Riverbank filtration: Effect of ground passage on NOM character

Research was conducted to explore the effect of underground travel on the character of the natural organic matter (NOM) originating from river water sources during riverbank filtration (RBF) at three Midwestern US drinking water utilities. Measurements of biodegradable dissolved organic carbon (BDOC) and assimilable organic carbon (AOC) showed significant reductions (50 to 90%) in the biodegradable portion of NOM at two of the sites. Specific UV-absorbance (SUVA) values suggested preferential reduction (26% reduction in SUVA) in UV-absorbing NOM at one of the sites but negligible changes in SUVA were observed at the other two sites. XAD-8 characterization was carried out on the river and well waters to investigate possible changes in the character of the NOM. The distribution of dissolved organic carbon (DOC) between the XAD-8 adsorbing (‘hydrophobic’) and non-adsorbing (‘hydrophilic’) fractions was similar between the river and well waters (40 to 70% hydrophilic and 30 to 60% hydrophobic), indicating no significant, consistent, preferential removal of either fraction upon ground passage. SUVA measurements on the separate XAD-8 fractions similarly showed no significant change during bank filtration. Disinfection by-product (DBP) formation testing was performed on the various fractions, keeping the ratio of chlorine:DOC:bromide constant. DBP formation testing showed no preferential formation between the hydrophobic and hydrophilic fractions in either the river or well waters. While the overall concentrations of organic DBP precursors are effectively reduced during bank filtration, the reductions appear to be largely the result of the reduction in NOM concentration rather than a consistent change in NOM character.