Site investigation techniques for DNAPL source and plume zone characterisation

Establishing the location of the Source Area BioREmediation (SABRE) research cell was a primary objective of the site characterisation programme. This bulletin describes the development of a two-stage site characterisation methodology that combined qualitative and quantitative data to guide and inform an assessment of dense nonaqueous phase liquid (DNAPL) distribution at the site. DNAPL site characterisation has traditionally involved multiple phases of site investigation, characterised by rigid sampling and analysis programmes, expensive mobilisations and long decision-making timeframes (Crumbling, 2001a) , resulting in site investigations that are costly and long in duration. Here we follow the principles of an innovative framework, termed Triad (Crumbling, 2001a, 2001b; Crumbling et al., 2001, Crumbling et al. 2003), which describes a systematic approach for the characterisation and remediation of contaminated sites. The Triad approach to site characterisation focuses on three main components: a) systematic planning which is implemented with a preliminary conceptual site model from existing data. The desired outcomes are planned and decision uncertainties are evaluated; b) dynamic work strategies that focus on the need for flexibility as site characterisation progresses so that new information can guide the investigation in real-time and c) real-time measurement technologies that are critical in making dynamic work strategies possible. Key to this approach is the selection of suitable measurement technologies, of which there are two main categories (Crumbling et al., 2003). The first category provides qualitative, dense spatial data, often with detection limits over a preset value. These methods are generally of lower cost, produce real-time data and are primarily used to identify site areas that require further investigation. Examples of such "decisionquality" methods are laser induced fluorescence (Kram et al., 2001), membrane interface probing (McAndrews et al., 2003) and cone penetrometer testing (Robertson, 1990), all of which produce data in continuous vertical profiles. Because these methods are rapid, many profiles can be generated and hence the subsurface data density is greatly improved. These qualitative results are used to guide the sampling strategy for the application of the second category of technologies that generate quantitative, precise data that have low detection limits and are analyte-specific. These methods tend to be high cost with long turnaround times that preclude on-site decision making, hence applying them to quantify rather than produce a conceptual model facilitates a key cost saving. Examples include instrumental laboratory analyses such as soil solvent extractions (Parker et al., 2004)and water analyses (USEPA, 1996). Where these two categories of measurement technologies are used in tandem, a more complete and accurate dataset is achieved without additional site mobilisations. The aim of the site characterisation programme at the SABRE site was to delineate the DNAPL source zone rapidly and identify a location for the in situ research cell. The site characterisation objectives were to; a) test whether semi-quantitative measurement techniques could reliably determine geological interfaces, contaminant mass distribution and inform the initial site conceptual model; and b) quantitatively determine DNAPL source zone distribution, guided by the qualitative site conceptual model

Architecture, persistence and dissolution of a 20 to 45 year old trichloroethene DNAPL source zone

AbstractA detailed field-scale investigation of processes controlling the architecture, persistence and dissolution of a 20 to 45year old trichloroethene (TCE) dense non-aqueous phase liquid (DNAPL) source zone located within a heterogeneous sand/gravel aquifer at a UK industrial site is presented. The source zone was partially enclosed by a 3-sided cell that allowed detailed longitudinal/fence transect monitoring along/across a controlled streamtube of flow induced by an extraction well positioned at the cell closed end. Integrated analysis of high-resolution DNAPL saturation (Sn) (from cores), dissolved-phase plume concentration (from multilevel samplers), tracer test and permeability datasets was undertaken. DNAPL architecture was determined from soil concentration data using partitioning calculations. DNAPL threshold soil concentrations and low Sn values calculated were sensitive to sorption assumptions. An outcome of this was the uncertainty in demarcation of secondary source zone diffused and sorbed mass that is distinct from trace amounts of low Sn DNAPL mass. The majority of source mass occurred within discrete lenses or pools of DNAPL associated with low permeability geological units. High residual saturation (Sn>10–20%) and pools (Sn>20%) together accounted for almost 40% of the DNAPL mass, but only 3% of the sampled source volume. High-saturation DNAPL lenses/pools were supported by lower permeability layers, but with DNAPL still primarily present within slightly more permeable overlying units. These lenses/pools exhibited approximately linearly declining Sn profiles with increasing elevation ascribed to preferential dissolution of the uppermost DNAPL. Bi-component partitioning calculations on soil samples confirmed that the dechlorination product cDCE (cis-dichloroethene) was accumulating in the TCE DNAPL. Estimated cDCE mole fractions in the DNAPL increased towards the DNAPL interface with the uppermost mole fraction of 0.04 comparable to literature laboratory data. DNAPL dissolution yielded heterogeneous dissolved-phase plumes of TCE and its dechlorination products that exhibited orders of magnitude local concentration variation. TCE solubility concentrations were relatively localised, but coincident with high saturation DNAPL lens source areas. Biotic dechlorination in the source zone area, however, caused cDCE to be the dominant dissolved-phase plume. The conservative tracer test usefully confirmed the continuity of a permeable gravel unit at depth through the source zone. Although this unit offered significant opportunity for DNAPL bypassing and decreased timeframes for dechlorination, it still transmitted a significant proportion of the contaminant flux. This was attributed to dissolution of DNAPL–mudstone aquitard associated sources at the base of the continuous gravel as well as contaminated groundwater from surrounding less permeable sand and gravel horizons draining into this permeable conduit. The cell extraction well provided an integrated metric of source zone dissolution yielding a mean concentration of around 45% TCE solubility (taking into account dechlorination) that was equivalent to a DNAPL mass removal rate of 0.4tonnes per annum over a 16m2 cell cross sectional area of flow. This is a significant flux considering the source age and observed occurrence of much of the source mass within discrete lenses/pools. We advocate the need for further detailed field-scale studies on old DNAPL source zones that better resolve persistent pool/lens features and are of prolonged duration to assess the ageing of source zones. Such studies would further underpin the application of more surgical remediation technologies

Monitoring well utility in a heterogeneous DNAPL source zone area : Insights from proximal multilevel sampler wells and sampling capture-zone modelling

Groundwater-quality assessment at contaminated sites often involves the use of short-screen (1.5 to 3 m) monitoring wells. However, even over these intervals considerable variation may occur in contaminant concentrations in groundwater adjacent to the well screen. This is especially true in heterogeneous dense non-aqueous phase liquid (DNAPL) source zones, where cm-scale contamination variability may call into question the effectiveness of monitoring wells to deliver representative data. The utility of monitoring wells in such settings is evaluated by reference to high-resolution multilevel sampler (MLS) wells located proximally to short-screen wells, together with sampling capture-zone modelling to explore controls upon well sample provenance and sensitivity to monitoring protocols. Field data are analysed from the highly instrumented SABRE research site that contained an old trichloroethene source zone within a shallow alluvial aquifer at a UK industrial facility. With increased purging, monitoring-well samples tend to a flow-weighted average concentration but may exhibit sensitivity to the implemented protocol and degree of purging. Formation heterogeneity adjacent to the well-screen particularly, alongside pump-intake position and water level, influence this sensitivity. Purging of low volumes is vulnerable to poor reproducibility arising from concentration variability predicted over the initial 1 to 2 screen volumes purged. Marked heterogeneity may also result in limited long-term sample concentration stabilization. Development of bespoke monitoring protocols, that consider screen volumes purged, alongside water-quality indicator parameter stabilization, is recommended to validate and reduce uncertainty when interpreting monitoring-well data within source zone areas. Generalised recommendations on monitoring well based protocols are also developed. A key monitoring well utility is their proportionately greater sample draw from permeable horizons constituting a significant contaminant flux pathway and hence representative fraction of source mass flux. Acquisition of complementary, high-resolution, site monitoring data, however, vitally underpins optimal interpretation of monitoring-well datasets and appropriate advancement of a site conceptual model and remedial implementation

Architecture, persistence and dissolution of 20 to 45 old trichlorethene DNAPL source zone

A detailed field-scale investigation of processes controlling the architecture, persistence and dissolution of a 20 to 45 year old trichloroethene (TCE) dense non-aqueous phase liquid (DNAPL) source zone located within a heterogeneous sand/gravel aquifer at a UK industrial site is presented. The source zone was partially enclosed by a 3-sided cell that allowed detailed longitudinal/fence transect monitoring along/across a controlled streamtube of flow induced by an extraction well positioned at the cell closed end. Integrated analysis of high-resolution DNAPL saturation (Sn) (from cores), dissolved-phase plume concentration (from multilevel samplers), tracer test and permeability datasets was undertaken. DNAPL architecture was determined from soil concentration data using partitioning calculations. DNAPL threshold soil concentrations and low Sn values calculated were sensitive to sorption assumptions. An outcome of this was the uncertainty in demarcation of secondary source zone diffused and sorbed mass that is distinct from trace amounts of low Sn DNAPL mass. The majority of source mass occurred within discrete lenses or pools of DNAPL associated with low permeability geological units. High residual saturation (Sn > 10–20%) and pools (Sn > 20%) together accounted for almost 40% of the DNAPL mass, but only 3% of the sampled source volume. High-saturation DNAPL lenses/pools were supported by lower permeability layers, but with DNAPL still primarily present within slightly more permeable overlying units. These lenses/pools exhibited approximately linearly declining Sn profiles with increasing elevation ascribed to preferential dissolution of the uppermost DNAPL. Bi-component partitioning calculations on soil samples confirmed that the dechlorination product cDCE (cis-dichloroethene) was accumulating in the TCE DNAPL. Estimated cDCE mole fractions in the DNAPL increased towards the DNAPL interface with the uppermost mole fraction of 0.04 comparable to literature laboratory data. DNAPL dissolution yielded heterogeneous dissolved-phase plumes of TCE and its dechlorination products that exhibited orders of magnitude local concentration variation. TCE solubility concentrations were relatively localised, but coincident with high saturation DNAPL lens source areas. Biotic dechlorination in the source zone area, however, caused cDCE to be the dominant dissolved-phase plume. The conservative tracer test usefully confirmed the continuity of a permeable gravel unit at depth through the source zone. Although this unit offered significant opportunity for DNAPL bypassing and decreased timeframes for dechlorination, it still transmitted a significant proportion of the contaminant flux. This was attributed to dissolution of DNAPL–mudstone aquitard associated sources at the base of the continuous gravel as well as contaminated groundwater from surrounding less permeable sand and gravel horizons draining into this permeable conduit. The cell extraction well provided an integrated metric of source zone dissolution yielding a mean concentration of around 45% TCE solubility (taking into account dechlorination) that was equivalent to a DNAPL mass removal rate of 0.4 tonnes per annum over a 16 m2 cell cross sectional area of flow. This is a significant flux considering the source age and observed occurrence of much of the source mass within discrete lenses/pools. We advocate the need for further detailed field-scale studies on old DNAPL source zones that better resolve persistent pool/lens features and are of prolonged duration to assess the ageing of source zones. Such studies would further underpin the application of more surgical remediation technologies

Understanding complex LNAPL sites : Illustrated handbook of LNAPL transport and fate in the subsurface

The goal of the paper is to highlight the management of the complexities and risks for light non-aqueous phase liquid (LNAPL) sites, and how the “Illustrated Handbook of LNAPL Transport and Fate in the Subsurface” (CL:AIRE, London. ISBN 978-1-905046-24-9. http://www.CL:AIRE.co.uk/LNAPL; LNAPL illustrated handbook) is useful guidance and a tool for professionals to understand these complexities and risks. The LNAPL illustrated handbook provides a clear and concise best-practice guidance document, which is a valuable decision support tool for use in discussions and negotiations regarding LNAPL impacted sites with respect to the risks of LNAPL sites. The LNAPL illustrated handbook is a user-friendly overview of the nature of LNAPL contamination in various geological settings including unconsolidated, consolidated, and fractured rock environments to best understand its fate and behavior leading to the appropriate management and/or remedial approach of the two major risks associated with a LNAPL source. As a source term, LNAPL has chemicals that form dissolved- and vapor-phase plumes, which are referred to as composition-based risks; and being a liquid there is the risk that the source may expand impacting a greater volume of the aquifer, which are referred to as saturation-based risks. There have been significant developments in recent years on the understanding of the complex behavior of LNAPL and associated groundwater and vapor plumes; however, the state of practice has often lagged these improvements in knowledge. The LNAPL illustrated handbook aids the site investigator, site owners, and regulators to understand these risks, and understand how these risks behave through better conceptual understanding of LNAPL transport and fate in the subsurface