Chemical Signals of Critical Zone Processing: Quantification of Water and Sediment Sources During Individual Storm Events in the Christina River Critical Zone Observatory

Abstract:

Chemical signals of water and materials in catchment exports have long been studied as proxies for within-watershed processing. In the Christina River Critical Zone Observatory, we use the chemistry of water, in particular the oxygen-18 and chloride concentrations, and hydrograph separation to evaluate the contributions of different water sources to the stream discharge during a series of five storm events in 2011 and 2012. These events varied in magnitude, from 44 to 168 mm total precipitation, and precipitation chemistry, with δ18O values ranging from -5.38 to -11.06 ‰. The contribution of old water during the storm peak, determined by isotope hydrograph separation, varied from 0% in a spring storm of annual magnitude to 76% during Hurricane Sandy. Soil moisture data, available for all but one of our storms, indicates higher old water contribution at peak flow when the catchment has higher antecedent soil moisture.

Understanding differences in water sourcing to the stream during different events provides a basis on which we analyze the movement of critical zone processing with regard to erosion and the source of exported sediment. For example, sediment fingerprinting with fallout radioisotopes indicated variation in sediment source between events. For example, suspended sediment samples taken during Hurricane Irene (28 August 2011) contained between 0 and 11.4 Bq/kg cesium-137 (¹³⁷Cs) and 175 – 698 Bq/kg of beryllium-7 (⁷Be), indicating some level of recent surface erosion. Suspended sediment samples taken during Hurricane Sandy (29 October 2012) did not contain measureable activities of either ¹³⁷Cs or ⁷Be.