Glyphosate dissapes very very quickly in soil. Just use a formulation that is registered for use around ponds like Rodeo. Formulations contain surfactants (a lot like soaps) that are meant to help the glyphosate to stick to and penetrate leaf surfaces. Some of these are toxic to aquatic organisms. Glyphosate is the most widely used and most extensively tested (for safety) herbicide on earth. Just use the appropriate formulation.
