Understanding Conceptual Site Models

Husaini Samion is an Environmental Scientist with Simmonds & Bristow. In this article (his first for our website), Husaini walks us through ways to model the pathways that contaminants may take on their way to impacting the environment.

What are conceptual site models?

A Conceptual Site Model (CSM) is representation of the biological, physical and chemical processes that determine the ways that contaminants move from sources through the environmental media to environmental receptors. Environmental media includes soil, water and air that has the capacity to transport contaminants to sensitive receptors such as plants, animals and humans.

How are Conceptual Site Models developed? When should they be developed?

The development of a CSM comprises an iterative process of characterising site contamination on the basis of available information or data. It should be undertaken for every contaminated site, developed as early as possible within the site assessment program and progressively updated as additional information or data become available.

How does the process work?

A CSM is developed before the start of any intrusive site investigations based on the physical layout (both current and former) of a site as well as its historical land-use information. The first step is to gain an understanding of current site conditions and establish:

    • the boundaries of the site and potential off-site sources;
    • local topography, geology, hydrogeology, surface drainage patterns and possible preferential underground contaminant migration pathways;
    • current land-use activities for the site and immediate surrounds;
    • the location, extent and nature of existing infrastructure; and
    • history of site activities and immediately surrounding area.

What about sources of contamination and contaminants of potential concern (COPC)?

Based on the potentially contaminating activities identified, the likely source areas and contaminants of potential concern (COPC) can be determined. A CSM should define the potential source areas and consider implications for both on- and off-site impacts, and assess the environmental media (e.g. soil, groundwater, surface water, sediments) that may have been affected by the COPC. The possible implications for the COPC of a site or its immediate surrounds, should take account of factors such as:

  • timing and duration of the identified activity;
  • period of use of the COPC; and
  • likely behaviours of the COPC within the environmental media.

Understanding Sensitive Receptors:

The identification of possible sensitive receptors associated with the site and the surrounding area should include current and future receptors, such as proposed land-use activities as well as the actual site development process.

Possible human receptors may include:

  • current or future site users such as residents, visitors and workers (depending on land use);
  • on- and off-site construction or maintenance workers;
  • current or future users of surrounding properties; and
  • groundwater bore users that could have implications for human health.

Possible environmental receptors may include:

  • surface water bodies;
  • groundwater beneath, or in the vicinity;
  • groundwater bores; and
  • flora and fauna that may inhabit or migrate through the site.

What are migration pathways and exposure routes?

A mechanism for contaminant to migrate through the environment must be present in order for a source to be a concern. The source and the contaminant release mechanism must be linked to a receptor through a migration pathway that allows the COPC to move from the source to the receptor in order for a risk to exist thereby creating an exposure pathway.

Possible migration pathways may include:

  • leaching of contaminants through the soil profile to groundwater
  • downward migration between groundwater aquifers
  • transport of contaminants via groundwater to surface water
  • transport of contaminants via surface water
  • volatilisation from soil and/or groundwater to air (indoor and outdoor)
  • transport of contaminants by mechanical disturbance (e.g. earthworks), and
  • biomagnification along food chains.

Possible exposure routes may include:

  • direct contact (e.g. skin exposure) with contaminated environmental media
  • ingestion of contaminated environmental media or
  • ingestion of food stuffs grown or reared in contaminated media, and
  • inhalation of contaminated media (e.g. vapour, dust).

How can CSMs help?

In some cases a CSM can eliminate the need for additional investigations if it shows that exposure pathways are incomplete. A figure illustrating the site setting and key contaminant migration mechanisms represents a powerful tool for interpreting and conveying site information.

In summary, a CSM is a highly beneficial tool that provides the evaluation of human health and environmental risks and identifies any additional investigation requirements. The CSMs allows the facilitation of communication of site contamination issues with a wide range of stakeholders, including the public in manner that is easily understood.

What next?

If you think that a conceptual site model may be helpful to you, give our team a call today.