INFLUENCE OF ORGANIC CARBON LOADING AND SEDIMENT SURFACE–ASSOCIATED METAL OXIDE CONTENT UPON CRYPTOSPORIDIUM PARVUM REMOVAL AND TRANSPORT DURING RIVERBANK FILTRATION OPERATIONS, SONOMA COUNTY, CA

Oocysts of the protist pathogen Cryptosporidium parvum are ubiquitous in ground and surface waters and highly resistant to conventional water-treatment disinfection (e.g., chlorination) methods. Riverbank filtration (RBF) is a cost-effective, primary treatment technology practiced world-wide in removing pathogens of concern during water passage from rivers through underlying alluvial aquifer material to public water wells. In the US, the EPA grants treatment credits to RBF utilities demonstrating adequate removal for C. parvum oocysts. We found that metal oxide (97-99% Fe- and Al-oxyhydroxides) mineralogy on RBF sediment surfaces (Russian River, Sonoma County, CA) related strongly to oocyst and microsphere removal. Because grain size or sediment sorting inadequately predicted microsphere or oocyst removal, the primary colloid immobilization mechanism within these sediments is likely sorptive filtration rather than physical straining. Water management practices which alter water quality, combined with the environmental persistence of C. parvum oocysts, raise concerns about pathogen removal during RBF operations. We studied how modest dissolved organic carbon (DOC) additions affect pathogen removal within poorly sorted, iron-rich RBF sediments in both flow-through (bulk sediments and dual porosity) and static mini-column systems. An amendment of HPOA (hydrophobic organic acid) isolated from Russian River DOC to flow-through columns containing subsurface sediments (0.06 mg HPOA g^-1 sediment) resulted in less (several fold to over an order of magnitude) sorption of oocysts and (or) oocyst-sized microspheres. In static mini-columns, we found colloid removal efficiencies in sediments (0.3 – 15.2 meter [m] depth interval) decreased with depth and were related to metal oxide content. However, previously immobilized oocysts and oocyst-sized microspheres were reentrained after passing Russian River HPOA through columns of RBF sediments. These data suggest that oocyst attachment sites on grain surfaces were altered by DOC. Because modest organic carbon levels systemically reduce sediment effectiveness for pathogen removal at this RBF site, caution should be exercised in allowing land-use changes and activities within the watershed that would substantively increase DOC loads in the river.