Impact of Agriculture on Water Quality in the North Carolina Middle Coastal Plain

Water quality in the Middle Coastal Plain of North Carolina has been impacted by agriculture; however, the water quality impacts in these areas over time have not been studied in detail. The surface water quality of several streams in the Neuse River Watershed along the Middle Coastal Plain of North Carolina was monitored for approximately five years, while shallow groundwater in cropped fields and adjacent to drainage ditches was monitored for three years. Surface water samples were collected biweekly and analyzed for nitrate nitrogen $\left(\mathrm{N}{\mathrm{O}}_3\text{-}\mathrm{N}\right)$ , ammonium nitrogen $\left(\mathrm{N}{\mathrm{H}}_4\text{-}\mathrm{N}\right)$ , total Kjeldahl nitrogen, orthophosphate $(\mathrm{P}{\mathrm{O}}_4-\mathrm{P})$ , total phosphate, and total suspended solids, and approximately monthly groundwater samples were collected from wells and analyzed for $\mathrm{N}{\mathrm{O}}_3-\mathrm{N}$ . Trends relating to seasonal changes in stream water quality as a function of land use and soil type were analyzed, as well as long term changes. Generally, upstream sampling points showed low levels of all constituents. Nutrient and sediment concentrations increased along the streams draining the agricultural lands. Trends in surface water quality did not change under row crops, and $\mathrm{N}{\mathrm{O}}_3-\mathrm{N}$ averaged $3.4–4.1\mathrm{mg}∕\mathrm{L}$ , while $\mathrm{N}{\mathrm{H}}_4-\mathrm{N}$ averaged $0.1\mathrm{mg}∕\mathrm{L}$ . Surface water was impacted dramatically by seepage from an anaerobic swine wastewater lagoon. $\mathrm{N}{\mathrm{O}}_3-\mathrm{N}$ and $\mathrm{N}{\mathrm{H}}_4-\mathrm{N}$ downstream from the seepage plume averaged 1.5 and $12.7\mathrm{mg}∕\mathrm{L}$ , respectively. Directly down gradient of the lagoon, groundwater samples averaged $121\mathrm{mg}∕\mathrm{L}$ for $\mathrm{N}{\mathrm{H}}_4-\mathrm{N}$ . Groundwater $\mathrm{N}{\mathrm{O}}_3-\mathrm{N}$ concentrations decreased 30–76% due to a reduced zone along many of the ditches. Biomass production in the drainage ditches resulted in substantial amounts of organic nitrogen in the ditches. Surface water nitrate concentration averaged at or above the limit for new wastewater treatment plants on the Neuse River. Groundwater nitrate concentration in the shallow groundwater averaged $4.6–9.3\mathrm{mg}∕\mathrm{L}$ ; however, in several locations on the farm, groundwater nitrate nitrogen concentration was elevated above the $10\mathrm{mg}∕\mathrm{L}$ U.S. Environmental Protection Agency limit for drinking water due to fertilization of crops and pastures. Hurricanes in 1999 flushed nutrients out of the soil profile in areas that were flooded along the river.