COUPLED GROUNDWATER AND VOLATILE ORGANIC COMPOUND FLUXES THROUGH A STREAMBED CONTRIBUTE TO OFF-SITE MIGRATION OF CONTAMINANTS IN A COASTAL PLAIN STREAM, NORTH CAROLINA

Contaminant flux from groundwater to surface water is an understudied factor in the transport and fate of volatile organic compounds (VOCs) and creates the potential for off-site migration and subsequent exposure. We measured the flux of VOCs from a contaminated aquifer to an adjacent stream, Hominy Swamp Creek (HSC) in Wilson, North Carolina, using streambed sampling methods. We used a piezomanometer (pushable piezometer coupled with an oil-water manometer) at 26 streambed points to measure the vertical hydraulic head gradient (J) in the streambed and to sample groundwater below the hyporheic zone. At the same points, falling head tests in a field permeameter were used to measure hydraulic conductivity (K), allowing calculation of groundwater flux as v = KJ at each point. VOC flux through the streambed at each point was calculated as the product of v and groundwater VOC concentration [VOC]: f­_voc = v[VOC].

The groundwater analyses indicate thirteen VOCs discharge to HSC: tetrachloroethene (PCE), trichloroethene (TCE), 1,1-dichloroethene (DCE), cis-1,2-dichloroethene (cDCE), trans-1,2-dischloroethene (tDCE), vinyl chloride (VC), benzene (BZ), 1,3-butadiene (BT), 1,1,2,2-tetrachloroethane (PCA), 1,1-dichloroethane (1,1-DCA), 1,2-dichloroethane (1,2-DCA), 1,2,3-trichlorpropane (TCP), and 1,2-dichloropropane (DCP). Interpolated, contoured maps of f_voc­ reveal large spatial variability in the 52-m reach. Chlorinated ethenes and DCP primarily discharge on the right side of the stream channel, in the upper third of the reach near a dry cleaner. Chlorinated ethanes, BZ, BT, and TCP primarily discharge on the left hand side of the stream channel, in the middle third of the reach. The spatial variability suggests at least 2 plumes with different chemical sources. The highest f­_voc was that for cDCE, at 9.98 mg m^-2 d^-1, followed by BZ (6.56 mg m^-2 d^-1) and VC (1.81 mg m^-2 d^-1). Other f_voc values were less than 1 mg m^-2 d^-1. Higher f­_voc values generally occurred in areas of higher v. The annual mass losses of cDCE, BZ, and VC from the aquifer to the stream reach were estimated to be 1.11, 0.73, and 0.20 kg yr^-1, respectively. Annual mass losses for other VOCs were less than 0.02 kg yr^-1. The spatial variation of VOC fluxes gives quantitative insight into the impact of multiple plumes discharging to a single urban stream.