Contaminants

DNAPL

DNAPL is composed of chlorinated VOCs that have not dissolved in water at the site of large VOC leaks.

Chlorinated VOCs can be released into the subsurface as either aqueous-phase or non-aqueous phase liquids. Typical solvent releases include both aqueous rinseates and non-aqueous phase relatively pure solvents. If the non-aqueous phase liquid (NAPL) is more dense than water, the material is referred to as a “dense non-aqueous-phase liquid” or DNAPL.

Remediation of soil and groundwater contaminated by organic chemicals in the form of DNAPLs represents major technical, economic, and institutional challenges. The characterization and remediation of DNAPLs in groundwater can be problematic. Failure to remove residual (held under capillary forces and essentially immobile) or free-phase (mobile) product may result in continued, long-term contamination of the surrounding groundwater.

Even at low concentrations, the solute plume emanating from DNAPL can pose a threat to human health. Current conceptual DNAPL transport models suggest that when sinking free-phase DNAPL encounters a confining layer (e.g., a competent clay or bedrock zone), it can accumulate, or “pool,” and spread laterally until it encounters a fracture or an alternative path toward deeper zones. In addition, globules can enter pores and be held as a residual phase in capillary suspension. This complex mode of subsurface transport results in an unpredictable heterogeneous distribution of non-aqueous product.

Commonly used pump-and-treat remediation systems have not been effective in removing DNAPL from the subsurface or in restoring downgradient contaminated groundwater to desired levels of cleanup. However, over the past two decades, innovations in site characterization and remediation technologies have been developed and deployed at DNAPL sites. Several in situ technologies are available that can achieve substantial DNAPL source depletion either by extraction or destruction. These technologies can remove or destroy a significant fraction of the DNAPL mass in a short period. However, the efficiency of DNAPL extraction often decays exponentially with increasing mass removal.

Sources:

Kram, M.L., A.A. Keller, J. Rossabi, and L.G. Everett, 2001. “DNAPL Characterization Methods and Approaches, Part I: Performance Comparisons,” Groundwater Monitoring & Remediation, Fall.

U.S. Environmental Protection Agency, 1998. Technical Protocol for Evaluating Natural Attenuation of Chlorinated Solvents in Ground Water, Office of Research and Development, Washington, D.C., September.

U.S. Environmental Protection Agency, 2003. The DNAPL Challenge: Is There a Case for Source Depletion? EPA/600/R-03/143, National Risk Management Research Laboratory, Ada, OK, December.

Emerging Compounds

“New” contaminants are defined by regulatory agencies in an evolving process and are known as emerging compounds.

The U.S. Department of Defense has an Emerging Contaminants Program that defines these compounds as contaminants that (1) have a reasonably possible pathway to enter the environment, (2) present a potential unacceptable human health or environmental risk, and (3) either lack human health standards or have an evolving science and regulatory status. This definition includes largely unregulated compounds whose risks have only recently been recognized and previously regulated compounds which will likely have stricter regulatory standards in place because of an improved basis for setting those standards.

Some emerging compounds that Orion has researched, investigated, or remediated include:

• 1,2,3-Trichloropropane
• 1,4-Dioxane
• Hexavalent Chromium
• Naphthalene
• n-Nitrosodimethylamine (NDMA)
• Perchlorate
• Per- and Polyfluoroalkyl Substances (PFAS)

Sources:

Adamson, D. and C. Newell, 2014. Frequently Asked Questions about Monitored Natural Attenuation in Groundwater, ESTCP Project ER-201211, Department of Defense Environmental Security Technology Certification Program, Arlington, VA.

U.S. Department of Defense, 2009. “Emerging Contaminants,” Instruction Number 4715.18, 11 June.

LNAPL

LNAPL is petroleum that has not dissolved in water. When petroleum-contaminated sites are cleaned up, LNAPL often is encountered.

When released to the environment, petroleum hydrocarbons enter the soil and remain in the soil pore spaces as a separate liquid-phase mixture that is lighter than water and that does not mix with or completely dissolve into the pore water. This is known as a “light non-aqueous phase liquid.” The LNAPL occupies the pore spaces in the soil (or fractures in rock) along with water and air.

Several factors — including the volume of the release, rate of the release, hydraulic conductivity of the soils, depth to the water table, and adsorptive capacity of the subsurface materials — will determine whether LNAPL will ultimately migrate downward to the area of the capillary fringe and the water table or remain entirely in the vadose (unsaturated) zone.

As LNAPL passes through the unsaturated zone, some LNAPL will remain behind in a residual (immobile) state, having been trapped by capillary forces. This residual LNAPL in the vadose zone can be a source of dissolved-phase constituents to pore water and/or volatilized constituents to soil vapor. If a sufficient volume of LNAPL is released, it will migrate through the vadose zone, the capillary fringe, and to the water table.

Movement of LNAPL in the subsurface is very complex and is described in many publications. If enough LNAPL is present in the pore spaces to overcome the capillary forces, the LNAPL will be mobile and can potentially migrate; this is known as “mobile LNAPL” or “free LNAPL” or “free product.” Mobile LNAPL can migrate if a driving head is present; this is known as “migrating LNAPL.” If insufficient LNAPL is present in the pore spaces to overcome the capillary forces, the LNAPL will not be mobile and cannot migrate; this is known as “residual LNAPL” or “immobile LNAPL.”

The constituents, or chemicals, that compose the LNAPL may be removed over time by various mechanisms such as sorption, volatilization, and dissolution. If not removed, the LNAPL “body” can function as a potentially long-lived source zone for secondary impacts to adjacent soil, soil gas, and groundwater.

Once LNAPL has come into contact with water, individual constituents will dissolve into groundwater. The dissolved-phase petroleum constituents migrate with groundwater at a rate controlled by advection, hydrodynamic dispersion, sorption, and biodegradation.

LNAPLs are important because they are present in the subsurface at thousands of remediation sites across the country and are frequently the focus of assessment and remediation efforts. A sound LNAPL understanding is necessary to effectively characterize and assess LNAPL conditions and potential risks, as well as to evaluate potential remedial technologies or alternatives.

Sources:

Interstate Technology & Regulatory Council, 2009. Evaluating Natural Source Zone Depletion at Sites with LNAPL, Technology Overview, LNAPLs Team, April.

U.S. Environmental Protection Agency, 1998. Technical Protocol for Evaluating Natural Attenuation of Chlorinated Solvents in Ground Water, Office of Research and Development, Washington, D.C., September.

Metals

Metals are naturally occurring and some are needed for nutrition. However, at certain concentrations, they are toxic to humans and the environment.

Metals contamination is a persistent problem at many contaminated sites. The most commonly occurring metals at U.S. Environmental Protection Agency Superfund sites are lead, chromium, arsenic, zinc, cadmium, copper, and mercury. The presence of metals in soils and groundwater can pose a significant threat to human health and ecological systems.

The chemical form of a metal contaminant influences its solubility, mobility, and toxicity in groundwater systems. A detailed site characterization must be performed to assess the type and level of metals present and allow evaluation of remedial alternatives. Metals are relatively immobile in subsurface systems as a result of precipitation or adsorption reactions.

General approaches to remediation of metal contamination include isolation, immobilization, toxicity reduction, physical separation, and extraction. Treatment of contaminated soils or solids is commonly performed by excavation followed by disposal or ex situ treatment by solidification/stabilization.

While attenuation of organic compounds is predominantly based on biotic processes, attenuation of metal and radionuclide contaminants is predominantly based on abiotic processes that can be influenced by microbial processes. For metal contaminants in the subsurface, the interaction of groundwater with the soils and sediments in the saturated zone becomes very important because, in large part, the properties of the soils and sediments strongly control the attenuation processes.

Naturally occurring biotic or abiotic processes do not destroy metals, but can change their valence state, which in turn affects their solubility and mobility. For example, hexavalent chromium can be chemically or biologically reduced to the less soluble and less toxic trivalent chromium.

Techniques for removing or containing metal contaminants in groundwater are often inefficient and quite costly below certain concentrations — concentrations that may still be above regulatory criteria. Dealing with this dispersed low-level contamination may be especially challenging at many cleanup sites. In addition, intrusive remedial techniques may cause other environmental consequences and may expose workers to unacceptable risks. Therefore, understanding how and where attenuation processes may be effective in remediating a waste site is a significant issue for these sites.

Sources:

Evanko, C.R., and D.A. Dzombak, 1997. Remediation of Metals-Contaminated Soils and Groundwater. Ground-Water Remediation Technologies Analysis Center, Pittsburgh, PA.

Interstate Technology Regulatory Council, 2010. A Decision Framework for Applying Monitored Natural Attenuation Processes to Metals and Radionuclides in Groundwater. Attenuation Processes for Metals and Radionuclides Team, Washington, D.C.

Mulligan, C.N., R.N. Yong, and B.F. Gibbs, 2001. “Remediation Technologies for Metal-Contaminated Soils and Groundwater: An Evaluation.” Engineering Geology 60:1-4.

Pesticides

Pesticides are used widely in agriculture, industrial, and residential settings. They can be toxic to humans and the environment.

Pesticide contamination includes a wide variety of compounds resulting from manufacturing, improper storage, handling, disposal, and/or agricultural processes. Pesticides include insecticides, fungicides, herbicides, acaricides, nematocides, and rodenticides as well as any substance or mixture of substances intended for use as a plant regulator, defoliant, or desiccant.

Pesticide wastes are generally complex chemical mixtures that can include solvents, carriers, and other components that will have a direct effect on toxicity, mobility, transport, and treatment.

Remediation of pesticide contaminated soils can be a complicated process, as most products are mixtures of different compounds rather than pure pesticide. There are three principal options for dealing with pesticide contamination: containment/immobilization, destruction, and separation/concentration.

Selecting a remedial strategy includes considering the individual contaminant’s toxicity, persistence, migration pathways, and rate of transport from a site. The wide range of physical and chemical properties of pesticides also influences the selection of an appropriate remedial technology or combination of technologies (known as a treatment train). It is important to gain information specific to the contaminants present in order to effectively identify the treatment technologies that are most applicable and cost-effective.

Sources:

U.S. Environmental Protection Agency, 1994. Contaminants and Remedial Options at Pesticide Sites, EPA/600/R-94/202, Risk Reduction Engineering Laboratory, Cincinnati, OH, November.

Petroleum

Petroleum compounds are found at oil production fields, pipelines, terminals, refineries, auto repair shops, and gasoline service stations. Some hydrocarbons are toxic to humans and the environment.

When petroleum is released (“leaked”) into the subsurface, it is a separate-phase liquid that is a mixture of thousands of constituents. The complex mixture of constituents is unique and affects the movement and final disposition (“fate”) of those constituents in the subsurface.

Crude oil and refined petroleum products are primarily composed of hydrocarbon molecules. Hydrocarbons contain only carbon and hydrogen atoms. These hydrocarbon molecules are divided into two classes: Aliphatic compounds and aromatic compounds.

Hydrocarbon molecules range from simple to complex structures, with a number of different arrangements (or isomers) of molecules with the same number of carbons. In aliphatic compounds, carbon atoms are joined together in straight chains (normal), branched chains (iso), or non-aromatic rings (cyclic). They are joined by single bonds (alkanes), double bonds (alkenes), or triple bonds (alkynes). Alkanes are sometimes referred to as “paraffins,” and alkenes may be called “olefins.”

The simplest hydrocarbon molecule is methane (CH₄), which has one carbon atom (C) surrounded by four hydrogen atoms (H). Because of their molecular structure, aliphatic compounds have lower solubility than aromatic compounds and, except for normal hexane or n-hexane, are significantly less toxic than the aromatics.

Aromatic compounds are unsaturated ring-type (cyclic) compounds (i.e., the ring contains a double bond) and can contain a single ring (mono-aromatic) or multiple rings (polycyclic-aromatic). Because of their molecular structure, aromatics have higher solubility than aliphatics and are more toxic than the aliphatics. Benzene is the smallest single-ring aromatic compound; naphthalene is the smallest multi-ring aromatic.

Crude oil and some petroleum products (such as diesel fuel) can contain molecules that include nitrogen (N), sulfur (S), and oxygen (O) in their structure. These molecules, also called “heterocyclics,” are ring compounds where at least one of the compounds in the ring is not carbon; that is, they are not hydrocarbons. The proportion of heterocyclics present in products varies widely, usually in correlation with higher molecular weight.

Crude oil, which contains molecules with the widest range of sizes (e.g., from 2 to more than 40 carbons: C₂ to C₄₀+) is refined into petroleum products ranging from gasoline to asphalt. The approximate boiling-point range or carbon range of crude oil and common products is shown on the figure below. These refined products are composed of hundreds to thousands of aliphatic and aromatic compounds. The products are refined to meet specifications for either performance in engines (fuels) or specialty products (e.g., lubricating oils, cutting oils, etc.). Additives and blending agents are often added to fuels to improve performance and stability.

A number of properties, including solubility, vapor pressure, density, and viscosity can affect the mobility and partitioning of liquid-phase petroleum in the subsurface. In general, as the average molecular size and weight of a product increase, the density and viscosity increase, while the ability of the product to move through the subsurface materials decreases.

Petroleum constituents will dissolve and migrate with groundwater at a rate controlled by advection, hydrodynamic dispersion, sorption, and biodegradation. Hydrocarbons are relatively easily biodegraded and biodegradation can be a very significant factor for hydrocarbon plumes.

All of these factors contribute to the ultimate length and width of the plume as well as its concentrations over time. The combination of factors such as degradation and dispersion is known as natural attenuation. Even at sites where no active remediation has occurred, the dissolved hydrocarbon plume will eventually stabilize because of natural attenuation. A plume that is “stable” is a contaminant mass that has expanded to its maximum extent.

Sources:

American Petroleum Institute, 1996. A Guide to the Assessment and Remediation of Underground Petroleum Releases, Publication 1628, Washington, D.C.

California State Water Resources Control Board, 2012. Leaking Underground Fuel Tank Guidance Manual, Sullivan International Group, Inc., San Francisco, CA, September.

Gustafson, J.B, D. Vorhees, and W. Weisman, 1999. Human Health Risk-Based Evaluation of Petroleum Contaminated Sites: Implementing the Working Group Approach, Total Petroleum Hydrocarbon Working Group, Amherst Scientific.

U.S. Environmental Protection Agency, 1995. Ground Water Issue: Light Nonaqueous Phase Liquids, EPA/540/S-95/500, Office of Solid Waste and Emergency Response, Washington, D.C.

VOCs

Volatile organic compounds, especially those that contain chlorine, were widely used in machine and electronics manufacturing, automotive and engine repair, airplane servicing, dry cleaning, asphalt operations, musical instrument manufacturing, dye manufacturing, and many other industrial operations. In addition, these solvents have been used in household products such as paint thinners and septic tank degreasers. Many are known or suspected to cause cancer.

VOCs have been detected in groundwater since the 1970’s. Most of the releases to the environment took place during the period of industrial development after World War II.

Spills and leaks of chlorinated solvents have caused widespread subsurface contamination in the environment. These contaminants can be present in the subsurface in the form of non-aqueous phase liquids (NAPL, the bulk chemical product), as dissolved contaminants in groundwater associated with aquifer sediments, and as vapors in the unsaturated zone. Because the density of these solvents is greater than water, they tend to sink in groundwater systems, which results in complex dispersal and plume patterns.

Sources:

Schwille, F., 1988. Dense Chlorinated Solvents in Porous and Fractured Media: Model Experiments, Lewis Publishers.

U.S. Geological Survey, 2015. toxics.usgs.gov/investigations/chlorinated_solvents.html.