3) and Reduced N Fertilizer refers to NO3– produced by nitrification of applied NH3 (as NH3 or the NH4+ in NH4NO3) and urea.

View larger version (16K): [in a new window]   Fig. 3. Nitrate-N concentrations vs. (a) 15N and (b) 18O of NO3– for Mississippi River and tile drain samples. Letters indicate sample locations (D = Davenport, Q = Quincy, C = Chester). The gray hexagon represents the average composition of the four river samples collected in April 2004 at Quincy. The month of sample collection is shown for tile samples. Lines are linear regressions for river samples (r2 = 0.64 for 15N and 0.61 18O).

View larger version (23K): [in a new window]   Fig. 4. Detailed portion from Fig. 2, showing the two denitrification lines calculated from the two estimated initial 15N and 18O values (gray hexagons). Open symbols represent river samples, closed symbols represent tile drain samples. Letters indicate river sample locations (D = Davenport, Q = Quincy, C = Chester). Tick marks on the vectors indicate an estimation of the extent of denitrification (percentage of original NO3– lost, using values of –15.9 for 15N and –8 for 18O in Eq. [3] for NO3– having initial values of 4.4 for 15N and 6.4 for 18O [solid lines] and for NO3– having initial values of 6.1 for 15N and 7.2 for 18O [dashed lines]).