Tracing nutrient pollution from industrialized animal production in a large coastal watershed

One of the highest concentrations of swine and poultry concentrated animal feeding operations (CAFOs) in North America is located on the Coastal Plain of North Carolina, in which the Cape Fear River basin is located. The CAFOs produce vast amounts of manure causing loading of nutrients and other pollutants to receiving waters. With the Cape Fear River basin vulnerable to nutrient pollution, as are many other watersheds with CAFOs, δ¹³C and δ¹⁵N stable isotopic signatures were identified from water samples collected within the Northeast Cape Fear, Black, and lower Cape Fear River watersheds to trace nutrient sources and their distribution downstream. The spatial and temporal variability of nutrients and isotopic signatures were also identified to understand water quality impacts of animal waste spraying season and proximity to CAFOs. Our results showed that significantly enriched δ¹⁵N signatures characterized sites in close proximity to CAFOs as well as point-source wastewater discharge areas, while the opposite was true for an unimpacted control stream and two estuarine sites. Additionally, the impacted sites yielded significantly (p < 0.05) higher nitrate concentrations than control and estuarine sites. Statistical analyses demonstrated that nitrate concentrations were positively correlated with heavier δ¹⁵N signatures, suggesting that animal waste, as well as human wastewater, are relatively more important sources of N to this large watershed than fertilizers from traditional row crop agriculture. Our results also suggested that during appropriate hydrological conditions CAFO-derived N can be detected many kilometers downstream from freshwater sources areas to the estuary.