Brazos River Alluvium Aquifer - Conceptual Model Testing

Quantitative characterization of the dynamics of water exchange fluxes between rivers and aquifers is necessary for water resources management, water quality, environment and ecology of the river-aquifer systems. The main uncertain factors for predicting river–aquifer exchange fluxes are aquifer and riverbed properties. In this study, we characterize the flux exchange dynamics between Brazos River Alluvium Aquifer and Brazos River, TX, USA, using alternative conceptual models. Six alternative conceptual models for the connection between the river and the aquifer, having varying aquifer lithology and river incision levels and incorporating processes such as river bed clogging and seepage face flow, are numerically modeled in HYDRUS 2D using small-scale, high-resolution transects across the river. Modeled results are tested against observed heads in three wells and finally a best-fit conceptual model is used to quantify river-aquifer flux exchange dynamics. Additionally we focused on how factors such as aquifer lithology, river channel incision, water table conditions, seepage face boundaries, and low-conductivity river-bed effect hydraulic head distribution and the corresponding flux exchange dynamics. Our results demonstrate that only a small portion of the aquifer close to the river channel is well-connected with the river and a major portion of the aquifer is disconnected. The proposed conceptual model predicts a) much frequent flux reversals (changes between gaining and losing conditions) and b) much smaller amount of recharge and discharges compared to that of the conceptual model which has been assumed by earlier studies; a reduction of 151% in recharge and 116% in discharges. These results suggest that the magnitude and dynamics of water flux exchange between the river and the aquifer are independent of the hydraulic gradients in the wider disconnected aquifer and are determined by the hydraulic gradients in the connected aquifer close to the river. The results also demonstrate that river-aquifer flux exchange is sensitive to aquifer lithology, river incision depth, and river-bed clogging. While different settings of aquifer lithology and river incision can produce very similar heads in the wider aquifer, the hydraulic head distribution close to the river and hence the river-aquifer flux exchange varies quite drastically from model to model. River-bed clogging decreases the magnitude of fluxes and effects hydraulic head in the aquifer, especially in the vicinity of the river channel, depending upon the gaining and losing river conditions. Furthermore, seepage face flow could be of the same order as that of flows through river-bed depending upon aquifer lithology and corresponding river incision depth.

对河流与含水层（aquifer）之间的水交换通量动态进行定量表征，对于河-含水层系统的水资源管理、水质调控、环境保护及生态修复均具有重要意义。预测河-含水层交换通量的主要不确定性因素为含水层与河床的物性特征。本研究针对美国德克萨斯州布拉索斯河冲积含水层（Brazos River Alluvium Aquifer）与布拉索斯河之间的通量交换动态，采用多套备选概念模型开展研究：构建了6套涵盖不同含水层岩性、河道下切深度，并纳入河床堵塞、渗流面流等过程的河流-含水层水力联系备选概念模型，随后利用HYDRUS 2D软件，基于横跨河道的小尺度高分辨率断面开展数值模拟。将模拟结果与三口观测井的实测水头（hydraulic head）数据进行校验，最终选取最优拟合概念模型，以定量表征河-含水层通量交换动态。此外，本研究还聚焦于含水层岩性、河道下切、地下水位条件、渗流面边界以及低导度河床效应等因素，如何影响水头分布与相应的通量交换动态。
研究结果表明，仅河道附近的小范围含水层与河流存在良好水力联系，而绝大多数含水层处于水力断开状态。相较于早期研究采用的概念模型，本研究提出的概念模型预测得到：a) 通量反转（即补给与排泄状态间的转换）发生频率显著更高；b) 补给量与排泄量均大幅降低，其中补给量减少151%、排泄量减少116%。上述结果显示，河-含水层间水交换通量的大小与动态特征，与大范围断开含水层中的水力梯度无关，而是由河道附近连通含水层内的水力梯度所决定。
研究结果同时表明，河-含水层通量交换对含水层岩性、河道下切深度及河床堵塞均较为敏感。尽管不同的含水层岩性与河道下切参数设置，可在大范围含水层中产生极为相似的水头分布，但河道附近的水力头分布以及由此产生的河-含水层通量交换，在不同模型间存在显著差异。河床堵塞会降低水交换通量的大小，并影响含水层内的水头分布，尤其在河道附近区域，其影响程度取决于河流处于补给型还是排泄型状态。此外，根据含水层岩性与对应的河道下切深度，渗流面流的量级可与通过河床的水流量级相当。