# 3.2. Sources of chemicals

Author: Ad Ragas

Reviewer: Kees van Gestel

Learning objectives

You should be able to:

- characterize the origins of environmental pollutants;
- explain the relevance of emission assessment;
- characterize emission sources;
- explain how emission sources can be quantified.

Keywords: environmental pollutant, life cycle, point and diffuse sources, emission factor, emission database

Chemicals can be released into the environment in many different ways. In the professional field, this release is called the emission of the chemical, and the origin of the release is referred to as the emission source. The strength and nature of the emission source(s) are important determinants of the ultimate environmental exposure and thus of the resulting risk. This section explains the most important characteristics of emission sources and familiarizes you with the most common terms used in the field of emission assessment. It starts with a brief introduction on the origin of pollutants in the environment, followed by an explanation of the relevance of emission assessment, how an emission can be characterized, and how data on emissions can be gathered or estimated.

Origin of pollutants

Pollutants in the environment can originate from different processes. Within this context, we here distinguish between three types of chemicals:

1. natural chemicals that are naturally present in the environment, like (heavy) metals and natural toxins, and that can either be released into the environment by natural processes (e.g. the eruption of a volcano) or by human-induced processes (e.g. resource extraction and subsequent use and dispersal);
2. synthetic chemicals that are intentionally produced and used by society because of their useful characteristics, e.g. (most) pharmaceuticals, pesticides and plastics;
3. chemicals that are unintentional byproducts of human activities and production processes, e.g. dioxins and disinfection byproducts.

The latter category can overlap with the first, since the reaction products of natural processes, such as many combustion processes, can be considered natural chemicals. Polycyclic aromatic hydrocarbons (PAHs), for example, can be released from natural (e.g. a forest fire) as well as human-induced processes (e.g. a power plant). This emphasizes the role of the emission process in defining an environmental pollutant. When human activities are involved in either the production or the release of a chemical into the environment, this chemical is considered to be an environmental pollutant. Some synthetic chemicals are also naturally present in the environment as this is a specific field of research in organic chemistry, i.e. the chemical synthesis of natural products.

The relevance of emission assessment

Emission assessment of chemicals is the process of characterizing the emission of a chemical into the environment. Knowledge on the emission can be relevant for different purposes. The most obvious purpose is to assess the exposure and risks of a chemical in the vicinity of the emission source. This is typically done when a facility requires an environmental permit to operate, e.g. a discharge permit for surface water or a permit that involves the emission of pollutants into air from a smoke stack. Such assessments are typically performed locally.

At a higher scale level, e.g. national or global, one might be interested in all emissions of a certain compound into the environment. One should then map all sources through which the chemical can be released into the environment. For synthetic chemicals, this implies the mapping of all emissions throughout the life cycle of the chemical. This life cycle is typically divided into three phases: production, use and waste (Figure 1). Between the production and use of a chemical there may be various intermediate steps, such as the uptake of the chemical in a formulation or a product. And after a chemical – or the product it is contained in – has been used, it may be recycled. The life cycle of chemicals can be illustrated with the simple example of pharmaceuticals. These can be released into the environment during: (1) their production process, e.g. the effluent of a production plant that is being discharged into a nearby river, (2) their use, e.g. the excretion of the parent compound via urine and feces into the sewer system and subsequently the environment, or (3) their waste phase, e.g. when unused pharmaceuticals are flushed through the toilet or dumped in a dustbin and end up with the solid waste in a landfill.

Figure 1. The three main life cycle phases of a chemical: production, use and waste. After production, chemicals can be applied in a formulation or product. After the chemical (or product) becomes waste, it can be recycled to be used again in production, the formulation/product or use phase.

Instead of focusing on the life cycle of an individual chemical, it is more common in environmental assessments to focus on the life cycle of products and services. The life cycle of products and services has an extra phase, i.e. resource extraction. The focus on products or services is particularly useful when one wants to select the most environmentally friendly option from a number of alternatives, e.g. the choice between putting milk in glass or carton. This requires that not only emissions of chemicals are being included in the life cycle assessment, but also other environmental impacts such as the use of non-renewable resources, land use, the emission of greenhouse gases and disturbance by noise or odor. Similar techniques to assess and compare the environmental impacts of human activities include material flow analysis, input/output analysis and environmental impact assessment. The quantification of chemical emissions into the environment is an important step in all these assessment techniques.

Characteristics of the emission source

Emission sources can be characterized based on their properties. An important distinction that is often made is the distinction between point sources and diffuse sources. Point sources are emission sources that are relatively few in number and emit relatively large quantities of chemicals. The smoke stacks of power plants and the discharge pipes of wastewater treatment plants (WWTPs) are typical examples of point sources. Diffuse sources are many in number and emit relatively small amounts of chemicals. Exhaust emissions from cars and volatilization of chemicals from paints are typically considered diffuse emissions. The distinction between point sources and diffuse sources can sometimes be a bit arbitrary and is particularly relevant within regulatory contexts since point sources are generally more easy to control than diffuse sources.

Another important characteristic of emission sources is the compartment to which the chemical is being emitted, in combination with the matrix in which the chemicals are contained. Two important emission types are chemicals in (waste)water discharged into surface waters and chemicals in (hot) air released through a smoke stack. Other common entry pathways of chemicals into the environment are the spaying of pesticides (emission into air, soil and water), the application of manure containing veterinary medicines, the dumping of polluted soils (in soils or water), the dispersal of polluted sediments, and leaching of chemicals from products. Chemicals emitted to air, and to a lesser extent also water, will typically disperse faster into the environment than chemicals emitted to soils. An important aspect that influences dispersal is whether the chemical is dissolved in the matrix or bound to a phase in the matrix, like organic matter, suspended matter or soil particles. The fate of chemicals in the environment is further discussed in Sections 3.3 and 3.4.

The temporal dimension of the emission source is another important characteristic. Distinction is often made between continuous and intermittent sources. Wastewater treatment plants (WWTPs) and power plants are typical examples of continuous sources, whereas the application of pesticides is a typical example of an intermittent emission source. The strength of an emission source may vary over time. Distinction can be made between sources with: (1) a constant emission, (2) a regularly fluctuating emission and (3) a irregularly fluctuating emission. For example, WWTPs typically have a continuous emission, but the amount of a chemical in the WWTP effluent may show a distinct regular pattern over 24 hours, reflecting the diurnal and nocturnal activities of people. Production plants that only operate during the day typically show a block pattern, whereas pesticide emissions typically follow a more irregular pattern fluctuating with the season and the emergence of pest species. Irregular emissions such as from pesticides are typically characterized by peak emissions, i.e. the release of relatively large amounts with a relatively short time frame. Other typical examples of peak emissions are the release of chemicals after industrial accidents or intense rain events, e.g. pesticide runoff from agricultural fields after a long period of drought or combined sewer overflows (CSOs).

Emission data

Considering the importance of emission assessment for assessing the environmental impacts of human activities, it is not surprising that a lot of effort is put in the quantification of emission sources. Emission sources can be quantified in different ways. An important distinction is that between measurement and estimation. The continuous measurement of an emission source is also referred to as monitoring. Measurement often involves the separate determination of two dimensions of the emission, i.e. (1) the concentration of the chemical in the matrix that is being emitted, and (2) the flow of the matrix into the environment, e.g. the volume of wastewater or polluted air released per unit of time. The emission load (i.e. mass of chemical released per unit of time) is subsequently calculated by multiplying the concentration in the matrix by the flow of the matrix.

Measurement is often costly and takes a lot of time. It is therefore not surprising that approaches have been developed to estimate emissions. These estimations are often in essence based on measurement data, but these are then generalized or extrapolated to come to more large scale emission estimations. For example, measurements on the exhaust emissions of a few cars can be extrapolated to an entire country or continent if you know the number of cars. A rather coarse approach that is widely used for emission estimation is the use of emission factors. An emission factor quantifies the fraction of a chemical being used and that ultimately reaches the environment. It is often a conservative value that is based a worst case interpretation of the available measurement data or data on the processes involved in the release of the chemical. A related but more detailed approach is to estimate the emission of a chemical based on proxies such as the amount produced, sold or used in combination with specific data on the release process. Pharmaceuticals can again serve as a good example. If you know the amount of a pharmaceutical that is being sold in a particular country, you can calculate the average per capita use. You can then estimate the amount of pharmaceutical that is being discharged by a particular WWTP if you know: (1) the number of people connected to the WWTP; (2) the fraction of the pharmaceutical that is being excreted by the patient into the sewer system through urine and feces, and (3) how much of the compound is being degraded in the WWTP. You can even further refine this estimation by accounting for (1) demographic characteristics of the population since older people tend to use more pharmaceuticals than young people, and (2) the fractions that are not used by the patient and are either: (a) flushed through the toilet, (b) dumped in the dustbin, or, preferably, (c) returned to the pharmacy.

Emission data can be a valuable source of information for risk assessors. Data gathered locally may be relevant to obtain a picture of national or even global emissions. This insight has led authorities to set up databases for the registration of emissions. Examples of such databases include:

- The European Pollutant Release and Transfer Register (E-PRTR), containing data reported by EU member states on releases to air, water and land as well as the transfers of pollutants in waste water for 91 substances and across 65 industrial sub-sectors, and the transfer of waste from these industrial facilities;
- Waterbase, containing data on the status and quality of Europe's rivers, lakes, groundwater bodies and transitional, coastal and marine waters, on the quantity of Europe's water resources, and on the emissions to surface waters from point and diffuse sources of pollution;
- The Toxics Release Inventory of the United States Environmental Protection Agency, containing data on how much of each chemical is released to the environment and/or managed through recycling, energy recovery and treatment as reported by different industry sectors;
- The data from California's pesticide use reporting (PUR) program, containing data on pesticide use in California;
- The Dutch Watson database (in Dutch), reporting measurements of chemicals in the influent and effluent of Dutch WWTPs.