In this paper, we explore the use of unsteady transit time distribution (TTD) theory to model pollutant removal in biofilters, a popular form of nature-based or “green” stormwater infrastructure (GSI). 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 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 with or without first-order decay. The solution is calibrated and validated with breakthrough data from 17 simulated storm events (+/- bromide as a conservative tracer) at a field-scale biofilter test facility in Southern California. 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 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 treatment credit attributable to GSI, are inexorably linked to the age distribution of water stored and released by these systems.