Water exposure pathway

Ingestion of contaminants is the primary exposure pathway for drinking-water. Dermal absorption and inhalation of contaminants during bathing are other common pathways. When contaminated surface waters serve as recreational areas for children, accidental ingestion (water or sediment) and dermal contact become additional pathways for exposure. Finally, aquatic organisms can bioaccumulate contaminants in surface waters, which can lead to dietary exposure through the food-chain. 

Accessibility to safe drinking-water and management of waste-water are important health issues in developing countries. It is estimated that 2.4 billion people, including the poorest in the world, lack access to basic sanitation, and 1.1 billion people lack access to even improved water sources. In the less developed countries, only 

54% of the population in rural areas is using improved drinking-water sources (World Bank, 2001). 

---

A key component in the initial site assessment is the identification of human and environmental receptors potentially impacted by the site. An exposure pathway analysis relies on transport information to identify receptor or exposure points. For example, potentially significant transport and exposure pathways may include groundwater transport, vapor migration into buildings or utilities, etc. Current and potential future land use is identified as well as the potential for future installation of groundwater drinking water wells. If surface water has been impacted by the release, then appropriate surface-water exposure pathways will be identified. 

States have made substantial recent progress in the adoption, and EPA approval, of toxic pollutant water-quahty standards. Furthermore, virtually all states have at least proposed new toxics criteria for priority toxic pollutants since Section 303 (c) (2) (B) was added to the CWA in February of 1987. Unfortunately, not all such state proposals address, in a comprehensive manner, the requirements or Section 303 (c) (2) (B). For example, some states have proposed to adopt criteria to protect aquatic hfe, but not human health other states have proposed human health criteria that do not address major exposure pathways (such as the combination of both fish consumption and drinking water). In addition, in some cases final adoption or proposed state toxics criteria that would be approved by EPA has been substantially delayed due to controversial and difficult issues associated with the toxic pollutant criteria adoption process. 

For each category of land or water body use, one may envision a simplified scenario. In each scenario, only those activities most likely to lead to toxic exposures are considered. For example, In the Industrial scenario, Indoor workers would not be exposed to levels of dust bearing high concentrations of soli contaminants outdoor workers who stir up dry soli with heavy machinery, however, could expect to Inhale contaminant-laden dust. A scenario could Involve more than one exposure pathway. Thus, the Industrial worker might drink water from a contaminated well, In addition to breathing contaminated dust these exposures might represent not only different pathways but different sources. 

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]). 

Pb concentration in the environment, on the other hand, is strongly influenced by air concentration and rain rate, in addition to direct emissions to water. For this metal the exposure pathway via burning gains a high importance among the different informal recycling processes. 

Exposure Levels in Humans. Human exposure levels to acrylonitrile can only be estimated based on average concentrations in air, food and water. Direct studies of personal exposure levels for individuals with exposures judged to be average and above average (e.g., people living near industrial sources or hazardous waste sites) would be helpful in improving total dose estimates, and in identifying exposure pathways of concern. 

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... 

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. 

From the human perspective, HABs are problematic because they cause (1) risks to human health, (2) loss of natural or cultured seafood resources, (3) impairment of tourism and recreational activities, and (4) damage to noncommercial marine resources and wildlife. Exposure pathways include (1) consumption of toxic shellfish that have accumulated phytoplankton toxins filtered from the water, (2) consumption of tropical fish that have accumulated phytoplankton toxins (ciguatera), (3) inhalation of aerosolized toxins ejected from the sea surface, and (4) skin contact resulting in irritations due to allergy-like reactions. Harmful health effects from acute exposures have been relatively well studied. Less well known are the health effects resulting from chronic exposures to low toxin levels. This is of particular concern with regards to marine mammals and seabirds. 

Children are exposed to 1,4-diehlorobenzene primarily by inhalation of vapors from toilet deodorants, moth proofing crystals and moth balls used in the home or by consumption of moth balls. Consumption of 1,4-dichlorobenzene in foods (See Section 5.4.4) and drinking water (See Section 5.4.2) contaminated with 1,4-dichlorobenzene is thought to be a minor exposure pathway. There have been no body burden measurements made on children. 

Exposures of Children. Children will be exposed to chlorine dioxide and chlorite ions in the same marmer as adults in the general population (i.e., ingestion of water). Additional information about possible differences in exposure pathways for children versus adults would be useful. 

Condie et al. 1986), studies involving exposure in drinking water would be valuable, especially since this is a likely exposure pathway for residents using private wells near hazardous waste sites. 

According to McFarland [26], aquatic toxicity can be considered the result of penetration of toxicant into biophases and its interaction with one or more biochemical sites of action. Thus, he and others have postulated that toxicity is a function of the ability of the chemical to enter biophases and its ability to react with cellular compounds. Bioavailability of chemicals in fish has been shown to be related to chemical flux across fish gills [27], an identified exposure pathway. Flux across fish gills is in turn related to the ability of the chemical to partition between organic and aqueous phases, which is usually correlated with the its octanol-water partition coefficient (logPo/w) [28]. It is therefore not surprising that the acute toxicity of narcotic chemicals has been shown to be related to their propensity to accumulate in the membranes, and hence their logPe/w [29]. 

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. 

Characterize possible mechanisms of exposure to hazardous substances. The pathways by which hazardous substances released from a disposal facility can be transported through the biosphere and the resulting routes of human exposure are specified, often along with their respective probabilities. To estimate exposures of humans at assumed receptor locations, dilution of contaminants by transport in air or water as well as concentration by various means, such as precipitation and uptake by intermediate biological organisms consumed by humans, must be considered. An example of the potentially complex web of exposure pathways is shown in Figure 3.3. 

---