Nitrates from agricultural fertilizer could continue to leach into groundwater for at least 80 years after initial use, according to researchers who conducted a long-term study of nitrogen uptake.
Using isotope tracers, scientists followed the fate of nitrogen-based synthetic fertilizers applied to fields planted in France with wheat and sugar beets.
They found that three decades after the 1982 application, 61% to 65% of the nitrates had been taken up by plants. Much of the rest continued to reside in soil matter (12% to 15%) or was migrating into groundwater (8% to 12%).
In a paper published Monday in the online edition of the Proceedings of the National Academy of Sciences, researchers estimated that nitrates would continue to seep into groundwater for at least another five decades.
That is "much longer than previously thought," wrote the authors, led by Mathieu Sebilo of the Pierre and Marie Curie University in Paris.
Clean-up programs need to take that long-term release into account, the paper concluded.
Nitrate pollution is widespread in California's agricultural regions, particularly in the San Joaquin Valley, where contamination of municipal water supplies and domestic wells from agricultural runoff poses a threat to public health.
The nitrate's lingering nature means that even if steps are taken to cut the contamination, the effects of earlier use endure.