Abstract
AbstractIn this study, we explore the use of unsteady transit time distribution (TTD) theory to model solute transport in biofilters, a popular form of nature‐based or “green” storm water infrastructure (GSI). TTD theory has the potential to address many unresolved challenges associated with predicting pollutant fate and transport through these systems, including unsteadiness in the water balance (time‐varying inflows, outflows, and storage), unsteadiness in pollutant loading, time‐dependent reactions, and scale‐up to GSI networks and urban catchments. From a solution to the unsteady age conservation equation under uniform sampling, we derive an explicit expression for solute breakthrough during and after one or more storm events. The solution is calibrated and validated with breakthrough data from 17 simulated storms at a field‐scale biofilter test facility in Southern California, using bromide as a conservative tracer. TTD theory closely reproduces bromide breakthrough concentrations, provided that lateral exchange with the surrounding soil is accounted for. At any given time, according to theory, more than half of the water in storage is from the most recent storm, while the rest is a mixture of penultimate and earlier storms. Thus, key management endpoints, such as the pollutant treatment credit attributable to GSI, are likely to depend on the evolving age distribution of water stored and released by these systems.
Highlights
In this study, we explore the use of unsteady transit time distribution (TTD) theory to model solute transport in biofilters, a popular form of nature-based or “green” storm water infrastructure (GSI)
The fraction of inflow volume recovered at the outflow tank is inversely correlated with antecedent dry period (R2 = 0.82, Figure S6), consistent with the hypothesis that at least some of the unrecovered water goes to storage
Extrapolating the fractional water recovery back to an antecedent dry period of zero hours we estimate that, in both 2018 and 2019, approximately α = 46% of the water added to the biofilter is routed to the outflow tank while 1 − α = 54% is lost to lateral exfiltration (Text S7)
Summary
Green storm water infrastructure (GSI) provides many benefits beyond the retention and detention of urban storm water flows (Walsh et al, 2005, 2012), including improved water quality, urban heat mitigation, habitat creation in support of urban biodiversity, carbon sequestration, recreational opportunities, and mental health (BenDor et al, 2018; Engemann et al, 2019; Grant et al, 2012, 2013; Grebel et al, 2013; Keeler et al, 2019; National Academy of Sciences, Engineering, and Medicine, 2016; Raymond et al, 2017; Walsh et al, 2016). These vertically oriented systems filter water through planted soil or sand-based media and are integrated into the urban landscape over a range of scales (Grant et al, 2013; Roy-Poirier et al, 2010; Wong, 2006). Their possible features include: (1) a ponding zone that retains water prior to infiltration; (2) biological components including upright vegetation and naturally colonizing soil invertebrates and microorganisms; (3) engineered filter media (sand, sandy loam, or loamy sand with or without media amendments [e.g., biochar; Boehm et al, 2020; Mohanty & Boehm, 2014]); (4) a coarse sand transition layer; (5) a drainage layer consisting of coarse sand or fine gravel which can be lined or unlined and with or without an underdrain; (6) an overflow structure that releases excess storm water; and (7) a raised outlet to facilitate the formation of a permanently wet “submerged zone” 2009; H. Kim et al, 2003; Payne et al, 2015; Rippy, 2015)
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