GeoZentrum Nordbayern, Department of Geography and Geosciences, Friedrich-Alexander University Erlangen-Nuremberg, Schlossgarten 5, 91054 Erlangen, Germany.
GeoZentrum Nordbayern, Department of Geography and Geosciences, Friedrich-Alexander University Erlangen-Nuremberg, Schlossgarten 5, 91054 Erlangen, Germany.
Sci Total Environ. 2018 Sep 1;634:374-381. doi: 10.1016/j.scitotenv.2018.03.230. Epub 2018 Apr 6.
Besides gas-water-exchange in surface waters, respiratory consumption of dissolved oxygen (DO) in adjacent riparian groundwater may trigger the addition of so far hardly explored sources from the unsaturated zone. These processes also systematically influence stable isotope ratios of DO and were investigated together with Cl as a conservative tracer for water mixing in a near-river riparian groundwater system. The study focused on a losing stream section of the Selke River at the foot of the Harz Mountains (Germany). The study area exposed steep DO gradients between the stream water and riparian groundwater between April 2016 and May 2017. Our results indicated dominant influences of microbial community respiration with observed DO concentration gradients. These observations can be explained by DO from the river that is subject to fractionation by microbial respiration with a typical fractionation factor (α) of 0.982. However, with such respiration dominance, we expected a simultaneous enrichment of δO towards values that are more positive than the well-known atmospheric O signal of +23.9‰ versus the Vienna Standard Mean Ocean Water standard (VSMOW). Surprisingly, our measurements revealed much lower δO values between +22‰ and +18‰ in the near-river groundwater. Mass balance calculations revealed that the DO pool in the shallow and unconfined aquifer receives contributions of up to about 80% by diffusion of oxygen from the vadose zone with a distinctly lower isotope value than the one of the atmosphere. This finding about additional oxygen sources from the unsaturated zone has numerous ramifications for oxygen related processes in near-river environments including the oxidation of pollutants, nutrients and ecosystem health.
除了地表水的水气交换，相邻河岸带地下水的溶解氧（DO）呼吸消耗也可能导致迄今为止很少被探索的非饱和带来源的添加。这些过程还会系统地影响 DO 的稳定同位素比值，本研究以德国哈茨山脚下的塞尔克河（Selke River）的一条消耗性溪流河段为研究对象，利用 Cl 作为水混合的保守示踪剂，对近河河岸带地下水系统中的这些过程进行了调查。该研究区域在 2016 年 4 月至 2017 年 5 月期间暴露出河岸带地下水与河流水之间存在显著的 DO 浓度梯度。研究结果表明，微生物群落呼吸对 DO 浓度梯度的主导影响。这种观察可以用河流中的 DO 来解释，河流中的 DO 受到微生物呼吸的分馏作用，典型的分馏因子（α）为 0.982。然而，由于这种呼吸作用占主导地位，我们预计同时会伴随着 δO 的富集，使其朝着比已知的大气 O 信号（+23.9‰，相对于维也纳标准平均海水标准（VSMOW））更为正值的方向发展。令人惊讶的是，我们的测量结果显示，近河地下水的 δO 值在+22‰到+18‰之间要低得多。质量平衡计算表明，浅层和无压含水层中的 DO 库通过从非饱和带扩散氧气来获得高达约 80%的贡献，其同位素值明显低于大气中的值。这一关于非饱和带额外氧源的发现对近河环境中的氧相关过程具有诸多影响，包括污染物、营养物和生态系统健康的氧化。
