Extensive pumping of aquifer systems alters the water budget components for individual aquifer units. This study evaluated long-term changes in alluvial-bedrock groundwater exchange within a highly pumped region of the Denver Basin Aquifer System, Colorado (USA). For a data set spanning five decades, bedrock-aquifer water levels were compared to the elevation of mapped stream alluvium in three-dimensional space, providing a description of spatiotemporal changes in the relationship between the alluvial and bedrock aquifers. Results clearly show increased downward hydraulic gradients (from the alluvial aquifer to underlying sedimentary bedrock); thus, recharge to the bedrock aquifer has increased over time. Variably saturated flow modeling was performed to investigate the dependence of recharge on the bedrock aquifer’s water-table position. Model simulations with realistic geologic heterogeneity demonstrate the potential for unsaturated conditions within the bedrock aquifer, which may eventually produce a hydraulic disconnection between the alluvial and bedrock aquifers, limiting further pumping-induced changes in recharge. These results have important implications for understanding and forecasting long-term water-budget changes in aquifer systems subjected to high pumping stress, especially those in semi-arid regions.

大量抽水蓄水层系统改变了单个蓄水层单元的水预算组成。这项研究评估了美国科罗拉多州丹佛盆地含水层系统的高抽水区域内冲积层-地下水交换的长期变化。对于跨越五十年的数据集，将基岩-含水层的水位与三维空间中映射河流冲积层的高度进行了比较，从而描述了冲积层与基岩含水层之间关系的时空变化。结果清楚地显示出向下的水力梯度增加（从冲积含水层到下面的沉积基岩）；因此，随时间推移，向基岩含水层的补给量增加了。进行了各种饱和流模拟，以研究补给对基岩含水层地下水位的依赖性。具有实际地质异质性的模型模拟表明，基岩含水层内存在非饱和条件的可能性，这最终可能在冲积层和基岩含水层之间产生水力分离，从而限制了泵送引起的补给量的进一步变化。这些结果对理解和预测承受高抽水压力的含水层系统的长期水预算变化具有重要意义，特别是在半干旱地区。