HERBICIDE TRANSPORT IN A MANAGED RIPARIAN FOREST BUFFER SYSTEM

The effect of a riparian forest buffer system (RFBS) on transport of t wo herbicides, atrazine and alachlor, was studied during 1992-1994. He rbicides were applied to an upland corn crop in March of each year The buffer system was managed based on USDA recommendations and averaged 50 m in width. The system included a grass buffer strip immediately ad jacent to the field (Zone 3); a managed pine forest downslope from the grass buffer (Zone 2); and a narrow hardwood forest containing the st ream channel system (Zone I). After the first year of the study, the m anaged forest was clear-cut in 1/3 and thinned in 1/3 of Zone 2. The o ther 1/3 of Zone 2 was left as mature for est. Most of the herbicide t ransport in surface runoff occurred before 30 June with about 25 cm of cumulative rainfall after herbicide application. During this period o f higher herbicide transport, atrazine and alachlor concentrations ave raging 34.1 mu g L-1 and 9.1 mu g L-1 at the field edge, respectively, were reduced to 1 mu g L-1 or less as runoff neared the stream. There were generally no differences among the mature forest and the two tre atment areas (clear-cut and thinned) for either concentration or load in surface runoff Using precipitation data collected on site, the effe cts of dilution versus other concentration reduction factors (infiltra tion, adsorption) was estimated for surface runoff. Concentration redu ction was greatest per meter of flow length in the grass buffer adjace nt to the field. There was only minor transport of herbicides through the buffer system in shallow groundwater and little difference between the Zone 2 treatment areas. In 1992 and 1993, herbicide concentration s in shallow groundwater in the RFBS and at the edge-of-field were gen erally at or below detection limits. In 1994, well concentrations of b oth herbicides increased, probably in response to infiltration of surf ace runoff containing high herbicide concentrations. Average herbicide concentrations were at or below detection limits in groundwater near the stream for most of 1994. Keywords. Herbicides, Pesticides, Runoff Riparian forests, Grass buffer,strips.