On May 28, 2003, the pump on well #C-118A, located at 9699 E. Speedway, failed and the well was taken out of service. On June 4, during the removal of the submersible pump from C-118A, a seal on the well's pumping mechanism failed, releasing mercury onto the ground at the well site. The City's hazardous materials contractor removed the spilled mercury and the site has been cleared of mercury contamination and secured. Because the well had been taken out of service on May 28, and was completely isolated from the drinking water system, no customers were affected by the onsite mercury spill.

As a precaution, a water sample was taken at the site. Mercury was detected at levels exceeding 2 parts per billion (approximately 50 ppb in the pressure vessel). In response to these sample results, additional testing was conducted in the water line that had been delivering water from the well prior to May 28. The neighborhoods at Calle Cascada and Calle Bolivar were the first ones on the water line from well C-118. Tests outside of those two neighborhoods have not detected any mercury in the drinking water. The presence of mercury in the water line shows that the seal had at least partially failed prior to taking the well out of service and some mercury entered the drinking water system. Because a leaking seal would relatively quickly lead to pump failure, Tucson Water staff believe that the seal leak occurred within a short time frame prior to the pump failure. The line from the well has been isolated from the rest of the drinking water system. This was the only well pump in the system containing mercury as part of the sealing mechanism.

On June 6, inorganic mercury at a level exceeding 2 parts per billion (ppb) was found in a water line serving a small neighborhood of 10 homes at the 9400 block of Calle Cascada just west of Harrison Road. Later testing on June 6 also found levels of mercury above the standard at the entrance to the Rosewood Estates III (9400 block of Calle Bolivar west of Harrison Road and the 1400 block of north Placita Ombria).

Residents of both neighborhoods were personally contacted by Tucson Water staff, provided with a supply of bottled water, and given clear instructions on how to respond. The neighborhoods were disconnected from the pipeline that had served them and received water from a pipeline that was never exposed to the mercury contamination.

Samples have been taken repeatedly outside of those two neighborhoods, and no mercury has been detected in any of those samples. Residents who have not been personally contacted by Tucson Water have not been exposed to mercury and do not need to take any steps to address this issue. Their drinking water remains the same as before this incident. No residents outside of those two neighborhoods need to replace their tap water, and bottled water is not necessary.