Biochar-based filters offer a promising solution for removing pollutants from stormwater, yet their performance under environmental stressors remains insufficiently studied. This study evaluated the efficiency of biochar beds in retaining microplastics (MPs) and metals under prolonged dry conditions and with increased salinity. Results showed that MPs were well retained through entrapment in biochar's porous structure, with non-polar polypropylene (PP) fragments removed more efficiently (98–99%) than polar polyamide (PA) fragments (83–92%). The MP retention improved over time, highlighting biochar's long-term filtration potential. However, a five-week dry period lowered effluent pH, consequently increasing metal mobility, while higher salinity events enhanced the dissolution of some metals, reducing their total removal. To simulate real-world conditions, semi-artificial stormwater was created by mixing road dust with deionized water. This mixture, along with virgin MPs, was introduced into biochar beds twice weekly under first-flush conditions. Effluent analysis of metals and MPs via inductively coupled plasma mass spectrometry (ICP-MS) and Fourier transform infrared spectroscopy (µ-FTIR imaging), respectively, confirmed the preferential retention of non-polar MPs and shifts in metal mobility. These findings emphasize the importance of considering environmental conditions and polymer characteristics when assessing biochar's filtration performance in practical applications.