31-6 A Monte Carlo Framework for Estimating Background Geogenic Soil Lead in Cacao-Producing Countries

Session: Environmental Geochemistry and Health (Posters)

Poster Booth No.: 32

Presenting Author:

Kalin McDannell

Authors:

McDannell, Kalin T.¹, Slagowski, Naomi², Dugan, Thomas³, Boomhower, Steven R.⁴

(1) Gradient, Boston, MA, USA, (2) Gradient, Boston, MA, USA, (3) Gradient, Boston, MA, USA, (4) Gradient, Boston, MA, USA,

Abstract:

Elevated lead (Pb) concentrations reported in chocolate have raised food-safety concerns. One source of Pb in chocolate may be naturally occurring soil Pb, which can be transferred into cacao beans grown in various countries in West Africa, South America, and other regions. However, the contribution of natural (geogenic) bedrock Pb sources to potential Pb exposure from cacao beans remains poorly quantified in scientific studies, particularly for various cacao-producing countries.  We present a Monte Carlo framework that estimates background geogenic soil Pb attributable solely to bedrock geochemistry, providing a quantitative baseline against which measured field values can be compared. Country-scale lithologic areal fractions were derived from a published global lithologic map (GLiM; average scale 1:3,750,000), classifying geology into igneous/metamorphic, sedimentary, and volcanic categories.  Lithologic Pb concentrations were parameterized from Turekian and Wedepohl (1961) using log-normal distributions considering the positively skewed nature of Pb in geological materials.  For each of the 31 cacao-producing countries evaluated, 10,000 Monte Carlo iterations combined randomly sampled rock-type Pb concentrations, weighted by areal fraction, to generate a probability distribution of total geogenic soil Pb.  The resulting geometric mean soil Pb estimate for each country was then multiplied by an empirical soil-to-plant uptake factor for "protected" crops (i.e., plants with enclosed seeds, such as cacao), yielding a point estimate of geogenic Pb in cacao beans.  Modeled geometric mean total soil Pb across cacao-producing countries ranged from ~2.5 to 15 parts per million.  Corresponding plant-tissue Pb uptake estimates were two orders of magnitude below those values for all modeled countries.  These results provide a quantitative, geology-based estimate of the minimum expected Pb contribution from bedrock. Notably, this framework offers a transferable approach for establishing geogenic (i.e., naturally occurring) background in multiple contexts, such as human health risk assessment, food safety, and evaluating regulatory compliance. This work illustrates how public geologic and empirical bioavailability datasets can be combined with Monte Carlo uncertainty propagation to support risk assessment and evaluating regulatory compliance where direct farm-level sampling is unavailable.

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