Hydrochemical Analysis of Source Mixing and Stream-Aquifer Interactions in Urban Stream

Many communities have increasingly used recycled wastewaters for irrigation in urban landscapes due to drought and expanding development in southern California. This contributes to dry weather runoff in local streams. Our study in Malibu Creek Watershed looks at the possible presence of recycled wastewater runoff and associated solutes at a control tributary by analyzing urban runoff, surface water, groundwater, and hyporheic zone water. Samples were analyzed for stable isotopes of nitrate, oxygen, and hydrogen; nutrients; general minerals; and trace elements. The investigation applies parameter mixing analyses with multiple tracers to supplement the understanding of how nutrient and trace element geochemical processes operate in the urban tributary. Analysis of isotopic data suggests perennial input of urban runoff and mixing with groundwater baseflow in the tributary creek. The study demonstrates distinct redox variability between concrete-lined and unlined parts of the tributary channel. The findings of the study demonstrate diminished capacity of lined channels to remove undesirable solutes from runoff of recycled wastewater.