Effects of a managed three zone riparian buffer system on shallow groundwater quality in the southeastern Coastal Plain

ABSTRACT:

Riparian forest buffers can help improve agricultural water quality. USDA guidelines are for riparian forest buffers of three zones. Zone I is permanent woody vegetation near the stream. Trees can be harvested in Zone 2, which is upslope from Zone 1. Zone 3 is a grass filter upslope from Zone 2 at field edge. In order to test USDA guidelines, a site was established in the southeastern Coastal Plain near Tifton, Georgia, with an 8 m wide grass buffer (Zone 3) situated between a field and a mature Riparian forest. In the Zone 2 forest, mostly 50 year-old pine trees, one block was harvested by clearcut, one block was thinned, and one block was left as a mature forest control. Care was taken to minimize soil disturbance during the timber harvest operation. The Zone I forest [15 m wide (49 ft)] was Left undisturbed Shallow groundwater wells were used to monitor the effects of the managed Riparian forest buffer on N, P, and Cl concentrations. Groundwater nitrate concentrations decreased from 11 to 22 mg L-¹ adjacent to the field to less that 2 mg L⁻¹ at 5 m (16 ft) into the forest. Nitrate concentration de- creased under the grass filter strip as well as in the forest. Nitrate concentrations increased in one corner of the Riparian forest near the stream. This increase may be due to flow patterns of groundwater that bypasses the Riparian forest buffer. Chloride concentrations increased under the buffer indicating that the nitrate removal was due to biological processes such as plant uptake and denitrification rather than dilution. Concentrations of other potential pollutants such as ortho-p, ammonium, and organic N moved in veiy small quantities and did not show consistent spatial patterns. There was no effect due to harvesting of the Zone 2 forest on either nutrient concentrations or water table elevations. These results indicate that Zone 2 trees, along small streams in the southeastern coastal plain, can be harvested with little effect on groundwater nutrient movement to streams.