Contaminant Flushing From An Urban Fringe Watershed: Insight Into Hydrologic and Soil Dynamics During the Wet Season

Much attention has been given recently to alterations of the hydrological cycle due to anthropogenic affects and the coupled impact on watershed geochemical characteristics. Of particular concern has been the study of chemical constituent mobilization due to hydrologic processes and the resultant impact on stream water quality. However, our understanding of constituent flushing in response to hydrological conditions remains rather vague, particularly at the timescale of a single hydrological event. This study builds on previous work by focusing on the timescale of individual hydrological events to better understand the temporal behavior of chemical constituents in the undeveloped, upper reach of the Arroyo Seco watershed. This watershed, located on the northeastern (windward) edge of the Los Angeles basin in the San Gabriel Mountains, has high rates of deposition originating from the Los Angeles metropolitan region. Utilizing high-frequency measurements of stream water during rainfall events, this study aims to obtain a deeper understanding of the role of hydrological events in the stream water chemical dynamics. In addition, a soil survey was conducted in the riparian zone of the Arroyo Seco basin to evaluate geochemical profiles. Stream water chemical composition in the upper Arroyo Seco watershed exhibited high variability on the hydrological event time scale. Of the four solutes measured, nitrate was the only constituent which behaved consistently throughout the study period, exhibiting hydrologically enhanced behavior during every storm event. Fluoride, chloride, and sulfate exhibited hydrologically enhanced behavior for the first few storm events of the 2009 water year, then switched to dilution behavior late in the rain season. Results suggest these constituents collect extensively in the watershed during the dry season. A first flush mechanism then mobilizes the constituents from the atmosphere and basin surface and transports them to surface waters during storm events. Analyzed soil samples from the riparian zone confirm the abundance of nitrate on the soil surface that serves as a source for stream water nitrate. A stream concentration-discharge hyperbolic model that was developed for the Arroyo Seco on the seasonal time scale was used to model sulfate and nitrate concentrations during the 2009 water year. The poor performance of the model suggests that stream chemistry in the Arroyo Seco is dependent on not just discharge, but magnitude and intensity of hydrological event as well as length of the antecedent dry period, among other factors. A further understanding of these factors will help lead to our ultimate goal of improving empirical models for prediction of downstream chemical loads from the upper Arroyo Seco.