Enough w/ the pH crap (Score 4, Informative)

3.5 pounds of sodium metal would not have that drastic of a long term effect on a pond. If the pond was 1000 liters in volume and had a pH of 7 (unlikely) the pH would rise to approximately 12 (1000 liters ~= 275 gallons.) A larger pond lets say 10000 (again not a very large pond/lake) gallons, with an initial pH of 7, would experience a rise in the pH of approximately 4 units. Now lets consider the fact that the water in the pond is probably buffered to some degree, the result of the sodium metal reaction would have even less effect. If the water has any metal in it to speak of, like something uncommon like calcium, or iron, or magnesium, the hydroxide ions produced by the sodium metal reaction would precipitate the metals in the water and the pH would be even less effected.