Stream restoration strategies often designed to perform in baseflow may be ineffective in stormflow regarding nonpoint pollution abatement

Abstract

There is a significant need for studies focusing on restored streams under stormflow conditions in tropical areas to better understand whether stream restoration strategies have positive, neutral, or negative impacts on water quality as these hydrological regimes are often overlooked. Here, we evaluated the effects of structural in-situ restoration interventions (cross-vanes and riffles) on the water quality of an urban tropical stream (Southeastern Brazil) during rainfall events. Mass-volume [M(V)] curves and event mean concentrations (EMCs) of suspended solids and dissolved nutrients were compared between pre- and post-restoration periods. There were evidences of nonpoint source pollution in the studied stream regarding all the assessed water parameters in most of the monitored rainfall events. A substantial first flush of suspended solids was observed pre-restoration, a pattern not seen post-restoration likely due to hydromorphological changes in the stream channel. The EMCs measured post-restoration were either higher or similar to those recorded pre-restoration. While such interventions seemed to prevent the occurrence of the first flush for suspended solids, the added structures did not significantly reduce the overall downstream export of sediments and nutrients as evidenced by the EMCs. Therefore, it is essential to manage these structures specifically for solids removal and to align their functioning with processes that target the removal of specific nutrients. Restoration strategies should extend beyond the stream itself and encompass the entire watershed to achieve significant improvements in water quality. These insights are essential for guiding future tropical stream restoration projects.