The export of agricultural contaminants from agricultural landscapes of the US Midwest has contributed to the impairment of surface waters throughout the Mississippi River Basin and has been linked to various human health concerns. Natural treatment systems (wetlands, bioswales, bioreactors) can capture agricultural runoff and significantly reduce nutrient loading to downstream waters but there is a paucity of data on the effectiveness of these treatment systems to attenuate the suite of pollutants (nutrients and synthetic organics) typically found in agricultural runoff. This understanding is important given that the degradation of different pollutants involves metabolic pathways that often require different redox environments. As part of the Aquisafe-2 project, a bioretention swale comprising two treatment cells (a subsurface cell in series with a surface cell) was monitored, and its performance evaluated over a three-year period (2011 - 2013). Results showed that the bioswale was moderately efficient with regard to nitrate (NO3-; retention range: 16-58 %). N removal averaging 30 % was measured during a series of wetting events during which the bioswale operated at an estimated average hydraulic retention time (HRT) of 0.97 day. Spatial analysis of the data showed that almost all the NO3- removal occurred in the subsurface cell; however, N removal was also measured in the surface cell under low flow conditions (estimated HRT: 2.5 days). The highest rates of N removal (~ 58 %) were measured when the bioswale stayed wet for several days probably due to the development of a more optimum environment for denitrifying microbes. Nitrate removal capacity was limited by NO3- availability, short retention times during high flows, and the frequent fluctuation between oxic and anoxic conditions, but not by water temperature (8.3-16.6 oC) and dissolved organic carbon (DOC; 1.9 - 29.2 mg C L-1). The bioswale performance with regard to soluble reactive phosphorus (SRP) and atrazine was more variable, with net retention during some periods and net release at other times. The bioswale was a net source of P during most sampling periods with an average SRP release corresponding to 13 % of input, probably due to desorption of water soluble P from the topsoil applied during construction. This interpretation is supported by the progressive decline in P release observed between the first and third year of monitoring. The subsurface and the surface cells contributed almost equally to the fate of P in the bioswale. Likewise, the bioswale was at times a small/moderate sink (13-31 % retention) for atrazine, and a net source (-38 % to -15 %) during periods when the bioswale received overland runoff from the adjacent crop field which bypassed the subsurface cell. Results suggested that competition between atrazine and DOC for sorption sites is a possible mechanism affecting atrazine removal efficiency. Additional work is needed to compare the efficiency of the subsurface and surface cells with regard to atrazine, and elucidate the biogeochemical factors controlling its fate in the bioswale.

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