Herbicide transport in a restored riparian forest buffer system

Little is known about the effects of restored riparian forest buffers on transport of herbicides. The effect of a restored riparian forest buffer system (RFBS) on transport of two herbicides, atrazine and alachlor, was studied during 1993-1994. Herbicides were applied above a restored 3-zone riparian buffer system in April of 1993 and 1994. Bromide was applied as a tracer with the April 1993 herbicide application. The buffer system was managed based on USDA recommendations and averaged 38 tn in width. The system included a grass buffer strip immediately adjacent to the application area (zone 3), an area of planted pines downslope from the grass buffer (zone 2), and a narrow area of planted hardwoods containing the stream channel system (zone 1). Most of the herbicide transport in surface runoff occurred before June 30 with about 250 mm of cumulative rainfall after herbicide application. During this period of higher herbicide transport, atrazine and alachlor concentrations averaging 12.7 μg L-1 and 1.3 μg L-1, respectively, at the field edge were reduced to 0.66 μg L-1 and 0.06 μg L-1, respectively, as runoff neared the stream. The effect of dilution versus other concentration reduction factors (infiltration, adsorption) was estimated for surface runoff using the bromide concentration data. Concentration reduction was greatest per meter of flow length in the grass buffer adjacent to the field. There was only minor transport of herbicides through the buffer system in shallow groundwater. Average herbicide concentrations were at or below detection limits in groundwater near the stream for the entire study period. The restored riparian forest buffer had similar effects on herbicide transport as a mature buffer.