Evaluating regional patterns in nitrate sources to watersheds in national parks of the Rocky Mountains using nitrate isotopes

In the Rocky Mountains, there is uncertainty about the source areas and emission types that contribute to nitrate (NO₃) deposition, which can adversely affect sensitive aquatic habitats of high-elevation watersheds. Regional patterns in NO₃ deposition sources were evaluated using NO₃ isotopes in five National Parks, including 37 lakes and 7 precipitation sites. Results indicate that lake NO₃ ranged from detection limit to 38 μeq/L, δ¹⁸O (NO₃) ranged from −5.7 to +21.3‰, and δ¹⁵N (NO₃) ranged from −6.6 to +4.6‰. δ¹⁸O (NO₃) in precipitation ranged from +71 to +78‰. δ¹⁵N (NO₃) in precipitation and lakes overlap; however, δ¹⁵N (NO₃) in precipitation is more depleted than δ¹⁵N (NO₃) in lakes, ranging from −5.5 to −2.0‰. δ¹⁵N (NO₃) values are significantly related (p < 0.05) to wet deposition of inorganic N, sulfate, and acidity, suggesting that spatial variability of δ¹⁵N (NO₃) over the Rocky Mountains may be related to source areas of these solutes. Regional patterns show that NO₃ and δ¹⁵N (NO₃) are more enriched in lakes and precipitation from the southern Rockies and at higher elevations compared to the northern Rockies. The correspondence of high NO₃ and enriched δ¹⁵N (NO₃) in precipitation with high NO₃and enriched δ¹⁵N (NO₃) in lakes, 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.