Quantifying Longitudinal Variation in Fluvial Morphology, Metals, and Nutrients of Los Peñasquitos Creek, San Diego County, CA

Abstract:

In highly urbanized coastal regions, such as Southern California, rare open spaces are vital for providing ecosystem services. Rivers in semi-arid climates are directly influenced by local geographic and hydrologic conditions and impacted by modifications to hydrology via urbanization. Changes in flow volume and quality influence erosion, morphology, the ability to sustain habitats, and overall watershed health. Los Peñasquitos Creek, one of few perennial freshwater streams in San Diego County, lies within a coastal watershed and discharges into an estuary. To plan for future urban and climate change impacts, it is important to understand the current conditions of the watershed. In this study, we applied field surveying and sampling, and laboratory methods to quantify a) river characteristics, b) grain size distribution, total metal concentration, and organic matter (OM) in sediment and c) phosphate concentration in water samples. The depth, velocity, width, and discharge data differentiate the river into three distinct reaches (upper, middle, lower). While grain size decreases downstream (D50s:45mm-0.2mm), metal concentrations vary longitudinally. Copper (88% > 32 PPM) and Mercury (100% > 0.015 PPM) samples exceed Effects Range Low (ERL) screening concentrations. At over half of the sites, Arsenic and Zinc also exceed ERLs. Lead, Chromium, and Cadmium were below screening levels, but could pose a risk during a drought. OM is positively correlated with Zinc (R2:0.72, R2:0.77) and Lead (R2:0.58, R2:0.70) in and out of the channel. Phosphate concentrations at several sites near development were at or above screening values, possibly indicating an anthropogenic influence. Reach trends are likely influenced by local geomorphology and vegetation cover, which also impact water quality. Data-driven efforts like this will help Los Peñasquitos Creek management focus monitoring efforts. This study emphasizes the importance of analyzing the local and geomorphic influence on geochemical variability to improve small coastal watershed quality.