Modelling of seepage water composition from experiments with an acid soil and a calcareous sediment
K. MICHEL & B. LUDWIG,
Acta hydrochim. hydrobiol. 33:595-604 (2005)
A thorough assessment of changes in seepage composition with soil depth is required when studying the fate of nutrients or toxic substances and geochemical models may improve our understanding. Our objective was to determine whether the reactions considered in the model PHREEQC2 were appropriate to predict pH buffering and transport of dominant cations and selected heavy metals in an undisturbed subsoil and a sediment under unsaturated conditions. Samples of a calcareous sediment (CS) and undisturbed columns of the subsoil of a sandy Podzol (SP) were irrigated for two years with 4 mm day-1 at 8 oC with either 1 mM HCl (pH buffering experiment), small concentrations (HM1, 0.1 to 1 mg L-1) or increased concentrations (HM2, 0.3 to 3 mg L-1) of the heavy metals Pb, Cr(III), Cu, Ni and Cd. Pb and Cu were completely retained in the soil and sediment, whereas increased concentrations of Cd and Ni (SP) and Cr (CS) were found in the seepage water in the HM1 and HM2 experiments. The model included one-dimensional transport, inorganic complexation, dissolution/precipitation of Al(OH)3, CaCO3 and Cr(OH)3, multiple cation exchange and sorption isotherms for Cd and Ni. The HM2 experiments were used for a calibration whereby the parameter optimization program PEST was used to optimise solubilities within a range of reported values, pCO2, cation exchange coefficients and sorption parameters for Ni and Cd. The model described pH and concentrations well for HM2 and predicted the data well or satisfactorily in the pH buffering and HM1 experiments. The 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 was useful for the description and prediction of main cation and heavy metal concentrations in seepage water.