This project was completed in 2001. These pages are for historical purposes only.
In North Carolina, about 40 percent of the cropland requires drainage improvements to increase agricultural production in poorly drained soils. A common practice for improving drainage is to install ditches (Pictures 1, 2) and subsurface tile drains (Pictures 3, 4) to lower the water table beneath agricultural fields. These drainage improvements increase the amount of land available for cultivation; however, the process of redirecting shallow ground water beneath agricultural fields through tile drains to ditches can alter the quality of drainage water exiting the fields to receiving streams. Nitrate-nitrogen (NO3) concentrations ranging from about 5 to 50 milligrams per liter (mg/L) in tile drainage water have been noted in various studies.
In eastern North Carolina, excessive nutrient loadings have contributed to the degradation of surface-water quality in the Neuse River basin. The North Carolina Environmental Management Commission adopted rules in 1997 to reduce nitrogen loads in the Neuse River by 30 percent to support the Neuse River Nutrient Sensitive Waters Management Strategy. For agricultural land, several combinations of best management practices (BMPs), including nutrient management, controlled drainage, forested riparian buffers, and vegetative filter strips, were proposed for reducing nutrient loads. Because tile drains and ditches are constructed channels that artificially intercept the water table, they can allow ground water containing agricultural chemicals beneath cultivated fields to bypass natural streamside buffers and organic carbon-rich streambeds that normally would reduce NO3 in the ground water before it discharges to streams. Subsurface tile drains are considered a major pathway for NO3 transport to surface water in some agricultural watersheds.