MODELED WET WEATHER TRACE METAL LOADS TO BALLONA CREEK, CALIFORNIA

Watershed models, such as the Hydrological Simulation Program – Fortran (HSPF), are widely applied to investigate runoff dynamics and associated pollutant loadings, but applications in arid urban watersheds for trace metals are rarely observed. The efficacy of HSPF in arid urban watersheds is questioned since very short time scales (i.e. minutes) are needed to understand the processes associated with these flashflood events in highly impervious areas. In this study, HSPF is developed for simulating concentrations and loads of total suspended solids (TSS), copper, lead, and zinc at in the Ballona Creek watershed, which comprises an exceptionally urbanized section of Los Angeles, CA. The model was calibrated in small homogeneous land use sites and then validated at an instream site that received cumulative discharges from multiple land uses upstream. Model evaluation focused on the ability to predict storm event mean concentrations (EMCs) and time-concentration series, which measures the changes in TSS and trace metal concentrations during rising, peak and receding storm flows. HSPF was capable of reproducing the observed time-concentration series and corresponded closely to the measured EMCs for TSS, copper, lead, and zinc at each of the land use calibration sites. Similarly, the model reproduced the observed time-concentration series and corresponded closely to the measured EMCs for TSS, copper, lead, and zinc at the instream validation site. The most problematic trace metal to model was zinc, which typically overestimated the measured concentration. For all metals, the correlation between loading and rainfall varied by storm size. All four water quality constituents were significantly highly correlated to rainfall during large storm events (≥. 2.54 mm), but rainfall was a poor predictor of constituent loading during small storm events (