Riparian Buffer Effectiveness in Removing Groundwater Nitrate as Influenced by Vegetative Type

Abstract

Nonpoint source contributions of nitrogen, particularly from agriculture, have become a serious concern for many watersheds in North Carolina. Recent regulatory action has increased the implementation of various best management practices (BMPs), particularly riparian buffer zones, for the purpose of reducing groundwater NO3-N pollution. However, the best design for such buffers has been the subject of great debate. The objectives of this project were to evaluate the relative effects of buffer vegetation and width on groundwater NO3-N removal and to determine if denitrification was the process most responsible. The main project consisted of four identically-designed buffer replications located on a farm in the Coastal Plain of North Carolina.

The influence of vegetative type on buffer NO3-N concentration decreases were as follows; trees had an average decrease of 57% (from 8.79 to 3.78 mg NO3-N L-1), fescue had a decrease of 40% (from 6.33 to 3.77 mg NO3-N L-1), switchgrass had a decrease of 44% (from 5.52 to 3.09 mg NO3-N L-1), native vegetation had a decrease of 37% (from 6.47 to 4.07 mg NO3-N L-1), and the no-buffer control had a decrease of 27% (from 4.93 to 3.62 mg NO3-N L-1). These calculations are averages for each vegetation type from all of the wells from both widths and depths from all four buffer replications.

For the 8 m buffer width, a total average NO3-N concentration decrease of 12% (from 9.97 to 8.75 mg NO3-N L-1) was observed for the intermediate well depth, while a 54% (from 5.26 to 2.41 mg NO3-N L-1) was observed for the deep well depth. For the 15 m buffer width, a total average NO3-N concentration decrease of 59% (from 6.42 to 2.61 mg NO3-N L-1) was observed for the intermediate well depth, while a 75% (from 4.31 to 1.06 mg NO3-N L-1) was observed for the deep well depth.

Despite these apparent observed differences in the NO3-N concentration decreases, there were no overall statistically significant differences (p>0.05) between any of the vegetation types or between the two buffer widths or depths. The lack of significance is due to the variability of the results observed between the four buffer replications.

An evaluation for buffer dilution using NO3-N:Cl ratio comparisons revealed that dilution appears to be a slight, if not inconsequential, factor in observed NO3-N concentration decreases. Redox monitoring probe results revealed low redox potential (Eh) values, indicating that substantial denitrification potential was present in all three of the buffer replications evaluated for redox. Dissolved organic carbon (DOC) concentrations indicate that the site has relatively low carbon present (overall average 3.1 mg L-1) and is considered to be an important limiting factor in the overall nitrate removal ability of the buffers.

Additionally, a second riparian buffer study was conducted on a farm in the North Carolina Mountains to compare vegetative effect with the Coastal Plain. This buffer, with four vegetative treatment types of shrubs, fescue, native vegetation, and a no-buffer control was installed in April 2004 after one year of pre-buffer groundwater monitoring. Preliminary results are mixed and may be the result of significant preferential groundwater flow paths caused by the very rocky nature of the soil on site. However, the vegetation is not yet fully established in the buffer and the monitoring will continue in an effort to determine if any NO3-N removal trends develop between the vegetation types.