Assessment of Flow in the Field

A flowmeter log provides a record of the direction and velocity of groundwater movement in a drillhole, Flowmeter logging requires the use of a velocity-sensitive instrument, a system for lowering the instrument into the hole, a depth-measuring device to determine the position of the flowmeter and a recorder located at the surface. 

A flowmeter log, made while a drillhole is being pumped at a moderate rate or by allowing water to flow if there is sufficient artesian head, permits identification of the zones contributing to the discharge. It also provides information on the thickness of these zones and the relative yield at that discharge rate. Because the yield varies approximately directly with the drawdown of water level in a well, flowmeter logs made by pumping should be pumped at least at three different rates. The drawdown of water level should be recorded for each rate. 

Yield drawdown curves from pumping tests. 

When a tracer is injected via a drillhole into groundwater, it is subject to diffusion, dispersion, dilution and adsorption. Dispersion is the result of very small variations in the velocity of laminar flow through porous media. Molecular diffusion is negligible, unless the velocity of flow is unusually low. Even if these processes are not significant, flow through an aquifer may be stratified or concentrated along discontinuities. Therefore, a tracer may remain undetected unless the observation drillholes intersect these discontinuities. 

The vertical velocity of water movement in a drillhole can be assessed by using tracers. A tracer is injected at the required depth, and the direction and rate of movement is monitored by a probe. 

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