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Fate and Efficacy of Polyacrylamide Applied in Furrow Irrigation: Full-Advance and Continuous Treat... Page <br />irrigations showed that tail-ditch position influenced PAM concentration at the 2-h sampling time. The 2-h tail-ditch <br />PAM concentrations averaged 1.6 mg L-~ at top, middle, and bottom positions but had decreased to 0.28 mg L'~ at the <br />tail-ditch end (Fie. 4). <br /> <br />D ~^ <br />eap DSa Da «w <br />o.~ ti,-s-- - -- - --- - <br />D.z ~I <br />i}~.::.:, _. _. _. <br />O.D `~~~~'-- .~r1 <br />~.5 <br />to ~~I11 tr.65. ~ 0 l L \\\ <br />0.0 iii ..:y1~.~.~~ ~ ~ ~~T <br />0 Y50 100 150 200 500 <br />View larger version (56K): <br />[in this windowl <br />[in a new windowl <br />Fig. 4. Polyacrylamide (PAM) concentrations in tail-ditch streams by <br />sampling position and time for Irrigation 2 and Irrigations 4, 5, and 6. Note <br />break and change in x axis scale. MDL, method detection limit. <br />Polyacrylamide concentration patterns in the tail ditch were similaz to those for furrows: (i) Tail-ditch PAM <br />concentration declined rapidly following the reduction in furrow inflow PAM concentration; and (ii) PAM <br />concentration did not decrease as rapidly with distance downstream as the season progressed. This result supported <br />the hypothesis that the rate of PAM concentration diminution with flow distance declined as stream sediment <br />concentration decreased over the irrigation season (Table 4). However, it was noted that the PAM concentration did <br />not decline along the upper 90 m of Cont-1 fiurows during Irrigation 6 at 2 h (FiQ. 3), nor along the upper 90 m of the <br />tail-water stream during Irrigations 4, 5, and 6 at 2 h (Fie. 4). These responses were similar despite the fact that mean <br />sediment concentrations in the tail ditch were 10 times that for Cont-1 furrows (Table 4). This suggested that other <br />factors influenced the rapidity of PAM removal or adsorption from the furrow stream as it flowed downfield. Physical <br />and chemical characteristics of the stream flow can change with distance downstream as a result of infiltration and a <br />declining flow rate, and these may have influenced polymer dynamics. Or, it may simply be that the adsorption <br />capacity of sediment entering the tail ditch was already nearly saturated and thus had little effect on the dissolved <br />PAM it encountered in the stream. <br />Polyacrylamide Loss Rate in Furrow and Wastewater Streams <br />Statistics for PAM mass-loss rate paralleled those for concentration. Main effects, treatment, furrow field position, <br />and time significantly influenced PAM mass-loss rates (Table 3). Main effect interaction terms were also significant <br />for all irrigations, except interactions treatment by position and treatment by position by time for Irrigation 6. For any <br />given irrigation and sampling time, mass-loss rates decreased with distance downfield. As furrow streams traversed <br />the field, increasing infiltration opportunity produced flow-rate reductions. The flow-rate decrease with distance <br />downfield caused moderate declines in PAM mass-loss rates of Initial-10 furrows (Fi¢. 5) . Polyacrylamide mass-loss <br />rates for Cont-1 furrows decreased more rapidly with distance down furrow than for Initial-10. The reason was that <br />PAM concentration in continuously treated furrows decreased down furrow (Fi¢. 3), while downstream <br />concentrations in Initial-10 furrow streams were constant (FiQ. 2). <br />http://jegscijournals.org/cgi/contenUfulU31/2/661 9/21/2006 <br />