¹²⁹I, with a 15.7 million year half-life, has been recognized as a contaminant of concern at numerous federal and international facilities. In order to understand the long-term risk associated with ¹²⁹I at these locations, quantitative analysis of groundwater samples must be performed. However, the ability to quantitatively measure the ¹²⁹I radioactivity in groundwater requires specialized extraction and sophisticated radioanalytical techniques, which are complicated and not always available to the general scientific community. This paper highlights an analytical method based on mass measurement that can be used to directly quantify ¹²⁹I in groundwater samples at concentrations below the maximum contaminant level (MCL) without the need for sample pre-concentration or extraction. Samples were analyzed on a Perkin Elmer ELAN DRC II ICP-MS after minimal dilution using oxygen as the reaction gas. Analysis of continuing calibration verification standards indicated that the dynamic reaction cell (DRC) mode could be used for quantitative analysis of ¹²⁹I in samples below the MCL (0.0057 ng/ml or 1 pCi/L). The low analytical detection limit of ¹²⁹I analysis in the DRC mode coupled with minimal sample dilution (1.02x) resulted in a final sample limit of quantification of 0.0051 ng/ml. Subsequent analysis of three groundwater samples containing ¹²⁹I resulted in fully quantitative results in the DRC mode, and spike recovery analyses performed on all three samples confirmed that the groundwater matrix did not adversely impact the analysis of ¹²⁹I in the DRC mode. This analytical approach has been proven to be a cost-effective, high-throughput technique for the direct, quantitative analysis of ¹²⁹I in groundwater samples at environmentally relevant concentrations that reach below the current MCL.