# Determination of geochemical parameters that control the spatial distribution of potentially toxic elements released from excavated shale at a temporary storage pit

## TL;DR

This study examines how toxic elements from excavated shale are released and distributed, identifying key geochemical factors that influence their release.

## Contribution

The study identifies specific geochemical parameters controlling the release of different potentially toxic elements from excavated shale.

## Key findings

- The release of arsenic and selenium is mainly controlled by amorphous iron content.
- Lead and chromium release is influenced by inorganic colloidal particles under 0.45-μm.
- Sampling 10 mixed rock samples can estimate released PTE amounts with a 30% error.

## Abstract

Large quantities of rocks naturally containing potentially toxic elements (PTEs) are generally excavated from urbanized and mountainous areas for construction purposes and are subjected to treatments (e.g., chemical immobilization) so that they can be re-used. To design such treatments, it is imperative to determine the total PTE amounts released from the excavated rocks and consider their spatial distribution. Within this context, the present study aimed to clarify (i) the spatial distribution of PTE contents released from 100 m3 of excavated shale deposited in a temporary storage pit, and (ii) the geochemical parameters that control such distribution. A total of 50 shale samples were collected from a temporary storage pit of rocks. These samples were subjected to the analysis for the determination of the released and total PTE amounts, and other chemical properties. No linear relationship was detected between the released and total PTE amounts. An error of ± 30% was estimated based on a sample of 10 rocks. The geochemical parameters controlling the PTE contents released from the excavated shale differed depending on PTE types. The release of arsenic and selenium was mainly controlled by the amount of amorphous iron, while that of lead and chromium was controlled by the amount of inorganic colloidal particles with size under 0.45-μm. This study suggested that the preparation of 10 mixed rock samples collected from temporary storage pits leads to an estimation of released PTE amounts with a 30% error. In addition, the actual PTE amount released from excavated rocks containing high and low amounts of amorphous iron and inorganic colloidal particles, respectively, would differ little from the estimated amounts.

The online version contains supplementary material available at 10.1007/s10653-025-02577-8.