Evaluation, Design, and Construction of Amended Reactive Caps to Restore Onondaga Lake, Syracuse, New York, USA

摘要

Onondaga Lake is a 12-square-kilometer urban lake located in Syracuse, New York, USA. The legacy of industrialization and municipal development since the late nineteenth century resulted in impaired water quality and contamination of lake sediments. A comprehensive lake restoration plan was developed following two decades of intensive multi-disciplinary technical studies. The restoration plan includes removal of approximately 1.5 million cubic meters of lake sediments (completed November 2014, one year ahead of schedule); transport of the dredge slurry through a 6.5-kilometer-long, double-walled pipeline; and dewatering in geotextile tubes strategically placed in a dedicated 20-hectare upland confined facility. More than 160 hectares of the lake received an engineered cap to isolate the remaining contamination after dredging and provide a habitat restoration surface. The deepest portion of the lake will be monitored for ongoing natural recovery. The restoration and design process involved close coordination and discussions with multiple regulatory agencies and stakeholders and was supported by several technical studies, including: 1) development of sediment and fish tissue remedial goals; 2) detailed evaluations of capping, dredging and upland containment, natural recovery, and nitrate addition to the lower part of the lake water column (to limit production of methyl mercury); and 3) integration of habitat restoration into the lake-wide cleanup effort. As one of the largest sediment capping projects constructed to date, the multi-function cap design incorporated several innovative components, including site-specific biodegradation and reactive media (activated carbon [AC] for contaminant sequestration and siderite amendment for pH control) to ensure remedial performance requirements for long-term chemical isolation and habitat restoration were met. The innovative capping concepts were developed through multiple years of field investigations, laboratory and bench-scale studies, and field pilot testing. In addition, state-of-the-science contaminant migration and cap effectiveness modeling studies were conducted to evaluate the required layering of the cap and to optimize the amendment media and required dosage for the various layers. Successful integration and regulatory acceptance of these unique design features provided opportunities for significant optimization of raw material needs (cap thickness and amendment dose). The innovative study methods also resulted in streamlined and efficient construction, while retaining environmental effectiveness. Construction began with a field pilot project in 2011, followed by full-scale field implementation in 2012; sediment removal was completed in 2014 and lake capping was completed in 2016. Details of the study methodology, analysis of amendment and cap effectiveness (laboratory testing and modeling), and construction verification methods and monitoring results are presented in this paper.