For 30 hours, 66 t of nickel sulphate, from a metallurgical plant producing nickel cathodes and 50,000 t of nickel-based chemicals, spilt into the Kokemaki River. The highest concentration of nickel measured in the river was 8,800μg/l for an instant environmental standard of 34μg/l, and an average concentration limited to 1μg/l. Significant mortality of mussels was observed. Mortality of the species Unio crassus (listed in the Habitats Directive) occurred 5-7 days after the pollution release. It was estimated that between 1 and 1.1 million specimens were killed (15% of the population). Other species of mussels were also killed: Unio pictorum, Unio tumidus and Anodonta anatida. The total mortality of mussels was equivalent to 100 t of biomass.

The local environmental authorities, informed one day after the leak was stopped, were not aware of the extent of the spill. In addition, the authorities had difficulty finding experts during the summer months. The extent of the spill was reported by the press the next day. At the end of the week, when the massive death of the mussels was discovered, the media exerted intense pressure on the authorities. The accident took on a national dimension, and the authorities were accused of not communicating early enough and of not warning the population of the consequences. Social media played an important role in spreading information and rumours.

The accidental spill occurred after a maintenance intervention in which a poorly assembled heat exchanger triggered a leak in the cooling system. At the time of the accident, several measuring instruments were out of service. Other automatic measuring instruments were working, but employees did not check the results. Due to these malfunctions, it took 30 hours to identify and correct the problem.

Following the event, the Environmental Protection Agency took the following measures:

• modification and clarification of emergency instructions,
• design of a crisis management procedure and accident simulation exercises,
• implementation of a weekend on-call service.

In addition, the plant conducted a new risk analysis after the accident. This analysis led to technical changes:

• installation of a secondary circuit for the leaky cooling water system. All other open systems were checked and changed, as required,
• modifications to the control system: installation of additional alarms and improvement of their visibility,
• training of employees on controlling environmental impacts,
• improved cooperation with other plants in the area (during the accident, conductivity alarms had identified a problem in a nearby plant, but the information was not transmitted to the operator concerned).

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