Formatted Title
Evolution of an In Situ Chemical Reduction Approach for Remediating an Expansive Metals Plume
Background/Objectives
For a Superfund Site located within a National Preserve in New Jersey, a diffuse metals (and VOCs) plume required containment and remediation to protect a river at the downgradient site boundary. The site was a former waste recycling facility where six lagoons were used to process liquid industrial waste and convert recovered metals that could be used for construction purposes. An inability to market recovered metals resulted in facility closure. Lagoon water, impacted soil, and various buried containers were removed, and sediment was treated using an innovative soil washing process, but groundwater was impacted by beryllium, chromium, nickel, copper, zinc, and mercury (and various VOCs). A groundwater recovery and treatment system (GWRTS) was initiated in 1995 as the sole groundwater remedial approach but reasonably timed achievement of remedial action objectives (RAOs) became increasingly difficult because of GWRTS inefficiencies caused by ineffective beryllium treatment; reduced recovery well effectiveness; and aging GWRTS components that needed frequent repair or replacement. Because of these GWRTS inefficiencies, achievement of stringent groundwater RAOs was predicted to require GWRTS operation for an additional 47 years (from 2009) at an average operating cost of $500,000 per year. To expedite the remedial action (RA) and reduce associated costs, a refined Conceptual Site Model (CSM) and remedial approach involving GWRTS optimization and supplemental in situ chemical reduction (ISCR) were established with an intent to achieve expedited, justifiable GWRTS shutdown and plume reduction via ISCR.
Approach/Activities
To expedite GWRTS shutdown and RAO achievement at a reasonable cost, refined CSM implementation dictated a stepwise remedial approach involving GWRTS optimization (i.e., improving groundwater recovery and metals treatment) with simultaneous ISCR bench-scale testing, ISCR pilot testing, and source area and hot-spot plume ISCR applications. Uniquely challenging natural conditions at this site, including lack of aquifer buffering capacity and highly transmissive, naturally acidic, and sulfate-laden groundwater added complexity to the updated remedial approach. Optimal pH adjustment was established for the ex situ and in situ treatment of metals (via jar, bench, and pilot testing) and was critical to the success of both RAs. Various groundwater conditioning and ISCR amendments were utilized and evaluated throughout the bench and pilot tests to overcome natural site challenges and maximize ISCR effectiveness. Groundwater conditioning amendments included magnesium hydroxide and oxygen scavenger. Because the RAOs included the treatment of both metals and VOCs, ISCR amendments including various formulations of zero-valent iron, organic carbon, sulfate, nutrients, and activated carbon were utilized to arrive at an optimum treatment balance. These amendments were applied using various direct placement approaches to overcome changing site-specific conditions.
Results/Lessons Learned
Because of the successes achieved in optimizing GWRTS operations for approximately nine years and reducing metals (and VOCs) via ISCR approaches, the GWRTS reached a point of diminishing returns and was shut down in May 2019. As GWRTS optimization and ISCR applications were conducted, the CSM was continually adapted to the improving site conditions (including segmentation of groundwater plumes) and to overcome smear zone desorption after GWRTS shut down. Consequently, during CSM implementation over the past 13 years, naturally challenging conditions were beneficially adapted (through in situ natural sulfate reduction to sulfide) to shorten remedial timeframes, reduce overall remedial costs, and protect the downgradient river. By early 2023 (approximately 4 years after GWRTS shutdown), metals achieved RAOs or were exhibiting decreasing trends in 39 of the 47 on-site wells and the list of targeted metals was significantly reduced. An ISCR polishing step was implemented in 2023 to address remaining hot spots. This presentation will provide details regarding the evolution of the ISCR approaches, lessons learned in implementing each RA step, and results from each incremental approach, including the ISCR polishing step.