Contaminant migration pathways

Identify most likely pathways of contaminant migration... 

FIGURE 3.4 Photograph showing small-scale sedimentary structures that can significantly affect migration pathways of contaminants. 

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 approach usually involves a weighting of the amount and composition of hazardous material at each site, the contaminant characteristics of the site, the potential migration pathways, and the proximity of environmental or human populations. Variations of this approach have already been applied many times, either in a quasl-rigorous fashion or in a more general manner, as state and local authorities are faced with deciding where to concentrate limited resources. 

MEPAS uses mathematical algorithms and a coupled pathways analysis to predict the potential for contaminant migration from a waste site to important environmental receptors. Groundwater, overland, surface water (e.g., rivers and wetlands), and atmospheric pathways are considered. Using the contaminant transport predictions, computed... 

Connected with the adoption of a risk-based methodology is the source-pathway-target or contaminant-pathway-receptor philosophy. This philosophy states that, in order for harm to have occurred, or for there to be a risk of harm occurring, contaminants must have been able to reach relevant receptors via specific environmental migration pathways. Where such a relationship exists between a contaminant, a pathway and a receptor at a site, a pollutant linkage is said to exist. 

Recent tritium data from Wells 199-N-67 and 199-N-69 (Color Illustration 2-2), however, show a different pattern. These data indicate significant contamination at depth, and in both Wells 199-N-67 and 199-N-69 higher levels of contamination at depth than at water table. For example, tritium activity observed in Wells 199-N-67 and 199-N-69 in November/December 1989 was 42,600 pCi/L and 78,400 pCi/L, respectively. The higher tritium levels at depth in the vicinity of 116-N-l may indicate a deeper, preferential flow path for discharge from 116-N-3, particularly in the most direct pathway towards the river. The Sr activity associated with discharges to 116-N-3 is presumably not observed at this point in the migration pathway because it was previously removed through adsorption in the immediate vicinity of the crib. 

Contaminant transfer to bed sediments represents another significant transfer mechanism, especially in cases where contaminants are in the form of suspended solids or are dissolved hydrophobic substances that can become adsorbed by organic matter in bed sediments. For the purposes of this chapter, sediments and water are considered part of a single system because of their complex interassociation. Surface water-bed sediment transfer is reversible bed sediments often act as temporary repositories for contaminants and gradually rerelease contaminants to surface waters. Sorbed or settled contaminants are frequently transported with bed sediment migration or flow. Transfer of sorbed contaminants to bottomdwelling, edible biota represents a fate pathway potentially resulting in human exposure. Where this transfer mechanism appears likely, the biotic fate of contaminants should be assessed. 

Due to the capacity of deep wells to store injected waste for a long period of time, if the correct measures are taken in design, construction and operation, deep well injection can provide an effective and environmentally safe method of concentrate management. The major environmental concern for deep well injection is the potential for contamination of nearby aquifers, which may be used as a source of drinking water. Six pathways have been defined that describe the potential migration of concentrate that can cause contamination of aquifers (Shammas et al. 2009 United States Environmental Protection Agency 2002) ... 

The US EPA summarized the results of studies of potential pathways for the release of chemicals from Superftmd sites (US EPA, 1988). Migration to groundwater was cited as the primary pathway of contaminants at these hazardous waste sites, a trend confirmed by the data in Table 1 37% of sites involved releases to groundwater and 23% were responsible for releases to both groundwater and surface water. Other studies document the potential hazards of hazardous waste disposal. The EPA, in a survey of 466 public water supply wells, found that one or more volatile organic... 

Hazardous waste problems are frequently generated by mixmres of complex wastes that have been disposed of on land and that have migrated through the subsurface. One approach to assessing the risks of contaminated sites has been to divide the problem into three elements sources, pathways, and receptors (Watts, 1998) as noted in Table 2. The first step in assessing the risk at a hazardous waste site is to identify the waste components at the source, including their concentrations and physical properties such as density, water solubility, and flash point. After the source has been characterized, the pathways of the hazardous chemicals are analyzed by quantifying the rates at which the... 

Local flora and fauna analysis to permit determination as to whether the contaminants have entered the food chain and to assess the tendency of various species to concentrate or eliminate individual contaminants. In some cases, it is necessary to supplement the field investigations with controlled bench- or pilot-scale studies. These studies may be performed to simulate a mobilization or dispersion mechanism, or the complex chemical interactions between the waste form, surrounding matrix, or soil pathways, and/or the effectiveness of certain technologies in preventing migration or providing the required level of isolation. These pilot studies are often defined as feedback and obtained from the assessment of remedial alternatives. 

Problem formulation a qualitative evaluation of contaminant release, migration and fate identification of contaminants of concern, receptors, exposure pathways and known ecological effects of the contaminants and selection of end-points for further study. The term end-point is used to describe the expected or anticipated effect of a contaminant on an ecological receptor (USEPA, 1989c). 

Exposure assessment a quantification of contaminant release, migration and fate characterization of exposure pathways and receptors and measurement or estimation of exposure point concentrations (USEPA, 1989c). 

The migration and fate of contaminants in each transport pathway can be simulated using MEPAS components. The transport pathways are systematically integrated with an exposure assessment component that considers the type, time, and duration of exposure and the location and size of the population exposed. These various pathways and their interactions as considered by MEPAS are discussed in Droppo et al. (5). 

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