Catchment-scale variation in the nitrate concentrations of groundwater seeps in the Catskill Mountains, New York, USA

Forested headwater streams in the Catskill Mountains of New York show signi ficant among-catchment variability in mean annual nitrate (NO3-) concentrat ions. Large contributions from deep groundwater with high NO3- concentratio ns have been invoked to explain high NO3- concentrations in stream water du ring the growing season. To determine whether variable contributions of gro undwater could explain among-catchment differences in streamwater, we measu red NO3- concentrations in 58 groundwater seeps distributed across six catc hments known to have different annual average streamwater concentrations. S eeps were identified based on release from bedrock fractures and bedding pl anes and had consistently lower temperatures than adjacent streamwaters. Ni trate concentrations in seeps ranged froth near detection limits (0.005 mg NO3--N/L) to 0.75 mg NO3--N/L. Within individual catchments, groundwater re sidence time does not seem to strongly affect NO3- concentrations because i n three out of four catchments there were non-significant correlations betw een seep silica (SiO2) concentrations, a proxy for residence dine, and seep NO3- concentrations. Across catchments, there was a significant but weak n egative relationship between NO3- and SiO2 concentrations. The large range in NO3- concentrations of seeps across catchments suggests: 1) the principa l process generating among-catchment differences in streamwater NO3- concen trations must influence water before it enters the groundwater flow system and 2) this process must act at large spatial scales because among-catchmen t variability is much greater than intra-catchment variability. Differences in the quantity of groundwater contribution to stream baseflow are not suf ficient to account for differences in streamwater NO3- concentrations among catchments in the Catskill Mountains.