Granular and Dissolved Polyacrylamide Effects on Hydraulic Conductivity of a Fine Sand and a Silt Loam

The impact of a linear, high molecular weight, moderate negative charge density polyacrylamide (PAM) on soil hydraulic conductivity (HC) and the mechanisms involved are partially understood. Our objectives were to evaluate the impact of (i) electrolyte concentration of the PAM solution, (ii) mode of PAM application, and (iii) drying of the applied PAM solution on the HC of a fine sand and a silt loam. We exposed the two media to (i) leaching with deionized water (DW) or a 0.01 mol L-1 CaCl2 solution, both containing 25 mg L-1 PAM, and (ii) leaching with DW aft er addition of either 2 g m(-2) of dry PAM granules (with or without gypsum) to the soil surface, or an equivalent amount of PAM in solution (with or without drying) to the soil column before leaching. For both media, leaching the columns with DW containing PAM resulted in a significantly lower steady-state HC (HCss, <0.10 cm h(-1)) than the HCss obtained for leaching with DW only. In the silt loam, replacing the PAM in the 0.01 mol L-1 CaCl2 solution with DW resulted in a moderate decline in the HC. Leaching the soil with DW aft er adding granular PAM to the surface resulted in a HCss of 0.18 cm h(-1), while aft er adding a concentrated PAM solution the HCss was 0.11 cm h(-1). Drying of the added PAM solution before leaching led to a twofold increase in the HCss. The adverse impact of PAM on the HC was ascribed to the extension of the unadsorbed segments of the PAM chains into the pores, thus creating a drag on the flowing water.