The Honolulu Board of Water Supply and a trio of contractors face a possible $420,000 fine for polluting the Nuuanu Stream over an 18-day period last year.

The state Department of Health said Friday it issued a notice of violation and order against the Board of Water Supply, R.M. Towill Corp., SSFM International Inc. and Drayko Construction Inc. for discharging sediment from Nuuanu Reservoir No. 4 to upper Nuuanu Stream last spring and failing to report the action for 15 days.

The discharge turned the stream dark brown and affected a lot of people downhill, including some property owners with koi ponds, said Matthew Kurano, head of enforcement for the Health Department’s Clean Water Branch.

The department Friday also announced an additional notice of violation and order, this time against a property owner and its contractors for polluting Manoa Stream.

The Hayama Trust, HH Constructions Inc. and Structural Hawaii Inc. face a $40,000 penalty for dumping 193 cubic yards of dirt, gravel and construction materials into Manoa Stream beginning in May without state permits or Department of Health authorization.

The alleged polluters have 20 days to contest their notices and request contested case hearings.

Regarding the Nuuanu Stream incident, a Clean Water Branch investigation found that more than 2,000 cubic yards of sediment found its way into the stream after the contractors failed to completely finish dredging activities prior to draining water from the reservoir.

“We think it was preventable,” Kurano said.

The board and its contractors not only failed to report the discharges within 24 hours as required by law, but delayed reporting the problem for 15 days and only acted to stop the discharges after health officials became involved, officials said.

“If an unlawful discharge occurs, the BWS and its contractors are responsible for acting immediately to mitigate the impacts and report the incident to the DOH. In this case, neither the BWS nor its contractors acted as required by law,” Keith Kawaoka, deputy director of environmental health, said in a news release.

The notice of violation requires corrective actions to prevent additional discharges from the reservoir.

Kurano said the department is continuing to talk to the Board of Water Supply and is committed to resolving the matter.

Asked to respond, the Board of Water Supply said in a statement that it is reviewing the state’s notice of violation and order and would have no further comment at this time.

In an earlier news release, the Board of Water Supply said the Nuuanu Reservoir No. 4 project was designed to repair and rehabilitate the dam structure, reservoir intake tower and suspension bridge, reservoir drainage and maintenance road. The project was necessary, it said, “to maintain operational adequacy” of the dam.

The Board of Water Supply is a semi-autonomous agency that manages Oahu’s municipal water resources and distribution system. It is the largest water utility in the state, serving about 1 million customers on Oahu.