Evaluation of Water Isotopes and Nutrient Loading in Urban Runoff at a Southern California Residential Site

This study examines urban runoff at a residential site in southern California, which receives imported water from northern California that is isotopically distinct from locally recharged water sources. Stable water isotopes have been used in previous studies to determine percentages of imported water in local urban streams in southern California, but previous studies used pure imported tap water as the endmember mixing source. By sampling urban runoff in street gutters derived from lawn watering and comparing to tap water, this study provides a better understanding of water isotope signatures in imported water as a mixing source in urban streams. Stable water isotopes show modest-moderate evaporation enrichment between irrigation source water and lawn runoff, suggesting tap water endmember may be used with modest error to assess mixing between imported water runoff and local groundwater baseflow in urban streams. Runoff from urban sources often contribute significant amounts of nutrients that can negatively impact waterways. Previous studies in southern California are conflicting as to the source of nutrients, whether they are geologic or anthropogenic. For this study, the site is fertilized and then monitored weekly for one year to measure nutrient content in runoff without additional fertilization. The study sheds light on sources of nutrients in local urban streams and provides constrained information on lawn runoff as a factor in elevated concentrations of nutrients.