COUPLING OF INORGANIC, PHARMACEUTICAL, AND ORGANIC WASTEWATER CHEMISTRY, BOULDER CREEK, COLORADO

Determining processes that control the fate and transport of pharmaceutically active ingredients (API) and other organic wastewater compounds (OWC) in aquatic systems enables a more complete understanding of how these emerging contaminants affect our environment. Inorganic geochemistry can provide important insight into these processes. Using comprehensive water-quality data from the Boulder Creek Watershed, Colorado, processes were identified by comparing inorganic constituents with different geochemical properties, and these results were linked to downstream variations of API and OWC. Much of the year, secondary effluent from Boulder’s wastewater treatment plant (WWTP) dominates the chemistry of lower Boulder Creek because upstream flow has been diverted for municipal and agricultural uses and cannot provide instream dilution. A 13-km reach of Boulder Creek downstream of the WWTP was sampled at 5 sites. The rare earth elements (REE) provide a unique geochemical signature of the WWTP effluent because of an enrichment in gadolinium (0.068 µg/L), likely derived from its use in the medical industry as a contrasting agent in magnetic resonance imaging. This signature helps constrain the fraction of effluent in downstream samples. To quantify the effluent load downstream, solution chemistry is being modeled to determine if the REEs behave conservatively in this Ca-Mg-HCO₃ water. The dominant inorganic REE aqueous species are carbonate complexes. In addition to the inorganic species, a complex matrix of organic compounds was found, including elevated concentrations of the strong ligand ethylenediaminetetraacetic acid (EDTA; up to 210 µg/L) and dissolved organic carbon (up to 7.8 mg/L). These metal complexing compounds potentially have an important role in the REE solution chemistry. Inorganic constituents have been studied to identify geochemical processes in this reach, which include precipitation (e.g. Fe and Al), sorption (e.g. Cu, Pb, and Zn), sedimentation, and dilution by ground-water inflow. By utilizing the different chemical properties of inorganic constituents and modeling the solution chemistry, a clearer understanding of the processes at work can help constrain the fate and transport of API and OWC in aquatic environments.