Securing a supply of potable water is essential for human survival, and doing so aboard the ISS presents challenges distinct from terrestrial water safety challenges. Repeated recycling is the primary means of replenishing water aboard the ISS in order to minimize the need for costly re-supply of water. There is a critical need to continually monitor impurities in the water supply aboard the ISS in a manner that minimizes the sample volume. Existing technology can measure inorganic contaminants at concentrations established as safe in the Spacecraft Water Exposure Guidelines, but these methods are inappropriate for the ISS due to the required size and power of the equipment, the size of water sample required for testing, or some combination of these. Spectral Sciences, Inc. proposes to develop a novel water quality monitoring technology using x-ray fluorescence (XRF). In the proposed method, a small volume of water is diverted into a sampling chamber for XRF analysis, which takes place over a period of several minutes. XRF from the sample is detected by an energy-dispersive detector. The concentrations of contaminant elements in the sample are determined simultaneously from the fluorescence spectrum and compared to pre-determined threshold concentrations. After analysis, the water sample is returned to the main plumbing, and a warning is issued if any contaminant is present above the threshold concentration. This process would be entirely automated and require no crew interaction. The instrument would use commercially available parts common in portable XRF analyzers.