Formatted Title
LNAPL Transmissivity and Enhanced NSZD at a Petroleum Pipeline Release Site: Is it Time to Transition to a Nature-Based LNAPL Management Strategy?
Background/Objectives
Recent years have brought a new understanding of the importance of sustainable remediation and nature-based management strategies at petroleum NAPL sites. Development of a robust petroleum NAPL conceptual site model (CSM) is essential for site management decision-making when it’s time to transition away from active remediation to a nature-based strategy. Accordingly, the assessments of LNAPL transmissivity (Tn) and petroleum NAPL natural source zone depletion (NSZD) are becoming more standard considerations in petroleum NAPL CSM development. Contemporary LNAPL guidance such as that developed by the Interstate Technology and Regulatory Council (ITRC, 2009 and 2018) includes a discussion of these topics, along with other considerations, for LNAPL CSM development.
While numerous (and growing) instances of case studies at LNAPL sites involving use of LNAPL Tn and NSZD exist in the literature (e.g., NSZD in CRC CARE Technical Reports 44, 46 and 47), there are few published demonstrations of sustainable/low footprint efforts to enhance NSZD (eNSZD) and the decision making involved for implementation and/or termination. This presentation will examine a site in Western Canada where a pipeline release occurred, followed by initial remediation activities, additional site investigation activities such as LNAPL Tn and NSZD assessments, and capping of the primary area of residual LNAPL. Additionally, an update will be given on the implementation of the enhanced NSZD based on low-temperature heating with a goal of increasing treatment zone temperatures by 10-15̊ C in hopes of enhancing NSZD activity by a factor of 2-3. The presentation will focus on the use of LNAPL Tn and NSZD in the decision-making process to transition to eNSZD and in the planning process to transition away from eNSZD to NSZD.
Approach/Activities
The LNAPL Tn assessments were performed by GHD following methods from the American Society for Testing and Materials (ASTM) E2856-13: Standard Guide for Estimation of LNAPL Transmissivity and an associated workbook for analysis developed by the American Petroleum Institute (API). Results are quantitatively compared against widely accepted de minimis criteria to assess whether hydraulic recovery of LNAPL may be feasible, and/or provide some technical benefit in terms of mitigating migration potential via a tangible reduction in LNAPL saturation levels, or whether LNAPL is considered to be largely present at residual levels and hydraulically immobile/unrecoverable.
The NSZD assessments were performed by GHD through the concurrent use of multiple measurement techniques, including establishing an average pre-enhancement baseline NSZD rates in a capped environment. The confirmation of NSZD and estimation of rates is being accomplished using a mix of soil gas gradient, biogenic heat (long-term placement of temperature data loggers in wells) and CO2 efflux (Trap method) NSZD measurement techniques to produce multiple lines of evidence. Monitoring events were completed before and after installation of an impermeable cap and in advance of eNSZD system installation. Monitoring events will continue during eNSZD operation to confirm heating efficiency and assess any beneficial change in NSZD rates.
Results/Lessons Learned
LNAPL Tn was found to be of de minimis magnitude in tested wells, indicating that LNAPL was already largely immobile, and LNAPL mass recovery efforts would not provide a meaningful reduction in LNAPL saturation. This evaluation determined a more sustainable, composition-based remediation such as NSZD was appropriate for the site. NSZD assessments verified biodegradation activity was occurring at the site. Pre- and post-cap measured NSZD rates using surficial CO2 efflux-based varied by an order of magnitude or more, while subsurface methods showed more comparable results. The significance of the chimney effect led to the saturation of CO2 Trap samples and decreased sampling times were needed to overcome this issue. Results from multiple rounds of NSZD testing were used to establish baseline levels such that the magnitude of the enhancement of NSZD activity due to the eNSZD system could be approximated. This presentation will also discuss any performance monitoring NSZD measurements obtained during eNSZD to determine effectiveness of the low temperature thermal treatment.