Drainage Water Recycling Reduced Nitrogen, Phosphorous, and Sediment Losses from a Drained Agricultural Field in Eastern North Carolina, U.S.A

Abstract

An experimental study was conducted to evaluate the effect of drainage water recycling (DWR) on reducing nitrogen, phosphorous, and sediment losses from agricultural fields to downstream surface water bodies. The two-year study (May 2019-April 2021) was conducted at an agricultural field site in eastern North Carolina. Two treatments were implemented at the study site: DWR treatment and control treatment (CT). A reservoir existed between the two treatments was used to store subsurface drainage and surface runoff water from the field during wet periods, and provide supplemental irrigation to the DWR treatment during dry periods. On average, the reservoir retained 14% of received inflow, with a higher flow reduction in the dry year (2019-2020; 29%) than the wet year (2020-2021; 8%). The reservoir significantly reduced nitrate-nitrogen (NO 3 -N), ammonium-nitrogen (NH 4 -N), and total nitrogen (TN) concentrations by 1.48 mg L -1 (53%), 0.10 mg L -1 (51%), and 1.47 mg L -1 (40%), respectively. The mass loadings of NO 3 -N, NH 4 -N, and TN were reduced by 59%, 56%, and 47%, respectively. Orthophosphate (Ortho-P) concentration was significantly reduced by 0.07 mg L -1 (46%), while total phosphorous (TP) concentration reduction was not significant. In contrast, particulate phosphorous (PP) concentration in the reservoir outflow was significantly higher than that in the inflow by 0.02 mg L -1 . The mass loadings of Ortho-P and TP were reduced by 52% and 30%, respectively. Sediment concentration was significantly reduced by 273 mg L -1 (86%) and the sediment loading was also reduced by 87%. Results suggested that the removal efficiency of the reservoir would be highest during the summer and early fall months when the reservoir has a relatively smaller water volume, due to irrigation withdrawal and warmer temperature. In addition, the expected water quality benefits from the DWR systems would be highly correlated with the crop water requirements that affect the reservoir water storage. This study clearly demonstrated the potential of DWR for significantly reducing nitrogen, phosphorus, and sediment losses from agricultural land to receiving surface water bodies. Further research is needed to investigate the different physical, chemical, and biological processes that occur in the storage reservoir and affect the fate and transport of nutrients and sediment. The understanding of these processes will enable optimizing the treatment efficiency of DWR, which maximizes the system’ benefits and reduces construction cost.

Keywords: Subsurface drainage, Subirrigation, On-farm water storage, Drainage water reuse, supplemental irrigation, surface water quality