Depositional seals in polyacrylamide-amended soils of varying clay mineralogy and texture

Abstract

Purpose

Depositional seals, formed when turbid waters infiltrate into soils, lead to a reduction in soil hydraulic conductivity (HC) and enhance runoff and soil erosion. Since clay size particles constitute a dominant proportion of depositional seals, soil texture and clay mineralogy play a significant role in determining the seal’s hydraulic characteristics. Presence of high molecular weight anionic polyacrylamide (PAM) in suspension flocculates fine sediments, and therefore, its application to the soil surface may modify the characteristics of the depositional seal. The impact of PAM on the latter is expected to be influenced by soil properties. The aim of this study was to elucidate the effects of PAM application on clay flocculation and the HC of depositional seals formed in four soils varying in texture (ranging from loamy sand to clay loam), and diverse proportions of clay mineral constituents (kaolinite, smectite, and vermiculite).

Materials and methods

Soils from four physiographic regions of North Carolina, with different textures and clay mineral compositions, were used in the study. Clay size particles were extracted from each soil using common procedures and used for preparing 5 g L⁻¹ clay suspension. The effects of adding an anionic high molecular weight (12 × 10⁶ Da) PAM in various concentrations (0–10 mg L⁻¹) to 5 g L⁻¹ clay suspensions on sediment flocculation were studied with a nephelometer probe. The HC of depositional seals was studied by leaching soil columns with either deionized water (DW) or 5 g L⁻¹ clay suspensions in the presence or absence of PAM at the soil surface. PAM was applied either as dry granules to the soil surface (at a rate equivalent to 20 kg ha⁻¹) or by filling the overhead volume in the columns with a 0.5 mg L⁻¹ PAM solution.

Results and discussion

Even at a PAM concentration of 0.5 g L⁻¹, there was an increase of >50% in clay flocculation. Leaching the columns with DW in the presence of PAM caused a significant reduction in the HC. Conversely, during leaching with clay suspensions, addition of PAM in solution resulted in HC values (both initial and at apparent steady state) that were generally higher than those obtained in the absence of PAM. The impact of adding dry PAM varied with soil type. It had a negative impact on the HC of the depositional seals in the loamy sand and had no effect in the sandy loams; it did increase the HC of the seal in the clay loam from 3.6 mm h⁻¹ in absence of PAM to 9.9 mm h⁻¹ with PAM application.

Conclusions

The HC of the depositional seals studied depended on the combined effects of soil texture and clay mineral constituents. The effects of PAM on the HC of depositional seals depended on soil texture and on the mode of PAM application. Our results suggest that, in fine-textured soils, PAM is effective in improving the HC of depositional seals because it leads to the flocculation of the suspended material and thus to the formation of a less dense and more permeable seal on the soil surface. In coarse-textured soils, the lack of success of PAM in improving the permeability of depositional seals may stem from either the formation of a PAM layer at the soil surface with a distinct lower HC than that of the bulk soil and the depositional seal, or due to accumulation of the flocculated material in the pores at the upper few millimeters of the soil, thus forming a layer with a permeability even lower than that of the depositional seal itself.