Previous studies have suggested reaction with Monterey/Modelo geologic strata by percolating groundwater is the most important source of orthophosphate during dry weather flow conditions (2 weeks after any rain event) in Upper Malibu Creek Watershed, Southern California. Our studies have reexamined this model through extensive sampling of stream water, shallow groundwater, dry weather urban runoff, and hyporheic zone water. Water samples were analyzed for nutrients, standard inorganic constituents, and stables isotopes of oxygen, hydrogen, sulfur, and nitrogen. Our findings indicate that groundwater sourced directly from Monterey/Modelo strata contains less than 0.3 mg/L dissolved orthophosphate in more than 95% of groundwater samples, while urban runoff sourced from both tap and reclaimed water sources contains 0.2 to 9 mg/L dissolved orthophosphate. Mass balance analysis demonstrably points to the fraction of urban runoff (10 to 40%) to be an additive source of orthophosphate that at least doubles the concentration in the developed parts of the watershed. Orthophosphate loading from dry weather urban runoff blends with baseflow and undergoes complex processes in stream riffles and pools, and especially within the hyporheic zone, to both remove and release orthophosphate into pore water and stream water. Our studies conclude that hyporheic zone processes and urban runoff are more important sources of orthophosphate than loading from baseflow from the Monterey/Modelo strata proper.
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