Natural Source Zone Depletion (NSZD) represents an innovative and highly sustainable remedial option for LNAPL plumes which has the potential to reduce reliance on traditional, more carbon-intensive, remediation techniques. The majority of NSZD studies published to date have been based in the USA and Australia, in regions where the climate and geology (both key factors influencing NSZD) can be very different to those in the UK.
This project was the first in the UK to apply NSZD quantification methods at a large field scale. The trial to quantify the rate at which NSZD occurred was designed to assess the suitability of an attenuation based remedial approach for the LNAPL plume. The application of these novel techniques required considerable regulatory engagement before approval for the trial was gained.
As well as describing the conceptual model pillars supporting the application of NSZD, this article describes the three vapour flux monitoring methodologies by which LNAPL mass depletion was shown to be occurring, and identifies the monitoring technique that was preferred in this case study.
At the time of writing UK guidance relating to NSZD assessment is in production. The successful application of NSZD quantification in an operational UK setting represents a significant advance in establishing the more sustainable remedial options for LNAPL plumes.
Background & Context
The study site (client confidential) is an operational facility in the UK. Detailed characterisation and assessment of the LNAPL plume was undertaken over several years prior to the NSZD assessment, including intrusive ground investigation, LNAPL mass recovery, routine groundwater monitoring, detailed risk assessment and LNAPL characterisation.
The extensive dataset collected allowed development of a robust LNAPL conceptual site model, which indicated that NSZD may be applicable. The key aspects that were considered to support application of NSZD are set out below.
Nature and Extent
The source zone was approximately 1 hectare in extent. The underlying geology comprised unconsolidated granular material (sands and gravels).
The depth to mobile (at the pore scale) LNAPL ranged between 3.5 and 5.0m below ground level (bgl) with a seasonal variation in the depth to the saturated zone of between 1.0 and 1.5m. Ground surface in the source zone and downgradient areas was predominantly unsealed.
Analysis of data from multiple phases of LNAPL sampling identified the source as a mixed product predominantly in the gasoline range (C5-12).
Time series data demonstrated LNAPL chemistry changes indicative of active degradation (reductions in C17:Pristane ratios) within the source zone.
Gauging and groundwater sampling data collected over multiple years described the presence of highly stable LNAPL and dissolved phase plumes.
Detailed quantitative risk assessment demonstrated that ongoing risks to downgradient Controlled Waters receptors were acceptably low. A detailed Human Health DQRA targeting potential vapour exposure risks also did not identify unacceptable risks.
LNAPL recovery was first implemented at a single well during the plume delineation phase, and subsequently expanded to multiple wells. Average recovery rates from the expanded system (initially 100s litres/day) exhibited a declining trend over time with rates reducing to approximately 10 litres/day, prior to the NSZD trial. This declining rate of recovery was supported by site-specific transmissivity testing, which gave results near the lower threshold of published values for effective recoverability.
No formal assessment of degradation was undertaken prior to the NSZD assessment trial however, both field and analytical data indicated that the NAPL source had a high degradation potential.
On the basis of all of the above lines of evidence, the study plume was considered a good candidate for a NSZD trial, and with regulatory support, WSP selected a number of techniques to apply at the site in the assessment of active NSZD processes.
NSZD Assessment Trial
NSZD describes a suite of natural processes that result in depletion of chemical contaminant mass from an LNAPL body. Over the past decade, academic research focus has intensified around the evaluation of NSZD processes and the potential for the quantification of these rates to offer a highly sustainable, remedial solution option to either compliment or negate the requirement for more carbon intensive remedial solutions.
In 2017, the American Petroleum Institute (API) published guidance on the quantification of vapour phase mass flux. The vapour phase is commonly the dominant mass transfer medium. Three vapour mass flux assessment techniques were recommended in this guidance, utilising either surface monitoring of CO2 flux (passive traps and dynamic closed chamber (DCC)) or the measurement of subsurface soil vapour concentration profiles (gradient method). To date in Europe and the UK, application of these techniques has largely been limited to academic research and small scale pilots.
All three techniques recommended in the API (2017) guidance were trialled during the study, to appraise feasibility of application in a UK operational setting.
Use of 15 passive traps for two monitoring rounds, each lasting at least two weeks.
>2,000 CO2 flux readings collected using a Dynamic Closed Chamber (DCC), from 47 locations (including background locations), over a 12 month period.
Soil vapour samples were collected from three vapour well clusters. Clusters comprised adjacent soil vapour wells with discrete (15cm) response zones offset at 0.5m intervals within the unsaturated zone (Figure 1).
 CL:AIRE, 2019. Technical Bulletin 20, An Introduction to Natural Source Zone Depletion at LNAPL Sites
 American Petroleum Institute, 2017. Quantification of Vapour Phase-related Natural Source Zone Depletion Processes, Publication 4784
Figure 1: Vapour Well Cluster – Source Zone
NSZD - Design & Communication
Following a period of internal stakeholder engagement, utilising the LNAPL CSM, a proposal to cease LNAPL recovery and assess mass degradation attributable solely to NSZD was presented to the Environment Agency.
WSP and the client were acutely aware that the assessment and potential application of a novel low-intervention technique such as NSZD required detailed communication and planning, with particular emphasis on residual uncertainties within the CSM. To effectively manage plume status and any potential adverse changes in risk profile during the NSZD trial, contingency plans were developed and agreed with the regulator. These included increasing the frequency of groundwater monitoring downgradient of the source zone and development of a clear set of response actions together with associated trigger criteria.
NSZD Assessment Outcomes
The DCC system (Li-Cor 8100A, Figure 2) provided the most effective monitoring method in an operational setting. The inherent flexibility provided through the combination of short measurement duration and the ability to obtain data in real time in the field, was identified as a significant advantage for this project. It is however recognised that in differing project settings alternative available NSZD monitoring techniques may be preferred.
Being the first field scale application of the technique for the project team, limited precedent data existed against which to establish an expected baseline. The ability to review and rapidly react to data as it was collected in the field was a significant advantage inherent to the DCC technique. A significant commissioning period was required early in the study to identify suitable background locations (a challenging constraint in an operational environment) and quantify temperature induced diurnal variations in observed surface flux (typically 50-60%, at the project site).
Figure 2: CO2 Flux Monitoring with Dynamic Closed Chamber
The mass flux derived through DCC monitoring varied seasonally over an annual cycle between approximately 100 and 700 gal/acre/year (approximately 940 – 6,550 l/ha/yr). These rates were consistent with the lower end of the ranges recorded from published research. Mass flux estimates of a similar magnitude were obtained using the gradient method.
The significant seasonal flux variations recorded were expected given the wetter climate in the UK compared with continental US examples which typify where these techniques have been applied previously. Soil moisture and temperature logging on site, as well as soil vapour profiling all formed crucial supporting data for the interpretation of surface CO2 flux.
The key assessment outcomes were:
Concept successfully proven through the application of NSZD monitoring techniques on a large scale at an operational UK site.
The assessment derived mass depletion rates consistent with published data;
Regulatory acceptance was obtained for the study findings; and
NSZD proven to represent both a technically viable and a regulator accepted low-intervention remedial option for the large LNAPL source at the site.
As with all novel techniques, the trial process threw up a number of key learnings that should be considered in future UK application of this sustainable remediation option:
The vapour phase mass flux techniques recommended by the API can be applied in the UK, even in an operational setting;
Testing and evaluation of all monitoring approaches should be considered. The most suitable monitoring techniques will vary dependent on project setting and CSM;
The value of soil vapour profiling, to conceptually support (or challenge) interpretation when relying on surface monitoring data, should not be underestimated.
Future innovations in this field could include the use of telemetry on suitable sites to enhance real time data collection from DCC to refine the understanding of short term vapour flux variations.
This successful application of NSZD quantification at a large scale in an operational UK setting, together with UK regulator engagement and acceptance is considered to represent a key advance in the range of remediation options to consider for suitable NAPL plumes in the UK. With greater emphasis clearly being placed on the carbon intensity of all our activities, the inclusion of this potentially more sustainable technique in the UK to the list of options for review and testing will play an increasingly important role in the sustainable management and remediation of some challenging UK sites.
At this specific site, the successful large-scale trial now paves the way for the design and application of a formal NSZD remediation solution.
Garg S., Newell C., Kulkarni P., King D., Adamson D., Renno M., and Sale T., 2017. Overview of
Natural Source Zone Depletion: Processes, Controlling Factors, and Composition Change.
Groundwater Monitoring & Remediation