Reach-scale modeling of transport and reaction due to aerobic respiration in streams
Abstract
Paired tracer injection studies, such as a conservative solute and reactive tracer, provide simultaneous information on transport and reactivity in freshwater ecosystems . Tracer experiments are commonly combined with a reactive transport model to extend data observations and to parameterize transport and reactivity. For example, the hydrometabolic tracer resazurin-resorufin (Raz-Rru) has become a common tool in recent years to estimate aerobic metabolic activity, as quantified by the first-order transformation of Raz to Rru. Many present approaches either provide an integrated measure of metabolic activity across all reactive zones including the water column, benthos, and hyporheic zones, or only assume reactivity within the hyporheic zone. Recent advances in modeling theory more accurately characterize the transport and retention of solutes, notably by allowing for a wide range of residence times within stream transient storage areas, but require solution of a large system of equations. A tractable model framework that can incorporate both long residence times and allow for zone-specific estimation of metabolic rates is needed.
We present a mobile-immobile model framework, designed to model parent-daughter or cascade reactions which occur at different rates in the mobile (e.g. combined surface and benthic) and any number of immobile zones (e.g. hyporheic). The model incorporates the multiple reaction pathways of Raz (i.e., via aerobic metabolic activity and abiotic decay) and Rru (i.e., via Raz transformation and abiotic decay) to quantify the Raz-Rru transformation rate. The amount of time spent in each zone determines how much reaction occurs and transformation of the parent species creates a mass forcing of the daughter species. The novelty of the framework is in its ability to accurately describe this forcing by accounting for the residence time distribution in each zone. It requires numerical solution of a single equation, making it readily transferrable to other modeling frameworks (e.g. continuous time random walk). We use this model to interpret tracer breakthrough of Raz and Rru in field experiments. We demonstrate the improved capability and additional insight gained by using this framework to determine zone-specific stream metabolic activity, as compared to an integrated estimate.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.H23K2083S
- Keywords:
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- 0408 Benthic processes;
- BIOGEOSCIENCESDE: 0414 Biogeochemical cycles;
- processes;
- and modeling;
- BIOGEOSCIENCESDE: 1806 Chemistry of fresh water;
- HYDROLOGYDE: 1830 Groundwater/surface water interaction;
- HYDROLOGY