Systematic Identification of Analytical Indicators to Measure Soil Load on Plants for Safety Assessment Purposes

Abstract

Contaminants in soil can be transferred to humans and other biota when soil dust on plants is ingested as food. Measuring how much soil is on plants, in the absence of artificial tracers or contaminants, is difficult because of the very small amounts involved and the confounding effect of absorption of elements through roots. However, measurements of soil load are essential for assessment modelling. We compared the ability of several analysis strategies, involving naturally occurring elements, to predict the soil load on plants. Large samples of 12 vegetable and fruit crops were collected, along with corresponding soil samples. An independent measure of soil load on the crop samples was derived from a combination of gravimetric measurements, including loss on washing and the acid-insoluble ash content. The best agreement between the elemental-abundance and the gravimetric methods was for Sr in the plant acid-insoluble ash. Through a systematic process of elimination, we conclude that analyses of the acid-insoluble ash fraction of the plant samples for Al, Ba, Fe, Si, Sr and Ti are most reliable. If analyses are to be restricted to the full plant ash, then the choice of analytes is restricted to Al, Fe, Si and Ti. Soil loads in our study averaged 20 mg soil kg⁻¹ dry plant for leafy tissues and 2 mg soil kg⁻¹ for fruits, and washing decreased soil loads about 1.5 fold.

Key Words:

• Soil load
• mass loading
• dust loading
• soil ingestion