California Agriculture

Widespread contamination of surface water with nitrate-nitrogen (NO ₃ -N) has led to increasing regulatory pressure to minimize NO ₃ -N release from agricultural operations. We evaluated the use of wood chip denitrification bioreactors to remove NO ₃ -N from tile drain effluent on two vegetable farms in Monterey County. Across several years of operation, denitrification in the bioreactors reduced NO ₃ -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 NO ₃ -N concentration in the tile drainage (60 to 190 mg L ^-1 ), water discharged from the bioreactors still contained NO ₃ -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 NO ₃ -N concentration allowed nearly complete NO ₃ -N removal with minimal adverse environmental effects.