Soil exposure pathway

Contaminated soils may expose children to multiple contaminants at levels of health concern (ATSDR, 1994). Ingestion of contaminated surface soil is a primary exposure route. Inhalation of contaminated dusts and direct dermal contact with contaminated soils can also lead to elevated exposure. 

DDT is one of the 12 POPs being phased out under the Stockholm Convention on Persistent Organic Pollutants (UNEP, 

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Simcox NJ, Fenske RA, Wolz SA, et al. 1995. Pesticides in household dust and soil Exposure pathways for children of agricultural families. Environ Health Perspect 103 1126-1134. 

Role of Humidity in Soil Exposure Pathway Formation in Steppe and Desert Ecosystems... 

The content of heavy metals in Steppe soils is tightly connected with their contents in geological rocks. In formation of soil exposure pathways in Desert ecosystems, water-soluble forms of these metals play the most important role. We can see an analogy between the increasing content of elements in soil dead organic matter as a function of decreasing water excess in Forest ecosystems and the increasing content of water-soluble species of chemical elements in the soils of Dry Steppe and Desert ecosystems as a function of enhanced aridity. The accumulation of water-soluble species occurs in the upper horizon for almost all elements, with exception of strontium. The main factor responsible for the accumulation of water-soluble forms is connected with evapotranspiration. 

The HRS system is based on risk to health and the environment. The criteria examined include the groundwater migration pathway, surface water migration pathway, soil exposure pathway, and air migration pathway (Hen-drichs, 1991). The ranking attempts to quantify the risk each site poses on a relative scale. Only those sites placed on the NPL will receive CERCLA funds however, regulations in the CERCLA can still be applied to non-NPL sites. 

Affected Soils Exposure pathways and associated screening limits designated under applicable rules. Review applicable environmental regulations. 

The exposure pathways of concern identified during the baseline risk assessment include direct contact, with the possible ingestion of contaminated soil (1 x 10 3 4 associated excess cancer risk), and potential ingestion of contaminated groundwater in the future through existing or newly installed offsite wells (2 x 11 0 2 associated excess cancer risk). 

Environmental exposure (direct). Exposure through air inhalation, soil and dust ingestion, and dermal contact of soil and dust are the principal exposure pathways. Other exposure pathways (e.g., water dermal contact) can be taken into account in some scenarios. Monitoring campaigns or multimedia fate models are used to assess the exposure (see [4]). 

Table 5. MPC values for Cd in soil, based on 16 different exposure pathways (After de Vries and Bakker, 1998a). ...

Similarly to N, most S pools are found in organic form in forest floor and soil humus. However, unlike nitrogen, there are important abiotic processes, especially sulfate sorption processes, which play a critical role in regulating sulfate dynamics in forest ecosystems. An example of this type of exposure pathway was shown in the Habbard Brook whole-tree harvesting experiment, where the decrease in sulfate output from the watershed was attributed to sulfate adsorption, which was enhanced by soil acidification from nitrification (see above). 

Biological exposure pathway of sulfur movement in soils of forest ecosystems is related to microbial transformation of sulfolipids. Back conversion of sulfate-S into organic matter immobilizes the anion and potentially reduces soil cation leaching. Processes of sulfur mineralization and incorporation proceed rapidly in response to several factors, including temperature, moisture, and exogenous sulfate availability in soils and water. 

BIOGEOCHEMICAL FLUXES AND EXPOSURE PATHWAYS IN SOIL-WATER SYSTEM OF BOREAL AND SUB-BOREAL ZONES... 

In the southern direction the Podzols are transferred into Podzoluvisols and Distric Phaerozems. These soils have less acidity and less pronounced migration of various elements. Accordingly, this determines the exposure pathways of pollutants, like heavy metals, in soil compartments of forest ecosystems. 

A concept of anion mobility may be considered a useful paradigm for explaining the net retention and loss of cations from soils, and thus exposure pathways. This paradigm relies on the simple fact that total cations must balance total anions in soil solution (or any other solution), and, therefore, total cation leaching can be thought of as a function of total anion leaching. The net production of anions within the soil (e.g., by oxidation or hydrolysis reactions) must result in the net production of cations (normally H+), whereas the net retention of anions (by either absorption or biological uptake) must result in the net retention of cations. 

Strontium, barium, manganese, copper, molybdenum, and nickel are elements of strong accumulation in plant species of African Savanna ecosystems, in spite of different content in soils and soil-forming rocks. The Cb values are >1. The other elements, like beryllium, zirconium, titanium and vanadium, are less taken up by plants and their Cb values are less than 0.5. These refer to various exposure pathways to both microbes and plants as links in biogeochemical food webs. 

The biogeochemical fluxes and exposure pathways of various macro- and microelements are different from those shown for Tropical Rain Forest ecosystems. The chemical composition of leaves of tree species in Mangrove Forest ecosystems is connected with higher content of Mg, Cl and S-SO4- and lesser content of K and Si as compared to the leaves of trees from Tropical Rain Forest ecosystems. The content of A1 is 3-4 times higher than that of Si and this can be related to the values of hydrogenic accumulation of these elements in soils (Figure 3). 

Thus, in according to the concept of equilibrium distribution, the relation of an organic pollutant concentration in the soil solid and liquid phase is constant at any moment (Vasilyeva and Shatalov, 2004). The example of such an approach application for assessing exposure pathways of POPs to living biota is shown in Box 1. 

Exposure pathways were estimated using the modified soil module of the MSCE-POP model (http //www.msceast.org). At present the scheme is complemented with the fraction of dissolved organic matter (/doc) and with the fraction of the chemical non-equilibrium adsorbed by solid phase (/n0n-equii) or low available with individual degradation rate. The scheme of a pollutant distribution between different soil components is shown in Figure 16. 

Children might be exposed to 3,3 -dichlorobenzidine if they eat small amounts of soil contaminated with 3,3 -dichlorobenzidine. However, studies suggest that it is very difficult to release 3,3 -dichlorobenzidine once it becomes attached to soil. Exposure via contaminated soil may occur if they live in an area near a source of the chemical (such as a hazardous waste site that contains 3,3 -dichlorobenzidine). Children can also be exposed if the parents work at chemical facilities where 3,3 -dichlorobenzidine is handled and bring home contaminated clothing or tools or if they do not shower before coming home. There are no known unique exposure pathways for children. 

Okrent and Xing (1993) estimated the lifetime cancer risk to a future resident at a hazardous waste disposal site after loss of institutional control. The assumed exposure pathways involve consumption of contaminated fruits and vegetables, ingestion of contaminated soil, and dermal absorption. The slope factors for each chemical that induces stochastic effects were obtained from the IRIS (1988) database and, thus, represent upper bounds (UCLs). The exposure duration was assumed to be 70 y. Based on these assumptions, the estimated lifetime cancer risk was 0.3, due almost entirely to arsenic. If the risk were reduced by a factor of 10, based on the assumption that UCLs of slope factors for chemicals that induce stochastic effects should be reduced by this amount in evaluating waste for classification as low-hazard (see Section 7.1.7.1), the estimated risk would be reduced to 0.03. Either of these results is greater than the assumed limit on acceptable risk of 10 3 (see Table 7.1). Thus, based on this analysis, the waste would be classified as high-hazard in the absence of perpetual institutional control to preclude permanent occupancy of a disposal site. 

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