A preconcentrating minicolumn sensor for technetium-99 detection in water consists of a packed bed containing a mixture of anion-exchange resin and scintillating plastic beads. The column materials are contained in a transparent plastic flow cell placed between two photomultiplier tubes for radiometric detection. Upon retention of pertechnetate anions, the radioactive decay of Tc-99 results in detectable scintillation pulses that are counted in coincidence. In equilibration-based sensing mode, the sample is pumped through the packed bed until complete chromatographic equilibrium is achieved between the activity concentration in the water sample and the concentration on the anion-exchange resin. The analytical signal is the observed steady-state count rate at equilibrium. The sensitivity is related to a measurement efficiency parameter that is the product of the retention volume and the absolute radiometric detection efficiency. This sensor can readily detect pertechnetate to levels 10 times below the drinking water standard of 0.033 Bq/mL. The potential for other anions in natural groundwater and contaminated groundwater plumes to interfere with pertechnetate detection and quantification has been examined in detail, with reference to the groundwater chemistry at the Hanford site in Washington state. Individual anions such as nitrate, carbonate, chloride, and iodide, at natural or elevated concentrations, do not interfere significantly with pertechnetate uptake on the anion-exchange resin. Elevated chromate or sulfate anion concentrations can interfere with pertechnetate uptake by the resin, but only at levels substantially higher than typical concentrations in groundwater or contamination plumes. Nevertheless, elevated anion concentrations may reduce pertechnetate uptake and sensitivity of the sensor when present in combination. Chromate is retained on the anion-exchange resin from water at parts-per-billion levels, leading to an orange stain that interferes with pertechnetate detection by the absorption of scintillation light pulses (color quench). Radioactivity from radioiodine, tritium, and uranium is not expected to create a significant positive bias in groundwater analyses. A method of automated fluidic standard addition is demonstrated that corrects for matrix interferences leading to accurate analyses over a wide range of groundwater compositions. This method is developed for automated groundwater monitoring applications.