Estimation of bioaccessibility of lead in intensive vegetable growing soils in Sri Lanka and screening of the Mehlich 3 extraction method to estimate total and bioaccessible lead (Pb) concentration in soil

Accidental ingestion of soil is a significant pathway for lead (Pb) exposure, although it is secondary to soil-plant transfer and thus receives less focus in risk assessments. Measuring total, Mehlich 3-extractable, and bioaccessible Pb in routine soil tests could enhance risk evaluation related to the consumption of contaminated soils. This study examined Pb bioaccessibility in soils used for long-term vegetable cultivation across two cropping systems and soil types in three Sri Lankan agro-climatic zones. A physiologically based in vitro extraction test (PBET) at pH 2 and 2.5 estimated human Pb exposure through soil ingestion. The research also evaluated whether Mehlich 3 extractable Pb could reliably indicate both bioaccessible and total Pb levels. Surface soil samples (0–15 cm) were collected from: vegetable-only cropping system on Ultisols in Nuwara Eliya (n = 46), vegetable-paddy rotation on Ultisols in Badulla (n = 54), and intensive vegetable cropping system on Entisols in Kalpitiya (n = 48). Concentrations of bioaccessible, total, and Mehlich 3 extractable Pb were measured. Bioaccessible Pb in soils from Nuwara Eliya, Badulla, and Kalpitiya ranged from 01 to 2.8, 0.1-8.0, and 03–8 mg/kg, respectively, representing 0.04-35%, 2.5–34%, and 49–60% of total Pb. Mehlich 3 extractable Pb levels in these soils were 63–15, 054–15, and 0.6–2 mg/kg, respectively, while total Pb ranged from 7 to 57, 7–46, and 0–34 mg/kg. Significant correlations between Mehlich 3 extractable Pb, bioaccessible Pb, total Pb, and specific soil properties suggest soil characteristics influence Pb bioavailability. The findings indicate that Mehlich 3 extraction could serve as an alternative method for estimating both total and bioaccessible Pb in soils.