UC Santa Barbara

This study utilizes high temporal resolution 222Rn activity data to create a chemical hydrograph in order to better quantify the influx of groundwater, soil water, and rain water into a stream during precipitation events. Three tests were conducted during separate storm events within the area of Montecito, CA along San Ysidro Creek throughout the 2016 and 2017 Water Years. These tests analyzed 222Rn activities, δ18O and δ2H ratios and silica concentrations with the goal of testing whether 222Rn activities measured through the use of the RAD7 system would provide similar hydrograph data to the other two methods.

222Rn activities showed trends in a higher temporal resolution than the other methods used despite low activities caused by rapid diffusion of 222Rn gas from the stream into the atmosphere. 222Rn data, dissolved silica data, and stable isotope data all showed steady increases in groundwater during the first test (Mar. 5, 2016). No distinguishable trends were noticed in the second test (Apr. 10, 2016). Strong trends were shared between the 222Rn data, dissolved silica data, and stable isotope data during the third test (Mar. 21, 2017) which was conducted with higher resolution discharge data for a significantly longer duration. This shows a positive correlation between the 222Rn data and the other tracers used in this study. Results suggest that with additional testing restrictions: information regarding the definition of end members within the stream or a more stable study site which is a location of groundwater influx, 222Rn activities would work as a viable proxy for pre-event water during a storm. 222Rn would then provide a more cost effective and less time-consuming means of creating storm chemical hydrographs without having to collect, transport, and analyze multiple water samples.