Influence of melt‐freeze‐cycles on the radionuclide transport in homogeneous laboratory snowpack
Influence of melt‐freeze‐cycles on the radionuclide transport in homogeneous laboratory snowpack
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DOI：
10.1002/hyp.11110
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发表时间：
2017-03
影响因子：
3.2
通讯作者：
K. Hürkamp;Stefanie Tafelmeier;J. Tschiersch
中科院分区：
文献类型：
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作者：
K. Hürkamp;Stefanie Tafelmeier;J. Tschiersch
如果环境温度保持较低，特别是在冰川地区或高山地区，释放到环境中并与雪一起沉积或沉积在雪上的放射性核素可以长期储存。除非雪底发生融水径流，否则放射性核素将在冬季积聚在积雪中。它们会在春季融雪期间短时间内释放到地表水中。在实验室受控雪融化条件下的两项实验中，检查了融化-冻结循环期间放射性核素的迁移和径流。确定了 Cs-134 和 Sr-85 示踪剂在均匀雪柱中的分布及其在第一融水部分中的分馏和潜在的优先洗脱。运输与积雪成熟后环境温度高于 0°C 时融水的渗透有关。在 -2 至 4 °C 的环境温度下进行一个昼夜熔化-冷冻循环后，检查了两种核素在堆中的平均迁移速度，约为 0.5 cm hr−1。计算出的融水通量的中位数为 1.68 cm hr−1。高度污染的融水部分，其浓度因子相对于积雪中的初始本体浓度在 5 到 10 之间，以离子脉冲的形式与第一融水一起释放。铯和锶均未观察到优先洗脱。经过反复模拟的昼夜循环（−2 至 4 °C）后，80% 的两种放射性核素在 4 天内随前 20% 的融雪释放。 24 小时后，50% 的 Cs-134 和 Sr-85 已被释放。当冷冻循环期间融化速率最低时，由于剩余液体的浓缩过程，融雪包含最高的比活性，并通过融化冷冻循环增强。这意味着对于大量放射性核素释放后的天然积雪来说，霜冻期间放射性核素在雪中的长期积累，随后以低融化速率开始稳定的融水径流，将导致积雪中污染物最明显的去除。这种情况代表了水生环境中水质和辐射暴露影响的最坏情况。
Radionuclides released to the environment and deposited with or onto snow can be stored over long time periods if ambient temperature stays low, particularly in glaciated areas or high alpine sites. The radionuclides will be accumulated in the snowpack during the winter unless meltwater runoff at the snow base occurs. They will be released to surface waters within short time during snowmelt in spring. In two experiments under controlled melting conditions of snow in the laboratory, radionuclide migration and runoff during melt‐freeze‐cycles were examined. The distribution of Cs‐134 and Sr‐85 tracers in homogeneous snow columns and their fractionation and potential preferential elution in the first meltwater portions were determined. Transport was associated with the percolation of meltwater at ambient temperatures above 0 °C after the snowpack became ripe. Mean migration velocities in the pack were examined for both nuclides to about 0.5 cm hr−1 after one diurnal melt‐freeze‐cycle at ambient temperatures of −2 to 4 °C. Meltwater fluxes were calculated with a median of 1.68 cm hr−1. Highly contaminated portions of meltwater with concentration factors between 5 and 10 against initial bulk concentrations in the snowpack were released as ionic pulse with the first meltwater. Neither for caesium nor strontium preferential elution was observed. After recurrent simulated day‐night‐cycles (−2 to 4 °C), 80% of both radionuclides was released with the first 20% of snowmelt within 4 days. 50% of Cs‐134 and Sr‐85 were already set free after 24 hr. Snowmelt contained highest specific activities when the melt rate was lowest during the freeze‐cycles due to concentration processes in remaining liquids, enhanced by the melt‐freeze‐cycling. This implies for natural snowpack after significant radionuclide releases, that long‐time accumulation of radionuclides in the snow during frost periods, followed by an onset of steady meltwater runoff at low melt rates, will cause the most pronounced removal of the contaminants from the snow cover. This scenario represents the worst case of impact on water quality and radiation exposure in aquatic environments.