Stream Nitrate Concentrations Diverge at Baseflow and Converge During Storms in Watersheds with Contrasting Urbanization

Management of non-point sources is challenging because it requires adequate quantification of non-point fluxes that are highly dynamic over time. Most fluxes occur during storms and are difficult to characterize with grab samples alone in flashy, urban watersheds. Accurate and relatively precise measurements using in situ sensor technology can quantify fluxes continuously, avoiding the uncertainties in extrapolation of infrequently collected grab samples. In situ nitrate (NO₃-N) sensors were deployed simultaneously from April to December 2013 in two streams with contrasting urban land uses in an urbanizing New Hampshire watershed (80 km²). Nitrogen non-point fluxes and temporal patterns were evaluated in Beards Creek (forested: 50%; residential: 24%; commercial/institutional/transportation: 7%; agricultural: 6%) and College Brook (forested: 35%; residential: 11%; commercial/institutional/transportation: 20%; agricultural: 17%). Preliminary data indicated NO₃-N concentrations in Beards Creek (mean: 0.37 mg/L) were lower than College Brook (mean: 0.60 mg/L), but both streams exhibited rapid increases in NO₃-N during the beginning of storms followed by overall dilution. While baseflow NO₃-N was greater in College Brook than Beards Creek, NO₃-N at the two sites consistently converged during storms. This suggests that standard grab sampling may overestimate fluxes in urban streams, since short-term dilution occurred during periods of highest flow. Analyzing NO₃-N flux patterns in smaller urban streams that are directly impacted by watershed activities could help to inform management decisions regarding N source controls, ultimately allowing an assessment of the interactions of climate variability and management actions.