Enhancing infiltration water quality through reactive organic layers: field-scale evaluation of soil-aquifer treatment managed aquifer recharge using treated wastewater

Declining water availability and increasing water demand highlight the need to improve water supply security and adopt sustainable water management approaches involving alternative water resources. Managed aquifer recharge (MAR) uses alternative water resources, enhancing groundwater storage while ensuring its quality. This study presents results from a field-scale soil-aquifer treatment managed aquifer recharge (SAT-MAR) system, representing the first application in Portugal using basins to infiltrate treated wastewater from the Comporta Wastewater Treatment Plant (WWTP). By comparing the performance of different infiltration basins, the study verified infiltration water quality improvement during vadose zone passage and evaluated groundwater deterioration risks. The system performance was examined across three infiltration basins (B). B1 and B2 were ameliorated with reactive organic layers (ROLs), containing a mixture of sand, 30% biochar and 15% granular activated carbon (GAC), respectively, compared to B4 with plain sand. The ROLs, designed as nature-based and energy-free, allowed a sustainable tertiary treatment for alternative water resources. GAC-ROL achieved superior pharmaceutical compounds (PhCs) removal (77%), compared to B1 (15%) and B4 (12%), reducing hydrophobic and polar compounds below quantification limits, including hydrochlorothiazide and sulfamethoxazole. Compound-specific adsorption and biodegradation patterns, characterized by pK a and logK ow , confirmed the higher retention capacity of GAC-ROL. The receiving aquifer showed ∼33% reduction of organic matter indicators (TOC, COD, UV254). Median stable pH (∼6.2) and redox (∼87 mV), indicated system buffering capabilities. Nutrients surpassed the groundwater (APA, 2023) regulatory thresholds in both vadose zone and groundwater, emphasizing the need for enhanced optimization at the WWTP. This study provides strong evidence that integrating ROLs into SAT-MAR systems enhances performance and improves TWW quality.