Synthetic nitrate fertilizers used in large-scale farming may leave a legacy of pollution that persists for decades in the soil and groundwater, researchers in France and Canada said Monday.

Too much nitrate in the environment has been linked to contaminated drinking water and can cause rapid growth of algae that damages freshwater ecosystems and coastal marine life.

To find out how much nitrate was left behind over time from farming activities in one area, researchers focused on a field in France where the crops were wheat and sugar beets.

The study, which appears in the US journal the Proceedings of the National Academy of Sciences, tracked the levels of nitrate in the soil from 1982 to 2012.

Researchers found that 12 to 15 percent of the nitrate from fertilizers still lingered in the soil 28 years after it was applied.

About 61-65 percent had been taken up by plants, and eight to 12 percent already leaked toward the groundwater.

Some of the nitrate that remains in the soil "is expected to continue to be taken up by crops and to leak toward the groundwater in the form of nitrate for at least another five decades," said the study.

"It is often assumed that most of the nitrate contained in fertilizers is used by plants for their growth or quickly leached out of the root zone," it said.

Previous research in the Mississippi River Basin has shown that high levels of nitrates remain in the river despite a cutback in human inputs.

Therefore, efforts to restore land and waterways "must take into account the delay resulting from legacies of past applications of synthetic fertilizers in agricultural systems" the authors said.