Nitrate contamination of groundwater in some of the state's most intensely farmed regions has grown worse in recent decades and will continue to spread, threatening the drinking water supplies of more than 250,000 people, according to a new study.
The research, conducted by UC Davis scientists, underscores the complexity of dealing with nitrate pollution, which is largely the result of nitrogen leaching into aquifers from fertilizers and manure applied to cropland. High nitrate levels have been linked to cancer and reproductive disorders and can be lethal to infants.
Examining groundwater data from the southern San Joaquin and Salinas valleys, the authors concluded that even if all farming operations ceased, nitrates would remain in water supplies and continue to spread for decades.
Almost all of the 2.65 million people living in the study regions rely on groundwater for drinking supplies, including the cities of Salinas, Fresno and Bakersfield.
Large water systems, such as Fresno's, have shut down contaminated wells. Researchers said the people most likely to be drinking tainted supplies were the one in 10 in the study area served by small, local systems that can't afford treatment or using private, domestic wells that are unmonitored.
Providing clean water for them will cost $20 million to $35 million a year. That is far less than the cost of substantially reducing the nitrates leaching from fields, the authors said. "You would have to shut down huge amounts of agriculture or greatly reduce its profitability," said study coauthor Jay Lund, director of the UC Davis Center for Watershed Sciences. "It would be devastating to a lot of these areas."
The use of synthetic fertilizer — one major nitrate source — has leveled off since the 1980s. But thanks to an explosion in the size of the state's dairy herds and an increase in the productivity of cows, a lot more manure is going onto fields.
"Not only has the number of cows been increasing, but the amount of manure per cow has been increasing," Lund said.
Monitoring data show that some of the most serious nitrate contamination in the Tulare Lake Basin, which encompasses the southern San Joaquin Valley, has been detected in areas with big dairy operations that use manure to fertilize the crops that feed the herds.
Because it can take years or decades for nitrate to migrate through aquifers, the authors warned that the effects of the rising volume of manure may not be evident for some time.
Nitrate leaching can be reduced by modifying irrigation and fertilizing practices and by switching to crops that take up more nitrogen. Another promising technique is to use nitrate-contaminated groundwater to irrigate fields, fertilizing the crops that would then absorb some of the nitrogen, partially cleansing the water.
The authors outlined a number of options for paying for the necessary drinking-water improvements, including imposing fees on fertilizer sales or groundwater pumping.
Danny Merkley, director of water resources for the California Farm Bureau Federation, called discussion of new agricultural fees premature. First, he said, the state should turn to unspent bond money. "We've got a number of clean water bonds with money that has not gone out," he said.
The study was commissioned by the State Water Resources Control Board, which will draw up recommendations and forward them to the state Legislature.