Upflow Reactors for Riparian Zone Denitrification

We used permeable reactive subsurface barriers consisting of a C source (wood particles), with very high hydraulic conductivities (∼0.1–1 cm s −1 ), to provide high rates of riparian zone NO 3 –N removal at two field sites in an agricultural area of southwestern Ontario. At one site, a 0.73‐m 3 reactor containing fine wood particles was monitored for a 20‐mo period and achieved a 33% reduction in mean influent NO 3 –N concentration of 11.5 mg L −1 and a mean removal rate of 4.5 mg L −1 d −1 (0.7 g m −2 d −1 ). At the second site, four smaller reactors (0.21 m 3 each), two containing fine wood particles and two containing coarse wood particles, were monitored for a 4‐mo period and were successful in attenuating mean influent NO 3 –N concentrations of 23.7 to 35.1 mg L −1 by 41 to 63%. Mean reaction rates for the two coarse‐particle reactors (3.2 and 7.8 mg L −1 d −1 , or 1.5 and 3.4 g m −2 d −1 ) were not significantly different ( p > 0.2) than the rates observed in the two fine‐particle reactors (5.0 and 9.9 mg L −1 d −1 , or 1.8–3.5 g m −2 d −1 ). A two‐dimensional ground water flow model is used to illustrate how permeable reactive barriers such as these can be used to redirect ground water flow within riparian zones, potentially augmenting NO 3 − removal in this environment.