Surface water and groundwater quality impacts at a swine CAFO with a capped lagoon system in eastern North Carolina

Abstract

Waste from concentrated animal feeding operations (CAFOs) is traditionally stored in open lagoons and the liquid wastewater is periodically applied to nearby sprayfields. Capping lagoons with an impermeable cover have been proposed to reduce environmental impacts by inhibiting the release of odors and accidental overflows, but there is a lack of data evaluating how capping a lagoon affects surface water and groundwater quality near the sprayfields. This study analyzed concentrations and masses of nitrogen in surface water downgradient of sprayfields compared to upstream reaches. Furthermore, this study investigates nitrogen concentrations in groundwater beneath and downgradient from sprayfields compared to background groundwater. Water samples were collected monthly to quantify concentrations and/or masses of total dissolved nitrogen (TDN) and other dissolved nitrogen species between October 2019 to January 2021. Results indicated that surface water downstream of the farm contained a median TDN concentration that was 14 times greater (6.65 mg L⁻¹) than upstream (0.48 mg L⁻¹). Median flux of TDN downstream of the farm (32.1 kg ha¹ yr⁻¹) was 11 times greater and statistically different than upstream (2.8 kg ha¹ yr⁻¹). In-situ, high frequency monitoring performed over 2 weeks (8/14/22-8/31/22) showed and elevated concentration and range of TDN downstream of the site (1.9-137.9 mg L⁻¹) compared to upstream (0.1-4.8 mg L⁻¹), which was attributed to baseflow inputs and spray events that occurred 6 days after deployment. Increases in surfaced water TDN were likely from wastewater applications to sprayfields. The lagoon wastewater had a median TDN concentration of 485 mg/L and TDN concentration reductions of 94.4-99.6% were observed between the lagoon and the nearby groundwater. The median TDN concentrations of groundwater under sprayfields (5.0 mg L⁻¹) was 12 times higher than background (0.42 mg L⁻¹). Groundwater beneath the riparian buffers was also elevated (6.4 mg L⁻¹) and 15 times higher than the background groundwater. Despite major reductions between the lagoon waste and the sprayfields where the waste is applied, TDN concentration in groundwater remained elevated compared to background levels. These data suggest that land application of wastewater contributed to elevated TDN concentrations in water resources downgradient of the farm. Strategies to ameliorate nitrogen in subsurface and surface discharge from the CAFO may be needed to reduce net nitrogen exports and impacts from the farm.