Contaminants

Contaminated land contains a huge variety of contaminants depending on the source, the age of the contamination and the type of activity, which has given rise to the contamination. The contaminated sites can be subdivided into a number of different types of sites, and knowing the type of site, the contaminants to be found at the site can be predicted to a certain degree. In Table 1, a number of site categories have been listed indicating the contaminants normally found at such sites.

Type of contaminated site	Selected potential contaminants to be found at the site
Oil, petroleum (refineries, etc.) and gasoline	Monoaromatic hydrocarbons (BTEX), PAH, n-alkanes (C₅-C₂₀), MTBE, Pb
Paint industries	Chlorinated and aromatic solvents, BTEX, alcohols, esters, glycolethers, ethylbenzene, various heavy metals (Cd, Pb, Cu, Zn, Fe, Cr), As
Asphalt and tar production	BTEX, phenols, naphthalene, PAH
Gas works, coke works, coal gasification	BTEX, phenols, PAH, Cyanide, Ammonia, sulphur compounds, As, Cr
Leather manufacturing (tanning etc.)	Chlorinated aliphatics, BTEX, Cr, As, Fe
Wood preservation	Phenols, chlorophenols (PCP), Cr, Zn, As, Cu, PCB, PAH,
Cleansing operation sites	Chlorinated aliphatics (TCE, PCE)
Textile and dye works	Chlorinated aromatics, polypropylene, polyethylene, TCA, Cd, Zn, Fe, Mn, Cr
Electroplating, Galvanisation	Various heavy metals (Cd, Cr, Cu, Ni, etc.), cyanide
Soap and detergent manufacturing	Detergents
Iron and steel works	Chlorinated aliphatics, various heavy metals
Plastics manufacturing	Phthalates, resins, polymers,�
Pharmaceuticals	Antibiotics, estrogens, cholesterols, clofibric acid, naproxen, ibuprofen
Pulp and paper	PCB, dioxins, furans, chlorinated phenols, organosulfur compounds, various heavy metals (Zn, Pb, Cr), Cyanide
Military sites, explosives manufacturing	Nitroaromatics, nitroglycerin, BTEX, aliphatic hydrocarbons
Chemicals manufacturing works	All contaminants
Landfills, hazardous waste disposal sites	All contaminants

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The soil groundwater contaminants, which have been in focus during the last 30 years since public concern arose on the threat from these types of contamination, comprise many different organic and inorganic compounds showing huge variety in terms of mobility, toxicity and persistence in the soil and groundwater environment. The most frequent compounds identified are different fractions of oil. These are low molecular aromatics like the monoaromatic hydrocarbons (benzene, xylenes, toluene and ethylbenzene � normally, normally abbreviated BTEX), aliphatic hydrocarbons, and high molecular polycyclic aromatic hydrocarbons (PAHs containing more than two aromatic rings), of which some are very potent carcinogens. Another group of compounds, which has attracted much attention, is the chlorinated compounds, which comprise chlorinated aliphatics, i.e. 1,1,1-trichloroethene, chlorinated aromatics, such as the chlorobenzenes, and the chlorophenols. Together with these two types of compounds, the pesticides constitute a group, which has been in focus for many years and in fact often has been the direct cause for closing down groundwater extraction wells. This is a very heterogeneous group of groundwater contaminants, which comprises compounds, which belong to many different categories. Chemically, these compounds cannot be described as belonging to one group, and since they are not considered to be a result of point sources, they are not very often seen as a contaminated land or brownfield issue. However, it has been shown that these contaminants often contaminate groundwater reservoirs as a result of filling operation or improper storage.

Also the heavy metals need to be mentioned in this context, since they from very early on attracted much attention as soil contaminants. They differ from the organic compounds in the sense that they are not degraded in the environment.

Processes governing the fate of contaminants in soil and groundwater

A number of processes governs the fate and transport of contaminants in the subsurface environment, of which biodegradation, adsorption/desorption, dissolution, volatilisation and dispersion are the most important ones. Also processes such as chemical oxidation, incorporation, diffusion, chemical speciation, precipitation, might for some substances play a role. Once a contaminant enters the environment, it will partition between the different phases (mineral surface, organic matter, pore water, air and bulk organics (NAPLs) if available) according to its physico-chemical properties (hydrophobicity, solubility, and volatility). It will then be transported by the media, if there is a flow. In soil they will be transported mostly downwards by infiltrating water or by the groundwater or upwards by airflow. During the flow they will be attenuated in the soil-water/air medium depending on its hydrophobicity relative to the water or air flow rate, and it will be transformed during the transport through the soil and in the groundwater according to its aerobic and anaerobic degradability or chemical reactivity. Since biodegradation is the most important process to effectively remove organic contaminants from the environment, this process is exploited in a number of in situ and ex situ remediation technologies. In Table 2 is indicated the potential aerobic and anaerobic biodegradability of a number of organic contaminant fractions.

Table 2. Potential biodegradability of organic point source contaminants under different redox conditions

Compound	Aerobic conditions	Denitrifying conditions	Sulfate-reducing conditions	Iron-reducing conditions	Methanogenic conditions
BTEX
� Benzene	++	-	+	-	+
� Toluene	++	++	+	+	+
� o-Xylene	++	+/-¹⁾	-	-	+/-
MTBE	+
PAH
� Naphthalene	++	+	+	+	-
� Phenanthrene	+	+	-	+	-
� Pyrene	+	-	-	-	-
� Benz(a)pyrene	+/-	-	-	-	-
Phenols	++	++	+	+	++
Chlorophenols	++	-	+		+
Chlorinated aliphatics
� 1,1,1-TCE	-/+¹⁾	-	+		+
� 1,1,1-TCA	-/+¹⁾		+		+
� PCE	-	-	+		+
Chlorinated aromatics	-/+				+
NSO-heterocyclic compounds
� Pyridine	++
� Quinoline	++
� Carbazole	+
� Dibenzothiophene	+
Detergents	+
� LAS	++	-	-	-	-
� Alcoholethoxylates	++	+/-	+/-		+/-

++: Fast degradation; +: Degradation; -: Persistence

¹⁾ Required sometimes primary substrate

Based on an integrated knowledge on these processes, the geochemistry and geology of the soil and the aquifer at the site, and the properties of the contaminant, the fate and transport and thus the potential exposure of a given receptor can be predicted. Furthermore, an understanding of the governing biogeochemical processes in soil and groundwater is a prerequisite for designing in situ remedial actions.

Toxicity of the contaminants

The toxicity of the most frequently found soil and groundwater contaminants varies considerably depending on their physio-chemical properties. Some of the compounds show high acute toxicity, i.e. chromium and some alkyl phenols, while others are more known for the carcinogenic properties, such as some of the PAHs, benzene, and 1,1,1-trichloroethene.

However, when considering the environmental or public health risk of the contaminants, it is equally important to take into account the potential of the contaminant to be transported to and reach whatever receptor is considered, being individuals, buildings, groundwater reservoirs, animals or other environmental indicators. As mentioned above, some of the contaminants are much more mobile in the environment than others. Thus, the (bio)availability is the key parameter for estimating the risk. The most critical contaminants are those, which are volatile or water soluble and therefore rapidly transported by water or air with minimum of attenuation, which are less degradable in the medium and which at the same time have high toxicological effect. Such compounds are benzene and 1,1,1-trichlorothene, while some of the toxic heavy metals or the PAHs are not very mobile in the environment.

Management and remediation

The different contaminants are given priority in the public regulation and the cleanup efforts according to their potential human health effects, although also ecotoxicity in many countries plays an important role. There are 3 basic approaches to risk reduction, which often are applied at the same time:

o Removing the contamination

o Pathway interruption

o Protection of receptors.

Removing the source might include either transferring the contamination from the medium to another to minimize exposure of targets, removing the medium containing the contamination, or destruction of the contamination either within the medium or after extraction from the medium. In some remediation technologies, a combination of removing the contamination and disruption of the pathway is applied (reactive barrier technology). Traditionally, remediation technologies have been divided up in technologies, which rely on excavation/mechanical handling of the medium before treatment, which are termed on site - or ex situ if the contaminated medium is transported away form the contaminated site � technologies, and in situ technologies which degrade, extract, or in other way treat the contamination where it is placed without disturbing the medium which contains it (Bardos et al. 2000). �

In this context, remediation is supposed to target both the original source, which is the area with some separate phase liquid remaining, and the plume area defined as the area contaminated by contaminants dissolved in the flow medium. The selection of remediation technology is dependent on the physio-chemical and geological characteristics of the site and the type of contamination. In most cases different types of contaminants are found in the same site, and the technology must then be selected according to its efficiency towards the most difficult contaminants, and in many cases a number of different technologies must be applied to reach cleanup criteria for all contaminants. In Table 3 are shown the feasibility of different techniques towards selected groups of contaminants.

Table 3. feasibility¹⁾ of selected remediation technologies towards different groups of contaminants in soil and groundwater

Compounds/-technology	Natural attenuation	*In situ* bioremedia-tion/bio-venting	SVE/air sparging	Thermal desorption	Reactive barrier	Solvent/-surfactant flushing	Soil washing	Contain-ment/-solidification	*On site/* *ex situ* bioremedia-tion	*On site/-ex situ* incinera-tion	*On site/-ex situ* extraction
BTEX	+	+	+	+	+	-	+/-	-	-	-	-
PAH	-	-	-	-	-	+	-	+	+/-	+	+
Phenols	+	+	-	-	+	-	-	-	-	-	-
Chlorinated aliphatics	-	-	+	+	+/-	+	+	-	-	-	-
Chlorophenols	+	+	-	-	-	+	-	+	+	+	+
PCB	-	-	-	-	-	+	-	+	+/-	+	+
Chlorinated aromatics	+	+	+	+	+/-	+/-	-	-	-	-	-
N-aromatics	+/-	+/-	-	+/-	+/-	+	-	-	-	+/-	+/-

Heavy metals	-	-	-	-	-	-	-	+	-	-	-

1) Whether the technology is likely to be beneficial for the group of compounds in question

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Relevant literature

Bardos, P, Morgan, P, Swannell, PJ (2000) Application of in situ remediation technologies � 1. Contextual framework. Land Contamination & Reclamation. 8(4), 301-322.

Calow, P (ed.) (1998) Handbook of Environmental Risk Assessment and management. Blackwell Science, Ldn..

Nathanail, P, Bardos, P (2004) Reclamation of Contaminated Land. John Wiley and Sons.

Page, W (1997) Contaminated sites and environmental cleanup. AP.

Schwarzenbach, RP, Geschwend, PM, Imboden, DM (1992) Environmental Organic Chemistry. Wiley, NY.