California Agriculture

Widespread contamination of surface water with nitrate-nitrogen (NO₃-N) has led to increasing regulatory pressure to minimize NO₃-N release from agricultural operations. We evaluated the use of wood chip denitrification bioreactors to remove NO₃-N from tile drain effluent on two vegetable farms in Monterey County. Across several years of operation, denitrification in the bioreactors reduced NO₃-N concentration by an average of 8 to 10 milligrams per liter (mg L^-1) per day during the summer and approximately 5 mg L^-1 per day in winter. However, due to the high NO₃-N concentration in the tile drainage (60 to 190 mg L^-1), water discharged from the bioreactors still contained NO₃-N far above the regulatory target of < 10 mg L^-1. Carbon enrichment (applying soluble carbon to stimulate denitrifying bacteria) using methanol as the carbon source substantially increased denitrification, both in laboratory experiments and in the on-farm bioreactors. Using a carbon enrichment system in which methanol was proportionally injected based on tile drainage NO₃-N concentration allowed nearly complete NO₃-N removal with minimal adverse environmental effects.