Vertical hydraulic gradient research in hyporheic zone of Beberibe river in Pernambuco State (Brazil)

ABSTRACT The interaction between groundwater and surface water occurs naturally and is dependent on the dynamics in the hyporheic zone. The hyporheic zone is the interface between the surface water source and the phreatic aquifer and it’s a system that also influences the water quality. An important feature is the ability to flux in this zone. This work aimed to evaluate the vertical hydraulic gradient in the hyporheic zone at two points in Beberibe river, Olinda-PE, to understand the hyporheic environment characteristics and water flow dynamics in experimental area, in addition to identify the existence of hydraulic connection between surface water and groundwater. The experimental phase consisted of infiltration tests in riverbed with cylinder infiltrometer and vertical hydraulic gradients readings with differential piezometer, complemented with grain size information, for an assessment of the water budget between the river and the aquifer. Analyzing the behavior of the interaction over the period of 10 months, it was observed that the Beberibe river (middle course) contributes to the groundwater recharge in most of the time. The average infiltration rate was 1.02 mm/min in point 1 and 0.85 mm/min in point 2. It was concluded that there is a variability in flow direction, which often is top-down, but may undergo change in the stream showing upstream after long periods of rainfall. Another conclusion is that grain size distribution of bed sediment, that is predominantly sandy, influences hydraulic conductivity of hyporheic zone and influences consequently the vertical flow rates.

摘要
地下水与地表水的相互作用属于自然过程，其发生依赖于潜流带（hyporheic zone）的动力学特征。潜流带是地表水水源与潜水含水层（phreatic aquifer）之间的界面，同时也是影响水质的关键系统。该区域的一项重要特征是具备物质通量能力。本研究旨在评估巴西奥林达市（Olinda-PE）贝贝里贝河两处测点的潜流带垂直水力梯度，以明确实验区域内的潜流环境特征与水流动态，并探明地表水与地下水之间是否存在水力联系。
实验阶段采用圆环入渗仪（cylinder infiltrometer）开展河床入渗试验、利用差动测压计（differential piezometer）获取垂直水力梯度数据，辅以沉积物粒度（grain size）分析，以评估河流与含水层间的水量平衡（water budget）。通过对10个月内的相互作用过程进行分析，结果显示贝贝里贝河（中游段）在绝大多数时段为地下水提供补给。1号测点的平均入渗速率为1.02 mm/min，2号测点为0.85 mm/min。研究得出两点结论：其一，水流方向存在变异性，常规流向为自上而下（即地表水补给地下水），但在长时间降雨后，水流方向可能反转，转为地下水补给地表水；其二，以砂质为主的河床沉积物粒度分布，会影响潜流带的渗透系数，进而对垂直流速产生影响。