Modelling cation transport and pH buffering during unsaturated flow through intact subsoils

Abstract

Prediction of cation transport during unsaturated flow through soils is complex, and chemical models are necessary for the purpose. Our objectives were to determine whether the reactions considered in the model PHREEQC2 were appropriate to predict cation transport in soils when small amounts of CaCl2 or acid are added and to investigate the usefulness of different parameterization approaches. Undisturbed columns of three subsoils were irrigated for 2 years with 4 mm day(-1) at 8 degrees C. The experiments were (A) irrigation with 1 mM and 5 mM CaCl2 and (B) irrigation with 1 mM HCl. The model PHREEQC2 was used to calculate one-dimensional transport, inorganic complexation, dissolution or precipitation of Al(OH)(3), AlOHSO4 and CaCO3 and multiple cation exchange. We compared three models: one with no adjustable parameters, a second with optimized solubilities within a reported range, pCO(2) and cation exchange capacity (CEC), and a third which also included an optimization of the exchange coefficients. The first was of little use to predict cation transport in soils. The second was appropriate for one of the soils, but an optimization of the exchange coefficients was required for an accurate description and prediction of cation transport for the others. The CEC accessible for seepage water varied between 50% and 100% of the total CEC. Our results indicate that for a reliable assessment of cation transport in soils during unsaturated flow, calibration experiments are required. The parameter optimization program PEST in combination with PHREEQC2 gave a better understanding of cation transport in undisturbed soil.