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Mechanism for transport of oil-contaminated groundwater into pink salmon redds

Groundwater movement from oil-contaminated intertidal beaches to surface and sub- surface water of salmon streams in Prince William Sound, Alaska, was studied to determine if trans- port of dissolved petroleum hydrocarbons to incubating pink salmon eggs (Oncorhynchus gorbuscha) was plausible. Beaches surrounding 31% of the streams in the Sound were extensively oiled in 1989; salmon egg mortality was elevated even though little oil was observed in stream gravel. In 2000, fluorescent tracer dyes injected into 2 of these beaches during ebb tides were subsequently observed throughout most of the intertidal portion of each watershed, including surface and subsurface (hyporheic) stream water. Mean horizontal groundwater flow was rapid through the porous gravel (4 to 7 m h -1 ) and was driven by hydraulic gradients within beach groundwater. When different dyes were simultaneously released at ebb tide on opposite sides of a stream, each dye was detected in the beach opposite release within the first tidal ebb. Dye was moved vertically upward at least 0.5 m by subsequent incoming tides. Thus, tidal cycles and resultant hydraulic gradients provide a mechanism for groundwater transport of soluble and slightly soluble contaminants (such as oil) from beaches surrounding streams into the hyporheic zone where pink salmon eggs incubate.