Water Quality Assessment of a Degraded Stream Prior to Restoration and Nitrate Reduction through Controlled Drainage

Abstract

Best management practices are often necessary to minimize non point source water quality problems. The Core Creek Project implemented and evaluated best management practices (BMPs) throughout the upper watershed to reduce nutrient loading to the Neuse River. A portion of the Core Creek Project, involving the evaluation and restoration of a degraded, channelized, pasture stream, was initiated in 2003. Beef cattle (Bos taurus) had unrestricted access to the waterway, severely degrading the stream bank and streambed. In preparation for stream restoration, the existing water quality was monitored and assessed. Near the end of the background evaluation study, the beef cattle were removed from the pasture.

The nitrogen concentrations downstream of the pasture were generally lower than those upstream, but these differences were not significantly different. The downstream phosphorus concentration was generally higher than the upstream concentration. This difference was significant for orthophosphate. Fecal coliform levels were significantly higher downstream than upstream, before cattle exclusion. After exclusion, there was a statistically significant reduction in fecal levels downstream, dropping an average of 83%. The average daily and maximum daily water temperatures at the downstream sampling location were significantly higher by an average of 2.5o C than the upstream temperatures. Macroinvertebrate sampling indicated that the quality of the stream was severely stressed, both during 2003 and 2004.

A monthly grab sampling scheme predicted a significantly different nutrient load than continuous weekly composite sampling. Total Kjeldahl nitrogen, ammonium, total phosphorus, orthophosphate and total suspended solids were 47%, 95%, 49%, 62%, and 54% lower, respectively, using the monthly sampling protocol. Nitrate was 51% higher when using the monthly sampling scheme.

An additional component of the Core Creek Project evaluated the reduction of NO3-N leaving the Core Creek watershed due to the implementation of controlled drainage and nitrogen management plans. DRAINMOD-N was calibrated and extrapolated on a field by field basis to predict long-term hydrology and nitrate losses based on BMPs implemented and crops grown throughout the watershed.

Predicted nitrate-nitrogen reduction was most effectively accomplished when controlled drainage and a nitrogen management plan were used in conjunction with one another. If implemented separately, a nitrogen management plan was predicted to be more effective than controlled drainage alone. The cropping system also impacted the drainage rate and nitrate loss from the fields. Potentially, a 30% and 75% nitrate reduction can be achieved growing cotton or soybeans, respectively, as compared to corn. Controlled drainage reduced the drainage outflow by 6.8 centimeters (21.3%), annually, compared to conventional drainage. This flow reduction accounted for 11.5% of the NO3-N reduction leaving the watershed.