Tracing sources of nitrate in snowmelt runoff using the oxygen and nitrogen isotopic compositions of nitrate

Abstract

Many upland catchments experience increased nitrate and hydrogen ion concentrations in stream water during the early snowmelt period. The dominant source of this nitrate is not well known, but likely contributors include atmospheric deposition of nitrate and ammonium in the snowpack, and soil-derived nitrate. Pilot studies initiated in three catchments during the 1994 snowmelt season show that these sources of nitrate have distinctive <5180 and <515N compositions. Almost all the stream samples have nitrate <5180 and <515N values within the range of the premelt soil and stream waters, suggesting that atmospheric nitrate eluted from the 1994 snowpack is a minor source of nitrate in early streamflow. Therefore, the nitrate eluted from the snowpack appears to go into storage, and the nitrate pulse in stream water during early melt appears to be largely derived from precipitation from previous months or years that is flushed from storage.