Evaluation of nitrogen isotopes as indicators of nitrate contamination sources in an agricultural watershed

Determining the relative contributions of nitrate contamination in agricultural watersheds characterized by applications of both mineral and animal waste nitrogen to fields is important in evaluating the environmental consequences of these practices. Here, the use of nitrate-nitrogen (NO3--N) stable isotopes to identify contaminant sources was examined in surface waters of an agricultural watershed to investigate whether NO3--(delta)15N fractionation processes obscure source signatures. This study was conducted in a 500 ha headwater agricultural watershed within the St. Lawrence lowlands where fertilization with mineral N and hog manure is a common practice. Sampling within the watershed was designed to monitor NO3concentrations and (delta)15N temporal trends at a series of locations that allowed evaluation of water flowing in watershed streams and entering these streams via different hydrological pathways. This required analysis of stream water, tile drain water, and streamside water transported through the soil matrix. These data demonstrated that NO3--(delta)15N values often exceeded the maximum (delta)15N source of approximately +12.5‰ that was associated with hog manure in the watershed. The (delta)15N values and NO3- concentrations during such periods were consistent with a pattern of denitrification between precipitation events within the watershed, with values repeatedly exceeding +15‰ in tile drain and stream samples during extended periods with little rainfall. Tile drain NO3- concentrations were consistently elevated (4-38 mg l^-1 N) relative to streamside soil and shallow groundwater exiting the same field (0.013-0.812 mg l^-11 N). These data suggest that NO3--N stable isotopes cannot be used as contaminant source indicators in the surface waters of this watershed between precipitation events, and that further evaluation of their use during storm events is required.