Utilizing distributed temperature sensors in a single well with a heating line source to simultaneously estimate the direction and velocity of groundwater flow

The primary objective of groundwater analysis is to determine the direction and velocity of water flow, which are essential for effective groundwater resource management and contaminant investigation. Conventional methods of evaluating groundwater flow direction, such as using solute or thermal tracers, require the installation of multiple observation wells and are typically laborious, expensive, and time-consuming. Furthermore, the uneven distribution of observation wells and the heterogeneity of aquifers often lead to inaccurate estimations of groundwater flow velocity and direction. Accordingly, this study proposes a novel approach: the thermal vector distributed temperature sensor (TV-DTS) method, combined with a heated line source, to overcome these challenges. The TV-DTS apparatus consists of a single heated fiber and four sensing fibers. The heated fiber functions as the heat source, while the sensing fibers are used to measure temperature changes. These measurements are then used to determine the direction and velocity of water flow by the analytical solution derived from the heat transfer with a heated line source. This method employs only a single-well heating test to estimate both the direction and velocity of groundwater flow, eliminating the need for multiple wells and significantly reducing the time and financial resources. Besides, the TV-DTS has several advantages, such as the ability to provide continuous spatial-temporal temperature data, ensuring reliable and high-resolution monitoring. Two groundwater contamination sites in northern and southern Taiwan have be selected to demonstrate the effectiveness of TV-DTS. The preliminary results showed that at the northern site, the flow direction was predominantly northeast to southwest, with velocities ranging from 0.25 - 0.34 m/day at different depths. In contrast, at the southern site, the flow direction was mainly toward west with higher velocities of 0.1 – 8.0 m/day. The estimated directions and velocities from both sites aligned with previous studies; however, uncertainties were higher at the southern site due to greater velocities observed. This method provides a high-resolution, cost-effective approach for hydrogeological investigations and contaminated sites assessment, serving as a valuable reference for the future investigation and evaluation of hydrogeological characterization.

Keywords: groundwater flow direction, groundwater flow velocity, heat transfer, distributed temperature sensors, borehole, uncertainty, contamination