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Fate and Efficacy of Polyacrylamide Applied in Furrow Imgation: Full-Advance and Continuous Treat... Page 2 of 16 Polyacrylamide was adsorbed to soil and removed from solution as the streams traversed the soil-lined channels. The removal rate increased with stream sediment concentration. Stream sediment concentrations were higher when PAM concentrations were <2 mg L-1 a.i., for early irrigations, and when untreated tributary flows combined with the stream. In these cases, PAM concentration decreased to undetectable levels over the flow lengths used in this study. When inflows contained >6 mg L-1 PAM a.i., stream sediment concentrations were minimal and PAM concentrations did not change down the furrow, though they decreased to undetectable levels within 0.5 h after application ceased. One percent of applied PAM was lost in tail-ditch runoff This loss could have been eliminated by treating only the furrow advance or not treating the last two irrigations. Abbreviations: Cont-1, 1 mg L-` polyacrylamide product (0.8 mg L-' a.i.) applied continuously to furrow inflows Initial-10, 10 mg L-1 polyacrylamide product (8 mg L-I a.i.) applied to initial irrigation inflows only • PAM, water- soluble anionic polyacrylamide - INTRODUCTION AN INCREASED AwARENSSS and heightened state and federal scrutiny of agriculture-related nonpoint-source contributions has encouraged producers in the western USA to reduce irrigation-induced erosion and runoff losses coming from their fields. Increasingly, farm managers aze adopting PAM technology as an effective, convenient, and economical means of reducing erosion and improving runoff water quality from furrow-irrigated and sprinkle-irrigated fields. One ~ TOP ~ ABSTRACT • INTRODUCTION ~ MATERIALS AND METHODS ~ NOTES ~ RESULTS AND DISCUSSION ~ CONCLUSIONS ~ REFERENCES practice recommended as a Natural Resources Conservation Service (MRCS) conservation standard applies 10 mg L-1 PAM product to irrigation water inflows only during the initial advance of water across the field, then untreated water is used to finish the remainder of the irrigation. We term this approach as the Initial-10. The Initial-10 treatment reduces runoff sediment, P, and N losses by 85 to 99%, lowers levels of chemical and biological oxygen demand in runoff by 83% (Lentz et al., 1992, 1998; Lentz and Sojka, 1994; Bahr et al., 1996), and decreases soil-sorbed pesticide losses in furrow runoff (Aeassi et al., 1995; Bahr et al., 1996). Polyacrylamide also reduced microbial biomass in furrow streams (Sojka and Entry, 2000). An alternative application strategy, termed Cont-1, continuously adds 1 mg L-1 PAM product to irrigation inflows during the entire irrigation period. The Cont-1 approach was initially employed in California and was preferred because it applied PAM to irrigation water at low concentrations and PAM's continual presence in the furrow stream may have better prevented soil loss during the late hours of the irrigation. Lentz and Soika (2000) reported that Initial-10 and continuous PAM applications of 1 to 2 mg L-1 controlled fiurow erosion similarly on 1.5% sloping fields. Polyacrylamide has low toxicity to aquatic and terrestrial organisms at concentrations used in this agricultural application (Barvenik, 1994; Deskin, 1996). Concerns about the use of PAM in irrigated agriculture persist, however, because it is not known whether applied PAM is transported via irrigation return flows to natural surface waters. Often, irrigation runoff from individual fields enters a wastewater ditch, which collects runoff and sediment from several farms. Some of the wastewater may be used by downstream irrigators. Some may enter a main irrigation return-flow channel, which ultimately conveys this water and runoff from other "subwatersheds" in the irrigation districtto anatural surface drainage. The linear PAM molecule assumes the form of a hydrated random coil when dissolved in water. Solvated PAM molecules in the firrrow stream collide with soil particles when treated water infiltrates into soil or when turbulent http://jeq.scijournals.org/cgi/content/fulU31/2/661 9/21/2006