Stormwater control measures (SCMs) are designed to mitigate deleterious effects of urbanization on river networks, but our ability to predict the cumulative effect of multiple SCMs at watershed scales is limited. The most widely used metric to quantify impacts of urban development, total imperviousness (TI), does not contain information about the extent of stormwater control. We analyzed the discharge records of 16 urban watersheds in Charlotte, NC spanning a range of TI (4.1 to 54%) and area mitigated with SCMs (1.3 to 89%). We then tested multiple watershed metrics that quantify the degree of urban impact and SCM mitigation to determine which best predicted hydrologic response across sites. At the event time scale, linear models showed TI to be the best predictor of both peak unit discharge and rainfall-runoff ratios across a range of storm sizes. TI was also a strong driver of both a watershed's capacity to buffer small (e.g., 1-10 mm) rain events, and the relationship between peak discharge and precipitation once that buffering

Insights into the effects of stormwater control measures (SCMs) on urban stream hydrology and in-stream processes are required to understand their effectiveness in mitigating the environmental problems associated with urbanization. Stable water isotopes were applied to understand processes occurring within SCMs and their effects on water sourcing in urban streams. We sampled ten events from June to November 2013 at four locations along a 360-m headwater stream reach in North Carolina and at four SCMs (two ponds, one wetland and one bioretention) that contribute to the reach. We used streamflow upstream of the SCMaffected reach and outflow from an intensively sampled retention pond as endmembers to quantify contributions of this pond's outflow to streamflow. Synchronous sampling revealed that SCM outflows have different isotopic signatures, likely a function of evaporation and mixing within the storage volume of each SCM. The SCMs also have distinctive isotopic signatures relative to the receiving stream. The isotopic signature of discharge from the intensively sampled pond reveals varying residence times (hours to weeks) within the structure. At sampled timepoints during ten events, this pond, which drains 25% of the watershed's impervious area, contributed an average of 10% (0-21%) of the streamflow on the rising limb and 12% (0-19%) of discharge at peak flow. During recession, this pond contributed an average of 32% (11-54%) of the stream's discharge, reflecting the SCM's design goals of temporarily storing and delaying run-off, mitigating the effects of impervious surface on peak flows. Based on this study, isotopes appear to be a robust tool for examining stormwater-stream dynamics.Where values are not provided, uncertainty bands spanned 0-100%. Negative values occur when the stream sample was outside the range of the end members 5302 A. J. JEFFERSON ET AL.

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Urban-use pesticides are of increasing concern as they are widely used and have been linked to toxicity of aquatic organisms. To assess the occurrence and treatment of these pesticides in stormwater runoff, an approach combining field sampling and watershed-scale modeling was employed. Stormwater samples were collected at four locations in the lower San Diego River watershed during a storm event and analyzed for fipronil, three of its degradation products, and eight pyrethroids. All 12 compounds were detected with frequency ranging from 50 to 100%. Field results indicate pesticide pollution is ubiquitous at levels above toxicity benchmarks and that runoff may be a major pollutant source to urban surface waters. A watershed-scale stormwater model was developed, calibrated using collected data, and evaluated for pesticide storm load and concentrations under several management scenarios. Modeling results show that enhanced stormwater control measures, such as biochar-amended biofilters, reduce both pesticide storm load and toxicity benchmark exceedances, while conventional biofilters reduce the storm load but provide minimal toxicity benchmark exceedance reduction. Consequently, biochar amendment has the potential to broadly improve water quality at the watershed scale, particularly when meeting concentration-based metrics such as toxicity benchmarks. This research motivates future work to demonstrate the reliability of full-scale enhanced stormwater control measures to treat pollutants of emerging concern.