Estimating Pollutant Loading from Industrial Activities in an Urban Watershed

In urbanized watersheds, storm water discharges from industrial facilities potentially major but poorly understood source of pollutants. Estimates of pollutant load from industrial facilities may vary with many factors, but must be refined if watershed based regulations, such as the Total Maximum Daily Load (TMDL) program, are to be implemented fairly and effectively. Storm water runoff from industrial facilities under the NPDES program are regulated through a general permit, which among requires facility operators to implement a limited monitoring program. This research used monitoring data reported by industrial facilities to estimate the contribution of pollutant load into the upper San Gabriel watershed in the Los Angeles region. First, we attempted to control for data variability caused by factors such as facility size, impervious portion of surface area, rainfall intensity, and seasonal antecedent rainfall, but no relationships were identified. Next, we estimated total loads from industrial facilities in the watershed. Industrial facilities constituted less than one percent of the watershed area, but contributed between 10% and 70% of total copper load and between 15% and 60% of total zinc load. Load estimates were strongly affected by variability in concentration data and estimates of the number of non-compliant facilities, but less strongly by spatial variability in rainfall.