title: Tracking nonpoint source nitrogen pollution in human-impacted watersheds.
abstract: Nonpoint source nitrogen (N) pollution is a leading contributor to U.S. water quality impairments. We combined watershed N mass balances and stable isotopes to investigate fate and transport of nonpoint N in forest, agricultural, and urbanized watersheds at the Baltimore Long-Term Ecological Research site. Annual N retention was 55%, 68%, and 82% for agricultural, suburban, and forest watersheds, respectively. Analysis of δ(15)N-NO(3)(-), and δ(18)O-NO(3)(-) indicated wastewater was an important nitrate source in urbanized streams during baseflow. Negative correlations between δ(15)N-NO(3)(-) and δ(18)O-NO(3)(-) in urban watersheds indicated mixing between atmospheric deposition and wastewater, and N source contributions changed with storm magnitude (atmospheric sources contributed ∼50% at peak storm N loads). Positive correlations between δ(15)N-NO(3)(-) and δ(18)O-NO(3)(-) in watersheds suggested denitrification was removing septic system and agriculturally derived N, but N from belowground leaking sewers was less susceptible to denitrification. N transformations were also observed in a storm drain (no natural drainage network) potentially due to organic carbon inputs. Overall, nonpoint sources such as atmospheric deposition, wastewater, and fertilizer showed different susceptibility to watershed N export. There were large changes in nitrate sources as a function of runoff, and anticipating source changes in response to climate and storms will be critical for managing nonpoint N pollution.
authors.affiliation: Department of Geology and Earth System Science Interdisciplinary Center, University of Maryland College Park , 5825 University Research Court, College Park, Maryland 20740, United States. skaushal@umd.edu
authors.forename: Sujay S
authors.initials: SS
authors.lastname: Kaushal
authors.forename: Peter M
authors.initials: PM
authors.lastname: Groffman
authors.forename: Lawrence E
authors.initials: LE
authors.lastname: Band
authors.forename: Emily M
authors.initials: EM
authors.lastname: Elliott
authors.forename: Catherine A
authors.initials: CA
authors.lastname: Shields
authors.forename: Carol
authors.initials: C
authors.lastname: Kendall
mesh_terms: D000383:Agriculture; D001272:Atmosphere / Q000737:chemistry; D015142:Baltimore; D002947:Cities; D055867:Cyclonic Storms; D017753:Ecosystem; D006801:Humans; D008956:Models, Chemical; D009566:Nitrates / Q000032:analysis; D009584:Nitrogen / Q000032:analysis*; D012424:Rural Population; D014867:Water / Q000737:chemistry*; D014874:Water Pollutants, Chemical / Q000032:analysis*; D014876:Water Pollution / Q000032:analysis*
chemical_list: D009566:Nitrates; D014874:Water Pollutants, Chemical; D014867:Water; D009584:Nitrogen