The influence of urbanization on runoff generation and stream chemistry in Massachusetts watersheds
The conversion of forested and agricultural land to suburban and urban landscapes is a dominant land use change dynamic in the United States and has implications for watershed hydrology and water quality. Here I evaluate the effect of integrated landscape features (e.g., percent residential or developed) and watershed-scale attributes influenced by urbanization on stream nutrient concentrations in headwater catchments in Massachusetts. In addition, I evaluate the importance of surface versus subsurface flow paths during rainfall events in stormflow generation in a small urban catchment. The percentage of residential land explains 52% of the variability in mean annual nitrate (NO3) concentrations in headwater catchments of the Ipswich River watershed, but is not correlated with mean annual phosphate (PO4) or dissolved organic nitrogen (DON) concentrations. A multiple regression of wetlands plus open water percentage and septic density explains 51% of the variability in NO3 concentrations and highlights the potential importance of wetlands (sinks) and septic wastewater (sources) at the watershed scale. Stream DON concentrations are best predicted by the percent wetlands in the study catchments (r2 = 0.56) and in a compiled dataset of northeastern U.S. watersheds (r2 = 0.60; n = 158 watersheds). Hydrograph separation in an intensively-studied 3.9 km 2 catchment indicates that surface flow paths are critical to stormflow generation during rainfall events in urbanizing catchments. Elevated discharge is largely composed of new water, with total precipitation depth describing most of the variability in new water runoff volumes. However, only about 20% of the impervious surface area contributes direct runoff to the stream during hydrologic events with the other 80% presumably exported from the watershed, evaporated or entering the groundwater. Impervious surfaces increase surface runoff of water and contaminants to streams, but may also result in reduced groundwater recharge. Reduced recharge may decrease wetland abundance and denitrification potential, in addition to increased runoff bypassing wetlands. Discharge from septic systems may compensate by providing some recharge, but with elevated subsurface NO3- inputs below the rooting zone. Understanding the simultaneous and interacting influence of these components will be critical for managing the impacts of urbanization on stream hydrology and water quality.
Biogeochemistry, Environmental Sciences, Engineering, Environmental
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