Transport of a conservative and "smart" tracers' in a first-order creek: role of transient storage type

Authors: YAKIREVICH, ALEXANDER - Ben Gurion University Of Negev; Shelton, Daniel; HILL, ROBERT - University Of Maryland; KIEFER, LYNDA - Bluewave Microbics; STOKER, MATTHEW - Orise Fellow; BLAUSTEIN, RYAN - University Of Florida; KUZNETSOV, MIKHAIL - Ben Gurion University Of Negev; McCarty, Gregory; Pachepsky, Yakov

Submitted to: Water
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/28/2017
Publication Date: 7/2/2017
Citation: Yakirevich, A., Shelton, D.R., Hill, R., Kiefer, L., Stoker, M., Blaustein, R., Kuznetsov, M., McCarty, G.W., Pachepsky, Y.A. 2017. Transport of a conservative and "smart" tracers' in a first-order creek: role of transient storage type. Water. 9(7):485.

Interpretive Summary: Microbial quality of recreation and irrigation water sources depends on the microbial exchange between bottom sediments and the water column. Respiration of microbes is one of the most efficient measures of microbial activity in sediments. The “smart tracer” resazurin was used to characterize microbial activity in sediments based on its rate of decomposition in water. A delay in tracer decomposition was observed; this was attributed to the existence of zones in the stream where the tracer entered and exited slowly. The objective was to determine whether such slow exchange zones are in the water column or in the bottom sediment. We ran experiments on tracer release and recovery in a creek and developed a novel fate and transport model to simulate slow exchange zones both in sediment and in the water column. Modeling showed that the assumption of a slow exchange zone in sediment provided for better correspondence between simulated and measured concentrations of the smart tracer and the product of its decomposition in water. Results of this work will be useful to microbial water quality researchers in that they provide evidence of the existence of zones of slow sediment exchange with the water column that contain substantial microbial populations potentially affecting microbial water quality assessment.

Technical Abstract: Quantification of microbial fate and transport in streams has become one of most important topics in studying biogeochemical properties and behavior of stream ecosystems. Using "smart" tracer such as resazurin (Raz) allows assessment of sediment-water interactions and associated biological activity in streams. In this study we conducted tracer experiment in the perennial first-order Beaver Dam Creek Tributary in a riparian zone, Maryland, USA, by injecting conservative (Br) and reactive (Raz) tracers. Flow and transport during the experiment were simulated using the Saint–Venant equations to calculate water depth and discharge while a stream solute transport models accounted for advection-dispersion, effect lateral inflow/outflow, exchange with transient storage (TS), reactions of decay and transformation. Two approaches were compared to simulate the effect of transient storage (TS) on the transport. The first approach considered TS as composed of metabolically active (MATS) and metabolically inactive (MITS) compartments, while the second model approach accounted for the surface transient storage (STS) and hyporheic transient storage (HTS). Reach-specific model parameters were estimated by solving the inverse problem using observed time series of concentrations at two weir stations. Results show that second approach led to more accurate simulation of Br transport. Both approaches performed similarly in simulations of Raz and its metabolic product resofurin (Rru) fate and transport in the case when decay and transformation rates of these tracers in the stream water column, MITS and STS were adopted from literature. However, breakthrough tails in this case were not fitted well. Simulations with the second approach in a run when reaction parameters were calibrated for all model compartments provided good fit for the whole observed BTCs, however, the rates of Raz to Rru transformation remain unreliable due to large uncertainty. Modeling fate and transport of the resazurin-resorufin system in creeks can provide additional parameters to characterize habitats of microbial water quality indicator organisms.