PREDICTING LATERAL HYPORHEIC EXCHANGE AND GENX FLUXES IN THE LOWER CAPE FEAR RIVER USING RIVER MORPHOLOGY

We determined groundwater-surfacewater exchange across a 170 kilometer section of the lower Cape Fear River (CFR) in North Carolina using river planform based hyporheic exchange model. Valley slope and sinuosity values were extracted from a LIDAR based DEM at sections representing the overall river valley morphology. Results of the model showed that water exchanged at any one point along the river ranges from 0.0421 m d^-1 to 0.1528 m d^-1. The net volume of exchanged water ranges from 14,314 m³ d^-1 to 51,192 m³ d^-1 which represent 0.3 to 1.3 percent of the lower CFR discharge. GenX concentrations on 2 groundwater samples adjacent to the banks of CFR show values of 344 and 492 ng L^-1 which is higher than the current levels observed in the CFR surface water. These concentrations are the first reported values in the riparian zone along the CFR. Assuming these levels represent higher end concentrations along the banks, the GenX flux entering the lower CFR could be as high as 7.04 x 10⁹ ng m^-2 d^-1 to 2.56 x 10¹⁰ ng m^-2 d^-1. Considering the relatively low velocity flow in groundwater and the undetermined extent of the GenX plume infiltration in CFR banks, our work suggests that GenX and other associated emerging contaminants may linger in the CFR.