An accelerated incubation method for the identification of acid sulfate soils

Sulfide bearing sediments occur worldwide in many coastal and inland settings and if exposed to oxidation, these sediments will develop into acid sulfate (AS) soils (pH <4) mobilizing acidity and soluble metals into watercourses with serious environmental consequences and structure/infrastructure damages. Thus, management techniques to minimize these hazards are important but rely above all on a correct identification of AS soil materials. The incubation pH method (Creeper et al. 2012) is one of the most preferred, reliable, easy-used and low-cost AS soil identification method, and simulates the natural oxidation behaviour of possible existing sulfidic materials by letting a wet soil sample oxidise in room temperature for a maximum of 19 weeks. An AS soil is identified if the amount of acidity produced during soil oxidation exceeds its acid-neutralizing capacity and thereby lowers the mineral soil pH to <4 (ΔpH >0.5). For organic-rich soil materials (LOI >20%) a lower identification limit, a pH <3 (ΔpH >0.5), is suggested as organic matters are naturally more acidic due to humic acids (Visuri et al., 2021; Nystrand et al. 2021, Hadzic et al. 2014). An identification duration of 19 weeks is, however, in many cases considered as to time-consuming and the main goal in this study was to further develop and improve the incubation method in such a manner, that the outcome would be a substantially reduced incubation time. To ensure the usability of the method, different type (e.g., grain sizes, sulfur concentrations, sample locations and sample depths) of known well homogenized sulfidic soil materials were chosen along the Finnish coastline. The intention was to increase the oxidation rate and, thus, decrease the time needed for incubation. Enhanced oxygen availability was trialed by optimising the soil sample thickness (alternatives tried were 2 mm, 5 mm and 10 mm thick samples) and by stirring the samples during the incubation three times per week. Temperature dependency on the microbiologically mediated oxidation was trialed at different temperatures (in 4 °C, 22 °C, 30 °C, 40 °C and 50 °C). In addition, trials were made to quick start the microbiologically mediated oxidation by “doping” samples with microbes from previously oxidized samples. Another quick start ”doping” agent tested was ozonated water. In all trials, pH was measured three times per week. The incubation time was significantly reduced (generally by 50%) by: 1) optimising the sample thickness to 2 mm, 2) increasing the air penetration by stirring the sample, and 3) adding heat to 30 °C. Potential mineral acid sulfate soils could by that already be identified in an average of 3 weeks and potential peat samples in an average of 4 weeks. For most of the mineral soil samples an incubation time of ≤2 weeks was enough to identify AS soils, but in clay-rich material the incubation time was prolonged to 5 weeks. Conclusionary, results suggest that the accelerated incubation method enables reliable identification of AS soil materials in a maximum incubation time of 5 weeks.