QUANTIFYING ATTENUATION OF HUNDREDS OF ORGANIC STORMWATER CONTAMINANTS BY A RESTORED URBAN HYPORHEIC ZONE

In the average year, 79% of coho salmon returning to Thornton Creek (Seattle, WA) die from stormwater pollution before they have a chance to spawn. In 2014, Seattle Public Utilities (SPU) completed a hyporheic and floodplain restoration project on Thornton Creek to improve water quality and reduce flooding, but quantifying hyporheic contaminant attenuation is extremely complex. 3D hyporheic flow fields and stormwater contaminant concentrations both vary rapidly in space and time. In short, it is not enough to measure contaminant concentrations at upwelling or downwelling locations – one must know where specifically a given parcel of water entered the hyporheic zone, the chemical compositions at the time of downwelling, and the amount of time it took to return to the stream. We applied a Lagrangian approach favored by numerical modelers (i.e., particle tracking), using downwell injections of dye/salt tracers to delineate specific hyporheic flowpaths and seepage meters to sample them. Samples collected at baseflow and during a fall storm were analyzed using high resolution mass spectrometry and non-target analysis. Nearly 1,200 different organic chemical peaks were found in the stormwater samples, of which 84 have been identified. Each hyporheic flowpath attenuated approximately half of the contaminants by >50%, and attenuation was stronger for longer hyporheic residence times and more hydrophobic compounds. Further, many of the attenuated contaminants are correlated with coho PSM. We estimate that the total hyporheic flow capacity is comparable to the baseflow surface discharge at the restored site. Altogether, these results show that hyporheic restoration may provide meaningful treatment for coho prespawn mortality during late summer and early fall coho runs. However, hyporheic treatment likely decreases in winter as wet weather increases surface discharge and causes regional groundwater upwelling. Our hyporheic monitoring method needs further refinement to make real-time adjustments for changing flowpaths, but presents a step forward in measuring fluxes across the dynamic stream-hyporheic interface.