For. and For. Prod. Res. Inst, 1 Matsunosato, Tsukuba, Ibaraki, 305-8687, Japan.
For. and For. Prod. Res. Inst, 1 Matsunosato, Tsukuba, Ibaraki, 305-8687, Japan.
J Environ Radioact. 2020 Dec;225:106418. doi: 10.1016/j.jenvrad.2020.106418. Epub 2020 Oct 7.
In an experimental watershed located around 120 km southwest of the Fukushima Daiichi Nuclear Power Plant with a drainage area of 59.9 ha, suspended solids (SS) and radioactive cesium discharge from a forested headwater catchment were monitored before and after line thinning. The lower slopes in the experimental watershed were covered with plantation conifer trees (Japanese cedar), while the upper slopes were covered with deciduous trees. In 2012, line thinning was carried out at a thinning rate of 35% across 17% of the northeastern part of the watershed and across the remaining part in 2013. Spur roads were constructed along all tributaries without water, and logged trees were dragged and grappled using forestry machinery and transported along these roads to timber yards using forwarder-type forestry vehicles. A V-notch weir and a water level gauge were installed at the watershed outlet and stream water was sampled twice a month during base flow, whereas during flood flow, stream water samples of 1 L were collected every hour using an automatic water sampler. These samples were filtered through 0.5 μm glass fiber filters to measure the SS concentration. SS concentration data was collected for 21 floods before thinning and for 37 floods after thinning. A time-integrated SS sampler was installed in the stream close to the weir and SS samples were collected every two or three months to measure their Cs-137 concentrations. SS concentrations before (from July 2010 to August 2012) and after thinning (from October 2013 to December 2018) were compared, where the maximum SS concentrations before and after thinning were 211 and 790 mg L, respectively. It was discovered that some SS concentrations during flood flow were higher after carrying out thinning than before. Some ΣLss values (specific cumulative load of SS in a flood event) also showed the same results as the SS concentrations. Thus, it was clear that SS discharge immediately increases after thinning, but as it increases Cs-137 export is limited. This is related to a change in SS source brought about by the process of thinning, a decrease with time in the Cs-137 concentration in organic solid expected from that in litter, and a regrowth of vegetation on spur roads, protecting them against soil erosion. Therefore, it was concluded that thinning does not drastically increase Cs-137 export from a forested watershed.
在福岛第一核电站西南约 120 公里的一个实验流域，流域面积为 59.9 公顷，对森林集水区的悬浮物 (SS) 和放射性铯的排放进行了林分疏伐前后的监测。实验流域的下部斜坡覆盖着人工种植的针叶树（日本雪松），而上部斜坡则覆盖着落叶树。2012 年，在流域东北部分 17%的地区以 35%的疏伐率进行了疏伐，并于 2013 年在剩余部分进行了疏伐。沿所有无水支流修建了支线道路，并使用林业机械将原木拖曳和钩住，然后沿这些道路使用集材型林业车辆将原木运往木材场。在流域出口处安装了 V 形缺口堰和水位计，并在基流期间每两个月采样两次，而在洪水期间，则使用自动水样采集器每小时采集 1L 的水样。这些样品通过 0.5μm 玻璃纤维过滤器过滤，以测量 SS 浓度。在疏伐前收集了 21 次洪水的 SS 浓度数据，在疏伐后收集了 37 次洪水的 SS 浓度数据。在堰附近的溪流中安装了一个时间积分 SS 采样器，每两到三个月收集一次 SS 样本，以测量其 Cs-137 浓度。比较了疏伐前后（2010 年 7 月至 2012 年 8 月和 2013 年 10 月至 2018 年 12 月）的 SS 浓度，疏伐前后的最大 SS 浓度分别为 211 和 790mg/L。发现一些洪水期间的 SS 浓度在疏伐后比疏伐前更高。一些 ΣLss 值（洪水事件中 SS 的特定累积负荷）也显示出与 SS 浓度相同的结果。因此，很明显，疏伐后 SS 排放量会立即增加，但随着时间的推移，Cs-137 的排放量会受到限制。这与疏伐过程带来的 SS 源变化有关，即枯枝落叶中有机固体的 Cs-137 浓度随时间的降低，以及支线道路上植被的再生，防止了土壤侵蚀。因此，结论是疏伐不会大幅增加森林流域的 Cs-137 排放量。
