Formatted Title
Remediation of a Former Wastewater Lagoon with a Forested Wetland
Background/Objectives
The Site includes three lagoons that originated as borrow pits during construction of a flood control levee. From approximately the 1950s through the 1970s the lagoons were utilized as part of a wastewater treatment system for a nearby petroleum facility. Two of the lagoons received the majority of the wastewater and the third lagoon had less flow-through.
The three lagoons were investigated to characterize the extent, distribution, and potential for migration of light non-aqueous phase liquid (LNAPL) impacts in the surface water sediments. The investigation concluded that the lagoons were constructed in a competent clay with no hydraulic connectivity to the groundwater. The only connectivity to the river is during river flood events. The floods are depositional, depositing clean sediment in the lagoons at the top of the sediment column. Consistent with the operation of the former wastewater treatment system, LNAPL was observed in sediment for the first two lagoons, but no LNAPL greater than a sheen was observed in the third lagoon. Similarly, ebullition-facilitated LNAPL flux (EFT) was consistently observed in the first two lagoons, but only one EFT event was observed in the third lagoon during the investigation.
The third lagoon, with comparatively minimal LNAPL impacts is the focus of this presentation. The remedy feasibility evaluation indicated that a nature-based remedy would be feasible and would appropriately contain any LNAPL impacts present and prevent discharge of surface water or sediment with LNAPL sheens to the river during flood events.
Approach/Activities
The LNAPL impacts in the lagoon will be addressed by dewatering the lagoon and establishing a vegetated soil cover that mimics the surrounding forested wetlands. The soil barrier will prevent direct contact with impacted sediment. It will also prevent erosion and lateral migration of the sediment during rainfall and flooding events. The barrier will be densely vegetated utilizing native wetland floodplain plants capable of surviving periodic inundation. The native plants possess extensive root systems compared to non-native plants. The root systems will bind to the soil serving as an additional measure to prevent erosion of the soil cover.
Results/Lessons Learned
The forested wetland remedy will control all potential exposure routes to potential human or ecological receptors. It will achieve this goal in a manner that is more sustainable than alternate engineered remedies such as dredging or subaqueous capping. The remedy will consume fewer resources, produce fewer wastes, and be less disruptive to the surrounding community. It will also improve the habitat in the vicinity of the lagoon. Two invasive species currently reside in the lagoons and will be removed, with native species relocated to nearby ponds. The forested wetland will then create additional habitat for an endangered species, the decurrant false aster.
Construction is scheduled to begin in 2024.