Table 1 - Summary of groundwater velocity measurement techniques discussed in the text. Within each of the method categories, the range of measurements can extend from a few centimeters per day to tens of meters per day, though this full range of performance is both tool specific and site specific, depending on conditions encountered. Cells colors are matched to Figure 2.

Method Scale Examples Instrumentation/Description Comments Application for best advantage
Darcy-based methods
  • generally, ~10 m to ~100m separation between wells
  • local to regional investigations common
  • conventional site investigation based on water level survey and estimation of hydraulic conductivity (K)
  • wells and water level tapes or sondes
  • measure head in wells for gradient across domain and obtain domain K value. Data collection requires minutes per well
  • scale dependent on the number and spacing of K measurements
  • scale dependent on the method of K measurement
  • limited by measurable differences in water levels, heterogeneity between wells, and hydraulic connectedness between wells
  • generalized flow characterization
  • forecasting of overall plume migration
  • application of Darcy’s Law in digital models
  • computer
  • match field distribution of hydraulic head in a computer model through calibration informed by field data on geology, hydraulic conductivity - generally, requires days to weeks to complete
In-well velocity techniques
  • centimeter to meter scale measurements from single wells
  • larger scale flow patterns possible with multiple wells and complimentary information (e.g., geophysical, Darcy, modeling)
  • point dilution and finite dilution point dilution (FVPD) methods
  • Drost et al., 1968; Brouyere et al., 2008
  • pump, packers, tracer injection system
  • inject solute tracer into test interval in well and, with mixing, measure concentration decline - requires minutes to hours to complete
  • measured flow depends on possible interferences from filter packs (if present), disturbed zone in the borehole outside well casing, and the well screen
  • best results expected in wells that have been developed extensively
  • PFM measured time averaged fluxes over days to weeks, other methods return minutes to hours for measurements
  • Some techniques can be coupled with other sensors or sampling ports
  • local flow patterns
  • verification of Darcy’s law calculations
  • identification of preferred flow zone in vertical profiles
  • direct velocity measurements in cohesive sediments (silt and clay content), or high gravel fraction
  • passive flux meter (PFM)
  • Hatfield et al., 2004
  • PFM instrument supplied by vendor
  • deploy instrument into well or borehole and leave for days to weeks - recover instrument and send to laboratory for analysis
  • in-well point velocity probe (IWPVP)
  • Osorno et al., 2018
  • in-well probe, tracer injection pump, datalogger
  • deploy in well or borehole, release tracer (saline, deionized water, or heat) - reposition and repeat for profiling - requires minutes to hours to complete each test
  • colloidal borescope
  • Kearl and Roemer, 1998
  • in-well instrument with camera, up-hole monitor and computer
  • deploy in well or borehole, allow flow to re-equilibrate, track colloids in water as they pass through the instrument in the well - requires minutes to hours to complete
  • heat pulse flowmeter (HPF)
  • Kerfott and Massard, 1985
  • probe supplied by vendor must be packed in glass beads and a ‘fuzzy packer’, up-hole control panel
  • deploy in well or borehole, activate heater, record temperature changes at thermistors. Interpretation may require expert assistance - requires minutes to complete
  • direct velocity technique (DVT)
  • Essouayed et al., 2019)
  • in-well device, up-hole tracer injection and detection system
  • deploy in well or borehole, release tracer into window drain tube at known rate while monitoring outflow concentrations - requires minutes to hours to complete
Dedicated borehole techniques
  • centimeter to meter scale measurements from single boreholes
  • larger scale flow patterns possible with multiple boreholes and complimentary information (e.g., geophysical, Darcy, modeling)
  • point velocity probe (PVP)
  • Labaky et al., 2007
  • probe(s) attached between lengths of casing, tracer injection system and datalogger
  • deploy instrument as multilevel stack or single in dedicated borehole that is allowed to collapse around the casing - release tracer (e.g., saline, deionized water, heat) and track as it moves on the perimeter of the instrument - requires minutes to hours to complete
  • require borehole dedicated to the instrument
  • subject to interferences related to disturbed zone surrounding borehole
  • scale depends on number of instruments deployed
  • PVP can be coupled with other sensors or sampling ports
  • vertical flow measurable in principle but ISPFS vertical flow data should be interpreted with particular caution
  • local flow patterns
  • non-cohesive sediments (usually high component of sand)
  • permanent installations suitable for time series measurements
  • multilevel deployment useful for mass discharge monitoring
  • in-situ passive flow sensor (ISPFS)
  • Ballard, 1996
  • instrument supplied by vendor, up-hole control panel
  • deploy the instrument in dedicated borehole that is allowed to collapse around the casing - warm the outside surface to steady state and measure final temperature distribution on surface - requires minutes to hours to complete