Groundwater quality sampling guidance typically requires representative samples to be obtained. Such guidance is not always clear what this means and which sampling methods are most appropriate. The situation is complicated by increasing well screen/open interval length. Uncertainty, resulting particularly from observations of vertical flow in wells has led to calls for the use of long-screen (> 3 m) wells to be abandoned for groundwater quality monitoring. Here, four complementary field and modelling studies at various scales are used to examine appropriate groundwater quality sampling in such wells. Numerical modelling demonstrates that literature reported vertical flows in wells < 10 m in length are sufficient to bias pumped groundwater quality sampling. Bias starts for vertical well flow rates less than 50 % of the pumping rate. Vertical flow measurements explain differences and similarities in historical passive sampling between four boreholes and allow vertical aquifer concentration distributions to be quantified. However, such quantification requires per-borehole flow measurement. New technology (Active Distributed Temperature Sensing) provides a versatile alternative to existing borehole flow characterisation methods under ambient and pumping conditions. Data from contrasting field environments demonstrate that even without comprehensive flow investigation long-screen wells can still provide useful information about groundwater concentrations and trends.