RESAZURIN AS A “SMART” TRACER FOR INVESTIGATING HYPORHEIC BIOGEOCHEMICAL PROCESSES

Use of resazurin (Raz) as a tracer in streams is encouraging because it may elucidate the relationship between transient storage and many biogeochemical processes. Raz is useful because it irreversibly transforms to resorufin (Rru) in the presence of aerobic bacteria. The region of stream-groundwater exchange, or the hyporheic zone (HZ), represents a key transient storage zone and known biogeochemical “hot spot.” Important questions regarding the use of the Raz tracer for HZ investigations are: How do Raz transformation dynamics seen at the reach scale relate to transformation dynamics in the HZ?, and Does Raz transformation correlate with HZ nutrient and chemical conditions? We used a whole-stream steady-state Raz and conservative tracer (Cl^-) addition to investigate the spatial and temporal hydraulic and physiochemical conditions controlling the Raz-Rru and nutrient dynamics in the HZ of an agricultural stream. We measured solute concentrations (Raz, Rru, NO₃^-, NH₃, DOC, DO, SRP, Cl^-), and hydraulic transport parameters (head, flow rates, flowpaths, and residence times) across the reach and along HZ flowpaths of a highly characterized gravel bar well network. To date this assessment of Raz presents the largest HZ flowpath lengths and time scales investigated as well as the most fully characterized water. HZ exchange was observed across the entire gravel bar (all 12 wells) with flowpath lengths ≤ 6.1 m and corresponding median residence times ≤ 28.4 h. Along sampled flowpaths (head to tail) and residence times (short to long), declines were seen in Raz (from 11.86 to 0.27 ug L^-1), Rru (from 3.02 to 0.01 ug L^-1), NO₃^- (from 0.53 to 0.31 mg-N L^-1), DO (from 9.77 to 0.66 mg-O L^-1) and DOC (from 1.94 to 0.76 mg-C L^-1). HZ residence time positively correlated with Raz and Rru loss, DO deficit, NO₃^- removal, and DOC depletion. Raz transformation and decay occurred at much greater rates in the HZ than it did in the surface water (i.e., across the reach). These results are consistent with previous Raz experiments and indicate that HZ Raz-Rru dynamics can be related to complex biogeochemical processes (e.g., denitrification). However, the observed net loss of Raz and Rru mass in this HZ indicates that Raz may not be an ideal tracer for studying HZ processes that operate at large time scales.