The concentrations of contaminants generated from discrete source areas are critical to urban nonpoint Source Load and Management Model (SLAMM) loading calculations to Lake Superior. This study summarizes data-collection efforts during 12 storms in October 1993 and from May through August 1994, in which stormwater data were collected concurrently at 33 sites representing the eight major source areas in a 117-hectare urban basin in Marquette, Mich. For the 12 storms, commercial rooftops produced the highest geometric mean concentrations of dissolved metals such as lead (20 mg/L), zinc (263 mg/L), cadmium (0.71 mg/L), and copper (17.8 mg/L). Parking lots produced the highest concentration for all of the individual polycyclic aromatic hydrocarbon (PAH) compounds (summation equal to 64 mg/L). Residential lawns generated the highest concentrations of total kjeldahl nitrogen (9.3 mg/L) and total phosphorus (2.3 mg/L). A mass-budget approach, in which summed source area loads are compared to those measured at the basin outlet for individual storms, provided the basis for an error analysis to identify unreliable concentration data. Of the 611 concentration samples used in the mass budgets, 59 were identified as unreliable. Seventy-six percent of the unreliable samples came from collection bottles that were filled prior to the end of runoff. These full bottles may have captured a first-flush effect or acted as a sediment trap. The relative importance of an individual source-area load to the overall basin-outlet load varied according to the individual constituent. Parking lots were a major contributor of total zinc (30 percent), total cadmium (25 percent), total copper (22 percent), and all the PAH compounds (=64 percent); whereas low-traffic streets were a major producer of total suspended solids (27 percent), nitrate plus nitrite (21 percent), and total cadmium (25 percent). Grass areas were a major producer of total kjeldahl nitrogen (31 percent) and total phosphorus (26 percent), even though the water volume generated from grass areas was low (5.8 percent of the total water volume generated).