As the Canadian North Boeing 737, (C-GCNV), operating as MPE9420, was levelling at 32 000 feet following departure from Edmonton on April 17, the crew observed decreasing hydraulic quantity and requested a return to Edmonton. The hydraulic quantity continued to drop throughout the descent until approximately 1/4 gallon remained, at which point System A lost hydraulic pressure. The flight landed without incident, with ARFF standing by, and fluid was observed to be leaking from under the left wing, around the left engine. The aircraft was shut down on Taxiway A1 and towed to a hangar. Maintenance examination determined that the No. 2 leading edge slat actuator, P/N 65-44651-16, was spraying fluid out of one port on the assembly. The actuator was changed, an overweight landing inspection was performed, and the aircraft was returned to service. A passenger had observed fluid leaking prior to departure and had taken photos; however, no crew members had been advised until deplaning.