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Ground-water/surface-water interactions and MTBE contaminant plume discharge were investigated in a low-order river that experiences episodic river stage fluctuations. Results show that the hydraulic gradient fluctuates hourly to monthly due to river stage changes, water table recharge events, and reservoir gate adjustments. Hyporheic exchange driven by channel morphology creates small-scale gaining, losing, and parallel flow systems along the mostly gaining reach. During precipitation events, infiltrating rainfall rapidly saturates the extended capillary fringe and the shallow floodplain water table rises forming a ground-water ridge or mound and causing a steepened hydraulic gradient towards the river. The system response is magnified by watershed characteristics which control the river stage hydrograph including stormflow lag and flashiness. Results of this study suggest that a ground-water plume discharging to surface water may have several discharge locations related to transient water-table configurations. Under conditions of a low hydraulic gradient, the MTBE plume is deflected away from the river by hyporheic flow toward a downstream discharge location. When the gradient toward the river steepens in response to precipitation and gate closure, the small-scale hyporheic exchange systems are overcome and the plume discharges along the entire reach. Under these conditions, a high influx of contaminated ground water is discharged from the floodplain to the river, temporarily elevating river contaminant concentrations. During site investigation and monitoring, these transient spatial and temporal relationships could easily be missed by traditional site monitoring strategies.

Assessing the Effects of Fluctuating River Stage on an MTBE Containment Plume Discharging to Surface Water, Pascoag, RI