Using stable isotopes to resolve transit times and travel routes of river water: a case study from southern Finland^†

The stable isotopic composition of two rivers, the Vantaanjoki River and the Kokemäenjoki River, in southern and southwestern Finland was studied to resolve the transit times and travel routes of the river water in the two different catchments. The Kokemäenjoki River is dominated by great lake basins whereas the Vantaanjoki River has been reported having a significant groundwater component. The mean residence time of the young surface flow component could be resolved by sine function fitting onto the annual fluctuations of the isotopic signal, and the amount of base flow was estimated by using the isotopic composition of the river and groundwater. In this study, we found that the methods work for simple two component catchments. In more complex cases with three different components mixing, the solution becomes increasingly difficult and requires more study.本研究针对芬兰南部与西南部的万塔河（Vantaanjoki River）及科凯迈基河（Kokemäenjoki River）的稳定同位素组成展开分析，旨在明确两条不同汇水流域内河水的运移时间与运移路径。科凯迈基河流域以大型湖盆为主要地貌特征，而已有研究显示万塔河流域地下水占比显著。通过将正弦函数拟合至同位素信号的年度波动特征，可求得年轻地表径流组分的平均驻留时间；结合河水与地下水的同位素组成，可估算基流占比。本研究证实，该方法适用于简单的双组分流域。而在包含三种混合组分的复杂流域场景中，求解过程将愈发复杂，需开展更多相关研究。