Feasibility of Using Woodchip Bioreactors to Treat Legacy Nitrogen to Meet Chesapeake Bay Water Quality Goals.

Abstract

The United States Environmental Protection Agency has established aggressive nutrient reduction goals to achieve water quality objectives for the Chesapeake Bay estuary. Nitrogen (N) reduction goals are proving particularly difficult to meet with an additional 20.4 million kg of annual nitrogen reductions needed by 2025, and many of the easily achievable and low-cost N reductions have been realized. We assess the feasibility of employing woodchip denitrifying bioreactors to treat legacy N derived from spring discharge in the Mid-Atlantic region. We estimate that in excess of 6100 kg of soluble N is discharged daily from United States Geological Survey identified springs in four Mid-Atlantic states within the Chesapeake Bay watershed. Based on typical bioreactor removal efficiency (30-55%) and potentially treatable flows (<6000 m3/d), widespread adoption of bioreactors to treat legacy N from 231 springs could conservatively result in 420-770 kg N removed per day, while strategic adoption targeting 48 springs with N concentrations of at least 3 mg/L and flows of at least 500 m3/d could result in 322-590 kg N removed per day more cost-effectively and with far fewer installations. A cost analysis indicates bioreactors can be a cost-effective N removal strategy, generally removing N for less than $5/kg·y. Relative to other nonpoint source pollution control practices, bioreactors also offer the ability to remove larger quantities of N per installation and are more easily monitored to quantify N reductions.