Every year from April to October, winegrowers in Northern California spray sulfur-based chemicals all over their vineyards to prevent powdery mold from growing. And while sulfur is extraordinarily good at fighting mildew, it may be harming ecosystems in surrounding areas, suggests a new study.
It was the first study to look at how sulfur use on vineyards might be affecting the environment. The findings suggest that sustainability-minded winegrowers might want to start looking for alternative ways to battle fungus.
"By and large, there has been a big movement among winegrowers trying to do it really well and sustainably," said Eve-Lyn Hinckley, an ecosystem biogeochemist at the University of Colorado, Boulder. "This is one more thing that we haven't been paying attention to that we probably need to. It's the responsibility of this industry to know what the consequences of sulfur are."
Scientists have long known that sulfur-containing compounds can be harmful to the environment. Much of that knowledge comes from research on acid rain, which contains sulfuric dioxide produced by the combustion of fossil fuels.
Among other negatives, acid rain has been linked to declines of certain plants, fish and insects. Despite that evidence, Hinckley said, very little attention has been given to sulfur's use in agriculture, where it has a variety of purposes -- from battling fungus to making soils more acidic for certain crops.
Hinckley and colleague Pamela Matson chose to study sulfur use on vineyards partly because winegrowers in many places use a fairly large amount of it. During the dry season from April to October, which is also the growing season, winegrowers use tractors or airplanes to fumigate their fields with tiny particles of sulfur in its most basic, elemental form. The sulfur ends up coating the plants and the soil.
To begin their study, the researchers took soil samples just before, just after and within a week or two after spraying at two sites in Napa Valley. Their analyses, published today in the journal Proceedings of the National Academy of Sciences, showed that on soil, elemental sulfur turned almost immediately into a form called sulfate, which is easily moved by water.
Next, the scientists followed the sulfate trail for three years by measuring its concentrations in plants, in soils and in water that ran off from vineyards. They looked both at irrigation water used during the growing season and at water from rainstorms during the dormant season.
Results showed that the amount of sulfate that drained off of vineyards just about equaled the amount of sulfur applied to the fields. And while most of the sulfur was applied during the growing season, most of it washed off during the rainy dormant season.
Although the researchers don't yet know what the consequences of all that runoff might be, they suspect that sulfate from Napa's vineyards may end up in downstream rivers, marshes and wetlands throughout the Napa River Basin and the San Francisco Bay. Among other worries, sulfate can turn benign forms of mercury into forms that are highly toxic to fish and people.
Compared to other crops, grapevines are fairly low on the scale of environmental impact, said Andrew Waterhouse, a wine chemist at the University of California, Davis. They require little water and few pesticides. In dry places like California, he added, sulfur is often the only substance applied to vines.
For eco-conscious wine-drinkers, there is no easy way to know what the environmental impact of your Cabernet is at this point. Sulfur is approved for use as a fungicide by organic vineyards, so a label won't tell you if or how much sulfur was used on the grapes you're drinking.
For now, perhaps the best thing a concerned wine connoisseur can do is talk to the managers of the vineyards they buy from about sulfur and other environmental impacts.
"Trying to give someone direction here is extremely difficult, because there is no way for consumers to tell whether wine has been made in a particularly sustainable manner at this point," Waterhouse said. "It would be great if there was some labeling system out there that could help guide consumers, but it doesn't exist."