A Siderite-Amended Reactive Cap for High-pH Sediments of Onondaga Lake, New York

摘要

Background/Objectives. Contaminated sediments in some areas of Onondaga Lake arecharacterized by elevated porewater pH (greater than 10 and up to 12.5). The finalremedial design includes capping of approximately 170 acres of the lake bed for habitatrestoration. Siderite, a naturally occurring iron ore, was identified as the most suitable capamendment for long-term pH control and buffering. A series of laboratory and modelinginvestigations were undertaken to determine site-specific performance and effectiveness,develop spatially optimized siderite doses to meet multiple performance criteria, andassess sensitivity of cap performance to, and develop methods for evaluating, deviationsfrom the design specifications during construction. Construction of the siderite-amendedcap was initiated in 2012.Approach/Activities. Batch tests were conducted to investigate porewater pH bufferingand neutralization rates by siderite. Preliminary cap modeling using PHREEQC, withreaction rates calibrated to batch test data, indicated a relatively low siderite dose(～1 lbs/ft~2 of cap surface) would meet cap performance requirements. The low sideritedemand also pointed to a potential concern that pore-scale bypassing could impact bulkreactivity at field-scale and that the cap model may have overpredicted effectiveness.Column studies were subsequently carried out to assess whether pore-scale flow couldimpact macroscopic pH neutralization rates under cap conditions. Reactive transportmodeling was also performed to develop optimal siderite doses on an area-specific basis,considering spatial distributions of porewater pH, upwelling velocities and expectedconsolidation following cap construction. Sensitivity analyses were also conducted toevaluate the sensitivity of cap performance to uneven vertical distribution of amendmentwithin the chemical isolation (CI) layer due to segregation during emplacement. Fieldmethods qualitatively verifying the presence of siderite within the sand cap matrix duringconstruction were also developed.Results/Lessons Learned. Column tests confirmed that pore-scale effects had anegligible impact on bulk reactivity for siderite doses as low as 1 lbs/ft2 distributed withina 0.5 ft thick CI layer. Transient simulations show that cap consolidation flux stronglyinfluences initial porewater pH neutralization, and indicate that pH above the CI layercould remain elevated for up to several years after cap construction, as the initiallyrapidly expressed high pH porewater is flushed by neutralized porewater. Long-termperformance is tied to porewater pH and average groundwater upwelling velocities.Optimized area-specific doses taking these factors into account ranged from 1.0 to 3.5