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 delta N-15-NO3- and delta O-18-NO3- values between February and June of 2001 and 2002 and compared them to those of throughfall, bulk precipitation, snow, and groundwater. Stream total A1, 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 32.4mm day(-1) during snowmelt, and element concentrations increased from 33 to 71 mu mol L-1 for NO3-, 3 to 9 mu mol L-1 for total A1, and 330 to 570 mu mol L-1 for DOC. Discharge in 2002 was variable, with a maximum of 30mm day(-1) during snowmelt. The highest NO3-, A1, and DOC concentrations were 52, 10, and 630 mu mol L-1, respectively, and dissolved Si decreased from 148 mu mol L-1 before to 96 mu mol L-1 during snowmelt. Values of delta N-15 and delta O-18 of NO3- in stream water were similar in both years. Stream water, atmospherically-derived solutions, and groundwaters had overlapping delta N-15-NO3- values. In stream and groundwater, delta O-18-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 delta O-18-NO3- indicating nitrification, increase in A1 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.