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SOIL PHYSICS

Clay Mineralogy, Ionic Composition, and pH Effects on Hydraulic Properties of Depositional Seals

^a Faculty of Sciences, Univ. da Coruña, A Zapateira s/n 15071, Spain
^b Inst. of Soil, Water and Environ. Science, The Volcani Centre, Agricultural Research Organization, P.O. Box 6, Bet Dagan 50250, Israel

^* Corresponding author (mlado{at}udc.es).

Formation of a depositional seal is common in soils subjected to rainfall or sprinkler or surface irrigation; however, studies on its hydraulic properties are scarce. In this study, the effects of clay mineralogy, exchangeable sodium percentage (ESP), electrolyte concentration, and pH on the hydraulic properties of depositional seals were investigated. A silt loam packed in columns was leached with 5 g L^–1 suspensions of reference montmorillonite, illite, and kaolinite with various ESPs (0–100), electrolyte concentrations (<2–20 mmol L^–1), and pH (7–11). Deposition of the clay particles on the soil surface formed seals, and their hydraulic resistance (R_c), which is the ratio between the thickness of the seal and its hydraulic conductivity, was measured. The R_c values of the seals developed by all clays with ESP 0 was low (<10.1 h). Conversely, at ESPs 10 and electrolyte concentrations 20 mmol L^–1, the R_c values of illitic seals ranged from 0.3 to 9.5 h and that of montmorillonitic seals ranged from 0.6 to 443.7 h. In both clays, increasing the ESP of the clay increased the seal R_c. A decrease of the electrolyte concentration increased the R_c of the montmorillonitic seals but had no effect on the R_c of the illitic seal. The R_c of Ca- and Na-kaolinitic seals was low at pH <9.0. Increasing the pH to 11 increased the R_c of the Na-kaolinite seal due to greater clay edge negative charge and associated dispersion. Conversely, in the Ca-kaolinite seal, the increase in pH decreased R_c due to CaCO₃ precipitation and clay cementation.

Abbreviations: ESP, exchangeable sodium percentage • SAR, sodium adsorption ratio