Dolomite dissolution, pH neutralization, and potentially toxic element immobilization in stormwater bioretention beds

The importance of pH in stormwater bioretention beds cannot be overstated since it impacts plant and microbial populations and removal of potentially toxic elements (PTEs) from stormwater runoff. This study investigated the effects of dolomite amendment on pH neutralization and subsequent PTE immobilization in bioretention media. To assess dolomite dissolution, pH neutralization, and PTE immobilization, engineered bioretention media was amended with different dolomite ratios and samples of dolomite-amended media were collected from two bioretention beds, one and two months after installation. The effects of inflow conditions and operational time on dolomite dissolution and PTE immobilization were investigated through a column study. Laboratory batch experiments revealed that pH neutralization was fast and reached the recommended pH range of 5 to 8 within 5 min. Among the 1-D column conditions, intermittent inflow had the highest porewater Mg and Ca concentrations, indicating greater dolomite dissolution. Batch experiments on field-collected media showed that pH neutralization was substantial within 1 month, and continued during the second month, due to dolomite dissolution. Dissolved (batch experiments) and column porewater Mg and Ca concentrations supported instantaneous Ca and a relatively slow Mg dissociation during dolomite dissolution. Among the monitored PTEs, dissolved and porewater concentrations (μg/L) were found to decrease with time, in the order Mn > Fe > Zn > Cu > Pb > Cd, with Cr mainly undetected in all experimental conditions. However, as pH became neutralized to slightly basic (pH ~ 8), dissolved and porewater As concentrations increased. The study suggests that pH neutralization and PTE immobilization depend on the soil-to-dolomite ratio and hydrological properties such as inflow rate, dry-wet cycle, and soil-water contact time. Dolomite amendment to engineered media can be an effective measure for intercepting contaminants, improving ecosystem health, and enhancing biochemical contaminant breakdown, though consideration should be given to bioretention bed design features and system hydrology.