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• Measuring in-well flow regimes improves key hydrogeological data and insight. • Efficient new field method and numerical model for a single-well tracer test. • Pumped flow profile quantified by inverting tracer dilution profiles. • Steady-state approach is suited to a wide range of pumping flow rates. • Analysis of mixed convection helps assess and avoid fluid/tracer density issues. 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. [ABSTRACT FROM AUTHOR]

Hydraulic head differences across the screened or open interval of a well significantly influence the sampled water mixture. Sample bias can occur due to an insufficient pumping rate and/or due to native groundwater displacement by intraborehole flow (IBF). Proper understanding of the sampled water mixture is crucial for accurate interpretation of environmental tracers and groundwater chemistry data, and hence groundwater characterization. This paper uses numerical modeling to quantify sample bias caused by IBF in an un‐pumped high‐yield well, and the influence of pumping rate and heterogeneity on the volume of pumpage required to purge an IBF plume. The results show that (1) the pumping rate must be at least an order of magnitude greater than the IBF rate to achieve permeability‐weighted yield, (2) purge volume was 2.2 to 20.6 times larger than the IBF plume volume, with the ratio depending on plume location relative to hydraulic conductivity and head distributions, and (3) after an example 1000‐day un‐pumped period, purging required removal of at least three orders of magnitude more water than the common practice of three to five well volumes. These results highlight the importance of knowing the borehole flow regime to identify IBF inflow and outflow zones, estimate IBF rates, and to develop a strategic sampling approach. Article impact statement: This paper provides new insight, cautionary advice, and an informed approach for using long‐screened wells in groundwater studies. [ABSTRACT FROM AUTHOR]