SOURCES OF NITRATE IN SNOWMELT DISCHARGE: EVIDENCE FROM WATER CHEMISTRY AND STABLE ISOTOPES OF NITRATE

摘要

To determine whether NO_3 concentration pulses in surface water in early spring snowmelt discharge are due to atmospheric NO_3~-, we analyzed stream δ~(15)N-NO_3~- and δ~(18)N-NO_3~- 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 32.4mm day~(-1) during snowmelt, and element concentrations increased from 33 to 71 μmol L~(-1) for NO_3~-, 3 to 9 μmol L~(-1) for total Al, and 330 to 570 μmol L~(-1) for DOC. Discharge in 2002 was variable, with a maximum of 30 mm day~(-1) during snowmelt. The highest NO_3~-, 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 δ~(15)N and δ~(18)O of NO_3~- in stream water were similar in both years. Stream water, atmospherically-derived solutions, and groundwaters had overlapping δ~(15)N-NO_3~- values. In stream and ground water, δ~(18)O-NO_3~- values ranged from +5.9 to +12.9 per thousand and were significantly lower than the +58.3 to +78.7 per thousand values in atmospheric solutions. Values of δ~(18)O-NO_3~- indicating nitrification, increase in Al and DOC, and decrease in dissolved Si concentrations indicating water flow through the soil suggested a dilution of groundwater NO_3~- by increasing contributions of forest floor and mineral soil NO_3~- during snowmelt.