Major oil companies’ multimillion-dollar payment to 153 public water systems to cleanse wells of a potentially carcinogenic gasoline additive could grow if more tainted wells are discovered over the next 30 years.

Companies including ConocoPhillips; Chevron Corp.; BP PLC’s BP America Inc.; Royal Dutch Shell PLC’s Shell Oil Co.; Valero Energy Corp.; and Marathon Oil Corp. agreed to pay $422 million to settle lawsuits claiming wells were contaminated by MTBE, a chemical that was once a key gasoline ingredient.