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Migration Behavior of Cesium in the Subsurface Environment After Wildfires
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Forest fires produce various particulate organic matters (POMs) derived from the incomplete combution process of biomass. The POMs deposited in soil and sediments can affect the physicochemical properties of the subsurface environments. This study investigated the sorption and transport behavior of cesium (Cs) in soil-groundwater environment after wildfire. Soil samples were collected at two locations (GS1 & GS2) in Gangwon Province, Korea, at different depths (~5, ~20, and ~40 cm). The sampling site, where a large-scale forest fire occurred in 2017, was damaged almost 252 ha of forest. The soil characteristics were determined by X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning electron microscopy (SEM), total organic carbon (TOC) analysis and organic petrography, and batch and fixed-bed column experiments were performed to evaluate the Cs uptake and retardation. The XRD patterns of the soils indicated that the mineral compositions of soils were quartz, feldspars (e.g., orthoclase & albite) with minor muscovite/illite. Quartz and feldspars were abundant in all studied soils, and GS2 sample contained higher feldspars and phyllosilicate minerals than the GS1. The TOC contents were high (7–8wt%) in the topsoils, decreasing with depth. The SEM and organic petrographic analyses showed that various organic carbon particles such as textinite, ulminite, fusinite (charcoal) and char existed. Presence of charcoal and char is the evidence of wildfires, even though their amount was few. Batch sorption experiments revealed that the Kd value decreased non-linearly as the Cs concentrations increased, and the sorption isotherms were fitted well with the Freundlich model. The Kd values of each soil were much greater in topsoils compared to subsoils at all experimental Cs concentrations. In particular, the GS1 topsoil had higher sorption capacity for cesium than GS2 subsoils, although it had low phyllosilicate mineral contents with realtively rich organic matter. The breakthrough curve of column experiments with high concentration (C0 ≈ 1×105 μg·L−1) also exhibited remarkable Cs retardation phenomena in topsoils. Their retardation factors (Rf,Cs) were max. 4 times greater than those of subsoils, showing Rf,Cs ≈ 43 to 45 for topsoils. At low concentration (C0 ≈ 1×104 μg·L−1), the Rf,Cs of topsoils (≈ 284 to 374) was slightly greater than that of subsoils (≈ 270 to 271). These results imply that POMs caused by wildfires can play important role on the Cs sorption and transport in the subsurface environments.
