Guidance on Natural Source Zone Depletion by Geosyntec and CL:AIRE
Shortlisted for Best Research or Application in the Advancement of Science, Technology or Digital Innovation in the Brownfield Sector
Introduction
Natural source zone depletion (NSZD) is a relatively new term that extends concepts underpinning monitored natural attenuation (MNA) to source zones to manage liabilities involving petroleum hydrocarbons in many environments. This method is rapidly evolving and is receiving increasing attention in the UK as stakeholders seek sustainable ways to remediate contaminated sites while meeting key performance indicators.
NSZD refers to naturally occurring processes that act together to reduce light non-aqueous phase liquid (LNAPL) mass. Recent research and industry experience have demonstrated that mass depletion occurs at much higher rates than were previously understood. These studies indicate that natural depletion of LNAPL is a viable alternative to, or can be a significant component of, active remediation.
The April 2024 CL:AIRE Guidance on NSZD compiles the latest information and practical considerations to create a holistic, up-to-date resource to help practitioners, regulators, and liability owners manage land impacted by LNAPLs. The CL:AIRE guidance was prepared by this author and steered by a collaborative technical review group comprising industry professionals, consultants, and regulators across the devolved administrations. CL:AIRE acknowledges the Environment Agency for their seed funding for this project, which was otherwise completed voluntarily.
The author and technical reviewers recognised the critical need for new guidance to capture recent scientific and technological advances in NSZD and for a decision-making framework to support practitioners interested in sustainably managing LNAPL sites in the UK. In a LinkedIn poll on the NSZD state of practice, a main reason cited for not considering NSZD in options appraisal was the lack of a regulatorily backed process to select, implement, and close NSZD-based monitoring programmes. The team tailored the CL:AIRE guidance to address these specific stakeholder-led needs.
Background
When petroleum hydrocarbons are released into the environment in the form of LNAPL, NSZD begins immediately. Biodegradation typically accounts for >90% mass depletion, occurring directly on the LNAPL and on vaporised and dissolved phase constituents. Methanogenesis is a key mechanism for NSZD, occurring both above and below the water table. Methane outgassing and ebullition into the unsaturated zone can react with oxygen, generating carbon dioxide and heat. As well as depleting source zone mass, NSZD processes cause compositional changes to the LNAPL, reducing its solubility, volatility and mobility over time (Figure 1).
Recent technological advances in analytical and monitoring methods allow the gaseous, aqueous, and compositional expressions of NSZD to be measured, from which mass depletion rates can be estimated. Rates of NSZD can be in the order of thousands to tens of thousands of litres of LNAPL per hectare per year, depending on the oil or fuel type, multiphase transport, and other processes. NSZD has been demonstrated to be occurring at significant rates at several LNAPL sites in UK and Europe and will provide an underlying driver to achieve remediation objectives.
Because assessing or monitoring NSZD can be used to support many kinds of decisions about sustainable site management, it should be a major consideration throughout a project’s life cycle:
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Advancing conceptualisation of potential contaminant linkages from generic to detailed quantitative risk assessment (such as Land Contamination Risk Management [LCRM] Stage 1 Tier 2 to Tier 3)
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Comparing NSZD with other potential remediation technologies in an Options Appraisal (such as LCRM Stage 2)
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Supporting stand-alone implementation or transition from active remediation to passive risk management during remediation and verification (such as LCRM Stage 3)
The NSZD guidance succinctly describes the state of practice, including biogeochemical processes; monitoring technologies; and the role of NSZD in the remediation project life cycle. A three-stage decision-making framework supports shortlisting of sites where NSZD could be a viable, complementary, or alternative remediation option. This framework can also guide NSZD-based monitoring programmes. Deeper dives into NSZD processes and monitoring technologies are appended. The final part of the appendix is dedicated to developing the LNAPL conceptual site model (CSM) for NSZD. This appendix presents considerations for characterising LNAPL sites to achieve data quality objectives; assessing risk regarding LNAPL and its gas-, vapour-, and dissolved-phase plumes; and predicting mass depletion rates at increasing levels of complexity and confidence, from screening-level to advanced modelling.
Promoting exemplary best practice: innovation to develop robust, cost-effective, and
defensible solutions
Before selecting, designing, and implementing NSZD to manage risk, an advanced CSM for LNAPL must be developed to demonstrate understanding of relevant NSZD processes and associated risks to receptors. The NSZD guidance outlines how such a CSM will likely evolve through each stage of the decision-making framework. The initial NSZD CSM provides a preliminary understanding of mass depletion processes to help characterise potential contaminant linkages associated with LNAPL migration, gas accumulation, vapour intrusion, and groundwater-plume migration. With increased quantitative understanding of NSZD, the CSM can advance to demonstrate NSZD with sufficient confidence to estimate LNAPL depletion rates using suitable techniques. Finally, it can evolve to support design and verify performance of a NSZD-based monitoring strategy that adequately manages risks and achieves remediation objectives.
For the CSM to evolve as described above, data collection and analysis must meet data quality objectives. Data should be relevant, sufficient, reliable, and transparent. Setting data quality objectives is a logical process that informs planning for data acquisition. This process includes prioritising data gaps and developing hypotheses; defining data collection objectives; and establishing what types of data are required, how data will be used, and whether data are acceptable for this purpose. This process helps practitioners understand whether sufficient information has been gathered to make appropriate decisions about risks and remediation of LNAPL. The guidance furthermore highlights the roles and integration of various LNAPL site characterisation approaches in developing the CSM for complex sites.
UK guidance (e.g., LCRM) does not stipulate remediation for all non-aqueous phase liquids (NAPLs). The only NAPL sites that require intervention are those posing direct risk of migration into a receptor, those involving indirect risk related to migration of gas, vapour, or groundwater plumes, or those involving both kinds of risk.
If CSMs for potential contaminant linkages are informed only by preliminary or generic quantitative risk assessments, they could misrepresent potential risks to receptors. They might rely on generalised assumptions about LNAPL mobility and permanence, or use generic assessment criteria that do not adequately describe LNAPL partitioning, omit natural attenuation processes, and/or misrepresent gas and vapour transport.
The CL:AIRE guidance introduces concepts and tools to support holistic risk assessment of LNAPL, integrating up-to-date technical guidance on NAPL migration (American Society for Testing and Materials [ASTM]), gas risk (CL:AIRE), vapour intrusion (United States Environmental Protection Agency [USEPA], Energy Institute), NAPL dissolution, and reactive solute transport (USEPA) with established UK procedures for quantitative risk assessment.
This decision-making framework is a first for LNAPL site management and responds directly to industry and regulatory requirements (Figure 2). It is intentionally presented in a familiar format, like other guidance for MNA. This is designed to make NSZD assessment and implementation more accessible. The initial screening stage provides risk-based constraints (red flags) and other factors to assess whether NSZD occurs at significant rates or could likely pose risks to receptors. The demonstration stage outlines how advanced data collection can close gaps and uncertainties in the evolving CSM and inform the scope of monitoring. The implementation stage describes the monitoring plan and data-driven decisions to either continue monitoring (with a view to achieving objectives within a designated timeframe), cease monitoring, or instigate contingency actions. Because each stage is underpinned by proactive and regular engagement, stakeholders and regulators must endorse decisions throughout.
Adopting NSZD within an adaptive risk- management strategy, as described in the guidance, is a versatile concept. This method helps develop robust CSMs that integrate advanced data collection, risk assessment, and prediction, to more sustainably manage LNAPL sites. The effort expended to develop an understanding of NSZD also informs conceptualisation for remediation alternatives and advances demonstration of the viability of MNA in the groundwater plume, if this is required. Some of the concepts within the CL:AIRE guidance also translate to complex sites involving other forms of NAPL.
Compliance with legislation, codes and guidance and adherence to diversity practice
To voice and incorporate the varied requirements and opinions of industry, practitioners, and regulators, a collaborative technical review group steered preparation of this guidance. This document was developed with reference to British and international standards, technical guidance, and peer-reviewed literature. The final document was reviewed and supported by the Environment Agency, Natural Resources Wales, Scottish Environment Protection Agency, and the Northern Ireland Environment Agency, an agency within the Department of Agriculture, Environment and Rural Affairs. The document was also peer reviewed by CL:AIRE’s Technology and Research Group.
Real environmental and social benefits
The NSZD guidance highlights the significance of naturally occurring mass depletion rates observed at many sites and the denudation of potential risks from LNAPL over time. While not suitable as a stand-alone solution for every site, NSZD has been shown to achieve timebound objectives where remediation alternatives are infeasible or cease to be efficient at reducing LNAPL mass.
By considering NSZD, the scope, cost, and timeframe for active remediation are reduced, shortening the time needed to transition to monitoring. Monitoring land contamination is a low-impact strategy to manage risk for brownfield areas while also effectively and efficiently using, improving, or returning them to other long-term uses. While petroleum hydrocarbon biodegradation is an unavoidable contributor to greenhouse gas emissions, the rates of emission are low compared to remediation alternatives and can be efficiently offset with strategic beneficial reuse of brownfield land where NSZD-based risk management is being used. Case studies demonstrate the resilience of NSZD to environmental stressors, such as storms and periodic changes in climatic conditions. By adopting the NSZD guidance, liability owners can achieve the environmental and social benefits they seek while pursuing sustainable management of LNAPL sites.
Effective stakeholder engagement, broad potential impact, and value of application
To support guidance being as widely adopted as possible, this publication was prepared with careful consideration to the motivations for developing it. It provides information to preside over an uptick in regulatory consultations from industry regarding NSZD and to address the need for a regulatorily backed decision-making framework.
Publication of the NSZD guidance on 30 April 2024 has had a big impact already. In only four weeks, it has prompted 13,000 impressions and 1,000 engagements on LinkedIn. It has been downloaded from CL:AIRE’s website over 750 times at the time of writing.
CL:AIRE has also given permission for the report to be downloadable from the charity The Groundwater Project (https://gw-project.org), a global platform that advances groundwater education, particularly in the developing world. The author is continuing to work with CL:AIRE and the Environment Agency to provide training sessions in-person and online to
share knowledge and upskill practitioners, regulators, and other stakeholders.