The knowledge of the sources of base cations in stream water is a prerequisite to assess potential effects of changing environmental conditions such as changing rainfall, weathering or groundwater flows on cation export with stream water. This study use stable Mg isotopes to identify potential sources in the well-studied catchment of Krycklan located on gneissic bedrock covered by quaternary sediments in Sweden. Samples were collected from open filed rain, throughfall, stream, soil, rock and litterfall. The δ²⁶Mg values of these samples was determined and the contributions of different sources to Mg fluxes in the stream were determined from the variation of the Mg isotope and Sr / Mg ratios.

The results show an overall variation of 1.10‰ between all samples. In addition, Magnesium isotope ratios varied little in the streamwater and in soil solution, except during snowmelt periods during which a large portion of the annual runoff occurs. Magnesium in the streamwater is explained as a mixture of three pools (open field rain, soil solution and groundwater) with the latter two influenced by catchment processes. Outside the snowmelt period, Mg in streamwater mainly derived from the groundwater, assumed to be mineral weathering signature in this catchment, with a contribution ranging from 12 to 63% to Mg fluxes. Open field rain dominates Mg fluxes in streamwater during spring flood (0 to 78%) and may contribute significantly during larger summer and autumn rainfall events. Soil solution input to streamwater range from 16 to 59% of Mg fluxes in streamwater. Our results demonstrate that δ²⁶Mg values together with Mg concentrations and Sr/Mg ratios can be used to constrain the Mg sources of stream water and quantify weathering release rates.

了解河流水中碱性阳离子的来源是评估不断变化的环境条件（如降雨、风化或地下水流量的变化）对河流水中阳离子输出的潜在影响的先决条件。本研究使用稳定的镁同位素来确定位于瑞典第四纪沉积物覆盖的片麻岩基岩上的 Krycklan 集水区的潜在来源。从露天雨、直落、溪流、土壤、岩石和凋落物收集样品。确定了这些样品的 δ ²⁶ Mg 值，并根据 Mg 同位素和 Sr / Mg 比率的变化确定了不同来源对流中 Mg 通量的贡献。

结果显示所有样品之间的总体变异为 1.10‰。此外，在河流水和土壤溶液中的镁同位素比率变化很小，除非在融雪期间发生了很大一部分年径流。河流中的镁被解释为三个池（露天雨、土壤溶液和地下水）的混合物，后两者受集水过程的影响。在融雪期之外，河流中的镁主要来自地下水，被认为是该流域的矿物风化特征，对镁通量的贡献范围为 12% 至 63%。春季洪水期间，露天降雨在河流中的 Mg 通量中占主导地位（0 至 78%），并且可能在较大的夏季和秋季降雨事件中做出显着贡献。输入到河流中的土壤溶液占河流中镁通量的 16% 到 59%。我们的结果表明，δ²⁶ Mg 值连同 Mg 浓度和 Sr/Mg 比率可用于限制河流水的 Mg 来源并量化风化释放率。