Stormwater volume reduction and water quality improvement by bioretention: Potentials and challenges for water security in a subtropical catchment.

Climate change scenarios tend to intensify extreme rainfall events and drought in Brazil threatening urban water security. Low Impact Development (LID) practices are decentralized alternatives for flood mitigation and prevention. Recently, their potential has increasingly been studied in terms of stormwater harvesting. However, there is still a lack of knowledge about their potentialities in subtropical climate regions. Therefore, this study evaluated the behavior of a bioretention cell in a Brazilian city, during the dry period, which is critical in terms of pollutant accumulation and water availability. In addition to the runoff reduction and pollutant removal efficiency, this paper analyzed the potential for water reuse in terms of the stored volume and water quality guidelines. The results obtained show an average runoff retention efficiency of 70%. Considering only the water availability aspects, the potential stored runoff could be reused for non-potable purposes, reducing the water demand in the catchment by at least half during the dry season. On the other hand, the bioretention presented two different conditions for pollutant removal: Condition A - the concentration values are within the recommended limits for water reuse. The parameters found in this condition were NO3, NO2, Zn, Mn, Cu, Cr; Condition B - the pollutant concentrations are above the guideline limits for water reuse and cannot be directly used for different purposes. The parameters found in this condition were Fe, Pb, Ni, Cd and color. Considering water reuse, an additional treatment is required for parameters in this second condition. Further studies should evaluate the design aspects that can allow collection of LIDs effluent, additional treatment if necessary, and reuse in the catchment.

Learning from the operation, pathology and maintenance of a bioretention system to optimize urban drainage practices.

LID practices for runoff control are increasingly being used as an integrated solution in urban drainage, helping to achieve hydrological balance close to the pre-urbanized period and decrease the diffuse pollution transported to urban rivers. Regarding bioretention, there is already broad knowledge about the detention of peak flows and their treatment capacity for many pollutants. However, there are still few field studies in microdrainage scale, which analyze the actual operation of these devices and raise common problems found, especially in subtropical climate. Therefore, this study aims to show what was learnt from the field operation of a bioretention cell on a micro-drainage scale, located in an urban catchment of a Brazilian city, suggesting maintenance actions as adaptations to the pathologies found. Five rainy events were monitored during the dry season, in order to carry out a preliminary analysis for critical conditions in terms of maintenance and diffuse pollution accumulation. From the first water balance results, low storage and low infiltration capacity of the soil were found as main pathologies. They led to a great amount of runoff passing directly through the cell surface and at a high velocity, resulting in soil erosion and low water retention efficiency. To overcome these problems, some structural adaptations were made over the cell, highlighting the semi-direct injection. The maintenance and adaptations proposed were suitable to avoid the erosion process, increasing the storage and improving the water retention efficiency in bioretention. They should be considered from the very initial stages, to using sites with low permeability.