Combined UV/H2O2 and biochar processes for enhanced removal of contaminants of emerging concern in dry wells

• Existing dry wells with bioswales are ineffective for hydrophilic CECs. • UV in combination with biochar efficiently removes hydrophilic CECs in runoff. • Estimated field lifespan in Los Angeles is more than four years with 10 wt% biochar. • The insecticide imidacloprid was the limiting contaminant for the system's lifespan. • The UV/biochar design supports stormwater reuse via dry well recharge. Dry wells are neighborhood-scale stormwater infiltration systems increasingly used in drought-prone areas for stormwater capture and groundwater recharge. These systems bypass the low permeability surface soil to maximize infiltration rates. However, hydrophilic contaminants of emerging concern (CECs) in urban runoff pose potential groundwater contamination risks. Field monitoring in this study confirmed the presence of CECs and the inability of current dry wells to remove these compounds. To address this, we explored stormwater treatment systems that: (1) can be easily operated in dry wells; (2) effectively remove hydrophilic CECs under realistic infiltration rates; and (3) offer multi-year field lifespan. Batch isotherm and kinetic studies were conducted on a large-grain biochar (d 50 = 2.24 mm) to assess removal efficiency for seven hydrophilic CECs (logK ow ≤ 4). Two column experiments evaluated biochar filters (5 wt%) with and without UV/H 2 O 2 pre-treatment under continuous high-throughput conditions. Results showed that biochar filters effectively removed five of seven CECs, with reduced efficiency for anionic compounds. The UV/H 2 O 2 process with approximately 1500 mJ/cm 2 UV dose degraded all CECs by >50 %, except dicamba, with direct photolysis as the dominant mechanism due to strong hydroxyl radical scavenging by dissolved organic carbon (DOC). Combining UV and biochar reduced CEC breakthrough concentration by 4 – 92 %. Based on the results, a contaminant transport model was calibrated and used to estimate the system lifespan in dry wells in Los Angeles. Findings showed that the insecticide, imidacloprid, is the limiting contaminant for the system's lifespan. Without UV/H₂O₂ pre-treatment, the system could last about four years with 10 wt% biochar in the filter. With pre-treatment, the lifespan will be determined by other operational factors rather than CEC removal. This study developed effective stormwater treatment systems for dry wells, supporting future local stormwater capture projects while safeguarding groundwater quality.