Soil ingestion pathway risk assessment

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). 

A reference dose (RfD mg/kg/day) was originally designed for estimating noncancer health risks at CERCLA (Comprehensive Environmental Response, Compensation, and Liability Act of 1980) Superfund sites (USEPA, 1989). As such, it is an essential component of the site risk assessment used to assess potential long-term exposures to contaminated media such as soil, where RfDs address the pathway of incidental ingestion of soil particles (see Dourson, 1994 Cicmanec et al, 1996 Abernathy et al, 2004 USEPA, 1989). 

A baseline risk assessment is conducted to assess the potential human health and environmental impacts associated with soil contamination. The primary exposure pathways evaluated for explosives contaminated surface soils are dust inhalation, soil ingestion, and dermal absorption. Reasonable Maximum Exposure (RME) concentrations are based on the 95% upper confidence interval (UCI) on the arithmetic mean of soil sampling data. The land use scenarios quantitatively evaluated may include industrial and residential use, utilizing EPA standard default exposure parameters. 

Incidental soil ingestion by children is an important pathway in assessing public health risks associated with exposure to arsenic-contaminated soils. Incidental ingestion of soil represents the principal direct pathway for exposure to nondietary sources of As in contaminated areas. The importance of soil ingestion by children as a health issue has been reported by numerous researchers and fully illustrates the importance of this pathway in terms of subsequent chemical exposure (4-8). 

PTMs-contaminated soils pose a human health risk on the basis of the potential of the contaminant to leave the soil and enter the human bloodstream. In order to assess human health risk, several pathways of transfer of metals from soil to humans have to be taken into account. The most important metal intake takes place via the food chain in which plants or meat of animal play a key role. The direct ingestion of soil can be a major route of exposure for humans to many low mobile soil contaminants, particularly for small children through putting hands into the mouth (Gupta et al., 1996). The contribution from the inhalation of particles smaller than 10 pm and from dermal contact with soil have little meaning compared with oral ingestion and are found to be less than 1% and 0.1% of the total intake, respectively (Paustenbach, 2000). 

The Twenty-Fifth Annual Conference on Trace Substances in Environmental Health was held at the University of Missouri, May 20-23, 1991. Findings and recommendations conclude that (1) lead is ubiquitous in the human environment (2) any lead exposure assessment must analyze all potential sources and exposure pathways and (3) acceptable soil lead levels can be determined only through an approach that takes into account numerous factors including but not limited to the population at risk, the physical availability of the lead, and the bioavailability of the lead, which is the amount of lead that can be ingested into the blood stream. 

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