| Ever-increasing urban development in southern California coastal watersheds has resulted in significant impacts on their water quality. However, we currently have no basis for differentiating water quality problems from natural variability. Observing high levels of constituents that occur naturally in water does not automatically indicate the water is polluted, since the constituents might have high natural background levels. This study investigated natural background water quality in streams from natural coastal watersheds of southern California and environmental factors to control the natural background water quality for dry weather and wet weather. Twenty-one sites were selected for inclusion in the study. They are located across six counties and 11 watersheds. Data were collected from each of the selected sampling sites during both dry weather and wet weather. Water samples were collected and analyzed for pH, TDS, TSS, hardness, TOC and DOC, nitrate, nitrite, ammonium, TKN, TP, OP, and total recoverable metals (As, Cd, Cr, Cu, Fe, Pb, Ni, Ni, Se, and Zn). The results of this study yielded important conclusions: (1) Concentrations in natural catchments are typically between one to two orders of magnitude lower than in developed watersheds, (2) Wet-weather fluxes of nitrate+nitrite, TKN, and TSS in natural catchments are not significantly different from those in developed catchments, (3) Differences between natural and developed catchments are greater in dry weather than in wet weather, (4) Dry-weather loading can be a substantial portion of total annual load in natural catchments, (5) Concentration and load peak later in a storm in natural catchments than in developed catchments, and concentrations and loads spread out widely over the course of a storm, (6) Metal concentrations in natural catchments are below existing water quality standards in both dry and wet weather, with the exception of copper in wet weather, (7) Concentrations of both wet- and dry-weather TP and dry-TN in natural catchments are similar to or lower than the EPA proposed nutrient criteria with wet-weather TN concentration is three-fold higher than the criteria, (8) Catchments underlain by sedimentary rock generally produce higher constituent concentrations than those underlain by igneous rock, (9) This study produced regionally applicable flux estimates for natural catchments during both storm and non-storm conditions. |
