Traverse Mountain Soil Sample Results
To address the Traverse Mountain community concerns regarding the safety of vegetables grown in secondary water containing thallium, the Environmental Epidemiology Program (EEP) conducted soil sampling in June, 2012.
Samples were collected from 15 different residential yards and were analyzed by the Chemistry Laboratory at the Utah Unified Public Health Lab. Results from the Traverse Mountain soil sampling event are presented in table 1. To ensure the privacy of the residents, the address of the sample location was replaced with a sample number. If you participated in this study, you will receive a postcard from the EEP that indicates which sample number corresponds to your residence. The soil sample results are given in parts per billion (ppb).
|Sample #||Thallium (ppb)|
As indicated in table 1, the mean thallium concentration for all soils sampled was 218.5 ppb. The geometric mean (an average that more accurately represents a population when the sample size is small and there is a wide range of values) for this group of samples is 169.9 ppb. Thallium is naturally occurring in the earth's crust at an average concentration of 700 ppb (ATSDR, 1992; Wexler, 2005).
The source of thallium contamination in Traverse Mountain soil is from the secondary irrigation water. Exposure from thallium contaminated soil can occur through direct contact with the soil (i.e. gardening, playing in soil, etc.) resulting in accidental ingestion or dermal absorption.
It is also possible to be exposed to thallium in soil by eating food grown in contaminated soils. Without data to indicate how much thallium is taken up by plants grown in this soil, dose calculations focus on accidental ingestion and dermal absorption of thallium from soil.
The EEP performed exposure dose calculations using the Agency for Toxic Substances and Disease Registry's (ATSDR) equations for soil ingestion exposure and soil dermal contact exposure (ATSDR, 2005). Both equations are based on contaminant concentration, amount of contaminant consumed/body area contacted, duration of exposure, and body weight of the exposed individual. Body weight used for adults was 70 kilograms (kg) (or 155 pounds lbs.) and 16 kg (35 lbs.) for children. The highest thallium soil concentration found in the samples, 965 ppb, was used in the equations. The exposure duration assumes 180 days a year of soil exposure.
The US Environmental Protection Agency (EPA) establishes a reference dose (RfD) based on available scientific studies estimating the daily lifetime dose of a substance that is unlikely to cause harm in humans. The RfD for thallium is 1 x 10-5 milligrams per kilogram per day (mg/kg/day) (EPA, 2009). Exposure doses for children and adults are presented in table 2. All potential doses calculated are less than the EPA's established RfD. This means that accidental ingestion and dermal exposure to the soil in Traverse Mountain alone does not pose an apparent health hazard to adults and children. It should be noted that these are the greatest possible exposures to the very highest thallium concentration recorded. Actual experiences are expected to result in far smaller exposures.
|Route of Exposure||Potential Child Exposure Dose
|Potential Adult Exposure Dose (mg/kg/day)||EPA Chronic RfD
|Notes: mg/kg/day = milligram per kilogram per day, EPA = Environmental Protection Agency, RfD= Reference Dose|
|Dermal||2.82 x 10-6||9.36 x 10-7||1.0 x 10-5|
|Ingestion||6.03 x 10-6||6.89 x 10-7|
|Total||8.85 x 10-6||1.63 x 10-6|
Plants and vegetables may accumulate thallium when grown in soils containing high contaminant concentrations (LaCoste et al., 2001). Different vegetables accumulate thallium at different rates and in different areas of the plant. Roots of plants are the best accumulators of thallium, as concentrations increase from leaves to roots in vegetables (Queirolo et al., 2009). Not all plants uptake thallium at the same rate; potatoes, turnips, radishes, and brassicaceous plants (cabbage family) have been found to have higher concentrations of thallium than other vegetables (Pavlickova et al., 2005; Queirolo et al., 2009). Vegetables with lower thallium intakes include green beans and tomatoes. It has been shown that vegetables in field/garden conditions have a lower thallium uptake than plants grown in pots, due to a lower root density (LaCoste et al., 2001).
Vegetables grown in soils with a thallium concentration below 700 ppb are not considered to pose any health risk (LaCoste et al., 2001). Concerns are raised when certain vegetables with higher thallium uptakes (potatoes, turnips, cabbage) are grown in soils greater than 700 ppb. Vegetables with a lower thallium uptake (green beans, tomatoes, onion, peas) are not expected to pose a potential health risk until the soil is above 3,500 ppb (LaCoste et al., 2001).
In conclusion, the soil sampling data collected in June of 2012 does not indicate that an apparent health hazard is presented by the thallium concentrations found in residential garden soil in Traverse Mountain. Although more sampling may provide a more accurate assessment of the thallium levels present in these soils, the conservative exposure dose calculations presented here give strong support to our conclusions.
Agency for Toxic Substances and Disease Registry [ATSDR]. Public Health Assessment Guidance Manual (Update). January 2005.
ATSDR (1992). Public health statement for thallium. Atlanta: U.S. Department of Health and Human Services. Retrieved from: http://www.atsdr.cdc.gov/PHS/PHS.asp?id=307&tid=49
LaCoste, C., Robinson, B., & Brooks, R. Uptake of thallium by vegetables: its significance for human health, phytorediation, and phytomoining. Journal of Plant Nutrition. 2001.
Pavlickova, J., Zbiral, J. Smatanova, M., Habarta, P., Houserova, P., & Kuban, V. Uptake of thallium from naturally-contaminated soils into vegetables. Food Additives and Contaminates. May, 2006.
Queirolo, F., Stegen, S., Contreras-Ortega, C., Ostapczuk, P., Queirolo, A., & Paredes, B. Thallium levels and bioaccumulation in environmental samples of Northern Chile: human health risks. Journal of Chilean Chemical Society. 2009.
U.S. Environmental Protection Agency [EPA]. Integrated risk information system, Thallium (I), Soluble salts; CASRN various. 2009. Retrieved from: http://www.epa.gov/iris/subst/1012.htm
Wexler, 2005 Encyclopedia of Toxicology, Second Edition. Oxford. Elsevier Ltd., 2005.