A Study of Chloride and Rhodamine Dye Tracer Movement through an Urban Stream and Raingarden

The University of Georgia (UGA) main campus is dominated by impervious surfaces that produce stormwater runoff during rain events. Campus runoff drains directly into local streams, primarily through storm drains. The effects of organic and inorganic contamination on local streams requires mitigation and management practices, which are regulated by the Athens-Clark County MS4 program (ACC 2015). The primary objective of this study is to observe and better understand the hydrology of stormwater within Lily Branch, one of three campus watersheds (Fig. 1). A mixture of sodium chloride (NaCl) and Rhodamine were used to characterize velocities and travel times through a section of Lily Branch. NaCl was selected because it is conservative, inexpensive, easily applied, readily detectable using water-quality meters (i.e., specific conductance), and the chloride-ion concentration can determined using subsequent chemical analysis by ion chromatography (NMSU 2006). Rhodamine was paired with NaCl to provide a visual signature that is helpful for collecting samples. Rhodamine concentrations can also be determined using fluorometric techniques (Smart and Smith 1976). A secondary objective is to evaluate stormwater movement through a raingarden that receives water from an impervious parking lot and deck, which then flows into Lily Branch. NaCl is also a potential contaminant that results from road-salt application during icy weather. As such, the background and peak concentrations in local streams are of concern, as is the ability of raingardens to mitigate the adverse impacts of watershed salt loading. Data collected from these natural and introduced tracer tests provide insight into the pathways of NaCl de-icers and how contaminants travel through an urban stream and raingarden. Figure 1: UGA campus watershed map.