Capping a swine farm wastewater lagoon: preliminary evaluation of water quality impacts in eastern North Carolina

Concentrated animal feeding operations (CAFOs) have at times been linked to the degradation of water quality in nearby ground- and surface waters. Wastes produced by animals at swine CAFOs are often stored in an open lagoon (or pit) with effluent periodically extracted and applied to sprayfields. There has been a move toward covering lagoons to prevent the release of odors, which may also reduce or inhibit accidental overflows as well as nitrogen volatilization and atmospheric redeposition. Few studies have investigated how capping a lagoon may impact nitrogen concentrations near the lagoon and sprayfields receiving land application of biosolids. The goal of this study is to quantify total dissolved nitrogen (TDN) concentrations in surface and groundwater and watershed nitrogen exports from a CAFO site where the lagoon was capped in 2008. Ongoing monthly data collection began in October 2019. During each sampling event, water samples were collected and analyzed for TDN among other constituents. The median TDN concentration of samples collected downstream of the farm is 6.91 mg L^-1, which is 16 times greater than for samples collected upstream (0.43 mg L^-1). Groundwater samples collected downgradient of the lagoon and sprayfields contain a median TDN concentration of 4.85 mg L^-1, which is 40 times greater than background groundwater collected in a nearby area assumed to be unimpacted by the lagoon and sprayfields (0.12 mg L^-1). Mass fluxes of nitrogen in a stream adjacent to the farm were substantially greater downstream (32.8 kg yr^-1 ha^-1) than upstream (1.73 kg yr^-1 ha^-1) of the study site. These preliminary data suggest that the farm is a potential source of nitrogen to the watershed, despite the historical capping of the lagoon. Additional nitrogen amelioration practices, such as saturated buffers, vegetated buffers, wood chip bioreactors, etc., may be necessary to reduce nitrogen concentrations and mass fluxes from this and other farms using capping systems. Future work will include analysis of isotope data to identify possible nitrate sources and estimates of groundwater mass transports.