State water officials Wednesday faced off against farmers and area elected officials over how best to reduce one of the nation’s worst water pollution problems — runoff from irrigated farmland.

The Central Coast Regional Water Quality Control Board held a daylong hearing in San Luis Obispo to decide whether to adopt sweeping new rules intended to curb nitrate and pesticide pollution. The meeting was expected to go well into the evening and could be continued to today for a decision, Chairman Jeffrey Young said.

U.S. Rep. Sam Farr, D-Carmel, state Sen. Sam Blakeslee, R-San Luis Obispo, and other local members of the state Legislature urged the water board to work with farmers to come up with more collaborative and less regulatory solutions to reduce pollution.

“This is not going to work if it is done in a penalistic way,” Farr said. “If this is going to work, it’s going to need more carrots and less sticks.”

The new rules would require that some of the larger farms that use fertilizers and pesticides and are close to bodies of polluted water establish buffers and adopt plans to manage runoff. They would also have to closely monitor and report pollution discharges from farmland.

If the new rules are adopted, it will take decades for pollution levels to be reduced to safe levels. Environmental advocacy and justice groups urged the board to take action and not draw the process out any longer.

Other farmers would have to comply with less stringent requirements or none at all depending on their size and how much fertilizer and pesticides they use. This tiered approach was one of many changes made to the rules based on input from the agricultural community, water board staff said.

Water board staff estimates that 103 farms or 3 percent of the farms in the Central Coast region would have to comply with the most stringent requirements. Most of them are located in the Salinas Valley and Santa Maria areas.

Water board staff said that many drinking water wells in those areas are tainted with nitrates from fertilizers and surface waters, particularly the Salinas and Santa Maria rivers, are tainted with pesticides. They urged the board to adopt the rules to deal with a significant public health risk that potentially affects 80 percent of the 1.5 million people who live in the Central Coast district that stretches from the Monterey Bay area through Ventura.

“We’ve been arguing about this for 31⁄2 years,” said Michael Thomas, the water board’s assistant executive officer.

The water board has held seven hearings on the rules since 2008 and has done four major revisions based on more than 2,000 comment letters.

Studies have shown, including a recent one from UC Davis, that 96 percent of nitrates come from agriculture. Most at risk are lower income people, particularly those living in agricultural areas.

However, farmers told the board that the new rules would be so onerous and expensive that it would force some local farmers out of business. One farm group estimates that the most stringent rules would cost a farmer an additional $600 per acre per year.

Mike Stoker of the United Agribusiness League said the new rules are another layer of regulation that make it harder to do business in California.