Depending on the nature of the assessment, a range of information may be required to characterise sources of environmental stressors.  In the case of contaminants, this includes information on:

• source location and distribution
• source activity
• source intensity

Source location and distribution

The spatial characteristics of sources are important because they determine to a large extent the distribution both of emissions and environmental concentrations. Spatial location and distribution, however, can be defined in different ways, depending on the type of activity concerned (e.g. whether it is localised or dispersed; a discrete entity such as a factory or a diffuse source such as an agricultural zone), the scale of analysis, and the data sources used to identify the sources.   In many inventories, for example, businesses are represented as points, defined by the address or map co-ordinates; in statistical data, however, they are often attributed to administrative areas or zones.  These differences can be vital, for they change the apparent relationship with the surrounding population, and thus affect estimates of exposures.  Care is also needed, especially with business enterprises, because the stated location is often that of the head office, rather than the premises where the activity is carried out.

Source activity

Sources are best distinguished in terms of the activity they perform, for this determines to a large extent their potential impacts on the environment.   There is, however, no single way of defining 'activities', and thus no single system for classification.  Instead, different classification systems have been developed, depending on their purpose, and the type of survey technology being used.  In the EU, the NACE system ( Nomenclature générale des Activités économiques dans les Communautés Européennes) is used as the standard for defining economic activities for statistical purposes: this comprises a 4-level hierarchy, containing 21 broad sectors at the highest level and recognising some 500 detailed activity classes at the lowest level.  In contrast, land cover maps based on satellite imagery provide discrimination of only a few tens of land cover types (and fewer land use classes).  The categories recognised thus depend on the source of the data, and matching between the source categories from different data sets can be problematic.

Source intensity

This refers to the level or magnitude of the activity undertaken at any location.  Measures of intensity vary, not only according to the type of activity, but also the way this has been represented spatially and the purpose of the assessment.  The intensity of industrial activities, for example, may be defined in terms of the inputs, throughputs or outputs, and in terms of mass, volume or value.  Agricultural activities are often measured in terms of area (e.g. under specific crops), livestock numbers, workforce, yield or value of production.  In each case, considerable differences (and ambiguities) may arise, depending for example on whether the production measures include or exclude waste materials, the averaging time used (e.g. the definition of a 'day') and whether they represent average or maximum (e.g. total permissible) levels of activity.