Evaluating Spatial Variability in Nitrate Deposition Sources to Watersheds in the Rocky Mountains using O and N in Nitrate

Leora Nanus
U.S. Geological Survey,
Lakewood, Colorado USA

Nitrate deposition can adversely affect sensitive aquatic habitats of high-altitude watersheds. Results of analyses of nitrogen isotopes in water samples from 37 lakes and 7 precipitation sites were used to evaluate regional patterns in sources of nitrate deposition across five National Parks. The analyses indicate that nitrate (NO3) concentrations in lakes sampled during baseflow in 2004, ranged from the detection limit (~1 ?eq/L) to 38 ?eq/L, ?18O (NO3) values ranged from -5.7 to +21.3 permil, and ?15N (NO3) values ranged from -6.6 to +4.6 permil. ?18O (NO3) in precipitation ranged from +71 to +78 permil. The ranges in values of ?15N (NO3) in precipitation and lakes overlap; however, ?15N (NO3) in precipitation is more depleted than ?15N (NO3) in lake water, with values ranging from -5.5 to -2.0 permil. The values of ?15N (NO3) are significantly related (p < 0.05) to estimates of inorganic N, sulfate, and acidity in wet deposition, suggesting that the spatial variability of ?15N (NO3) over the Rocky Mountains may be related to source areas of these solutes. Regional patterns show that NO3 concentrations and ?15N (NO3) values generally are more enriched in lakes and precipitation from the Southern Rockies and at higher elevations, compared to values at lower elevations and in the Northern Rockies. The correspondence of high NO3 concentrations and enriched ?15N (NO3) values in precipitation with high NO3 and enriched ?15N (NO3) values in lake waters, suggests that deposition of inorganic N in wetfall may affect the amount of NO3 in lakes through a combination of direct and indirect processes such as enhanced nitrification.