Mineralogical evidence of unfavorable geochemical interactions between urban runoff pollutants and filter materials in low-impact systems

We report on the geochemical interactions between a synthetic urban runoff (SUR) and the minerals of materialsused in a multi-layered column filter (soil, sand, gravel, and tezontle) at the laboratory scale, which mimicked anunvegetated low-impact development (LID) system. After five 8 h infiltration cycles using the SUR, the averageconcentrations of Pb and Mn decreased slightly at the column outlet, as did HCO3 ?, SO4 2-, and Na+, whereas Mg increased and Cl?, Ca2+, and K+ were only detected at the outlet. The filter materials were comprised ofsilicates, Mn-bearing oxides (hausmannite and manganite), carbonates (calcite), chlorides (sylvite), and sulfates(anglesite, lanarkite, barite, and epsomite). PHREEQC modeling allowed the identification of the geochemicalprocesses that occurred in the filter. The results showed the removal capacity of the filter materials through theformation of secondary minerals such as rhodochrosite (MnCO3) and the over-saturation of anglesite (PbSO4),and also showed that they may mobilize ions from the upper to the interior layers (as Mg2+ from epsomite,MgSO47H2O, and Ba2+ from barite, BaSO4). We highlight the importance of knowing the geological nature offilter materials used in LID systems because they may lead to the geogenic mobilization of toxic contaminants tothe environment.