Ecological studies indicate that impervious cover (IC) greater than approximately 5%-20% may have adverse effects on receiving-stream ecology. It is difficult to separate the effects of runoff quality from other effects of urbanization on receiving streams. This study presents the results of a numerical experiment to assess the effects of increasing IC on water quality using the Stochastic Empirical Loading and Dilution Model (SELDM). Hydrologic and physiographic variables representative of southern New England were used to simulate receiving water quality in a basin with IC ranging from 0.1% to 30%. Simulation results mirror the results of ecological studies; event mean concentrations (EMCs) of total phosphorus (TP) increase proportionally to the logarithms of imperviousness for a given risk percentile. Simulation results indicated that commonly used stormwater treatment methods may be insufficient for mitigating the effects of imperviousness. Therefore, disconnection, rather than treatment, may be needed to protect water quality, and efforts to preserve undeveloped stream basins may be more effective than efforts to remediate conditions in highly developed basins. Results also indicate that commonly used water-quality criteria may be too restrictive for stormwater because TP EMCs frequently exceed these criteria, even in minimally developed basins.

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vii 18. Graph showing range of chloride concentrations estimated from specificconductance values measured at 1-minute intervals in highway runoff during runoff events with one or more estimated values that exceed the 230-milligramsper-liter criterion continuous concentration for chloride in receiving waters at U.S. Geological Survey monitoring station 423027071291301 along State Route 2 in Littleton, Massachusetts, during the period from

The Wachusett Reservoir in central Massachusetts was modeled using the two‐dimensional CEQUAL‐W2 hydrodynamic and water quality model to simulate a conservative contaminant spill from a roadway and analyze contaminant behavior at the Cosgrove drinking water intake 12.5 km (7.8 mi) from the spill site. Modeled calendar years 2003–09 were developed using meteorology, bathymetry, inflow quantity and quality, and outflow quantity data. Scenarios investigated included the influence of season, spill density, additional water transferred from Quabbin Reservoir, and wind on contaminant transport. Contaminants arrived at the intake much earlier than the mean hydraulic residence time. Contaminant concentrations were greater and more variable during the summer. Turning the Quabbin transfer off decreased the variability in concentration at the intake, especially during the summer. Model results provide useful information to reservoir operators for how to respond to potential contaminant spills under various scenarios.