Estimates of Horizontal Groundwater Flow Velocities in Boreholes

Abstract

Currently, monitoring tools can be deployed in observation boreholes to better assess groundwater flow, flux of dissolved contaminants and their mass discharge in an aquifer. The relationship between horizontal water velocity in observation boreholes and Darcy fluxes in the surrounding aquifer has been studied for natural flow conditions (i.e., no pumping). Interpretation of measurements taken with dilution tests, the colloidal borescope, the Heat Pulse Flowmeter, and other techniques require the conversion of observed borehole velocity u to aquifer Darcy flux q . This conversion is typically done through a proportionality factor α = u/q . In experimental studies as well as in theoretical developments, reported values of α vary almost three orders of magnitude (from 0.5 to 10). This large variability in reported values of α could be explained by: (1) unclear distinction between Darcy flux and water seepage velocity, (2) unclear definition of water velocity in the borehole, (3) effects of well screen and the presence of the measurement device itself on the observable velocities, and (4) hydraulic conditions in the borehole annulus. We address (1), (2) from a conceptual/theoretical perspective, and (3) by means of numerical simulations. We show that issue (1) in low porosity aquifers can yield to order-of-magnitude discrepancies in estimates of q ; (2) may result in discrepancies of up to 50%, and (3) can cause differences up to 20% of water velocity in the borehole void space compared to the theoretical case of an open borehole.

Seepage Velocity: Large Scale Mapping and the Evaluation of Two Different Aquifer Conditions (Silty Clayey and Sandy)

“…Despite the importance of seepage velocity in engineering applications, such as heat transfer [34,37,51] and aquifer pollution [104][105][106], few published papers exist that consider the mapping of seepage velocity (direction and magnitude) at a large scale. A large number of studies have been conducted to simulate seepage velocity at a small scale [52,107,108], but no previous studies (with the exception of those conducted by the authors of the current paper) have investigated seepage velocity on a large scale. Therefore, this study presents a straightforward approach to mapping seepage velocity at a large scale.…”

Numerical Modeling of Head Losses in Different Types of Well Screens

“…Modeling flow fields through pumping wells requires moving away from traditional Darcy's law approach for porous media. In previous work, Bayer‐Raich et al (2018) presented coupled models to simulate laminar flow in observation boreholes through typical PVC screens with horizontal slots. In this work we analyze the same problem through pumping wells, where water velocity is much higher.…”