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[AGE27-P03] Evaluation of radioactive cesium-bearing microparticles discharged with suspended solids from a small forested watershed
Keywords:erosion, radioactive cesium, cesium ball, forest
Introduction
A large amount of radionuclides were released to the environment by the accident at the Fukushima Daiichi Nuclear Power Plant (FDNPP). After the accident, in Fukushima Prefecture, forest area which accounts for 70% of the total area of the prefecture was excluded from the decontamination work. However, it is still important to understand dynamics of 137Cs in forests. 137Cs deposited on the forest floor is considered to move slowly downward. It adsorbs to the surface soil and discharged into rivers by surface runoff during rainfall. Previous studies estimated the annual discharge of 137Cs from forests to rivers as about 0.2% of the initial deposition.
Various types of radioactive particles have been isolated from forest. One of them is a weathered biotite adsorbed 137Cs and another is a radioactive Cs-bearing micropaticle (CsMP). Previous studies reported that CsMP is composed mainly of the silicate and its radioactivity concentration (Bq g-1) is approximately 105~6 times of the weathered biotites. While weathered biotites can desorb 137Cs by acidic compounds, CsMP is thought to have low solubility in acid solutions due to its chemical composition. It was also reported that CsMP dissolves by heating in artificial seawater. Therefore, quantification of CsMP based on the selective dissolution with acid solution and artificial seawater is expected to be utilized.
Thus, we aimed to eavluate discharge of 137Cs and CsMP from a forest to a river by using acid and artificial seawater treatment.
Methods
Our study site was a small forest watershed in Iitate Village, Fukushima Prefecture, located about 30 km northwest from FDNPP. Initial deposition of 137Cs in the area was about 1 MBq m-2. We installed an automatic water sampler, a turbidity meter, and a water level gauge with a system that sampled river water when the water level exceeded a certain level.
Each water sample was filtrated through a 6 μm pore size filter paper and a 0.45 μm pore size membrane filter to separate suspended solids (SS). After each filtration, we passed the filtrate through a zinc substitute prussian blue cartridge to adsorb dissolved 137Cs and measured the radioactivity of particulate 137Cs from suspended solids and dissolved 137Cs from the cartridge with a germanium semiconductor detector.
After filtrations, the filter paper was cut into pieces and immersed in 250 mL of 10 mM HCl together with the suspended solids. They were heated at 90°C for 48 hours. After the treatment, we separated solids and liquid, and measured the radioactivity of the liquid. Then, the solids were immersed in 250 mL of artificial seawater and heated at 90°C for 48 hours. After the treatment, we measured the radioactivity of both of solids and liquid. We also conducted the same treatments against 0.25 g of forest soils collected at 5 cm on the surface layer in the study site.
Results
We estimated the discharge of particulate 137Cs by the regression equation between SS concentration (mg L-1) and turbidity (NTU) of water samples, between SS concentration and particulate 137Cs concentration (Bq L-1), river water turbidities, and river water flow rates. We also estimated the discharge of dissolved 137Cs by the average dissolved 137Cs concentration (Bq L-1). As a result, the total discharge of 137Cs was 1.98 GBq (particulate 137Cs: 1.89 GBq, dissolved 137Cs: 82.6 MBq), which was 0.355% of the initial deposition.
From the suspend solids in water samples, 12.7% of the total particulate 137Cs were desorbed by the artificial seawater treatment. This was equivalent to the ratio of CsMP (RCsMP) in suspended solids. Meanwhile, in the forest soils, RCsMP was 12.3%. By SS concentration and RCsMP of water samples, the discharge of CsMP was estimated to be 15.4 MBq. The deposition of CsMP was also estimated to be 685 GBq by RCsMP in forest soils. Therefore, the ratio of CsMP discharged was calculated to be 0.0224% of the CsMP deposited.
A large amount of radionuclides were released to the environment by the accident at the Fukushima Daiichi Nuclear Power Plant (FDNPP). After the accident, in Fukushima Prefecture, forest area which accounts for 70% of the total area of the prefecture was excluded from the decontamination work. However, it is still important to understand dynamics of 137Cs in forests. 137Cs deposited on the forest floor is considered to move slowly downward. It adsorbs to the surface soil and discharged into rivers by surface runoff during rainfall. Previous studies estimated the annual discharge of 137Cs from forests to rivers as about 0.2% of the initial deposition.
Various types of radioactive particles have been isolated from forest. One of them is a weathered biotite adsorbed 137Cs and another is a radioactive Cs-bearing micropaticle (CsMP). Previous studies reported that CsMP is composed mainly of the silicate and its radioactivity concentration (Bq g-1) is approximately 105~6 times of the weathered biotites. While weathered biotites can desorb 137Cs by acidic compounds, CsMP is thought to have low solubility in acid solutions due to its chemical composition. It was also reported that CsMP dissolves by heating in artificial seawater. Therefore, quantification of CsMP based on the selective dissolution with acid solution and artificial seawater is expected to be utilized.
Thus, we aimed to eavluate discharge of 137Cs and CsMP from a forest to a river by using acid and artificial seawater treatment.
Methods
Our study site was a small forest watershed in Iitate Village, Fukushima Prefecture, located about 30 km northwest from FDNPP. Initial deposition of 137Cs in the area was about 1 MBq m-2. We installed an automatic water sampler, a turbidity meter, and a water level gauge with a system that sampled river water when the water level exceeded a certain level.
Each water sample was filtrated through a 6 μm pore size filter paper and a 0.45 μm pore size membrane filter to separate suspended solids (SS). After each filtration, we passed the filtrate through a zinc substitute prussian blue cartridge to adsorb dissolved 137Cs and measured the radioactivity of particulate 137Cs from suspended solids and dissolved 137Cs from the cartridge with a germanium semiconductor detector.
After filtrations, the filter paper was cut into pieces and immersed in 250 mL of 10 mM HCl together with the suspended solids. They were heated at 90°C for 48 hours. After the treatment, we separated solids and liquid, and measured the radioactivity of the liquid. Then, the solids were immersed in 250 mL of artificial seawater and heated at 90°C for 48 hours. After the treatment, we measured the radioactivity of both of solids and liquid. We also conducted the same treatments against 0.25 g of forest soils collected at 5 cm on the surface layer in the study site.
Results
We estimated the discharge of particulate 137Cs by the regression equation between SS concentration (mg L-1) and turbidity (NTU) of water samples, between SS concentration and particulate 137Cs concentration (Bq L-1), river water turbidities, and river water flow rates. We also estimated the discharge of dissolved 137Cs by the average dissolved 137Cs concentration (Bq L-1). As a result, the total discharge of 137Cs was 1.98 GBq (particulate 137Cs: 1.89 GBq, dissolved 137Cs: 82.6 MBq), which was 0.355% of the initial deposition.
From the suspend solids in water samples, 12.7% of the total particulate 137Cs were desorbed by the artificial seawater treatment. This was equivalent to the ratio of CsMP (RCsMP) in suspended solids. Meanwhile, in the forest soils, RCsMP was 12.3%. By SS concentration and RCsMP of water samples, the discharge of CsMP was estimated to be 15.4 MBq. The deposition of CsMP was also estimated to be 685 GBq by RCsMP in forest soils. Therefore, the ratio of CsMP discharged was calculated to be 0.0224% of the CsMP deposited.