Tracking Nitrate Leaching in the Idaho Upper Snake Rock Watershed.

Identifying nitrate (NO₃-N) sources leaching into groundwaters of the irrigated Upper Snake Rock watershed, in south central Idaho, is of paramount importance for maintaining or improving water quality.  Monitoring efforts have focused attention on locations where groundwater emerges to the surface, including drain tunnels and cliffband seeps/falls from the Snake River canyon walls within the watershed. A combination of NO₃-N determination, age-dating via the tritium/Helium-3 ingrowth method, 15N and 18O isotopic analysis, and source tracking of bacteroides species, is being used to help identify sources. Nitrate concentrations cycle annually, with maximum concentrations in October and minimum in May coinciding with changes in the irrigation season.  Differences in NO₃-N content from seeps/falls along the canyon walls can vary drastically within short distances (<100 m). Age-dating suggests that drain tunnel waters are young (<50 years old) while the waters from Snake River canyon seeps/falls are a mixture of young and old (> 50 years old) water. Isotopic and source-tracking analysis suggests that NO₃-N is organic or a mixed N source, yet no clear fertilizer or human/animal signature was observed.