Limitations of the Stream Tracer Approach for Hyporheic Investigations

Stream tracer studies and subsequent analyses with transient storage models have long been a mainstay of hyporheic investigations. These stream tracer studies are appealing. They are easier than methods that rely on direct observations from wells and appear to provide estimates of the size of the hyporheic zone. Stream tracer methods have many limitations, however, such as the problem known as the “window of detection”. The "window of detection" problem is revisited by comparing results from tracer-based methods with well-network based methods. The two methods yield disparate results. Observations from well networks show that the hyporheic zone is characterized by long residence time exchange, with median residence times exceeding 10s of hours. In contrast, tracer-based studies are only sensitive to very short residence time exchange flows, with the average of reported transient storage residence times from 101 published studies of only 20 minutes. Clearly, tracer-based methods fail to characterize portions of the residence time distribution important to many biogeochemical processes that occur at long residence times (e.g., denitrification). Stream tracer studies can be improved by (1) conducting mass balances on recovered tracer mass and (2) using well networks to document the spatial extent of the hyporheic zone actually measured in any given tracer study.