Predicting Unsteady Pollutant Removal in Green Stormwater Infrastructure with Transit Time Distribution Theory

Abstract:

We explore the use of unsteady transit time distribution (TTD) theory to model pollutant removal in biofilters, a popular form of nature-based, “green” stormwater control measures. TTD theory elegantly addresses many unresolved challenges associated with predicting pollutant fate and transport in 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 treatment networks and catchments. From a solution to the unsteady age conservation equation under uniform sampling, we derive an explicit expression for solute breakthrough with or without first-order decay. The solution is calibrated and validated with breakthrough data from conservative tracer experiments carried out at a field-scale biofilter test facility in Southern California. The biofilter (approximate volume 2 m³) was packed with a homogeneous mixture of loamy sand and planted with two species of a common sedge (Carex spp.). Over two summers we simulated 17 separate storm events (+/- bromide as a conservative tracer). TTD theory closely reproduces measured bromide breakthrough concentrations, but only when subsurface lateral exchange is taken into account (by letting the size of the biofilter be a free variable). The model predicts that, at any given time, 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. Because pollutants can linger in a biofilter over multiple storms, measurements of their treatment performance will likely depend on the timescale over which pollutant removal is observed.