In-situ monitoring of potentially toxic elements in mining water samples is of special relevance in order to evaluate the environmental impact of abandoned mining activities. However, it is difficult to develop suitable analytical methods for this purpose considering the complexity of mining water samples matrix as well as the limited sensibility of conventional filed portable X-ray fluorescence spectrometry systems (pXRF). In the present contribution, a simple, sustainable and reliable triaxial pXRF method (3pXRF) is proposed for multielemental analysis of mining water samples. In this method, several microliters of sample are deposited onto a commercial filter paper retainer and after drying, the loaded filter is directly analyzed in the field using a laboratory pXRF with orthogonal triaxial geometry (p3XRF). This geometry reduces the background of the measured spectra improving peak-background ratios in comparison with conventional pXRF systems. Using the best analytical conditions (sample deposition volume of 200 μL and 600 s as measurement time), limits of detections were in the low mg∙L− 1 range and therefore suitable considering the expected high metal concentration in mining water samples. Acceptable accuracy and precision of the results was obtained in comparison with laboratory techniques such as inductively coupled plasma optical emission spectrometry and total reflection X-ray fluorescence spectrometry but with the possibility of in-situ measurements in the field. The developed method was applied to the analysis of different types of mine water samples (stream waters, drainage water tanks and mining lake waters) collected in the mining districts of Tinoca (Portugal) and Cartagena-La Unión (Spain).