Evaluating of the effect of thinning on suspended sediment runoff in a cypress and cedar plantation forest using Fukushima-derived Cs-137, Cs-134 and Pb-210ex

Skid trail and heavy machinery for forest practice becoming more common for effective forest practices, but these causes soil disturbance in the forest, leading to a sudden increase in the amount of suspended sediment during and post thinning. The discharged sediment can flow into the river and may cause downstream water pollution. To evaluate the effect of thinning on sediment production, sediment fingerprinting techniques can be an effective tool for proper forest practices. In Tochigi prefecture in Japan, in addition to the Cs-137 by global fallout and Pb-210ex, additional FRN, the Fukushima-derived Cs-137 and Cs-134 on March 2011 are available, but few studies are available for combining use of Fukushima-derived radiocesium and fallout Pb-210ex. Therefore, the objective of this study is to determine the transport of the fine sediment in the forest pre- and post- thinning with using fingerprinting techniques.

The study area is Mt. Karasawa, located 180 km southwest of the Fukushima nuclear power plant in Tochigi prefecture. The fallout inventory of Cs-137 and Cs-134 is 8 kBq/m²(Kato et al, 2012). The study site has two catchments which are called K2(17ha) and K3(9ha) respectively and the observation period was from August 2010 to August 2019. In K2, strip thinning was performed with heavy machines from June to October 2011 while randomly thinning without heavy machines was applied for K3 from January to March 2013.Soil samples were collected from the slope surface, skid trail and stream bed, which are the possible sources of suspended sediment. The suspended sediment concentration was measured based on the data of the turbidity censor installed in the stream. The particle size distribution and radionuclide concentration of sediment collected from SS sampler and soil samples are also measured. Hysteresis analysis based on suspended sediment concentration and flow rate and fingerprinting using Fukushima-derived Cs-137, Cs-134, and Pb-210ex was applied to determine the contribution of the slope surface layer and streambed to suspended sediment. By using the difference in the depth distribution of Cs-134 and Cs-137, the production source depth of suspended sediment was estimated.

In the K2 catchment where strip thinning was performed with heavy machinery, suspended sediment concentration during high flow period was rapidly increased up to 2833 mg/L during thinning period and then decreased down to 503 mg/L. On the other hand, in the controlled catchment(K3), no increase in suspended sediment concentration was observed during the same period. By using End- Members Mixing analysis, we found that the contribution of suspended sediment from hillslope increased (from 22% to 50%) more than the stream bed (30%) in the thinning period. Since 2014, the trend has reversed and in 2019, the contribution from the streambed is dominant (50%), and the contribution from slope is decreasing (28%).