Sources of nitrate in snowmelt discharge: Evidence from water chemistry and stable isotopes of nitrate

To determine whether NO₃⁻ concentration pulses in surface water in early spring snowmelt discharge are due to atmospheric NO₃⁻, we analyzed stream δ¹⁵N-NO₃⁻ and δ¹⁸O-NO₃⁻ values between February and June of 2001 and 2002 and compared them to those of throughfall, bulk precipitation, snow, and groundwater. Stream total Al, DOC and Si concentrations were used to indicate preferential water flow through the forest floor, mineral soil, and ground water. The study was conducted in a 135-ha subcatchment of the Arbutus Watershed in the Huntington Wildlife Forest in the Adirondack Region of New York State, U.S.A. Stream discharge in 2001 increased from 0.6 before to 32.4 mm day⁻¹ during snowmelt, and element concentrations increased from 33 to 71 μmol L⁻¹ for NO₃⁻, 3 to 9 μmol L⁻¹ for total Al, and 330 to 570 μmol L⁻¹for DOC. Discharge in 2002 was variable, with a maximum of 30 mm day⁻¹ during snowmelt. The highest NO₃⁻, Al, and DOC concentrations were 52, 10, and 630 μmol L⁻¹, respectively, and dissolved Si decreased from 148 μmol L⁻¹ before to 96 μmol L⁻¹ during snowmelt. Values of δ¹⁵N and δ¹⁸O of NO₃⁻ in stream water were similar in both years. Stream water, atmospherically-derived solutions, and groundwaters had overlapping δ¹⁵N-NO₃⁻ values. In stream and ground water, δ¹⁸O-NO₃⁻ values ranged from +5.9 to +12.9‰ and were significantly lower than the +58.3 to +78.7‰ values in atmospheric solutions. Values of δ¹⁸O-NO₃⁻ indicating nitrification, increase in Al and DOC, and decrease in dissolved Si concentrations indicating water flow through the soil suggested a dilution of groundwater NO₃⁻by increasing contributions of forest floor and mineral soil NO₃⁻ during snowmelt.