The key is that when it melts naturally it doesn’t all go in at the same spot. Dumping all of the snow in a single place in a body of water can create a “dead zone”, but a gradual runoff isn’t as big a problem because it is both diluted and dispersed. As a poster on another board once said, “The solution to pollution is dilution”.
Has the lake only one edge?
All road treatment products are mined from seawater; even Torricelli figured that out after he had already built his first thermometer.
If it melts into the soil where it’s piled, plants won’t grow so well but eventually it will be washed to the lowest level which remains the sea.