You can sue your neighbor for nuisance.  Nuisance is a serious and unreasonable interference with your use and enjoyment of your property.Damages (monetary compensation in your lawsuit) may be an inadequate remedy because since the sewage polluting your property is an ongoing situation, multiple lawsuits would be filed.  Damages are also inadequate because land is unique.  Therefore, instead of damages, your remedy would be an injunction to stop the sewage flowing onto your property and into your pond.  A court will balance the benefits and burdens of the parties in order to determine whether or not to grant an injunction.  The benefit to you is of course preventing the contamination of your property which is also a health issue.  This will be weighed by the court against the burden (cost to your neighbor of stopping the sewage flow).  If the court grants your injunction, a temporary restraining order will be issued which will be in effect until a preliminary hearing at which time the court may issue a preliminary injunction to halt the sewage flow until trial at which time the court will decide whether to grant a permanent injunction to permanently stop your neighbor from allowing sewage to flow onto your property and into your pond.