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The potential for DNAPLs to be a major hazard for the Teesworks site are well understood from reading the Environment Agency's An illustrated handbook of DNAPL transport and fate in the subsurface.
DNAPLs present a different level of contamination risk due to their mixed nature and localisation within the ground, so they require specific approaches. The handbook starts with “Dense non-aqueous phase liquids (DNAPLs) such as creosote, coal tar' coal tar at other sites, chlorinated solvents and polychlorinated biphenyl oils represent a particular class of soil and groundwater contaminant that exist as a separate liquid phase in the presence of water. DNAPLs come to rest in the subsurface as disconnected blobs and ganglia of liquid referred to as residual DNAPL, and in potentially mobile distributions referred to as 'pools.”
The mixed nature of DNAPLs means that the concentration of contaminants doesn't do what is expected over time, i.e. it can not just decrease over time but actually increase, why? DNAPLs are a mixture of a wide range of materials in the case of coal tar mainly hydrocarbons, these hydrocarbons will have different levels of solubility. So taking a simple example a DNAPL which contains 90% of a highly toxic but low solubility hydrocarbon and 10% of less toxic but more soluble hydrocarbon, then sampling the water around the DNAPL would initially show only a very low concentration of the highly toxic component, but once the more soluble component has been removed then the concentration of the highly toxic component would shoot up. So the ground could present more of a hazard to humans and nature decades or even centuries after the original contamination event. This also makes it more difficult to find the DNAPL, hence special approaches have to be taken to remediation of sites which due to previous uses are expected to contain DNAPLs and South Tees with over 140 years of coke production is definitely one of these.
DNAPLs are explicitly mentioned in only a small number of the risk assessment, despite many parts of the site either having previously housed coke ovens or being adjacent to coke oven site. DNAPLs should have been assumed to be present unless it could be proven by consideration of the underlying strata that it would have been impossible for them to migrate to a particular location on the Teesworks site.
Below is a simple simulation which shows how coal tar ends up in the River Tees - click on the Green Flag and then slide the Auto slider to the right to start the simualtion.
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R/2021/0713/CD - Compliance Officer Report
R/2021/0713/CD - Planning Application
R/2021/0713/CD - Detailed Quantitative Risk Assessment: South Bank Area A - May 2021
Executive Summary
“The modelling shows that sufficient time may not have yet elapsed for contaminants to have reached a steady state equilibrium with groundwater (in relation to the groundwater source). A timeframe assessment for the groundwater source predicated that measured concentrations may increase by around an order of magnitude from present day (assumed around the 50yr timeframe modelled) to steady state (approximately 190yrs or greater). Given this and the current measured concentrations in the sentinel wells, measured sentinel well concentrations will still remain well below their SSAC (considering dilution in the River Tees).”
“DNAPL has been measured on site. This may require consideration as part of remediation works. However, dissolved phase concentrations indicate that DNAPL is not presenting a risk to water resources, although does have potential to act as an ongoing source of contamination.”
“A potential pathway due to piled foundations was identified under the pollutant linkages. Based on the findings of the contaminant distribution and underlying ground conditions, it is considered unlikely that this pathway could represent a significant risk to water resources. Although contamination may locally be able to enter the underlying natural deposits around piles, lateral migration within the unit is unlikely to be significant.”
Page 16: GQRA Conclusions and CSM: Water Resources
“Dense Non-Aqueous Phase Liquid (DNAPL) was measured in BH110 in two of the three groundwater monitoring visits. This, along with other CoC could not be fully delineated as part of the ESA due to the presence of an exclusion zone on site relating to the instability of the ammonia scrubbers on the SBCO site to the south.
The proximity of the site to the River Tees (approximately 20m north of the site boundary) has resulted in a tidal influence in some of the monitoring wells and this should also be considered.
The Royal Haskoning April 2021 report also shows detections of the same CoC highlighted within the Arcadis March 2021 report within the groundwater in the wells running along the northern site boundary. They also highlight a potential for DNAPL to be present within BH2025, although this was not measured or sampled during the works but rather based on observations during drilling and contaminant concentrations within the soil and groundwater. The source of the impacts in BH2025 is not known as is speculated to perhaps be an off-site source.”
Page 17: Potentially Active Pollutant Linkages: Water Resources
“A DNAPL source within the groundwater has been measured in BH110 towards the southern boundary of the site. While the DNAPL has not been measured downstream, a dissolved phase plume located north towards the River Tees from the BH110 area is present. The DNAPL was found resting on the boundary between the Slag Made Ground and the Tidal Flat deposits. Due to the granular nature of the Made Ground and groundwater levels underlying the site, a migration pathway from this groundwater source to the River and estuary is also considered potentially active.
To summarise, the following pollutant linkages are considered potentially active;
* Leaching of CoC from soil into groundwater
* Migration in groundwater towards the surface water receptor (River Tees Estuary)
* Vertical migration of impacts down relic foundation piles is also considered potentially active.”
Page 19: Site Characterisation: Environmental Setting: Sources: Non-Aqueous Phase Liquid (NAPL)
“Dense Non-Aqueous Phase Liquid (DNAPL) was identified resting at the base of the Made Ground on the Tidal Flats deposits in BH110 at the southern boundary of the site. DNAPL thickness was measured on two occasions in October 2020 resting between 7.44 and 7.50m bgl (6cm thickness), and 7.16m and 7.50m bgl (34cm thickness).
NAPL was not identified in the Royal Haskoning April 2021. Hydrocarbon odours were noted in BH2025 and BH2021 (in the northwest and northeast of the site respectively) which the Royal Haskoning report states may be due to the presence of DNAPL.
Investigations undertaken by Enviros in 2004 (Enviros, 2004) also identified NAPL in two locations in the south of the site, approximately 30m from the SBCO boundary.”
Page 21: Site Characterisation: Environmental Setting: Sources: Modelled Source Area
“In groundwater, a more distinct plume is evident and does not appear to be associated with concentrations in unsaturated soils. The highest groundwater contaminant concentrations were measured in BH110 adjacent to the southern boundary, which separates the site from the SBCO facility (predominantly hydrocarbons and phenolics, and also cyanide). Relatively high hydrocarbon concentrations, particularly benzene, were measured in nearby location BH105 and downgradient BH103. BH110 also had a measured thickness of DNAPL on two of the three groundwater monitoring visits undertaken in October and November 2020 resting at the base of the Slag Made Ground above the Tidal Flat Deposits.
Based on the above, two distinct sources have been considered:
* Made Ground from across the site – considered to comprise a single diffuse soil source associated with Made Ground and slag.
* Groundwater in the vicinity of BH110 – considered to represent impacts associated with an off-site source to the south of the site (SBCO).”
Page 30: Water Resource Risk Evaluation
“DNAPL has been measured on site. This may require consideration as part of remediation works however, dissolved phase concentrations indicate that DNAPL is not presenting a risk to water resources, although it does have potential to act as an ongoing source of contamination.”
R/2021/0713/CD - Detailed Quantitative Risk Assessment: South Bank - July 2021
Page 18: Water Resources
Dense Non-Aqueous Phase Liquid (DNAPL) was measured in SBA_AUK_BH110 in two of the three groundwater monitoring visits. This, along with other CoC, could not be fully delineated as part of the initial SBA ESA due to the presence of an exclusion zone on site relating to the instability of the ammonia scrubbers on the SBCO site to the south. This was highlighted as a potential data gap in the SBA DQRA. The 14 additional trial pits advanced in the SBCO area following the SBA DQRA provide further assessment of the soil quality in this previously investigated area.
The proximity of the site to the River Tees (approximately 20m north of the site boundary) has resulted in a tidal influence in some of the monitoring wells and this should also be considered.
The Royal Haskoning April 2021 report also shows detections of the same CoC highlighted within the Arcadis 2021a, 2021b & 2021c reports within the groundwater in the wells running along the northern site boundary. They also highlighted a potential for DNAPL to be present within BH2025, although this was not measured or sampled during the works but rather based on observations during drilling and contaminant concentrations within the soil and groundwater. The source of the impacts in BH2025 is not known as is speculated to perhaps be an off-site source.
Page 28/29: Summary of Contaminant Distribution Findings
The assumption from no spatial distribution trends that Made Ground is uniform across the site seems to be fundamentally flawed, surely no spatial distribution trends implies exactly the opposite that the Made Ground is in fact highly heterogeneous. In the context of detected DNAPLs this means that there are likely to be localised areas of DNAPLs which will not be easily picked up by regular sampling, instead have to be removed by flushing.
In unsaturated soil, in most cases, no significant spatial distribution trends have been identified, suggesting Made Ground is of a similar composition across the site and should be considered as a single source. As such, contaminants measured in Made Ground are not generally due to primary contaminant sources associated with infrastructure and historical industrial processes on-site.
In groundwater, a more distinct plume is evident in the vicinity of the SBCO area and southern SBA boundary, in the same area as the highest saturated soil impacts. The highest groundwater contaminant concentrations were measured in SBA_AUK_BH110 adjacent to the southern boundary of SBA, which separates SBA from the SBCO facility (predominantly hydrocarbons and phenolics, and also cyanide). Relatively high hydrocarbon concentrations, particularly benzene, were measured in nearby locations around the SBCO area. SBA_AUK_BH110 also had a measured thickness of DNAPL on two of the three groundwater monitoring visits undertaken in October and November 2020 resting at the base of the Slag Made Ground above the Tidal Flat Deposits.
Given that the Made Ground source material is in part saturated, the ground is uncovered, the permeable nature of the Made Ground, and the significant length of time the Made Ground has been present, partitioning into groundwater is likely to have occurred already. Based on this, groundwater is considered to provide the best representation of the potential risk to water resource receptors.
R/2021/0713/CD - Enabling Earthworks and Remediation Strategy Report, August 2021
Page 6- Materials Impacted with Non-Aqueous Phase Liquids
This states that NAPLs should not be reinstated, but says nothing about flushing out DNAPLs and so does not appear to be an appropriate approach.
Although limited evidence of NAPL and tar have currently been identified within the wider South Bank landholding, it is reasonable to anticipate that it may be encountered locally, in line with adjacent plots of the wider South Bank site. In the wider South Bank site NAPL and Tar have been identified primarily within the Made Ground and associated with subsurface or former above ground structures and plant. Further consideration of the NAPL with respect to the risk to human health will be needed as part of the remedial strategy.
Materials impacted with NAPL and tar should not be reinstated due to being a primary source of contamination. The impacted materials will be required to be consigned to a treatment process to remove the NAPL element or disposed of at an appropriate waste facility under duty of care.
Page 17: Remediation Objectives
Manage the contamination in excess of screening levels and that are likely to be present following completion of ongoing ground investigation, including NAPL containing soils.
Page 22: Removel of NAPL on Groundwater
If free phase NAPL is encountered on the groundwater during excavation works, the Contractor shall undertake recovery prior to groundwater discharge. The Contractor shall continue the NAPL recovery process until no visible NAPL is observed or further recovery is not reasonably practicable (evidenced by diminishing recovery quantities i.e. base of asymptotic curve).
Where there is evidence of the presence of NAPL in the unsaturated zone, excavations will be extended to expose the groundwater table and identify if it is impacted by the above material and if groundwater treatment is required.
Page 23/24: Compliance Sampling Frequency
Excavation Extents in areas without NAPL: Ensure that soils remaining in-situ do not contain contaminant concentrations in excess of the remediation and reclamation criteria: Composite soil samples do not exceed the Remediation Criteria. Samples collected at the following frequency:
* One sample per 50 linear metres of excavation; and,
* One sample per stratum or at 1.0m vertical intervals (whichever is the greater)
* One sample per 100x100m extent of excavation base
* One sample per 2,000m3 of stockpiled excavated Made Ground.
* One sample per 2,000m3 of stockpiled crushed site aggregate
Accumulated NAPL: Ensure that no NAPL is present on groundwater as far as is reasonably practicable: No visible NAPL to be recorded on groundwater or accumulated water as far as reasonably practicable (To consider that further free phase recovery is not reasonably practicable, it should be demonstrated that free phase recovery rates have diminished to asymptotic conditions.)
Page 25: NAPL Impacted Materials
Materials impacted with NAPL are likely to be excavated as part of the enabling earthworks. The most sustainable use of these materials is to facilitate their reuse on site and as such treatment is required to make them suitable for use such that they do not represent a potential ongoing source of contamination. The volumes of materials for treatment have been estimated at between 0-5% of the predicted soil arisings, however due to the discrete nature of these impacts, materials will potentially be generated throughout the works and in variable amounts.
The exact approach to treatment of materials impacted with NAPL will be influenced by a number of variables including volume of material, contaminant loading, contaminant properties and timescales. A number of remediation technologies are available that could technically, operationally and commercially be employed to meet the remediation objectives and make the NAPL impacted soils suitable for re use at the site, these include;
* Ex situ bioremediation;
* Stabilisation
* Soil Washing;
* Ex situ thermal remediation (smouldering combustion or thermopile); and,
* Excavation and disposal
Where soils are visibly impacted with NAPL or onsite screening / testing indicates the presence of NAPL they shall be consigned for treatment either on site or within a wider project treatment hub
R/2021/0713/CD - Enabling Earthworks and Remediation Strategy Report - August 2021
To address the South Bank Initial Freeport Area - remediation of the site Arcadis produced the document Enabling Earthworks and Remediation Strategy Report - [10035117-AUK-XX-XX-RP-ZZ-0348-01-Initial_Freeport_Rem_Strat
It should be noted that DNAPLs are not mentioned in the report' and as such DNAPLs can not be considered to have been addressed in the strategy proposed. NAPLs are extensively mentioned, but LNAPLs (light non-aqueous phase liquids) and DNAPLs require different approaches to remediation. 'No evidence of DNAPL appropriate remediation is present in the strategy.
Page 12 - Materials Impacts with Non-Aqueous Phase Liquids
Although limited evidence of NAPL and tar have currently been identified within the wider South Bank landholding, it is reasonable to anticipate that it may be encountered locally, in line with adjacent plots of the wider South Bank site. In the wider South Bank site NAPL and Tar have been identified primarily within the Made Ground and associated with subsurface or former above ground structures and plant. Further consideration of the NAPL with respect to the risk to human health will be needed as part of the remedial strategy.
Materials impacted with NAPL and tar should not be reinstated due to being a primary source of contamination. The impacted materials will be required to be consigned to a treatment process to remove the NAPL element or disposed of at an appropriate waste facility under duty of care.
Page 17 - Remediation Objectives
The relevant objective only talks of directly dealing with NAPL soils, no approach to dealing with DNAPL sources etc..
Manage the contamination in excess of screening levels and that are likely to be present following completion of ongoing ground investigation, including NAPL containing soils.
Page 18 - Enabling Earthworks
Treatment of soils impacted with NAPL (if present) in order to make them suitable for reuse.
Page 22-23 - Removal of NAPL on Groundwater
The explicit assumption that an asymptotic curve has been taken to show that all NAPLs have been removed is inappropriate unless all NAPL components have identified in the DNAPL source and their relative quantities ascertained, which is not possible. A different approach should have been taken for a site containing DNAPLs, required active removal of the DNAPL source or the installation of an impermeable barrier with active groundwater removal and treatment around the whole site.
If free phase NAPL is encountered on the groundwater during excavation works, the Contractor shall undertake recovery prior to groundwater discharge. The Contractor shall continue the NAPL recovery process until no visible NAPL is observed or further recovery is not reasonably practicable (evidenced by diminishing recovery quantities i.e. base of asymptotic curve).
Where there is evidence of the presence of NAPL in the unsaturated zone, excavations will be extended to expose the groundwater table and identify if it is impacted by the above material and if groundwater treatment is required.
Page 23 details a DNAPL ignorant approach to sampling, as DNAPL sources will localised and randomly distributed across contaminated areas of the site.