Effectiveness of nitrate reduction in riparian buffers: A riparian buffer hydrologic and biogeochemical evaluation

Defining ideal placement of buffers enrolled in NC CREP could maximize stream miles protected, to improve downstream water quality in sensitive streams and estuaries. During the past five years effects on nitrate reduction efficiency of a riparian buffer enrolled in the North Carolina Reserve Enhancement Program (NC CREP) has been evaluated. The buffer width was 40-45 m, and groundwater monitoring well nests were installed in three transects within the buffer. Each well nest contained a shallow (1.5-2.1 m) and deep (2.7-3.4 m) well. Additional wells were installed in the deeper aquifer to measure the quality of that water for comparison to the more shallow wells. Upslope agricultural practices have included soybeans, peanuts, cotton and corn. Nitrate decreased through the buffer from Zone 3 (grassed filter strip) to Zone 1 (stream edge) with average nitrate concentrations of 12.7 to 5.9 mg/l and 13.0 to 1.4 mg/l for deep and shallow wells respectively. Water table measurements, nitrate to chloride ratios, deep well water quality analyses, topography, redox measurements and dissolved organic carbon (DOC) were used to determine whether the primary mechanism for these decreases was denitrification or groundwater dilution. Preliminary results show that both contributed to nitrate reductions in the system, but denitrification was the main reducing mechanism. An advanced understanding of the hydrologic and biogeochemical factors in riparian buffers will lead to design recommendations that will enhance pollutant reduction in these treatment systems.