U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings
of the Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993,
Water-Resources Investigations Report 94-4015
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Use of 15N to Trace Movement of Nitrogen Fertilizer at a Field
Plot
by
Dale W. Blevins (U.S. Geological Survey, Independence, MO), Donald
H. Wilkison (U.S. Geological Survey, Independence, MO), Steven R. Silva
(U.S. Geological Survey, Menlo Park, CA), and Brian P. Kelly (U.S. Geological
Survey, Independence, MO)
Abstract
A simplified, lower-cost technique forpreparation
of water samples for nitrogen isotope ( 15N) analysis
of nitrate (NO3) is being tested. Anion
exchange resins were used to concentrate nitrate in
the field and samples were prepared for 15N - NO3 (ratio of 15N
to 14N in NO3 , relative to atmospheric
N, expressed in per mil) analysis using a modification
of the Dumas combustion method. Initial
test results of the 15N - NO3
technique indicate that a precision of 0.05 per mil
can be achieved. More than 99 percent of
nitrate in solution is sorbed onto the resins.
Hydrochloric acid was used to elute 95 to 100
percent of the nitrate from the resins. Chloride and
organic molecules can interfere with 15N - NO3
values, but not at concentrations detected at the
field plot.
The 15N-labeled fertilizer was applied to a 0.1-acre
field plot in May 1992 to determine the quantity
of nitrogen fertilizer reaching ground water.
Twenty-two percent of the labeled fertilizer was removed
from the plot in harvested grain. Soil-gas analyses
indicated no significant denitrification of the
applied fertilizer. Labeled fertilizer accounted for 34
percent of the nitrate in runoff in the first storm after
application. Small quantities of labeled fertilizer
were detected 1.5 feet below ground surface 1 month
after application, but evapotranspiration prevented
further downward movement until November 1992.
Labeled fertilizer was detected in shallow wells
within 6 days of the first recharge event in November
1992. The 15N - NO3 values
in deep wells increased 1 month later. Labeled
fertilizer accounted for as much as 0.74 milligram
per liter of the nitrate in ground
water.
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