The logistics and mechanics of conducting tracer injection experiments in urban streams

Field-based environmental tracer studies are commonly used to investigate hydrological and ecological processes in flowing waters. These studies involve injecting a conservative tracer into a stream or into a near-stream well and monitoring the surface and subsurface waters at downgradient locations. Results have been used to quantify stream velocity, inflow, outflow, dispersion, and transient storage exchange processes. However, no single source provides a detailed methodology for conducting these tests in streams within urbanized watersheds. Working in urban watersheds brings with it unique problems such as private property access, vandalism, encounters with police and the lay public as well as long-term, intermittent and ephemeral hydrologic modifications. We present such a methodology based on results of 20 tests conducted in streams with urban watersheds ranging in size from 0.39 km² to 60 km² in Pennsylvania and Maryland. The tracer injection period ranged from instantaneous to 24 h with monitoring lasting from 8 h to 5 days. The methodology is demonstrated with a 5-day tracer test in which sodium bromide was injected into Dead Run, Baltimore, Maryland for 24 h.