Hydraulic Conductivity of Polyacrylamide-Treated Depositional Seals in Soils Varying in Clay Mineralogy and Texture.

Depositional seals in soils are the result of accumulation at the soil surface of fine soil particles, carried in runoff from eroding areas. We studied, using column experiments, the effect of anionic polyacrylamide (PAM) addition on the hydraulic conductivity (HC) of depositional seals in soils from 4 different regions of NC representing different clay mineralogical compositions and texture. Depositional seals were formed by leaching a given soil with a suspension containing 5 g L^-1 of clay size particles extracted from that soil. Dry PAM at a rate of 20 kg ha^-1 or PAM solution at a concentration of 0.5 mg L^-1 was applied to the surface of the soil in the columns prior to leaching with the clay suspensions. PAM introduced in solution improved the HC in all four soils compared to the untreated seals; this favourable effect increased as the soil texture became finer. When PAM was added to the soil surface in the form of dry granules it had a negative or no effect on the HC of the depositional seal in the three soils with 10% clay and a positive effect on the HC in the soil with 30% clay.  It is suggested that PAM has two opposing effects on the permeability of depositional seals: (i) reducing the HC due to viscosity induced resistance to flow and/or the formation of a thin PAM layer at the soil surface with a distinct lower permeability than that of the bulk soil, and (ii) flocculating the suspended particles in the leaching suspension and thus leading to the formation of a depositional seal of a greater permeability. The observed impact of PAM on the permeability of depositional seals emphasizes the need to consider both mineralogical composition of soil-clays and soil texture for enhanced PAM effectiveness.