Developing a Conceptual Site Model- Contaminant Pathways and Receptors

Groundwater Testing, Historical Site Process Audit, Historical Site Records Search, Soil Testing

Developing a Conceptual Site Model- Contaminant Pathways and Receptors

When developing a conceptual site model (CSM), the focus moves from the initial review of background information to the identification of potential contaminants of concern.

Investigation into the site’s historical background should identify past site owners and operators, which would identify contaminants likely to be present at the site. For example, if past operations on a site included a chrome plating operation, chromium contamination in its many forms should be investigated in the areas thought to contain the plating operations. In this case, possible chromium contamination would be listed as a data gap until an investigation could rule it out.

Following the identification of potential contaminants, it is the investigator’s responsibility to provide detailed characterization to the site. This would include, but not be limited to answering the following:

What contaminants are actually present on the site
Where is the contamination
Volume/area/mass of contamination
Is the contamination in soil, groundwater, sediment
Is the contamination likely to migrate
Where would it migrate

Characterization of contamination may come from a brand new investigation or the pairing of past investigations with supplemental investigations where needed to close existing data gaps (link to past post). Analytically, it is very important to consider one or more indicator analytes – compounds that can serve as “representative” for other contaminants of concern. This allows the team to save significant laboratory expense, as they’ll only be looking closely at one or a few analytes instead of testing every sample for every compound. AEL is able to assist with determining what analyte or analytes would be appropriate for this purpose.

When data has been gathered to the satisfaction of the investigator, it can be analyzed for trends, or potential receptor exposures. Analysis of data may reveal a contaminant is naturally occurring, due to discharge from off-site sources, due to specific on-site operations, etc. Identification of the source of a contaminant is important when trying to track its potential impacts and pathways to receptors.

Migration pathways are then evaluated and incorporated into the CSM. Migration pathways investigate how the contaminants move through media and how they may impact receptors, such as the environment and human health. Evaluated media includes soil, groundwater, surface water, sediment, vapor and air.

Subsequently, the identification of human and ecological receptors or potential receptors is a key function of a CSM. These receptors are identified by their presence in the area of investigation, or presence along a potential migration pathway for a contaminant. Expected exposure rates and doses are evaluated based on the future site use. For example, a site designated for commercial use will be evaluated for human health risk differently than a site designated for residential.

Human Health Risk is evaluated by considering:

The characteristics of onsite contaminants in the media types found on site
Migration pathways of each contaminant and how they may be exposed. Ie will they be contacted through touch, ingested, inhaled
Who may be potentially exposed (ie, sensitive populations such as children, pregnant women, size of population)
Likely exposure/dose rates

Ecological Risk is evaluated by considering: