Anticipated effects of an unlined brackish-water canal on a confined multiple-aquifer system

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The major source of fresh water at Southport, North Carolina, is a confined multiple-aquifer groundwater system that can be affected in several ways by the proposed unlined cooling-water canal for the Brunswick nuclear power plant. The canal will route brackish water three miles from the Cape Fear Estuary to the plant condensers. The heated brackish effluent will then be routed six miles from discharge to the Ataantic Ocean. To evaluate the impact of the canal on the groundwater system, a mathematical model was used to describe steady-state responses resulting from a variety of conceivable hydrologic stresses. Numerical solutions were obtained by means of finite-difference approximations and a successive line overrelaxation solution technique. The model apprdximated three- dimensional saturated flow in aquifers of variable thickness by assuming two- dimensional flow in each aquifer with interaquifer transfer throughout the groundwater system. Simulation results indicated that saatwater from the estuary will eventually intrude upon Southport's municipal wells regardless of the presence of the proposed Brunswick canal. The canal system will not significantly decrease the time required for estuarine saltwater contamination of the wells. A major portion of the brackish water that downwells from the discharge canal appears to upwell in a parallel drainagemore » channel west of the canal and in Dutchman Creek east of the canal. However, a low potential gradient toward the Southport wells is introduced by the downwelling portion of the discharge canal. This analysis did not consider the effect of the downwelling on the water quality of the Southport wells. However, saltwater movement along this gradient will require an order of magnitude more time to reach the wells than will saltwater intrusion from the estuary. Magnitudes and locations of upwelling and downwealing and the areal extent of the depression cones for the Southport wells wiaa be affected considerably by reducing the proposed operational water surface elevation in the discharge canal from +4.5 ft MSL to 0.0 ft MSL. (auth)« less