Chemistry and mineral stability in saline alkaline soil environments

Salinization processes can be expected to cause significant changes to the chemical characteristics of a soil but the effects on mineral stability are poorly understood. The primary objectives of this study were to characterize the chemistry and mineralogy of salt-affected soils in Alberta and to investigate clay mineral weathering in solutions of varying salinity and alkalinity in laboratory-controlled dissolution studies. A survey of 50 salt-affected soils in Alberta showed salt efflorescences were usually dominated by sodium and magnesium sulfates. No mean accumulation of trace elements with the exception of Br and Cl were measured, but trace element concentrations between sites were extremely variable, with potentially toxic levels at some sites. Soil solutions extracted from four salt-affected soils ranged in pH from 7.8 to 10.6 and were dominated by Na and SO;A preliminary survey of the clay mineral characteristics indicated that smectites were degraded in many saline soils. Subsequent analysis showed this apparent degradation was a result of interaction with organic matter or from formation of a heat and acid unstable interlayer material. After traditional pretreatments to remove organic matter and carbonates, the clay mineralogy of salt-affected soils was found to be similar to that of non-saline soils. Subsequent layer charge analysis of the smectites showed that the charge distribution was heterogeneous, with the mean layer charge lower than previously observed in Alberta soils. More than half of the clay fraction of one saline soil was composed of well-ordered, authigenic dolomite.;The weathering of clay minerals in laboratory dissolution studies varied considerably with the chemistry of the solution, with minerals generally most stable in saline solutions. Changes in characteristics for most samples were noted after equilibration for a year or more. The direction of change of solution characteristics for most samples was frequently away from equilibrium, with mineral saturation indices becoming increasingly positive with time. Because of the differences in the products of clay mineral dissolution in different chemical environments, a single dissolution reaction that is valid for a mineral in all soils cannot be written. The prediction of clay mineral stability in complex soil environments using simple thermodynamic models may therefore be inappropriate.