ABSTRACT Groundwater discharge into lakes is an important component of the fluid and nutrient budgets, and a possible route for contaminant transport. However, groundwater flow beneath lakes is difficult to investigate due to the need for drilling deep boreholes. In 2012, a 2,000 m deep borehole was drilled in Lake Vättern, the second largest lake in Sweden. A continuous temperature profile was collected from the borehole. The geothermal gradient in the upper 180 m is highly non-linear, and not controlled by variability in the measured thermal properties of the sediments and rocks. The anomalous temperature profile is best explained by fluid flow into the borehole and subsequent vertical flow of warm waters towards the lake floor. Combining the temperature profile with stratigraphic information from drilling logs and seismic data, we find that fluid flow into the borehole occurs in glacial and glaciofluvial sediments deposited on top of a large sandstone aquifer (the Visingsö Group). The warm waters flowing through the glacial and glaciofluvial sediments are likely sourced from the underlying Visingsö Group sandstones. There is no evidence for substantial vertical migration of these waters through the overlying glacial and postglacial sediments. We speculate that they escape either along lake margins where overlying sediments become thinner, or along faults that are known to exist in the deeper basin.These results highlight an important hydraulic transport pathway between recognised regional aquifers and Lake Vättern. Further work is needed to evaluate the significance of groundwater discharge on the water and nutrient budget of the lake.

中文翻译：

---

地下水流经瑞典韦特恩湖下方基岩含水层上的第四纪沉积物的地热证据

摘要 地下水排入湖泊是流体和养分预算的重要组成部分，也是污染物运输的可能途径。然而，由于需要钻深孔，很难调查湖泊下方的地下水流。2012 年，在瑞典第二大湖 Vättern 湖钻了一个 2,000 米深的钻孔。从钻孔中收集了连续的温度分布。上层 180 m 的地温梯度是高度非线性的，并且不受沉积物和岩石热特性测量变化的控制。异常温度分布最好通过流体流入钻孔和随后温水垂直流向湖底来解释。将温度剖面与钻井测井和地震数据的地层信息相结合，我们发现流体流入钻孔发生在沉积在大型砂岩含水层（Visingsö Group）顶部的冰川和冰川河流沉积物中。流经冰川和冰川河流沉积物的温水很可能来自下伏的 Viisingsö Group 砂岩。没有证据表明这些水域通过上覆的冰川和冰后沉积物发生了大量的垂直迁移。我们推测它们要么沿着上覆沉积物变薄的湖边缘逃逸，要么沿着已知存在于更深盆地中的断层逃逸。这些结果突出了公认的区域含水层和 Vättern 湖之间的重要水力输送路径。需要进一步的工作来评估地下水排放对湖泊水和养分收支的重要性。