Nutrients on asphalt parking surfaces in an urban environment

Amounts of readily soluble nutrients on asphalt parking lot surfaces were measured at four locations in metropolitan Phoenix, Arizona, U.S.A. Using a rainfall simulator, short intense rainfall events were generated to simulate 'first flush' runoff. Samples were collected from 0.3 m² sections of asphalt at 8 to 10 sites on each of four parking lots, during the pre-monsoon season in June-July 1998 and analyzed for dissolved NO₃^--N, NH₄⁺-N, soluble reactive phosphate (SRP), and dissolved organic carbon (DOC). Runoff concentrations varied considerably for NO₃^--N and NH₄⁺-N (between 0.1 and 115.8 mg L^-1) and DOC (26.1 to 295.7 mg L^-1), but less so for SRP (0.1 to 1.0 mg L^-1), representing average surface loadings of 191.3, 532.2, and 1.8 mg m^-2 respectively. Compared with similar data collected from undeveloped desert soil surfaces outside the city, loadings of NO₃^--N and NH₄⁺-N on asphalt surfaces were greater by factors of 91 and 13, respectively. In contrast, SRP loads showed little difference between asphalt and desert surfaces. Nutrient fluxes in runoff from a storm that occurred shortly after the experiments were used to estimate input-output budgets for 3 of the lots under study. Measured outputs of DOC and SRP were similar to those predicted using rainfall and experimentally determined surface loadings, but for NH₄⁺-N and particularly for NO₃^--N, estimated rainfall inputs and surface runoff were significantly higher than exports in runoff. This suggests that parking lots may be important sites for nutrient accumulation and temporary storage in arid urban catchments.