Saline water intrusion adjacent to the Fraser River, Richmond, British Columbia

During periods of average to low flow in the Fraser River, seawater from Georgia Strait migrates with a rising tide up the river channel, reaching distances up to 16 km from the river's outlet to the ocean. This relatively dense water infiltrates through the river bed and circulates as a stable wedge of saline water within permeable deltaic deposits adjacent to the river. At the Kidd2 site in Richmond, the wedge extends approximately 500 m inland from the river. The top of the transition zone between fresh and saline water is 10 m below ground surface. The bottom of the saline wedge is located at the contact with underlying low-permeability delta slope deposits, resulting in a wedge with a vertical extent of approximately 10 m. Maximum salinity values observed in the wedge (16–17 parts per thousand) reflect the time-averaged response to density-driven flow under a complex sequence of daily and seasonal variations in salinity within the Fraser River and mixing with fresh river water that may enter the aquifer during the summer freshet. Under the assumption of dynamic equilibrium, simulation of the saline wedge at the Kidd2 site suggests (1) the effective, long-term average salinity at the base of the river channel is approximately 19 parts per thousand; (2) the velocity of groundwater, directed inland in the central portion of the wedge, is in the range from 0.5 to 6 m/year; and (3) fresh water in the confined deltaic sands moves from inland areas toward the Fraser River under a hydraulic gradient of approximately 3 × 10–4.Key words: salt water intrusion, deltaic sediments, modeling.