Accelerated Incubation Method for Identification of Boreal Soils With Hypersulfidic Material

ABSTRACT Sulfide‐bearing sediments (i.e., soils with hypersulfidic material) occur worldwide in many coastal and inland freshwater settings, that if exposed to oxidation, will transform to acid sulfate soils (ASS; a soil with sulfuric material) mobilizing acidity and metals into watercourses with serious environmental consequences and damage to infrastructures. Thus, management techniques to minimize these hazards are important but rely on correct identification of hypersulfidic material. The incubation pH method is one of the most reliable, easy‐to‐use, and low‐cost methods for this purpose. The method simulates the natural oxidation behavior of possible existing hypersulfidic material by exposing the soil to the atmosphere at room temperature while maintaining it moist over a period of time (often between 8 and 19 weeks) for acid‐generating oxidation reactions to take place. The soil material is considered hypersulfidic if the amount of acidity produced during oxidation exceeds its acid‐neutralizing capacity and thereby lowers the mineral soil pH to < 4 (ΔpH > 0.5) and the organic‐rich soil (LOI > 20%) to < 3 (ΔpH > 0.5). For practical reasons, an incubation time of up to 19 weeks is, however, in many cases too long and the main goal in this study was to reduce the required incubation time by optimizing oxidation/acidification conditions with the intention to recommend an accelerated incubation method. To ensure the method's usability, several trial experiments were conducted on various types of soil materials containing sulfidic materials, including peat, sandy/loamy soils, and heavy clays. The trial included testing different temperature ranges, optimizing the sample thickness, determining the effects of stirring versus non‐stirring the samples during incubation, and using “quick start” doping agents. The incubation time needed was reduced up to 50% by optimizing the sample thickness from 5 to 2 mm according to the most recent recommendations and, most importantly, by applying heat to 30°C. In mineral soil samples, the incubation time was additionally reduced by stirring the samples. Mineral soils with hypersulfidic material could generally be identified in less than 2 weeks and organic soils with hypersulfidic material in around 4 weeks. Heavy clay soils needed, however, 5 weeks to be identified as hypersulfidic material. Consequently, the best set of recommended incubation methods, based on numerous trial experiments and conditions conducted, for faster identification of different textured soils with hypersulfidic material is as follows: (I) Mineral soils: use a 2 mm thick sample in chip trays; applying heat to 30°C; stirring the samples three times per week. (II) Mineral soils with heavy clay textures: use a 2 mm thick sample (or thinner) in chip trays; applying heat to 30°C; stirring the samples three times per week. (III) Peat/organic soils: use a 2 mm thick sample in chip trays; applying heat to 30°C.