Predicting Solute Transport Through Green Stormwater Infrastructure With Unsteady Transit Time Distribution Theory

Abstract

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). 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.

Limited Bacterial Removal in Full-Scale Stormwater Biofilters as Evidenced by Community Sequencing Analysis

“…(0.78–0.90 log10 reduction) and Clostridium perfringens (3.20 log10 reduction) . The performance of stormwater treatment systems is likely site-specific, owing to variations in influential factors including physiochemical characteristics, the selection of plants, incorporation of submerged zones, amendments to the medium, and operations under wet or dry conditions. , Also, our experiments were conducted at the field scale under realistic (transient) flow conditions and thus may be more representative of the treatment efficiencies likely to be achieved in practice. Indeed, removal rates observed in this study were more in line with field-based, rather than laboratory-based, studies in the literature .…”

“…The mixing tank was thoroughly cleaned and disinfected (using household bleach) between experiments. Detailed information on the experimental setup can be found in the work by Parker et al…”

“…The mixing tank was thoroughly cleaned and disinfected (using household bleach) between experiments. Detailed information on the experimental setup can be found in the work by Parker et al 25 Stormwater or mixed influents were applied to the four biofilters under transient flow conditions during three experimental phases in May−June, 2019 (Figure S3). In the first phase, a simulated storm event was conducted, whereby each of the four biofilters was individually infiltrated with stormwater.…”

“…Initial hydrological experiments using tap water and bromide indicated that the biofilter would outflow for around 2−3 h under transient flow conditions. 25 Thus, each transient flow experiment was planned to last approximately 2−3 h, and 6 to 10 flow-weighted composite effluent samples were collected during this period for each experiment, alongside influent stormwater, untreated sewage, or mixed influent samples. The composite stormwater effluent samples were collected using a peristaltic pump drawing from a port located at the discharge of the underdrain inside each biofilter.…”

“…Detailed information on the experimental set-up can be found in Parker et al 25 Stormwater or mixed influent were applied to the 4 biofilters under transient flow conditions during 3 experimental phases in May-June, 2019 (Fig. S3).…”