Nitrate-N removal using slowly released molasses barrier in a shallow aquifer: Obstacles from lab/pilot-scale results to field application

Abstract

This study was conducted to test the field applicability of a well-type barrier system containing Slowly Released Molasses (SRM) as an extra carbon and energy source to promote the indigenous bacterial denitrification activity. A total of 22 wells were placed as a grid system in the 4 m x 4 m square by 1-m interval in one alluvial area. A total of 70 SRM rods were placed in the center of the grid system to consist of a field- scale SRM system. Nitrate-N plume (72 mg N L-1 of nitrate-N) was introduced into the SRM system and change in nitrate-N concentration was monitored for 90 hours. Nitrate-N concentration decreased up to only 39% while 79-85% of removal efficiency was attained from lab/pilot experiments. Such a difference probably resulted from heterogeneity/anisotropy of the aquifer, DO concentration in the shallow groundwater, and a wide SRM rods-containing well distance. Spatial heterogeneity/anisotropy induced the preferential groundwater flow paths in which nitrate plume moved so fast and met the limited quantity of molasses molecules within or near the paths. Relatively aerobic condition of the shallow aquifer had not changed during the test period. Relatively far distance (100 cm) between the wells led to the incomplete mixing of the released molasses and nitrate-N plume. A detailed characterization of a target site should proceed before applying the SRM system for more effective nitrate-N treatment in the field.