Reducing Nitrate Loading From Tile Drain Effluent Through the Use of Denitrification Bioreactors.

The Salinas Valley on California’s central coast is the primary source of leafy greens production in the U.S.  Intensive production of these crops has been linked to elevated nitrate-nitrogen (NO₃-N) concentration in both surface water and groundwater in this region, and a nutrient total maximum daily load (TMDL) process has recently been initiated.  Tile drains are used extensively in parts of the Valley, and NO₃-N concentration > 50 mg L^-1 in tile drain effluent is common.  While substantial improvements over current irrigation and fertilization practices are possible, no realistic combination of agronomic practices will consistently reduce the NO₃-N concentration of tile drain effluent to the Federal drinking water standard of 10 mg L^-1.  In 2010 two pilot-scale denitrification bioreactors (DBRs) were constructed on commercial farms, using chipped construction wood waste as the source of labile carbon.  Tile drain effluent was continuously pumped through the reactors at a rate that achieved approximately two days residence time.  Averaged across sites the mean denitrification rate was approximately 6 g N m^-3 day^-1 during the 2011 crop production season.  The DBRs were carbon limited; injection of soluble carbon (from molasses) increased the denitrification rate > 40%.  Companion studies in laboratory-scale bioreactors showed that other carbon substrates (straw and fir bark) supported higher denitrification rates than did the chipped wood waste, but the economic and operational feasibility of utilizing the more active substrates is questionable.  We conclude that while DBRs can remove a substantial mass of NO₃-N from tile drain effluent, attainment of a 10 mg L^-1 environmental goal is impractical without significant reduction in tile drain NO₃-N concentration through improved agronomic practices.