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Abstract Resazurin (Raz) is a phenoxazine dye that can be reduced irreversibly to the daughter compound resorufin (Rru) by aerobic respiration. Previous hydrologic studies using the Raz‐Rru reactive tracer system to quantify water‐sediment interactions and metabolic activity have reported that dilution‐corrected masses of Raz and Rru recovered are smaller than the mass of Raz injected. This lack of mass balance closure has been reported as a nonideality of this tracer system and, to date, it is still unclear what drives incomplete recovery. We used controlled laboratory experiments varying the initial concentrations of Raz, the duration of the experiments, and the type of microbial communities present to quantify mass balances of Raz and Rru under conditions that removed other suspected causes of incomplete recovery in field experiments, i.e., sorption to sediments and photodecay. We used the summation of Raz and Rru concentrations over time to assess mass recovery and variability and found mass recoveries in the range of 85.6–110.4%, with a maximum standard deviation of 7.5%. In three of the four experiments, no strong temporal trend in mass recovery is present. In an experiment withBacillus subtilisbacteria, lower recovery and evidence of a temporal trend in recovery only occurred after 13 hr past the complete transformation of Raz (i.e., beyond the duration of most field experiments). These results suggest that the lack of mass recovery in field studies is likely associated with physical or chemical mechanisms rather than biological interactions with the Raz‐Rru tracer system.
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Sorption and transformation of the reactive tracers resazurin and resorufin in natural river sediments
Abstract. Resazurin (Raz) and its reaction product resorufin (Rru) have increasingly been used as reactive tracers to quantify metabolic activity and hyporheic exchange in streams. Previous work has indicated that these compounds undergo sorption in stream sediments. We present laboratory experiments on Raz and Rru transport, sorption, and transformation, consisting of 4 column and 72 batch tests using 2 sediments with different physicochemical properties under neutral (pH = 7) and alkaline (pH = 9) conditions. The study aimed at identifying the key processes of reactive transport of Raz and Rru in streambed sediments and the experimental setup best suited for their determination. Data from column experiments were simulated by a travel-time-based model accounting for physical transport, equilibrium and kinetic sorption, and three first-order reactions. We derived the travel-time distributions directly from the breakthrough curve (BTC) of the conservative tracer, fluorescein, rather than from fitting an advective-dispersive transport model, and inferred from those distributions the transfer functions of Raz and Rru, which provided conclusive approximations of the measured BTCs. The most likely reactive transport parameters and their uncertainty were determined by a Markov chain–Monte Carlo approach. Sorption isotherms of both compounds were obtained from batch experiments. We found that kinetic sorption dominates sorption of both Raz and Rru, with characteristic timescales of sorption in the order of 12 to 298 min. Linear sorption models for both Raz and Rru appeared adequate for concentrations that are typically applied in field tracer tests. The proposed two-site sorption model helps to interpret transient tracer tests using the Raz–Rru system.
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- Award ID(s):
- 1301346
- PAR ID:
- 10081632
- Date Published:
- Journal Name:
- Hydrology and Earth System Sciences
- Volume:
- 18
- Issue:
- 8
- ISSN:
- 1607-7938
- Page Range / eLocation ID:
- 3151 to 3163
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/hess-18-3151-2014
