Water Quality Assessment of the Los Angeles River Watershed, California, USA in Wet and Dry Weather Periods

River runoff in semi-arid urban watersheds may consist entirely of treated wastewater effluent (dry-weather) and/or urban nonpoint source runoff (wet-weather), which can be a source of nutrients, bacteria, and metals to receiving waters. The purpose of this study is to identify sources of potential pollutants and to characterize urban water quality along the Los Angeles (LA) River from its head to the mouth during dry and wet weather seasons. The LA River is an effluent-dominated water body during the dry season. The three wastewater treatment plants (WWTP) including the Tillman, Burbank, and Glendale waste water treatment plants discharge the majority of the volume flowing in the LA River during the dry and wet period. The WWTPs discharge chemicals such as chloride, nitrate, and sulfate to the river. The metals are more likely attributed to street runoff. In both cases, the contamination is dispersed through various water channels that carry semi treated effluent from various sources ending up into the ocean. To understand seasonal and spatial pattern of these contaminants, the water samples for chemical and physical analysis were collected along the LA River to assess the recent pollutant deposition processes in response to extensive human activity in wet and dry seasons. The general trend of the results shows that the concentrations of anions are higher during the dry season in compare to the wet season. Anion concentration values (ppm) in the dry season ranging from 5.5-16,027 (chloride), 0-1.0 (fluoride), 0-21(nitrate), 0-1.6 (phosphate), and 13.3-2,312 (sulfate); whereas the values (ppm) for anions in the wet season ranging from 3.4-5,860 (chloride), 0-0.66 (fluoride), 0-17 (nitrate), 0-0.67 (phosphate), 7.9- 745 (sulfate). Dry season concentrations values for trace metals were obtained with values (ppb) ranging from 0.9-10 (nickel), 0.8-62 (zinc), 1-4 (arsenic), 0-1 (lead) and 0-3 (selenium). As for the wet data for trace metals (ppb) ranging 0.001-0.008 (nickel), 0.000001-0.038 (zinc), 0.0016-0.016 (arsenic), 0.00099-0.0058 (lead), 0.000001-0.0093 (selenium). Data were used to calculate mean concentrations and loads for various sources. The most likely sources for chloride are the three WWTPs in the Sepulveda Basin, Burbank, and Glendale. Other source for higher chloride level may be the tidal effect in the region closer to the ocean. The source of nitrate loading is attributed to storm drains including lawn fertilizers, septic systems, WWTPs, and agricultural runoffs in headwater area. Comparison of wet vs. dry weather loading indicates that dry weather loading can be a significant source of metals. This study indicates that constituent loading during the dry weather period can comprise a substantial portion of the total annual load in LA River urban watersheds, such as those investigated in this study. Moreover, the risks posed by the metals were highest in the dry season due to dry season runoffs. In water-limited areas such as the Los Angeles basin, urban runoff is a water resource that could improve restricted water supplies and to enhance localized renewable groundwater resources. Thus, an assessment of this precious water resource is important for local cities and regulatory organizations.