Nitrate removal in riparian wetland soils: Effects of flow rate, temperature, nitrate concentration and soil depth

Riparian zones, located adjacent to intensely managed agricultural fields, are thought to play an important role in removal of nutrient contaminants including NO−3 from groundwater. We studied the effect of flow rate, NO−3 concentration and temperature on NO−3 removal in soil columns under saturated-flow conditions. Bibb (coarse-loamy, siliceous, acid thermic Typic Fluvaquent) sandy loam soil was collected from a riparian forest located in Nomini Creek Watershed, Virginia. Soils included in the study were a permanently inundated surface horizon, a seasonally saturated surface horizon, a shallow subsurface horizon and a deep subsurface horizon. Soil columns were infiltrated with NO−3 amended groundwater at concentrations from 14 to 36 mg NO−3-N L−1. Column operating temperatures varied between 8 and 20°C and flow rates between 0.01 and 0.09 mL min−1. Following a 48 h equilibrium period, effluent NO−3 and N2O concentrations were determined. Denitrification was the primary mechanism of NO−3 removal, with higher denitrification capacities found in the surface horizons. Effluent NO−3 concentrations could be described by a linear combination of temperature, flow rate and influent NO−3 concentrations. Low temperatures and increased flow rates reduced the denitrification capacity in all soils. Our results showed that the NO−3 removal capacity present in the Bibb soil should theoretically be sufficient to remove most, if not all, NO−3 from the groundwater at the Nomini Creek study site. However, on-site measurements of NO−3 concentration in receiving streams indicated that this capacity is not fully realized in the field, suggesting the importance of other factors such as local hydrology and groundwater flow patterns.