Catchment-Scale Variation in the Nitrate Concentrations of Groundwater Seeps in the Catskill Mountains, New York, U.S.A.

Abstract

Forested headwater streams in the Catskill Mountains of New York show significant among-catchment variability in mean annual nitrate (NO3-) concentrations. Large contributions from deep groundwater with high NO3-concentrations have been invoked to explain high NO3-concentrations in stream water during the growing season. To determine whether variable contributions of groundwater couldexplain among-catchment differences in streamwater, we measuredNO3- concentrations in 58 groundwater seeps distributed across six catchments known to have different annual average streamwater concentrations. Seeps were identified based on release from bedrock fractures and beddingplanes and had consistently lower temperatures than adjacentstreamwaters. Nitrate concentrations in seeps ranged from neardetection limits (0.005 mg NO3--N/L) to 0.75 mg NO3--N/L. Within individual catchments, groundwaterresidence time does not seem to strongly affect NO3-concentrations because in three out of four catchments therewere non-significant correlations between seep silica (SiO2) concentrations, a proxy for residence time, andseep NO3- concentrations. Across catchments, therewas a significant but weak negative relationship betweenNO3- and SiO2 concentrations. The large rangein NO3- concentrations of seeps across catchmentssuggests: 1) the principal process generating among-catchmentdifferences in streamwater NO3- concentrations mustinfluence water before it enters the groundwater flow system and 2) this process must act at large spatial scales becauseamong-catchment variability is much greater than intra-catchmentvariability. Differences in the quantity of groundwater contribution to stream baseflow are not sufficient to account for differences in streamwater NO3- concentrationsamong catchments in the Catskill Mountains.