Impact of grassed waterways and compost filter socks on the quality of surface runoff from corn fields.

Surface runoff from cropland frequently has high concentrations of nutrients and herbicides, particularly in the first few events after application. Grassed waterways can control erosion while transmitting this runoff offsite, but are generally ineffective in removing dissolved agrochemicals. In this study, we routed runoff from one tilled (0.67 ha) and one no-till watershed (0.79 ha) planted to corn (Zea mays L.) into parallel, 30-m-long grassed waterways. Two 46-cm-diam. filter socks filled with composted bark and wood chips were placed 7.5 m apart in the upper half of one waterway and in the lower half of the other waterway to determine if they decreased concentrations of sediment and dissolved chemicals. Automated samplers were used to obtain samples above and below the treated segments of the waterways for two crop years. The filter socks had no significant effect (P <or= 0.05) on sediment concentrations for runoff from the no-till watershed, but contributed to an additional 49% reduction in average sediment concentration compared with unamended waterways used with the tilled watershed. The filter socks significantly increased the concentrations of Cl, NO(3)-N, PO(4)-P, SO(4), Ca, K, Na, and Mg in runoff from at least one watershed, probably due to soluble forms of these ions in the compost. The estimated additional amounts of these ions contributed by the socks each year ranged from 0.04 to 1.2 kg, thus were likely to be inconsequential. The filter socks contributed to a significant (P <or= 0.05) additional reduction in dissolved glyphosate [N-(phosphonomethyl)glycine] (5%) and alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide] (18%) concentrations for the tilled watersheds, but this was insufficient to reduce alachlor concentrations to acceptable levels.