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Fate and Efficacy of Polyacrylamide Applied in Furrow Imgation: Full-Advance and Continuous Treat... Page 3 of flow drives the molecules against entrained sediment or the wetted soil perimeter. The dissolved high moleculaz weight polymers are readily adsorbed to soil particles via electrostatic, hydrogen, and chemical bonding, and by displacement of inner solvation-sphere water molecules (LaMer and Healy, 1963; Mortland, 1970; Jin et al., 1987; Malik et al., 1991; Laird, 1997). As a result, incoming PAM is bound to soil in the upper 1 to 5 cm of the profile (Malik et al., 1991). Dry soil adsorbs more polymer than wet soils because sorbed water reduces the number of potential soil binding sites (Chang et al, 1991). Polyacrylamide is adsorbed to and flocculates soils suspended in water. Polymer adsorption on soil occurs rapidly during the first minutes of exposure, but may continue at a reduced rate for several hours or days (Van de Ven, 1994). In batch tests (soil, water, and dissolved PAM mixed in a shaker), Nadler et al. (1992) reported that little or no polymer desorbed from the soil while it remained wet, and the polymer became irreversibly bonded to the soil upon drying. In flowing systems, Lee and Fuller (1985} found that polymer adsorption rate decreased with increasing velocity of flow. Polymer desorption did not occur under quiescent conditions, but was observed when the adsorbent material was subjected to flow shear. Desorption increased with increasing flow velocity (Lee and Fuller, 1985). When furrow inflows were treated with 10 mg L-I PAM and permitted to flow down the entire furrow, polymer concentration in runoff was 6 to 10 mg L-I PAM (Lentz and Sojka, 1996). To our knowledge, no published reseazch has described dissolved PAM transport within treated irrigation furrows or determined its fate in receiving tail-water ditches where it mixes with untreated runoff The objective of this study was to determine dissolved PAM concentrations and mass losses in treated irrigation furrows and tail waters, and relate furrow PAM concentration to associated furrow sediment loads and infiltration. We also wished to determine how PAM transport in furrows may differ when inflows were treated with an initial 10 mg L-I PAM application vs. a continuous 1 mg L-I PAM application. It was hypothesized that (i) PAM concentrations in treated-furrow irrigation inflows decrease with distance downstream from the application point; (ii) PAM does not desorb from treated soil, so furrow stream concentrations rapidly decline once the application ceases; and (iii) PAM effects on furrow erosion and infiltration are a function of its concentration in the furrow stream. / MATERIALS AND METHODS The study was done on a 0.34-ha field located near Kimberly, Idaho. Soil was a Portneuf silt loam. The silt loam surface horizon had 100 g kg I clay, 700 g kg I silt, and 10 to 13 g kg I organic matter; a cation exchange capacity of 190 mmol~ kg ~; saturated-paste-extract electrical conductivity (EC) of 0.07 S m ~; exchangeable sodium percentage (ESP) of 1.5; pH of 7.7; and calcium carbonate equivalent of 5%. Irrigation furrows were 167.2 m long with 1.5% slope. The field ~ TOP ABSTRACT ~ INTRODUCTION • MATERIALS AND METHODS ~ NOTES ~ RESULTS AND DISCUSSION CONCLUSIONS ~ REFERENCES plot was disked twice, roller-harrowed, and planted to bean (Phaseolus vulgaris L.). Irrigation furrows at 0.56-m spacing were formed in wheel-trafficked lanes using a v-shaped sled. Only every other furrow was watered during a given irrigation, resulting in an irrigation furrow spacing of 1.12 m. A commercially available granular anionic PAM with 18% charge density and moleculaz weight of 12 to 15 Mg moC' (Superfloc A-836; CYTEC Industries, Stamford, CT!) was added dry to irrigation water or used to produce aqueous stock solutions. Polyacrylamide stock solutions were added to furrow streams on awhole-product basis to attain target concentrations. Since PAM granules contained 80% active ingredient, actual furrow stream PAM concentration for the whole-product 10 mg L-I tazget was 8 mg L-~, and for the 1 mg L'I target, 0.8 mg L-1. http://jeq.scijournals.org/cgi/content/fulU31/2/661 9/21/2006