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_r) of 0.982. However, with such respiration dominance, we expected a simultaneous enrichment of δ¹⁸O_DO towards values that are more positive than the well-known atmospheric O₂ signal of +23.9‰ versus the Vienna Standard Mean Ocean Water standard (VSMOW). Surprisingly, our measurements revealed much lower δ¹⁸O_DO 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.

除了地表水中的气体-水交换以外，相邻河岸地下水中呼吸性溶解氧（DO）的呼吸消耗还可能触发增加迄今几乎从未开发的来自非饱和区的水源。这些过程还系统地影响DO的稳定同位素比，并与Cl-一起进行了研究^。作为近河沿岸地下水系统中水混合的保守示踪剂。该研究的重点是位于哈尔茨山脉（德国）脚下的塞尔克河（Selke River）流域。研究区域在2016年4月至2017年5月期间暴露了溪流水和河岸地下水之间的陡峭的DO梯度。我们的研究结果表明，观察到的DO浓度梯度对微生物群落呼吸的主要影响。这些观察结果可以用溶解氧从河流中进行解释，溶解氧通过微生物呼吸进行分馏，典型分馏因子（_αr）为0.982。然而，这种呼吸的主导地位，我们预期δ的同时富集¹⁸ Ø _DO向值是比众所周知的大气Ø更积极与维也纳标准平均海洋水标准（VSMOW）相比，+ 23.9‰的_2号信号。出人意料的是，我们的测量结果表明低得多δ ^18个Ø _DO在近河地下水+ 22‰和+ 18‰之间的值。物质平衡计算表明，浅层和无限制含水层中的溶解氧池通过氧气从渗流带扩散而获得的贡献高达约80％，同位素值明显低于大气层之一。关于来自非饱和区的其他氧气源的发现，对近河环境中与氧气相关的过程产生了许多影响，包括污染物，营养物质和生态系统健康的氧化。