Keywords

1.1 Introduction

Petroleum hydrocarbons (PHCs) are among the most common and widespread subsurface contaminants in urban and industrial environments. PHCs are primary compounds found in crude oil and its refined products. These organic liquids contain a large number of different hydrocarbons with the most common being alkanes, cycloalkanes (naphthenes), aromatic hydrocarbons, and more complex substances like asphaltenes. Some common products and liquid wastes containing PHCs include:

  • Gasoline/petrol;

  • Jet fuels (Jet-A, JP-4, JP-5);

  • Kerosene/heating oil no. 2;

  • Diesel fuel;

  • Heating oil no. 4 (Bunker A) and no. 6 (Bunker C);

  • Hydraulic oil, cutting oil, lubricating oil, mineral oil, dielectric fluid, dielectric mineral oil, and transformer oil;

  • Crude oil and other PHC mixtures;

  • Biofuels;

  • Waste oil, waste vehicular crankcase oil; and

  • Creosote, coal tar, and bitumen.

Residential, industrial, or transport activities that use these petroleum products may be sources of contamination through accidental or intentional spills, releases from above ground and underground storage tanks, and leaks from associated pipelines. Petroleum releases can occur at gas stations, refineries, bulk-product terminals, manufactured gas plant facilities (gasworks), airports, homes heated with oil-burning devices, pipelines, chemical manufacturing facilities, landfills, clandestine disposal sites, metal cutting facilities, automotive repair shops, oil drilling pads, and farms.

PHC products typically comprise complex mixtures of predominantly non-polar organic compounds that together form organic liquid fuels or oils. These liquids are commonly referred to as “non-aqueous phase liquids” (NAPLs), are relatively immiscible (sparingly soluble) with water, and can persist in the subsurface as sources of soil and groundwater pollution for decades. A defining property of the NAPL is its density relative to water. NAPLs are classified as either lighter than water (LNAPL) or denser than water (DNAPL). Most petroleum NAPLs are LNAPLs. As such, upon reaching the capillary fringe and the water table, LNAPL will laterally spread with limited invasion below the water table due to its buoyancy. A DNAPL such as some coal tars or creosotes, on the other hand, may penetrate vertically downward well beyond the water table through the saturated zone.

Mass transfer, natural attenuation, and natural source zone depletion (NSZD) phenomena are also critical to understanding the fate and transport of PHCs in the subsurface. Partitioning of LNAPL constituents into other phases (water, gas) result in aqueous and gas phase plumes of organic compounds that could pose risks to human health and the environment. Important classes of PHCs that are of environmental concern include the more soluble mono-aromatic hydrocarbons (i.e., benzene, ethylbenzene, toluene, and xylenes (BTEX)), polycyclic aromatic hydrocarbons (PAHs) (e.g., naphthalene, fluorene, and anthracene), and gasoline additives such as the fuel oxygenate methyl tert-butyl ether (MTBE). Microbial degradation (biodegradation) processes have been frequently found to play a pivotal role in mitigating risks derived from PHC contamination.

As conceptualized in Fig. 1.1, when LNAPL is released into an unconsolidated porous medium, it will tend to migrate downwards through the unsaturated zone under the influence of gravity with its flow path affected by variations in water content and the porous medium’s properties. For instance, the presence of a finer-grained layer acting as a capillary barrier will cause lateral spreading of the LNAPL at the interface between layers. If the LNAPL pressure builds and is able to overcome the non-wetting fluid capillary entry pressure of the finer-grained layer, then it will break through the finer-grained layer. Otherwise, it will flow along the top and eventually around the finer-grained layer.

Fig. 1.1
An illustration of the L N A P L release into the soil layers. It leads to perched L N A P L, mobile L N A P L, and entrapped L N A P L in the liquid-unsaturated zone with N A P L, liquid-saturated zone with free L N A P L, and liquid-saturated zone with entrapped L N A P L that produces various climatic effects.

Conceptual LNAPL architecture in the subsurface following a point release, along with petroleum hydrocarbons fate and transport mechanisms (central figure); LNAPL mobility as a function of relative saturations of air, water, and LNAPL (left circles); factors modulating in-situ petroleum hydrocarbon biodegradation (top right circle) (adapted from Lundy and Gogel 1988; Rivett et al. 2011; CL:AIRE 2014; Vila et al. 2019; García-Rincón 2020)

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If sufficient LNAPL volume is present, the LNAPL will continue to flow downwards to the water table. Once the LNAPL reaches the water-saturated capillary fringe, downward movement of LNAPL and buoyancy forces can displace some water in the capillary fringe and in the uppermost water-saturated zone below the water table. Lateral LNAPL spreading will occur due to density contrasts with some bias of the main LNAPL body down the hydraulic gradient. This would not preclude migration in other directions in response to geological heterogeneities and preferential migration pathways in the porous or fractured geologic media and influence of local gradients induced by LNAPL pressures. The LNAPL body will eventually stabilize as the LNAPL pressure heads and gradients weaken due to LNAPL redistribution and NSZD processes.

Water-table fluctuations due to daily and seasonal variations or local groundwater pumping induce vertical migration of mobile LNAPL and the creation of an LNAPL “smear zone”. As the water table falls, LNAPL mass will be transferred downward while leaving behind a trail of residual LNAPL (not occluded by water) retained in the pores. As the water table rises, part of the LNAPL will move upward, while other LNAPL mass becomes entrapped (occluded by water) due to capillary forces. Depending on the amplitude of the groundwater fluctuations, LNAPL may be trapped meters below the elevation of the mean water table in the saturated zone. In addition, residual LNAPL will be retained in pores that the LNAPL encountered as it migrated through the unsaturated zone. The amount of residual and entrapped LNAPL is a function of porous medium properties, LNAPL fluid properties, and fluid saturation history. The cross-sectional illustration of an LNAPL body in Fig. 1.1 has been shaped by one or more water-table fluctuations.

In general, water preferentially wets the geological formation with respect to LNAPL and air (which commonly are the intermediate-wetting fluid and non-wetting fluid, respectively) except in very dry soils, where water may be the non-wetting fluid. Released LNAPL to the subsurface is hence usually non-wetting preferring to occupy the more spacious pore body rather than pore throats, thereby minimizing contact with the solid grains that may remain water coated. With repeated spills, the LNAPL may transition from a non-wetting to a wetting fluid in time. Changes in LNAPL chemical composition may also change its wettability. As a result of all these factors, LNAPL architecture varies dramatically between sites and across geological settings. This is generally illustrated in the conceptual models of CL:AIRE (2014) depicting LNAPL behavior for a number of geologic settings, including mildly to highly heterogeneous unconsolidated porous media (e.g., sands to glacio-fluvial sands and gravel) and low and high matrix porosity fractured bedrock (e.g., granite to sandstone).

As shown in Fig. 1.1, a dissolved or aqueous phase plume will develop in the capillary fringe and in the saturated groundwater zone, and a vapor or gas phase plume will develop in the unsaturated zone. Since most LNAPLs are “multicomponent” comprising complex mixtures of PHC compounds, both the aqueous and gas phase plumes will likewise be multicomponent in nature. The composition of the aqueous phase plume is controlled by the dissolution rate and the effective solubility of each LNAPL component that is LNAPL composition-dependent. The composition of the gas phase plume is controlled by the volatilization rate and the effective partial pressure of each LNAPL component which is likewise LNAPL composition dependent. As the LNAPL weathers, the more soluble and volatile components in the LNAPL will become depleted, and the remaining LNAPL will become enriched with progressively less soluble and volatile compound constituents. The change in composition may influence physical properties such as LNAPL density and viscosity.

The significance of microbial degradation (biodegradation) to PHC sites cannot be overstated. While biodegradation of individual PHC compounds varies, most hydrocarbons are relatively biodegradable across a range of aerobic and anaerobic conditions ranging from nitrate reducing to methanogenic. The contribution of biodegradation to the natural attenuation of dissolved-phase hydrocarbon plumes has long been recognized and may cause sufficient attenuation of plumes prior to their reaching receptors at risk to justify the use of monitored natural attenuation (MNA) as a viable site management strategy (Wiedemeier et al. 1999; USEPA 1999b; ASTM 2004; CL:AIRE 2014; ITRC 2018). It has only been in recent years, however, that the significance of biodegradation in LNAPL source zone areas has been recognized leading to the emergent adoption of NSZD monitoring as a management option to complement or even replace active remedial interventions (Rivett and Sweeney 2019). In particular, the role of methanogenesis and direct outgassing from the LNAPL source circumventing the assumed requirement for hydrocarbons to dissolve to be biodegraded was overlooked for many years and may lead to significant vapor-based NSZD far exceeding the aqueous-based depletion (Smith et al. 2022).

Effective LNAPL problem conceptualization within specific PHC-contaminated site scenarios underpins the management of risks and appropriate intervention remedy selection. Recognition needs to be made of the breadth of risks or drivers at sites, including nuisance impairment of beneficial use of resources or aesthetic values, societal community concerns, and business factors including reputational risk and problem persistence leading to intergenerational equity issues (CL:AIRE 2014). One possible approach to risk evaluation discussed in CL:AIRE (2014) is to categorize risks by “LNAPL saturation” (largely controlling LNAPL mobility) or composition. “Saturation” risks occur when pore-space LNAPL saturations exceed residual or entrapped saturation thresholds enabling LNAPL mobility and risks of migration of the LNAPL body to receptors resulting in, for example, seepage of LNAPL oily films into surface waters, or LNAPL contamination of supply wells. Addressing saturation risks are often an initial site priority. Composition risks, on the other hand, derive from the LNAPL chemical composition, which may include constituents that have toxicity to human health or the environment and may pose direct or indirect, chronic, or acute risks. Certain constituents may also be highly volatile and flammable and therefore drive explosive risks. Composition risks hence include the migration of toxic constituents such as BTEX or PAHs within dissolved-phase plumes to supply wells or within vapor plumes migrating toward and accumulating with PHC degradation products such as methane in buildings and posing explosive risks. It is recognized within this simple bi-classification of risks that the mobility of the LNAPL will still be influenced by its composition as composition determines physical properties such as viscosity and interfacial tensions; however, such changes are usually very gradual.

The remainder of this chapter will provide a more detailed overview of challenges associated with:

  • Problem recognition and regulatory issues (Sect. 1.2);

  • Multiphase flow mechanics (Sect. 1.3);

  • PHC chemical makeup and interactions (Sect. 1.4); and

  • Geological and hydrogeological controls (Sect. 1.5).

1.2 Problem Recognition and Regulatory Environment

1.2.1 Problem recognition—The Case of Large Oil Spills

Contaminated site assessment and management has evolved to being a multidisciplinary field dealing with hydrogeological, chemical, biological, fluid mechanics, and management topics, often requiring subject-matter experts from a variety of backgrounds including environmental, chemical, and civil engineering, geology, microbiology, toxicology, statistics, legal, and project management.

Subsurface contamination by PHCs can be a result of leaking storage tanks and pipelines, accidental releases, or intentional disposals. Without accurate wet-stock reconciliation or leak-detection systems, subsurface releases can go unnoticed for years, an “invisible problem” that can be costly, time-consuming, and difficult to treat after it is discovered. Often, PHC cleanup reminds the general public’s mind of large oil spills, nevertheless, cleanup of local gasoline stations is the predominant type of events managed by the soil and groundwater PHC remediation industry.

Large oil spills galvanize significant media and public attention because they have catastrophic local environmental and human health impacts and, when on land, may create complex surface water and groundwater remediation problems that can take years of effort to manage the environmental impacts. Examples include the 1989 Exxon Valdez oil tanker collision into a reef offshore of Alaska, USA, that drew much media coverage and attention to the detrimental environmental impacts that occurred to 2000 km of the intertidal beaches and marshes along the shoreline of Prince William Sound (Xia and Boufadel 2010). The 2010 Deepwater Horizon explosion and oil rig blowout illustrates the potential consequences of not following established safety protocols. The heavy oil spill from the tanker Prestige in 2002 affected 1900 km of the coast in the northwest of Spain, devastating the local economy and demonstrating the gargantuan effort required to restore conditions to a pre-spill state. One of the earliest and largest oil spills on land is the seemingly forgotten 18-month long hydrocarbon eruption in 1910 at Lakeview Gusher #1 in California, USA, where oil-soaked soils are still present. The United Nations Environment Program (UNEP) released a report in 2011 documenting the environmental degradation of the land and waters of the Niger River Delta in Ogoniland, Nigeria, associated with operations of the multinational petroleum industry in the area since the 1970s. This document set out urgent recommendations for cleanup and protection of drinking water and health of exposed populations. The cleanup effort is nevertheless far from being completed. Other examples of oil spills associated with unstable sociopolitical environments include the Kuwaiti oil fires caused by Iraqi military forces during the Gulf War in 1991 and the multiple attacks to the Caño Limón–Coveñas crude oil pipeline in the Arauca Department, Colombia.

Some large oil spills have yielded significant research opportunity, a foremost example of relevance to the soil and groundwater remediation community being the terrestrial oil pipeline rupture near Bemidji, Minnesota, in 1979 (Fig. 1.2). The release consisted of 1,700,000 L of crude oil, of which 400,000 L remained in the subsurface after the initial cleanup works in 1980. In 1983, the United States Geological Survey (USGS) established the National Crude Oil Spill Fate and Natural Attenuation Research Site, which has produced decades of data and has been paramount for the current understanding of natural processes at PHC-contaminated sites. The ongoing USGS research project has four primary objectives: (I) characterizing the nature, toxicity, and prevalence of partial transformation products emanating from the crude oil source; (ii) evaluating the secondary impacts (such as arsenic cycling) of biodegradation; (iii) understanding the timeframe of natural attenuation of PHC source zones; and (iv) developing field tools, methods, and data that support evaluations of environmental health effects of natural attenuation of crude oil (USGS 2021). The numerous research outcomes produced at Bemidji illustrate the importance of establishing experimental sites for the study of subsurface contamination phenomena.

Fig. 1.2
A photograph of a wetland has 2 barrels connected to pipes with 3 people near them. The ends of the pipes are submerged in the water.

Crude oil reached a wetland near Bemidji after the rupture of an oil pipeline. Initial efforts to remove the floating oil were conducted by the pipeline company in 1979 using pumps and other machinery (photo courtesy of Jared Trost, USGS)

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The great proportion of PHC-contaminated sites requiring cleanup are relatively small sites in industrialized countries. In the United States alone, there are 135 reported operating refineries and over 133,000 gas stations. Between 1998 and 2021, 564,767 petroleum releases from underground storage tanks (USTs) were reported, and 85% have been addressed to meet regulatory requirements (i.e., “closed”). (UST FAQ www.epa.gov/ust, accessed on May 28, 2022 https://www.epa.gov/ust/frequent-questions-about-underground-storage-tanks#gen11).

1.2.2 Regulatory Frameworks

In the context of a country’s regulatory framework, oil spills on land are sources of soil and groundwater contamination often brought to light during environmental assessments driven by real estate transactions, suspicion of chemical leakage from existing tanks or historical land use, or as a precautionary action prior to future land use. The need to meet regulatory compliance and achieve cleanup standards agreed to mitigate risks is often a key remediation driver. Risks do not just relate to human health but also the environment, assets, reputation, and financial aspects.

Environmental law is often created in response to high-profile events in which the press generates public awareness (Wargo 2009). Once an issue gains societal recognition as a problem, it may get on the docket of government institutions through organized interest group pressure and eventually may receive enough attention by governing policymakers for decisional action (Kraft and Vig 2010). For example, the wide distribution of Rachel Carson’s famous book Silent Spring in the early 1960s, warning about the compounding effect pesticides have on the environment focused public opinion sharply and likely incentivized the creation of the United States Environmental Protection Agency (USEPA) that now sets corrective action guidelines with respect to contaminated soils and groundwater in the United States. Extraordinary events, such as the Cuyahoga River in Ohio catching fire thirteen times between 1868 and 1969 due to petroleum NAPL on its surface, prompted establishment of the National Environmental Policy (NEPA), which mandates that federal agencies assess the potential environmental impacts of their actions and the Clean Water Act to reduce emissions from industrial facilities (Wargo 1998). Arguably the most pivotal has been the infamous New York State Love Canal (landfill) Incident in the 1970s soon after which groundwater contamination problems emerged throughout the United States where public interest and perception changed dramatically (Hadley and Newell 2012). The incident was an important catalyst for the cleanup of contaminated aquifers in the United States as it prompted the passing of the Superfund Act, technically known as the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA), which was passed in 1980. Its notoriety raised the profile of groundwater contamination internationally.

Parallels in contaminated land and groundwater legislations to manage PHC and most contaminants are evident amongst countries in North America, Europe, and in Australia. Although PHCs and LNAPL contamination are rarely directly specified within such legislations, oil/fuel storage and transport-specific regulations enable groundwater protection and are indirectly concerned with PHCs and LNAPL. Similarly, many countries have agency–government (or related national organizations) published non-statutory guideline documents specific to PHC- and LNAPL -contaminated sites that guide the assessment, characterization, and remediation of these sites specifically. By way of example, specific regulatory acts that influence PHC remediation practices in the United States are summarized below:

  • The Safe Drinking Water Act (SDWA) sets standards for drinking water quality which consider cost, technological feasibility, and health goals (USEPA 1999a). Despite the intention that maximum contaminant limits (MCLs) had been developed to evaluate public water supply systems, the urgency of addressing the ever-increasing scope of groundwater contamination soon led to widespread use of MCLs as cleanup goals for all aqueous groundwater plumes, even those not threatening water supply systems (Hadley and Newell 2012). These stringent goals are often hard to achieve with the technologies available and our current understanding of subsurface processes.

  • The Resource Conservation and Recovery Act (RCRA) determines corrective actions that individual states adopt, adapt, and enforce through their own agencies to address the release of hazardous chemicals. USEPA guidelines under RCRA state that cleanup should “limit the risk to human health first and then (…) ensure protection [to humans and aquatic receptors] based on maximum beneficial use of the groundwater at a particular facility [such as drinking, agriculture, discharge to adjacent groundwater or surface water bodies]” (USEPA 2004).

  • Directly relevant to NAPL risk management is regulation regarding the condition of underground storage tanks (USTs). This relies on local enforcement to find and clean up current and past leaks. For example, the code of Federal Regulations (CFR) pertaining to UST sites requires the removal of NAPL, which is referred to as “free product,” to the maximum extent practicable (MEP), a definition actually left to the discretion of the “implementing agency (40 CFR §280.64).”

  • The oil spill prevention program is set up to assist facilities to prevent spills and to implement countermeasures to control spills should they occur through the Spill Prevention, Control and Countermeasure Plan (SPCC).

  • The Small Business Liability Relief and Brownfields Revitalization Act stems from the USEPA’s 1995 Brownfields Program that, through the use of grants, helps states, tribes, communities, and other stakeholders work together to prevent, assess, clean up, and sustainably reuse properties that may be impacted by the presence or potential presence of hazardous substances (i.e., Brownfield sites).

  • The Superfund Act is a federal program to investigate and remediate contaminated sites that have been placed on a National Priority List (NPL) based on complexity, a preliminary Hazard Ranking System (HRS), or a health advisory from the Agency for Toxic Substances and Disease Registry (ATSDR).

Similar types of legislation are mimicked internationally to varying degrees. In some countries, contaminated land regulations are based on laws approved at national level and their implementation and the manner in which they are interpreted are often left to federal agencies. In contrast, in other countries jurisdiction for legislation may be at a state or regional level rather than national. Either way, both often create differences between regions and pose much of the burden of implementing environmental protection work on state and local governments. For example, the Brownfield Act in the United States relies on state voluntary response programs, through which states create incentives to assist in the cleanup and reuse of brownfield properties and ensure resources for long-term monitoring and other needs.

Globally, numerous published (often non-statutory) guidance developed nationally or at state/regional level exist for investigating, assessing, understanding, and addressing the presence and migration of NAPL releases. Plans typically require that sites achieve a permanent mitigation solution in terms of eliminating significant risk or removing oil and hazardous material to the most feasible extent. Some jurisdictions may set LNAPL recovery goals to the highest potential or “ceiling” values as determined from site-specific estimates of LNAPL and hydrogeological properties, while others determine that NAPL is of concern when the apparent LNAPL thickness measured in a monitoring well exceeds a certain thickness. Such approaches may not always be supported by the technical literature and current multiphase flow understanding.

1.2.3 Toward Improved Management and Regulation of PHC-Contaminated Sites

LNAPL treatment remedies focus on removing, destroying, or stabilizing LNAPL, particularly when this LNAPL poses risks if it is mobilized or can form dissolved-phase and vapor plumes of toxic constituents. Since removal is often technically and/or economically unfeasible, the industry seems to increasingly favor in-situ treatments addressing the LNAPL source zone or focusing on the aqueous or vapor-phase PHC plumes when the only objective is to create a barrier to protect sensitive receptors against contaminant exposure. In many cases, actively monitoring a contaminant plume suffices as a contamination management strategy, in which cases supporting evidence is typically gathered to show that risks are controlled.

Creating a regulatory framework is usually a political, long, arduous, and constantly evolving process. One of the factors influencing this evolution is the technical and economic challenges encountered by remediation practitioners. For example, the presence of NAPL in combination with unfavorable complexities has encouraged stakeholders to advocate to modify cleanup criteria or create technical impracticability waivers. In the United States, for example, there are various alternative cleanup end points, where instead of meeting a drinking water maximum contaminant levels, site-specific risks to potential receptors are evaluated and mitigated using both remediation techniques and engineering controls. Risk-based goals are sometimes a response to the economically prohibitive resources required to eliminate contamination. In other words, remediation objectives have evolved from practically complete removal to focus on interim stages that address site-specific needs that are evaluated in terms of intended land use and stakeholder requirements, which in turn requires effective stakeholder engagement. Hadley and Newell (2012) proposed functional remedial goals that incorporate contaminant mass discharge as a remediation metric.

The adoption of sustainability practices, including prioritizing smaller environmental footprints  and integrating concepts such as environmental health, social equity, and economic vitality in the assessment of cleanup methods, has been gaining popularity in the remediation industry. In countries like Australia, Brazil, South Africa, the United Kingdom, and the United States, among others, the management and remediation of contaminated soil and groundwater is sometimes incorporating sustainability concepts into their guidelines and/or risk-based regulatory frameworks (Defra 2012; Scott and McInerney 2012 as cited in CL:AIRE 2014). The design of resilient remediation systems is also gaining traction as there is greater awareness on how climate change, extreme weather events (ITRC 2021), and unstable sociopolitical environments may negatively impact existing systems.

Along with sustainability and resiliency, environmental justice and indigenous and nature rights are philosophies that may influence contaminated site management. Values brought forward by the environmental justice movement have gained momentum since it first emerged in the United States in the late 1980s after the publication of Toxic Waste and Race, a study exposing disparities in the burden of environmental degradation and pollution facing minority and low-income communities (Commission for Racial Justice of the United Church of Christ 1987). The United States government begun addressing this issue by creating environmental justice offices in the EPA and the Department of Justice. Similarly, values adopted by indigenous movements that may drive changes in regulations and potentially groundwater cleanup include nature’s rights and concepts such as Sumak Kawsay, a Quechua value embracing ancestral, communitarian knowledge and lifestyle. Based on this philosophy, the 2008 Constitution of Ecuador incorporated the concept of the rights of nature, as did the 2010 Law of the Rights of Mother Earth in Bolivia.

1.3 Multiphase Flow Mechanics

An understanding of multiphase flow processes in water-NAPL-gas systems (see Chaps. 2, 3, and 17) is paramount for the development of sound conceptual site models (CSMs) as well as the design of effective remediation strategies. As depicted in Fig. 1.3, NAPL, water, and gas coexist in the pore space. NAPL behavior is understood through the interaction of forces in the system that are a function of each fluid’s properties such as viscosity, density, surface and interfacial tension, wettability, and physical properties associated to a chemical composition.  For a comprehensive introduction to multiphase flow mechanics in porous media, the reader is referred to Corey (1994).

Fig. 1.3
An illustration has various color gradient particles of different sizes. The concentration of the particles is the strongest in the center.

Water-LNAPL-gas system in a sand tank where a fluorescent tracer was added to the LNAPL (photo and video courtesy of Julio Zimbron, E-Flux). See Electronic Supplementary Material for a video illustrating how some LNAPL mass became entrapped (occluded by water) after rising the water level in the tank (ESM_1)

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LNAPL behavior may sometimes be considered counterintuitive, and its oversimplification has historically led to costly consequences. Examples include the misconception that the LNAPL thickness in a monitoring well corresponds to a similar thickness of LNAPL uniformly saturating the vicinity of the monitoring well (as opposed to a more accurate elevation-dependent LNAPL saturation profile that penetrates below the water table). Similarly, in-well LNAPL thickness has often been assumed proportional to the NAPL mass in the subsurface without considering site-specific LNAPL-retention properties or the LNAPL distribution architecture.

The fate, migration, and stability of LNAPLs markedly vary by type of LNAPL, reflecting differences in their intrinsic fluid properties. Although a low-viscosity NAPL such as gasoline moves faster and perhaps further, it may still reach hydrostatic equilibrium relatively quickly. Viscous LNAPLs like a heavy oil or coal tar creosote, on the other hand, may not equilibrate within practical time frames as the LNAPL body continues to migrate very slowly. Weathering of the LNAPL may cause properties to change; for instance, viscosity and density can increase and borderline LNAPLs such as coal tar creosote may render them becoming borderline DNAPLs with time.

Rationalization of seasonal variations of observed LNAPL thickness in monitoring wells can be better appreciated when multiphase flow concepts are considered together with geological complexities. For instance, within unconfined aquifers the thickness of LNAPL in wells is usually observed to decrease as the water table rises. In contrast, within a confined aquifer, the thickness of LNAPL in wells increases as the potentiometric surface rises since the monitoring well provides a “pressure release” for LNAPL confined by the overlying aquitard. Predicting the volume of scattered LNAPL blobs and ganglia entrapped below the water table (discontinuous, water occluded LNAPL) represents a significant challenge, but is important as these represent long-term sources of contamination. The profile of LNAPL entrapped varies with water-table elevation and may lead to seasonal changes in the mass transfer of PHCs into the dissolved-phase and vapor-phase plumes.

It is possible to encounter LNAPL at significant depths below modern water tables. Such a scenario found in the LA Basin was ascribed to sites with older, historical releases of LNAPL and subsequent rising water tables since the 1950s (as also shown in the case study in Chap. 4). It is not unusual for many cities in developed nations to show rising water tables over recent decades, especially where groundwater was long used by industries that have since demised, for instance both London and Birmingham in the UK.

Quantifying subsurface LNAPL mass requires estimating LNAPL volumes at multiple locations. Site-specific uncertainties complicate this task, such as lack of knowledge of the LNAPL source and type, formation heterogeneity at the scale of centimeters, unknown historical water-table fluctuations, and delineating the depth intervals impacted by LNAPL. Such investigations when at depth may be expensive and challenging especially in consolidated aquifer systems (e.g., sandstone or fractured rock). Often three-dimensional spatial interpolation of data is required as characterization resources are usually limited to discern the detail of LNAPL architecture ideally sought.

A NAPL mass estimate represents a critical baseline parameter used to evaluate the efficiency of remediation technologies or, combining it with estimates of NSZD rates, the longevity of the LNAPL mass (see Chaps. 5 and 13). The scarcity of monitoring points, misinterpretation of in-well LNAPL thickness values and the sensitivity of multiphase LNAPL flow to permeability within intrinsically heterogeneous geological environments means that constraining LNAPL mass estimates can often be a rather “holy grail” challenge but nevertheless important to make efforts to constrain these estimates as far as possible and recognize associated uncertainties present.

1.4 Complexities Associated with PHC NAPL Composition

Refined fuel NAPLs consist of complex mixtures of potentially hundreds of compounds blended with biofuel components such as ethanol in gasoline or fatty acid methyl ester (FAME) in diesel, octane enhancers (such as MTBE), and low-volume additives (such as detergents and anti-foaming agents). Microbial products, metabolites, and other non-PHCs such as natural organic matter or additional organic contaminants like halogenated compounds can also be found at sites impacted by petroleum NAPL releases. This complicates the definition, analysis, and quantitation of bulk parameters such as total petroleum hydrocarbons (TPH) in environmental media and has led to the reliance on a few specific compounds like BTEX or certain PAHs and the adoption of fractionation approaches for risk assessment purposes (ITRC 2018). Analytical techniques such as silica gel cleanup, non-targeted two-dimensional gas chromatography with time-of-flight mass spectrometry (GCxGC-TOFMS), or high-resolution mass spectrometry, to name a few, are being increasingly employed to investigate the fate of PHCs and better inform risk assessments (Bekins et al. 2020; Bojan et al. 2021; O’Reilly et al. 2021). As discussed in Chaps. 10 and 11, molecular biological tools (MBTs) and compound-specific isotope analysis (CSIA) are also gaining popularity to forensically investigate contaminated sites, evaluate remediation performance, and/or identify the responsible party when multiple possible contamination sources exist.

The overall LNAPL composition, along with the age of the release and weathering processes (dissolution, volatilization, and biodegradation), dictates how the bulk properties of an LNAPL, such as density, viscosity, and toxicity will vary with time. Although the majority of petroleum LNAPLs or refined individual constituents (e.g., benzene solvent) are less dense than water and, in general, cease their downward migration upon reaching the vicinity of the water table the densities of coal tar and creosotes may evolve from being marginally less dense than water to more dense than water as discussed above.

Partitioning or mass transfer of individual constituent compounds from the NAPL to adjacent gaseous (air) or aqueous phases are critical processes leading to the continual formation of vapor phase plumes in the vadose zone and dissolved-phase plumes in groundwater. Mass transfer rates from a NAPL are highly dependent upon the chemical composition of the NAPL which varies substantially across the wide range of NAPL types. Both volatilization and dissolution of the individual compounds in a multicomponent NAPL will vary with time as the composition of the NAPL changes as the effective solubility and volatility of a compound depends on its mole fraction within the NAPL. Preferential dissolution or vaporization of more soluble and volatile compounds may leave the remaining NAPL progressively enriched in less soluble and less volatile compound constituents and result in an overall declining mass transfer of hydrocarbon mass with time. The most common volatile organic compounds found in PHC NAPLs that are monitored due to their known toxicity are BTEX, some PAHs, and fuel oxygenates (e.g., MTBE). The relatively high volatility and solubility of these compounds allows them to deplete from the NAPL mass fairly rapidly, eventually reducing the composition-based risk NAPL poses with respect to these compounds.

The spatial extent of dissolved-phase PHC plumes is limited by various depletion mechanisms such as dilution by dispersion and diffusion, volatilization, biodegradation, and sorption and retention. It is thus valuable to identify site-specific limitations to natural attenuation of PHCs. Because biodegradation of dissolved-phase constituents can be the dominant contaminant mass loss process under various groundwater conditions, it is critical to review potential limitations to biodegradation. These include extreme pH levels, low temperatures, insufficient electron acceptors, and the abundance of labile carbon or the presence of inhibitory compounds. Biodegradation mechanisms and rates are a function of biogeochemical conditions which can vary in time and space. For instance, conditions can relatively quickly become anaerobic or even methanogenic at the core of a contaminant plume where PHC are used by microorganisms as a carbon and energy (electron donor) source. Aerobic respiration processes thus deplete dissolved oxygen prior to degrading all the PHC and thus need to turn to nitrates, sulfates, or other reduced compounds in groundwater for terminal electron acceptors. Between the anaerobic core and a typically aerobic fringe of the plume intermediate redox zones may be dominated by sulfate-, manganese-, iron-, or nitrate-reducing metabolic processes. Nevertheless, not all compounds are readily biodegradable under certain redox conditions and biodegradation rates can be easily inhibited by lack of electron acceptors or micronutrients, pH outside of circumneutral conditions, extreme temperatures, or in the presence of inhibitory chemicals. The natural attenuation process knowledge base will also underpin effective selection of treatment technologies. For example, biodegradability of a specific PHC can determine the suitability of bioremediation approaches such as biosparging, bioventing, bioaugmentation, or anaerobic biostimulation.

With practitioner focus on quantifying natural attenuation of dissolved plumes, it has only been recently appreciated that natural depletion of LNAPL mass can be significant (Rivett and Sweeney 2019). Of key significance, has been the comparatively recent recognition and emerging acceptance that the vapor-based mass transfer from LNAPL may have been underappreciated and may be much more relevant than aqueous phase mass transfer limited by the low solubility of most hydrocarbons (Smith et al. 2022). In particular, the importance of LNAPL losses via volatilization and methanogenesis and subsequent aerobic biodegradation of methane in the overlying unsaturated zone that mitigate contaminant breakthrough at ground surface (Lundegard and Johnson 2006). Such direct losses imply that less soluble PHC components may be degraded circumventing the need for (and commonly held previous belief) these constituents to be dissolved first in order to biodegrade in the aqueous phase. Monitoring NSZD processes may represent a source zone management strategy or be used to evaluate the effectiveness of active remediation techniques against a baseline NSZD rate. In addition, the presence of microorganisms, extracellular polymeric substances, and bubbles formed during ebullition may influence LNAPL transport. For instance, bioclogging of the pore space limits the aquifer’s permeability. The evaluation of the dynamic influence on NAPL’s fate due to water-table fluctuations or shallow subsurface temperature variations across climatic zones represents research opportunities.

1.5 Geological and Hydrogeological Concepts that Help Tackle LNAPL Management Challenges

The heterogeneity of hydrogeological systems inevitably renders complex LNAPL distribution, and the likelihood of complex dissolved and vapor plume generation and fate. It is unsurprising then that the remediation of such NAPL architectures and the associated plumes is likewise challenging. CL:AIRE (2014) provides generic conceptual models for a variety of geological systems, ranging from porous unconsolidated sedimentary units to karst limestone and fractured bedrock of all types. While such conceptual models are illustrative of a specific (hydro)geology setting, site-specific characterization is required to develop a CSM to underpin effective remedy development. The most challenging geological settings are those with a high degree of heterogeneity, which can range in scale from centimeters (pore properties) to kilometers (type of sediment or rock body, tectonic setting, and climate).

The sensitivity of LNAPL migration to subsurface heterogeneity inevitably poses challenges to elucidating the detail of LNAPL migration and its retention. However, some general features may be predictable with preferential migration through coarser-textured materials and wider aperture fractures in fractured or dual porosity systems as shown by the various hydrogeological scenarios in CL:AIREe (2014). Pooling or confining of LNAPL by a system’s more contiguous, finer-textured units is also reasonably predictable.

In unconsolidated sediment, sedimentary rocks, and some extrusive igneous flows, primary physical factors are those developed at the time of initial deposition such as grain size, grain sorting, and grain shape may be determined from core logs. Secondary physical characteristics developed post deposition or eruption in response to surface weathering, burial, and movement of fluids are important to consider. Example processes affecting porosity and permeability include compaction (intergranular space reduction), cementation (pore filling by precipitation of minerals from groundwater), burrowing (mixing by organisms), dissolution (enlargement of pores or development of new pores through groundwater dissolution and removal), and fracturing (cracks or fissures from a variety of processes).

A particular challenge for remediation practitioners to overcome is the heterogeneous nature of porous media, not just in terms of porosity and permeability but also mineralogy, geochemistry, and biological microenvironments. The depositional environment of sedimentary units frequently results in sediments that vary greatly and abruptly both in the direction of sediment transport and perpendicular to it. For instance, fluvial systems typically consist of a mixture of channel bar deposits composed of sand and/or gravel interbedded with overbank mudstones, or perhaps silt or clay-rich units that may serve as aquitards restricting groundwater flows or act as capillary barriers impeding NAPL migration. This results in geological units that can be highly partitioned with interleaved aquifer–aquitard sequences with create a complex groundwater flow behavior (see Chap. 4 for further details on depositional environments).

Of concern to both contaminant fate and remediation efforts is the identification of preferential migration pathways as well as relatively stagnant or low-flow regions in which groundwater flow and contaminant transport are limited.

1.6 Summary

The ever-increasing value of groundwater as a freshwater resource, along with a better understanding of risks and sustainability issues associated to the release of PHC compounds will further justify the adoption of improved characterization and remediation approaches. By highlighting complexities encountered in this process, this chapter is not only a reminder to practitioners of the diligence required to carry out projects successfully but also invites the wider research community to continue to pursue underpinning science that helps address complexities encountered at the field scale.

Environmental policy is evolving by incorporating collaborative decision-making that involves multiple stakeholders, public–private partnerships, market-based incentives, and enhanced flexibility in rulemaking and enforcement (Kraft and Vig 2010). This complements the management and remediation of contaminated soil and groundwater increasingly being undertaken in the context of sustainable, risk-based regulatory frameworks.

Thousands of petroleum NAPL projects have successfully mitigated risk of exposure to hazardous chemicals, many sites classified as toxic have been restored to beneficial uses, and remediation has served as a profitable practice in the short and long term. The understanding of PHC contamination in the subsurface, the associated regulations protecting human health and the environment, and the associated cleanup methods and technologies have become more efficient over the years to overcome technical and economic challenges that have often delayed cleanups for significant periods of time.

Ultimately, CSMs take into account available information and the latest research about conditions and risks of contaminated sites. Because PHC remediation is complex, specialized characterization techniques are developed to locate and quantify contaminants, demonstrate degradation rates and mechanisms, forecast their persistence, and evaluate performance of remediation techniques. Management of PHC-impacted sites involves continued CSM updating, with the goal of mitigating risks by implementing site management strategies.

Still, much remains to be done. Site investigation and characterization can always be improved, risks better understood, implemented remedies more efficient, targeted, cost effective and sustainable, community awareness and education increased, and underpinning science advanced. These provide rationale for this volume and the subsequent chapters that seek to draw together a substantial part of our learning to date and identify future research required.