Phased Construction of a DNAPL Recovery System to Reduce Client Risk and Increase Third-party Property Owner Confidence
Marcus Byker, P.E., Managing Engineer, Ramboll
A 6-acre DNAPL body was identified adjacent to a former MGP and within approximately 200 feet of a surface water body. Potentially recoverable DNAPL was delineated on property owned by the utility, the city, railroad, local port district, and a paint manufacturer. In consideration of property ownership constraints, the United States Environmental Protection Agency (USEPA) issued an interim Record of Decision (ROD) to recover as much DNAPL as practicable. The ROD specified a physically enhanced recovery approach that involved groundwater extraction, treatment, and reinjection to facilitate migration and recovery of DNAPL. Initial plans involved implementation of the system on all properties in one mobilization. This approach would have included construction of a 150-gallon per minute groundwater treatment plant, 40 vertical wells, four horizontal wells, and related infrastructure. Given the complexities of long-term DNAPL recovery and in consideration of third-party property owner constraints, the project team implemented construction in two phases with the first phase focused on utility-owned property and serving as a pilot study. The goal of the pilot study was to validate design assumptions, evaluate material and component specifications, optimize groundwater treatment plant operations, and refine construction methodologies prior to expanding to third-party properties. The objective of this presentation is to demonstrate how a DNAPL recovery pilot study was used to optimize system design and construction prior to completing work on third-party properties.
The pilot study consisted of six vertical wells, a horizontal injection well, and a rented groundwater treatment plant. The pilot study was completed on utility-owned property, allowing greater flexibility and evaluation of construction approaches and modifications to system operations that would be carried forward for a more streamlined full-scale roll out. Prior to the pilot study, key design and operation criteria were compiled and a strategy was developed to validate those criteria during pilot study execution. Throughout operations, key system components were evaluated to optimize full-scale design. In addition, systematic modification of operations was completed to inform full-scale operational criteria. Operational modifications included batch and continual pumping of DNAPL recovery pumps, constant and pulse pumping of groundwater extraction pumps, and modification of heat trace settings.
The pilot study system operated between April and November 2020. From a design perspective, the pilot study identified the need for an alternate groundwater extraction pump, clarified selection of preferred DNAPL recovery pump, and confirmed the need for heat trace to facilitate conveyance of recovered DNAPL. From an operational perspective, the pilot study demonstrated that pulsed groundwater extraction was four times more effective at recovery of DNAPL and reduced influent treatment plant flow by approximately 50%. In addition, third party property owners were able to observe and understand construction and operations to gain comfort in execution of the full-scale system on their properties. The presentation will provide details on pilot study construction and operations and how lessons learned from the pilot study were used to reduce risk in the final design and gain confidence from third parties on expansion of the recovery system onto their properties.
Mr. Byker is a professional engineer with fourteen years of environmental remediation consulting experience with particular focus on feasibility study evaluations, concept and detailed design engineering, and regulatory agency interactions. Mr. Byker's recent experience has predominantly focused on characterizing and remediating coal tar non-aqueous phase liquid (NAPL)-affected soil, groundwater, and sediment. Mr. Byker is an active committee member involved with developing ASTM standards for evaluating NAPL mobility in sediments. Mr. Byker also is a leader of innovative thought in NAPL characterization and remediation at Ramboll, including development of techniques to aid in identification of NAPL and tools to improve recovery of NAPL.
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