What are the options to stop a sewer company from dumping underground next door?
A "nuisance" (also called a "private nuisance") is a condition or activity that unreasonably interferes with the use and enjoyment of private property. An objectionable odor may be found to be a nuisance.
A landowner may file a court action seeking a remedy from the responsible party for a nuisance. One remedy is damages (i.e., money) to compensate for harm resulting from the nuisance. Courts can also grant injunctive relief, such as ordering the responsible party to abate the nuisance. A court can also enjoin (block) a planned activity that has a high probability of causing a nuisance, such as building an incinerator in a residential area.
A suit to abate a nuisance by means of an injunction generally requires that without the intervention of the injunction, the activity will be ongoing, and irreparable harm without a remedy compensible in money damages will occur. Injunctions to abate such activity are granted only where necessary and where there is no adequate remedy at law.
To issue a preliminary injunction, the courts typically require proof that (1) the movant has a ‘strong’ likelihood of success on the merits; (2) the movant would otherwise suffer irreparable injury; (3) the issuance of a preliminary injunction wouldn't cause substantial harm to others; and (4) the public interest would be served by issuance of a preliminary injunction.
I suggest contacting the local building, land use and/or zoning authority to inquire about applicable regulations and ordinances. It may be possible to petition your local representative to enact a sewage ordinance if one doesn't already exist.