Biomat Resilience to Desiccation and Flooding Within a Shallow, Unit Process Open Water Engineered Wetland

Projections of increased hydrological extremes due to climate change heighten the need to understand and improve the resilience of our water infrastructure. While constructed natural treatment analogs, such as raingardens, wetlands, and aquifer recharge, hold intuitive promise for variable flows, the impacts of disruption on water treatment processes and outcomes are not well understood and limit widespread adoption. To this end, we studied the impact of desiccation and flooding extremes on demonstration-scale shallow, unit process open water (UPOW) wetlands designed for water treatment. System resilience was evaluated as a function of physical characteristics, nitrate removal, photosynthetic activity, and microbial ecology. Rehydrated biomat that had been naturally desiccated re-established nitrate removal consistent with undisrupted biomat in less than a week; however, a pulse of organic carbon and nitrogen accompanied the initial rehydration phase. Conversely, sediment intrusion due to flooding had a negative impact on the biomat's photosynthetic activity and decreased nitrate attenuation rates by nearly 50%. Based upon past mechanistic inferences, attenuation potential for trace organics is anticipated to follow similar trends as nitrate removal. While the microbial community was significantly altered in both extremes, our results collectively suggest that UPOW wetlands have potential for seasonal or intermittent use due to their promise of rapid re-establishment after rehydration. Flooding extremes and associated sediment intrusion provide a greater barrier to system resilience indicating a need for proactive designs to prevent this outcome; however, residual treatment potential after disruption could provide operators with time to triage and manage the system should a flood occur again.