"There are regional differences in the chemistry of the ground water that correlate with the incidence of heart disease," Pekka Puska, director general of the Finnish National Health Institute, told AFP.
"The study shows that the softer the water is, the more heart disease there is," he added.
While researchers have been aware of this phenomenon for some time, most studies done so far have been very general, Puska said.
The new research is the first detailed survey, covering 19,000 incidents of heart attacks across Finland, and then linking them to geochemical data of the drinking water in the communities where the incidents had occured.
Whether water is hard or soft depends on the quantity of mineral salts, typically magnesium, calcium, zinc, copper, aluminum, fluoride and iron, dissolved in it.
Normally hard water is described as containing more than 250 parts per million (PPM) of mineral salts, while mineral water typically has 500 PPM.
Research from the west and south of Finland, where the drinking water is hard, found far fewer incidents of cardiovascular diseases than was the case in the north and east of the country, where the drinking water is soft, Puska said.
The researchers did not single out which of the minerals contributed to the effect, Puska said.