A pollutant that has leached into California aquifers since farmers first began using synthetic fertilizer continues to accumulate and would not be removed from groundwater even if the state’s agriculture businesses abruptly quit using nitrogen-based materials to boost the productivity of their crops. That’s one of the themes of a new study from the UC Davis Agriculture Sustainability Institute that assesses the scale and sources of a kind of pollution that can harm infants if it seeps into groundwater and contributes to respiratory problems if it drifts into the air as a gas.