Human Bioaccessibility of Heavy Metals and PAH from Soil

Human Bioaccessibility of Heavy Metals and PAH from Soil

Summary and conclusions

Most soil quality criteria and cleanup levels for soil contaminants are based upon oral exposure and effect studies with soluble, highly bioavailable contaminant forms ingested with water or with food. When ingested with soil, metals and PAH are likely to be less available than in the toxicity studies underlying the soil quality criteria and cleanup levels.

Bioavailability of soil contaminants for humans depends primarily upon the ability of the stomach and the small intestine to dissolve the contaminant (bioaccessibility) and upon the ability of the intestinal membranes to absorb the contaminant. Bioaccessibility of the soil contaminants depends upon the contaminant chemistry, the soil properties and the chemical conditions in the gastrointestinal system.

Therefore, the current knowledge on bioaccessibility of 7 metals and 7 PAH has been reviewed.

A number of different in vitro test methods are available to measure bioaccessibility of soil contaminants. The results are not generally comparable between methods, and data on the quality of the bioaccessibility test methods are limited. Based upon reported bioaccessibility test methods, candidates for one common test method are proposed and requirements for method parameters and performance are given.

The overall picture from available data on bioaccessibility of soil contaminants is, that reduced soil bioaccessibility is very likely for cadmium, lead and chromium (III) (uptake in small intestine), likely for arsenic and chromium (VI), and possible for copper, nickel, zinc and PAH.

Bioaccessibility will impact human exposure if dissolution of the soil contaminants is rate limiting compared to absorption and this is suggested to be the case by studies of lead, arsenic and PAH. Reduced bioavailability has been reported for in vivo uptake studies with animals for at the least arsenic, cadmium, lead and PAH.

Reduced bioaccessibility and/or bioavailability has been taken into consideration in site specific regulation of cleanup levels for contaminated sites in the US, Canada and UK (regional specifik), in particular for mine waste and ore processing sites.

The general conclusion is that regulation of soil quality criteria and cleanup levels based upon reduced bioavailability/bioaccessibility of the contaminants is recommended after site specific risk assessment. Conversely, the data available at present does not allow for general regulation of soil quality criteria and cleanup levels for specific contaminants, soil types or sources.

A short term and a long term model for implementation of bioaccessibility in risk assessment of contaminated sites is suggested, and the requirement for their implementation is described.