The suburban stream syndrome: Evaluating land use and stream impairments in the suburbs

Development is known to impair stream water quality at moderate to high levels of urbanization, but the effects of low-density urban expansion, the kind occurring on the outskirts of many cities, remain unclear. We examined five suburban headwater streams in Duchess County, New York whose watersheds contained between 4.7% and 34% impervious surface cover. We measured Cl^- and nitrate-N (NO₃-N) concentrations in water samples taken at four to six sites on each stream in winter and summer. Even at low levels of population and impervious cover, concentrations of both Cl^- and NO₃-N exceeded reference levels found in cleaner streams in the region. Chloride levels were elevated in upper reaches and remained elevated or continued to increase downstream, with a linear response to impervious cover. Nitrate-N increased downstream in all watersheds, indicating that NO₃-N inputs exceeded natural denitrification and uptake in both winter and summer. Nitrate-N responded logarithmically to impervious surface cover, with steep increases at low levels of imperviousness. Per-capita inputs were also high in rural areas. Agricultural inputs were not sufficient to explain observed trends in NO₃-N; we interpret inputs to result chiefly from low-density exurban expansion. Widespread residential expansion has significant impacts on water quality that have not previously been acknowledged.