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Fate and Efficacy of Polyacrylamide Applied in Furrow Irrigation: Full-Advance and Continuous Treat... Page <br />M1~wl~~~ 6 <br />~~ _ ]~ ~_ <br />w~M~ <br />~' <br />b <br />~* <br />~~l- ~ <br />b ~~ <br />'•~ b ~A Ib <br />WuK~ p~ M1.1w IN <br />View larger version <br />(40K): <br />[in this windowl <br />[in a new windowl <br />The rate of decrease in PAM concentration with distance downstream was greatest during the first 2 to 3 h of an <br />irrigation relative to Hours 7 through 12, and greatest during Irrigations 2 and 4 than for Irrigations 5 and 6 (FiQ. 3). <br />Thus, by the 7-h sampling time in Imgation 6, we observed no change in the PAM concentration as the stream <br />traversed the furrow. This pattem of changing PAM concentration paralleled that of furrow stream sediment. On <br />average, sediment concentrations in Cont-1 furrow streams progressively decreased with time; from 7.7 g L-~ during <br />the initial hours of the irrigations to 4.2 g L-~ at irrigations' end, and from 12.6 g L-~ in Irrigation 2 to 0.9 g L-~ in <br />Irrigation 6 (Table 4). Less PAM would be adsorbed to soil solids and be removed from furrow stream flows as <br />stream sediment concentration decreased. Therefore, we hypothesized that the flattening of PAM concentration versus <br />flow-distance relationship was generally due to the decreased availability of adsorbent (sediment) in the furrow <br />stream. Evidence in Irrigation 6 suggests that a second process may also have influenced the decline rate of furrow- <br />stream PAM concentration over time. Furrow-runoff PAM concentration in Cont-1 furrow streams decreased <br />significantly between middle and bottom sampling positions at 3 h, but not at 7 h (Fi¢. 3). Yet, stream sediment <br />concentrations were the same at the two times, so the PAM concentration differences observed at 3 and 7 h were <br />apparently not caused by a difference in the availability of entrained sediment adsorbent. Polyacrylamide absorbance <br />may have declined in response to a number oftime-related factors: (i) A number of physical and chemical <br />chazacteristics of the stream flow probably changed with time as a result of changing flow rates, which may have <br />decreased adsorption of dissolved PAM onto soil surfaces; and (ii) soil-lined channels may have a finite capacity for <br />nonequilibrium adsorption of PAM at the time scale imposed here (<12 h), and this adsorption capacity was <br />progressively filled over the period of PAM application. Thus, fewer PAM molecules were absorbed to the soil- <br />wetted perimeter as time progressed, and more incoming dissolved PAM moved downstream. <br />Recall that runoff collected from treated and nontreated furrows flowed into the 530-m-long tail-water ditch, where it <br />was sampled at the top, middle, and bottom positions at 2, 3, and 7 h; and at the end position at 2 h during each <br />irrigation. The tail-ditch end was sampled at 3 and 7 h during only two in•igations. When Irrigations 2, 4, 5, and 6 <br />were analyzed together, only time (P = 0.0001), and not field position (P = 0.14), or the time by position interaction <br />term (P = 0.39) significantly affected PAM concentration in tail-ditch flows. Mean tail-ditch PAM concentration at 2 <br />h was 0.9 mg L-~, when polymer was being applied to both Initial-10 and Cont-1 furrows, but was one-tenth of this 2- <br />hvalue at the 3-h (<0.1 mg L-~) and 7-h (0.1 mg L-~) sampling times, when PAM was being applied only to Cont-1 <br />fun•ows. A large portion of vaziability among irrigation responses was contributed from Irrigation 2, which exhibited a <br />response pattern quite different than those for Irrigations 4, 5, or 6 (Fie. 4} . A sepazate analysis for the late-season <br />http://jegscijournals.org/cgi/content/full/31/2/661 9/21/2006 <br />