The reaction of sodium and water produces sodium hydroxide (lye) and hydrogen gas (thus the flames). Considering the volume of water in the lake and the fact that this happened 60 years ago, I don't think you need to be too worried.
hydrogen in the air will steal the hydroxide from sodium creating water and the sodium can go off to form a salt. thats why hydroxide pellets turn really wet when just left out a few minutes. over 60 years im sure the hydroxide levels have evened out to what they normally are.