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A constant rate salt tracer injection method to quantify pumped flows in long-screened or open borehole wells

Poulsen, David L., Cook, Peter G., Simmons, Craig T., McCallum, James M., Noorduijn, Saskia L., Dogramaci, Shawan
Journal of hydrology 2019
aquifers, chemistry, convection, drilling, equations, fluid mechanics, groundwater, models, planning, tracer techniques, wells
Quantifying vertical flows in long-screened or open wells is essential for their reliable use in all types of groundwater investigations. In ambient conditions, a flow profile shows the producing/receiving zones of head-driven flow, the relative vertical head gradient in the aquifer system and potential bias in chemistry samples. A flow profile while pumping can be used to quantify aquifer heterogeneity and the sampled water mixtures. This paper describes a novel approach to a single-well tracer test to quantify the flow regime in a pumped well, which is unique in its utility over a wide range of discharge rates. During constant pumping, a tracer is injected at the opposite end of the well and, as it is drawn towards the pump, the tracer is diluted in proportion to each inflow. A dilution model using the advection-dispersion equation is used to visually fit a flow profile that explains all tracer profiles (pre-injection, transient phase and steady-state). Results compare favourably to borehole EM flowmeter data, particularly if tracer density issues are correctly interpreted and head-loss in the flowmeter is avoided. A dimensionless Froude number is provided to assist both with understanding and minimising the role of free convection when planning all types of in-well tracer tests involving a density contrast. Like the flowmeter, this method is particularly suited to screened wells, where packers are ineffective. Used together or separately in existing wells, these in-well methods can provide considerable information on aquifer-well hydraulics without the cost of additional drilling.