CE 397: Environmental Risk Assessment

Department of Civil Engineering

The University of Texas at Austin

Homework #3 Solution - Developing Site Conceptual Models

By Lesley Hay Wilson

Goals of the Exercise


Goals of the Exercise

The site conceptual model is an important framework for the analysis of sources of chemicals of concern (COC) in the environment. The potential for exposure must be evaluated for all credible pathways from sources to human or ecological receptors. The site conceptual model is a method to characterize the situation being studied and a means to communicate that understanding.

This exercise is designed to help you work through the process of building an initial site conceptual model for the hypothetical case study described in the homework document. There is also an opportunity to identify a completely new scenario and define the site conceptual model for that situation.


The following sections outline the characteristics of a case study site that is a petroleum bulk storage facility (Dr. Maidment says go with what you know….). The narrative contains the type of information that you would normally have following a non-intrusive investigation (also known as a "paper audit" and site inspection). It is at this point in a corrective action project that as the project manager you would be required to develop the initial site conceptual model. Following the narrative there are specific questions to be turned-in.

You may wish to consult the BP Risk-Based Decision Process Guidance Document or the TNRCC Risk Reduction Program Guidance to complete this assignment. Both documents are on-reserve for the class in the Engineering Library (First Floor, ECJ).


(1). For the case study site as a whole, answer the following questions.

What are the potential source(s)?


What are the potential source area(s)?

In the areas of each of the sources identified above the following could be source areas:

What are the potential chemical(s) of concern?

What are the media likely to contain chemical(s) of concern?

Surface soils, subsurface soils, soil gas, and groundwater.

What are the potential transport mechanisms?

What is the current and reasonably potential future land use of the property?

The current land use of the property is commercial. Commercial use was selected since access to the property is not limited and there is the potential for customers to be present on the property.


The reasonably potential future land use is based on the proposed development. Approximate two-thirds of the property (on the south end) is proposed to be commercial land use and the remainder is proposed to be recreational land use.

What is the current and reasonably potential future ground water use?

The ground water in the area is considered to be a current and future source of potable water. Currently there is a potable water supply well on the property. There are also private potable water wells on properties north, south and east of the property; public potable water supply wells approximately 4000 feet south of the property; and irrigation water supply wells on the farm west of the property.

What are the human and ecological receptors?

The current on-property potential receptors include commercial workers and construction workers. Customers and trespassers could also be considered.

The current off-property receptors include child and adult residents, commercial workers and construction workers. Customers, trespassers on the adjacent commercial property and livestock and crops at the farm could also be considered.

The potential on-property future receptors based on the proposed future land include commercial workers, construction workers and child and adult recreational park users. Customers and trespassers could also be considered.

The potential off-property future receptors based on the proposed land use include child and adult residents, commercial workers and construction workers and child and adult recreational park users. Customers, trespassers on the adjacent commercial property and livestock and crops at the farm could also be considered.

What are the potential exposure routes?

(2). From the list of potential sources developed in question (1), choose a source on the case study site and develop the site conceptual exposure model for both the current and future land use for that source. Document the chemicals of concern that are applicable for that source. You may use the spreadsheet sitecon4.xls, or another method to document your decisions about exposure pathways.

The source areas, transport mechanisms, receptors and exposure routes itemized in the sections above generally apply to all of the potential sources. A reasonable interpretation of the potential exposure pathways associated with the selected source is expected. As an example the spreadsheet exsoln.xls has been prepared for the gasoline UST potential source and can be downloaded here. Remember that the initial site conceptual model will be revised when additional information is collected. Receptors and pathways may be added or removed from the model as our understanding of the site changes. There are several equally correct answers for any situation; the important idea is that the assumptions made in completing the exposure pathway flowchart are documented.

(3). Focusing on the potential source that you documented in question (2), what sampling or other field data should be collected to determine if the exposure pathways are complete?

The initial investigation will focus on the potential source areas to determine if concentrations of COC are present in the environmental media. In general, the field data that are necessary include surface soil, subsurface soil and ground water samples in the potential source areas. Field screening techniques should be used to identify soil samples to be sent to the laboratory for analysis. Also during the initial investigation, samples from the existing irrigation well on the Old Farm property and samples from the on-property water supply well should be taken to determine if there are any immediate impacts to these points of exposure. The samples should be analyzed for the COC identified for the source and source area. Although initially, all of the source area samples could be analyzed for all of the potential COC identified for the property.

During the installation of soil borings to collect soil samples, the site-specific soil types should be determined including a profile of soil types identifying the depth and thickness of any significant soil lenses. Ground water sampling locations should be placed to determine the depth to the uppermost saturated zone (e.g., first ground water encountered), the ground water flow direction and hydraulic gradient. A site-specific estimate of hydraulic conductivity of the uppermost saturated zone encountered should be made. Additional data may be necessary to evaluate the lower saturated zone and to determine the area of influence of the public water supply wells.

Based on the initial data available it is not clear what ecological pathways relevant to the farming in the area would be potentially complete. Additional information could be collected on farming practices. Also, the potential for off property impacts could be assessed following the initial soil and groundwater sampling.

(4). Use the spreadsheet to analyze an environmental exposure scenario that you define. Provide the details of the source, the chemicals of concern and complete the spreadsheet for the potentially complete pathways.

An example of the evaluation of the presence and migration of COC in the environment is expected here. A description of the source and potential COC should accompany the exposure pathway flowchart for the example. The reasonable potential exposures should be documented.


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