Non-Aqueous Phase Liquids (NAPL) are organic liquids with relatively low water solubilities that can exist in the sub-surface as a separate fluid phase. Their density, relative to water, determines whether they are considered Light Non-Aqueous Phase Liquids (LNAPL) or Dense Non-Aqueous Phase Liquids (DNAPL).
Most fuels (petrol, diesel, kerosene etc), hydraulic and lubricating oils, and some solvents such as benzene and toluene are examples of liquids that form LNAPLs whereas chlorinated solvents, polychlorinated biphenyl (PCB) oils, coal tars and creosotes form DNAPL. Most NAPLs are considered contaminants of concern with the ability to cause adverse effects on both human and environmental receptors.
Whilst direct surface spills happen, corroded or damaged storage tanks or pipelines are the source of most NAPL contamination. Site geology determines the initial migration of any NAPL spill, driven by gravity, until it reaches the water environment at which point the density has a significant bearing on the outcome.
LNAPL is somewhat predictable, settling on top of groundwater (or surface water) with route and rate of migration being directed by movement of the water body. As it migrates the LNAPL is an ongoing source of contamination created though volatilisation and dissolution as well as via direct contact.
DNAPL is much less predictable with relative permeability, cracks and fissures influencing the spread of the contamination.
This behaviour makes DNAPL much more difficult to detect with its presence often inferred from site data rather than physically identified.
NAPLs constitute an ongoing source of contamination, with pollutant linkages created though volatilisation and dissolution as well as via direct contact. They usually require detailed investigation, characterisation and remediation. This article focusses on remediation options. All of these techniques have been employed by ERS with strategies and detail designed on a site specific basis.
Direct LNAPL Recovery by Surface Water Skimming
Where LNAPL is present on a surface water body it can be directly removed from the water using a skimmer which sits on the water surface and removes mostly NAPL and only a little water. A variety of booms and barriers can be used to contain the LNAPL and assist recovery. This can also be used in excavations where the groundwater table is exposed and is particularly useful when groundwater is shallow and soil excavation is straightforward.
LNAPL Recovery from the sub-surface
With deeper groundwater or where excavation is impractical, LNAPL recovery can be achieved through the use of either trenches or wells. This can be achieved in a number of ways.
Skimming
NAPL can be skimmed from the surface of the well or trench using either oligophilic belt skimmers which attract the NAPL or hydrophobic pumping units which only permit the entry of the NAPL. This then transports the NAPL to a collection point for disposal.
Water Pumping
Skimmers will only remove the NAPL which accumulates, and it is often necessary to create a gradient which will enhance the recovery. In a trench groundwater movement may be enough, but in wells water pumps are used to create a hydraulic gradient toward the well and thereby promote NAPL migration. These pumps can either be Total Fluid Pumps which recover a mixture of NAPL and water, or water only pumps combined with a separate NAPL skimmer. These systems often require detailed controls to optimise recovery and, if Total Fluids Pumps are employed, an efficient oil / water separation system.
Vacuum Enhanced Product Recovery
Water pumping draws down the NAPL within the soil matrix creating or increasing a smear zone, and whilst this can be controlled there are some situations, such as in a fractured bedrock, where you wish to reduce it. Applying a controlled vacuum can create a pressure gradient towards a well whilst minimising drawdown. Again, this can be combined with Total Fluids Recovery or separate water / NAPL pumps.
Multi-Phase Extraction
Whilst Soil Vapour Extraction (SVE) can be used to remove residual NAPL from the vadose zone when the contaminants of concern are volatile, Multi-Phase Extraction (MPE) is preferred for NAPL removal. Here both fluids and vapours are recovered simultaneously, with the drawdown achieved by fluid recovery exposing more vadose zone for vapour recovery.
Enhancing NAPL recovery
A number of techniques can be used to enhance NAPL recovery. However, they all can increase mobility and therefore a detailed understanding of the site hydrogeology is necessary to avoid exacerbating the situation.
Water Flooding – injecting water into the periphery of a plume, often recycling treated water, can be used to enhance recovery by increasing the hydraulic gradient towards the recovery point.
Surfactant or solvent use – Surfactants or solvents can be used to increase mobility or solubility of contaminants increasing movement towards recovery points.
Heat – A variety of thermal remediation techniques can be used to decrease viscosity and thereby increase mobility, or can also be used to increase solubility or volatility permitting recovery by a pumping system.
Heat can also be used to directly destroy NAPL contaminants.
Bioremediation – Bacteria tend not to penetrate significant bodies of NAPL but can produce surfactants which enhance recovery as well as degrading dissolved phase and vapour phase contaminants, thereby potentially breaking significant pollutant linkages.
In-Situ Chemical Oxidation – In-Situ Chemical Oxidation (ISCO) can be used effectively to degrade organic contaminants but is usually considered too costly for more than a small quantity of NAPL and is usually considered more appropriate for residual NAPL and dissolved phase.
Containment
Where NAPL recovery is difficult it can be considered acceptable to break pollutant linkages by containment with a variety of techniques employed, from physical containment by in-situ barriers (sheet piles or concrete or slurry walls) or stabilisation where bentonite, cement and other agents are mixed directly with the NAPL contaminated soils. Activated carbon based products are also used to adsorbs NAPL to minimise migration and provide a matrix where bacteria can proliferate and gradually deplete the contaminants.
DNAPL Remediation
With the obvious exception of surface skimming, many of the techniques are suitable for DNAPL recovery. However, the difficulty in predicting the pathways taken by DNAPL mean that often recovery is incomplete and source depletion or pathway disruption techniques such as thermal with SVE, or in-situ chemical/biological treatments are often preferred,
With so many options, it’s important to ensure that you have an experienced contractor who can provide the correct solution for your site. In complex cases, it is often necessary to combine treatment methods. ERS has in-house engineers who can design bespoke treatment trains to meet the site requirements especially where remediation targets are onerous, for example in ecologically sensitive sites or where volatile plumes are present beneath occupied buildings.
Below are a few examples of NAPL recovery techniques employed by ERS:
New Street, Jersey
Hydrocarbon contamination was discovered during piling works at this retail development site in Jersey. Subsequent investigation and monitoring revealed a body LNAPL in the vicinity of former diesel tanks that had contaminated the groundwater and needed remediation. ERS designed and installed an automated free product removal system powered by compressed gas which removed NAPL and contaminated groundwater through installed extraction wells.. The system allowed the wider construction works to continue around the remediation area and ran for 4 months. This reduced the NAPL thickness by 95% and prevented further migration of the plume to the satisfaction of the local authority. Download the case study to learn more.
Dundee Distribution Yard
This was a geologically complex site, with a fractured sandstone aquifer and a major fault on the site boundary risking off-site migration of an extensive free-product plume. As this was also a live distribution depot with a call centre located on-site, much of the drilling was carried out at night and all headworks and pipelines were buried to minimise disruption. A vacuum-enhanced product recovery system (VEPR) was then installed to recover the free-phase hydrocarbons and treat contaminated groundwater above-ground. Use of the VEPR in combination with water draw down allowed recovery of product from the fractured bedrock and the overlying glacial till. Controlling groundwater during the treatment phase minimised the risk of the plume migrating off-site.
Nigg Yard
At this coastal site, underground fuel pipes had been constructed in an area of dredged, reclaimed soils with a high salt content, which led to corrosion and the loss of 250,000 litres of diesel. ERS was appointed to design and install a system to initially prevent migration of diesel towards the nearby dry dock and ultimately remove it from the site. First a site investigation was carried out to determine the extent of the NAPL. This was followed by groundwater modelling to design and optimise a groundwater control and NAPL recovery system. Around 150,000 litres of diesel was recovered. Treated groundwater was then re-injected along with bacteria (bioaugmentation) to enhance recovery through biosurfactants and continue to bioremediate dissolved phase diesel. Download the case study to learn more.
Printworks, Slough
Free-phase toluene was discovered at the site of a former printworks in Slough post-demolition. Further site investigations identified a large body of LNAPL impacting groundwater both on and off the site. ERS was designed, installed and operated a pump-and-treat system to meet the site remedial objectives. The complex geology required a combination of impeller pumps for groundwater manipulation and pneumatic pumps for recovery of toluene NAPL. The pumped water was then fed through an oil-water separator, with the water then further treated before discharge to the sewer under consent. Chemical agents were then injected into the sub-surface to remove residual toluene. Download the case study to learn more.
Hamilton – DNAPL
This former industrial site in Hamilton was being remediated after factory closure and in preparation for re-development for housing. Chlorinated solvent spills led to the presence of DNAPL and dissolved phase contamination impacting a nearby watercourse. ERS worked with a US based specialist to install a thermal treatment system which heated the site soils and groundwater, volatilised the contaminants and facilitated recovery though an MPE system. Download the case study to learn more.
Markinch – DNAPL
At this small site, coal tar from a former gas holder structure was discovered close to the surface in private gardens. Contaminated surface soils were removed, and a nearby burn was bypassed to allow remediation of the bank. Following this, the plan was to install extraction wells in the area of the old gas holder, however the discovery of a void halted drilling after only a few wells had been completed. To overcome this, our team engineered a new remediation solution using expanding geopolymer resin to stabilise the gas holder backfill and simultaneously displace the coal tar DNAPL towards the extraction wells, following which it was recovered by a combination of pumped and vacuum extraction. Read more about this challenging project on our website.
Current Projects
ERS is currently remediating NAPL at several sites across the UK. This includes a pump and treat system recovering a complex mixture of NAPLs from a former landfill site, and several borehole skimming systems recovering diesel, heavy fuel oil and hydraulic oil at a variety of sites including an active refinery, a council depot and a former manufacturing site. Keep an eye out on our social media channels for updates on these projects.
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