How does the heavy rains in 1986 relate to the increased incidence of waterborne disease from private water systems? Years with heavy rains caused the septic tank water to mix with the rain and entered the reservoir. Years of heavy rain caused the reservoir water to mix with the rain and entered the septic tanks. The floods caused the sewage lines to overflow into the reservoir. Years with heavy rains caused the septic tank water to mix with the private well water.