Denitrification drainage bioreactors are emerging as an innovative practice to address water quality concerns stemming from nitrate leaching from drained agricultural lands. Although installation of these systems has begun in farms in the Midwestern United States, the understanding of their design and in-field performance remains deficient. This study utilized a set of pilot-scale drainage bioreactors to evaluate the impact of bioreactor geometry on reactor hydraulic properties and to determine nitrate removal under steady state conditions and during a simulated storm event. Bioreactors with different cross-sectional geometries but similar depths and total volumes were evaluated. Percent reduction of the influent nitrate mass was linearly correlated to the theoretical hydraulic retention time (HRT) with 30 to 70% NO3--N removals observed within the four to eight h of retention time suggested for field installations. Tracer tests revealed that in-situ HRTs were substantially greater (i.e. at least 1.5 times as large) than theoretical HRTs.