Sample Conceptual Site Model

Finally, a chemical-specific dose is estimated for each receptor and complete exposure pathway identified in the conceptual site model. In this step, assumptions are made regarding the rate at which exposure could occur. For example, it is typically assumed that the average American adult drinks two liters of water daily from the tap, weighs 70 kg, and incidentally ingests 100 mg of soil each day. It is further assumed that a resident lives at the same house for thirty years, and a worker works at the same location for twenty-five years. These assumptions can change if there is more specific information available about the site. Using the gas station example, the owner of the station could be contacted to obtain records about the job duration for a typical employee, and the number of hours a typical employee is present at the station each day. 

The chemical concentrations identified in the data evaluation component are combined with all of these exposure or intake assumptions into equations to estimate a dose in units of milligrams chemical per kilogram body weight per day. These are the same units in which toxicity data are reported, as discussed in chapter 7. For the gas station example, the results of this step would include chemical-specific dose estimates for all complete exposure pathways for each receptor. These dose estimates are combined with the toxicity values discussed below in the risk characterization component of a risk assessment. 

In this component, toxicity values are identified for the chemicals at the site. Unlike the exposure assessment, toxicity values are the same regardless of the situation because toxicity can be defined only by the potency of the chemical. It Is the combination of toxicity and exposure that determines risk and the likelihood of toxic effects. The methods used to develop toxicity values for chemicals were discussed in chapter 7. 

Toxicity values are separately developed for different exposure routes. Typically, values are developed for the oral and inhalation routes of exposure because the majority of toxicity studies are based on these exposure routes. In most cases, the oral values are used for the skin exposure route, adjusted for decreased absorption across the skin relative to the oral route. Two types of toxicity values are currently used in risk assessment those describing cancer potency and those for non-cancer effects. Some chemicals are known to have both cancer and non-cancer effects. In these situations, both cancer and non-cancer values might be developed. Therefore, each chemical might have up to four toxicity values. Toxicity values for cancer and non-cancer effects are separately discussed below. 

A slope factor (SF is used to quantify the cancer potency of a chemical. An SF defines the rate at which effects increase with dose. The higher the SF, the more potent the cancer effects of the chemical. An SF assumes that there is no dose that is associated with no risk. Any amount of the chemical has the potential to cause cancer, but the chance of this occurring might be so small as to be unmeasurable. This assumes that there is no threshold dose below which cancer does not occur (chapter 7). We know this is true for some chemicals, and is likely not true for others. In general, this assumption ensures that we will not underestimate the potency of a potentially carcinogenic chemical to humans. These SF values are typically based on a conservative estimate of the cancer potency in test animals, and therefore include a safety margin in their derivation (see chapter 7). 

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Data analysis should focus on the development or refinement of the conceptual site model by analyzing data on source characteristics, the nature and extent of contamination, the contaminants transport pathways and fate, and the effects on human health and the environment. All field activities, sample management and tracking, and document control and inventory should be well managed and documented to ensure their quality, validity, and consistency. 

A conceptual site model is useful in helping to determine the type of environmental samples that is required. A conceptual model emphasizes the type and extent of the contamination, defines the pathways for contaminant migration, and identifies potential receptors (e.g., well users, surface water bodies, and food and feed material) (US EPA, 2002). 

This uncertainty has resulted in a patchwork of remedial projects. Although each project has been beneficial in finding and removing hazards or at least increasing the knowledge base about, waste and inefficiency have been major by-products due to the lack of a comprehensive plan. For example, most properties were sampled for arsenic. Now a repeat mobilization will have to be conducted to sample for a small handful of other toxic metals and constituents, which could have been sampled for in the first event. It is in an effort to minimize such future duplication of effort and data gaps that the District of Columbia has tackled the enormity of a comprehensive planning document with this Conceptual Site Model. 

Some basic knowledge of the directions and rates of groundwater flow at a site is important for deciding the sample collection and model conceptualization. The direction of groundwater flow determines the sequence in which the water will contact different mineral assemblages in the aquifers. Knowledge of the flow path ensures that observed... 

Perform a reconnaissance site investigation using exploratory sampling. This may include nondestructive geophysical methods and field screening tests to refine the conceptual model... 

The basic requirements for surveillance and operational monitoring network design are specified in the Ordinance on Water Quality Monitoring. They cover all main aspects of the monitoring cycle, i.e. the establishment of sites, the consideration of the conceptual model, parameter selection, duration and frequency of monitoring, methods for sampling and analyses, quality assurance, data management and publication of results. 

Firstly, sampling and testing should be chosen that addresses the specific hypotheses contained in the proposed conceptual model. The objectives of the investigation should link to questions such as Is the existence of assumed sources of contamination proven Are the assumed pathways between sources of contamination and identified receptors viable Is the extent and strength of the assumed sources sufficient to drive transfer of contamination to identified receptors The investigation is planned around these and other key questions, so that it gives information on the presence or absence of site features, the spatial extent and concentrations of contaminants, the presence of pathways and whether these are active or not, and the dynamics of water and gas movement. 

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