Removal mechanisms for organic micropollutants during artificial groundwater recharge

One of the major concerns regarding the use of surface water sources of impaired quality for drinking water supply is the survival and accumulation of organic micropollutants, such as endocrine disruptors (EDCs), pharmaceutical residues, personal care products, or disinfection by-products (DBPs). Riverbank filtration (RBF) or soil-aquifer treatment (SAT) have been recognized as potential barriers for these compounds. However, some organic micropollutants are not efficiently attenuated during soil passage by physical adsorption and have affected production wells at groundwater recharge facilities. The purpose of this study was to investigate the role that biological metabolism and adsorption play in the removal of selected hydrophilic trace organic contaminants in artificial groundwater recharge systems. Specifically, we investigated how different source water qualities and recharge operations promote the microbial breakdown of trace organic contaminants. The working hypothesis for this study was that the composition and concentration of organic carbon in recharged water introduced into an aquifer has a major impact on establishing soil biomass activity and a soil microbial community to enable the metabolic breakdown of certain trace organic contaminants. Several emerging micropollutants (representing pharmaceutical residues and personal care products) were selected for this study that differed in terms of physico-chemical properties such as molecular size and hydrophobicity (indicated by KOW), and their reported biodegradability. Removal of these compounds was studied in different soil column systems representing different redox regimes (anoxic vs. oxic). Column system influents were spiked with the selected micropollutants at environmental concentrations and different organic carbon fractions (bulk water, hydrophobic acids (HPO-A), hydrophilic carbon (HPI) and colloidal carbon). Column performances were monitored twice a week in terms of soil biomass activity (measured as phospholipids extraction and dehydrogenase activity), organic carbon removal, pH, conductivity, and trace compound removal. In parallel, the adsorption behavior of selected compounds was evaluated in abiotic column and batch tests under addition of sodium azide. Results of this study indicated that different organic carbon fractions were able to support different soil biomass activities and promoted different removal behavior for certain micropollutants. Oligotrophic conditions, which established in systems fed with more recalcitrant organic carbon fractions (HPO-A, HPI), led to a high degree of removal pointing to a highly diverse biocommunity responsible for removal. Findings of this study suggest that an effect of organic matter on sorption of intermediate hydrophobic organic micropollutants in RBF is not expected.