Efficiently measuring groundwater flow in bedrock aquifers is inherently challenging due to the irregular distribution and fine scale of fractures. Recent advances in Active Distributed Temperature Sensing (A-DTS) in boreholes temporarily sealed with liners have made it possible to quantify flow rates in such aquifers at many different depths using heat as a tracer, but until now only data collected under a single hydraulic condition have been published. This paper presents the first field data from multiple A-DTS field tests conducted under different hydraulic conditions to quantify groundwater flow redistribution within a bedrock aquifer. Three separate quasi steady state A-DTS tests were collected in a sealed borehole: (1) natural gradient condition where all boreholes were sealed with flexible and impermeable liners, (2) cross-connected condition where a nearby borehole was open allowing vertical flow within the borehole, and (3) forced gradient condition where the nearby open borehole was pumped at a constant rate of 54 L/min. The depth-discrete hydraulic head responses were also measured during the three tests using a string of transducers in a sealed borehole. Results provide quantifiable insights as to how the bedrock aquifer responds, including A-DTS-derived measurements of flow changes in fractures at multiple depths driven by changes in gradients. The results confirm that a single open borehole or long-screened well can significantly alter the site hydraulics and demonstrate that not all large or transmissive fractures show evidence of active flow and thus, transmissivity and aperture should not be used alone to infer active flow zones. Plain Language Summary Measuring groundwater flow in fractured bedrock aquifers is difficult because flow is primarily controlled by small and irregularly spaced fractures. Very few tools exist to measure the natural flow through fractures in these aquifers, which is essential for understanding contaminant transport flow paths. One emerging technique, called Active Distributed Temperature Sensing (A-DTS), uses a type of fiber optic sensor that can measure temperature at many different intervals along a fiber optic cable. This cable is lowered into a borehole, and a flexible inflatable liner is installed to prevent vertical flow within the borehole. The cable is then heated using integrated heating wires for an extended period, and the temperature response can be used to locate and estimate groundwater flow rates. This study collects field data under three different flow conditions at a site to demonstrate how the flow in a bedrock aquifer responds when it is stressed and the sensitivity of the A-DTS technique. Results demonstrate highly variable flow with depth and that having a single open borehole on a site can strongly affect the natural flow system. A-DTS allows efficient measurement of this variable flow with depth and provides a better understanding of these complex bedrock groundwater systems.

中文翻译：

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使用主动分布式温度传感测量由于裸眼和泵送条件导致的基岩含水层中的裂缝流量变化

由于裂缝的不规则分布和精细尺度，有效测量基岩含水层中的地下水流量本身就具有挑战性。用衬管临时密封的钻孔中的主动分布式温度传感 (A-DTS) 的最新进展使得使用热量作为示踪剂来量化许多不同深度的此类含水层中的流速成为可能，但直到现在只能在单一水力条件下收集数据已经发表。本文提供了在不同水力条件下进行的多个 A-DTS 现场测试的第一个现场数据，以量化基岩含水层内地下水流的重新分布。在密封钻孔中收集了三个单独的准稳态 A-DTS 测试：（1）自然梯度条件，其中所有钻孔都用柔性和不渗透的衬管密封，(2) 附近钻孔开放允许钻孔内垂直流动的交叉连接条件，以及 (3) 附近开放钻孔以 54 L/min 的恒定速率泵送的强制梯度条件。在三个测试期间，还在密封钻孔中使用一串换能器测量了深度离散的水头响应。结果提供了关于基岩含水层如何响应的可量化见解，包括由 A-DTS 衍生的对梯度变化驱动的多深度裂缝流量变化的测量。结果证实，单个裸眼井或长筛井可以显着改变现场水力，并证明并非所有大型或透射裂缝都显示出活动流的证据，因此不应单独使用透射率和孔径来推断活动流区。简明语言总结 测量裂隙基岩含水层中的地下水流是困难的，因为流主要由不规则间隔的小裂隙控制。很少有工具可以测量通过这些含水层裂缝的自然流量，这对于了解污染物运输流动路径至关重要。一种称为有源分布式温度传感 (A-DTS) 的新兴技术使用一种光纤传感器，该传感器可以沿着光纤电缆以许多不同的间隔测量温度。该电缆被放入钻孔中，并安装了柔性充气衬管以防止钻孔内的垂直流动。然后使用集成电热丝长时间加热电缆，温度响应可用于定位和估计地下水流量。本研究收集了现场三种不同流量条件下的现场数据，以展示基岩含水层中的流量在受到压力时如何响应以及 A-DTS 技术的灵敏度。结果表明，随着深度的变化，流动高度可变，并且现场有一个开放的钻孔会强烈影响自然流动系统。A-DTS 可以有效测量这种随深度变化的流量，并提供对这些复杂基岩地下水系统的更好理解。