Preferential flow and metal distribution in a contaminated alluvial soil from São Domingos mine (Portugal)

- Pedon-scale metal distribution in sediment is related to non-equilibrium water flow.
- Preferential flow acts as metal storage or enhance leaching depending on metals.
- As water flow rate increases, Cu and Pb increase but As and S decrease.

Preferential flow is the non-equilibrium movement of water and solutes through soils that bypasses a portion of the soil matrix. It limits the sorbing capacity of the soil and it has been related to a reduced solute residence time accounting for an increased risk of groundwater contamination. A dye tracer experiment and a subsequent soil sampling procedure in three adjacent alluvial soil profiles located within the river bank area of the São Domingos river downstream of the abandoned São Domingos Mine (SDM) (Beja District, South Portugal) part of the Iberian Pyrite Belt (IPB) were done to identify different flow domains and test whether preferential flow phenomena influence the metal content distribution, to study differences in selective sediment properties resulting from flow domains and, lastly, to investigate whether preferential flow induces changes in the metal solid phase distribution. The existence of non-equilibrium water flow conditions resulting in different flow domains is showed in this sediment. Greater water flow through preferential flow (PF) paths induced differences in mineralogical and element composition of the soils at the pedon scale. While water flow heterogeneity did not add any variability to pH values distribution in the profiles, amorphous Fe (Feox), Al (Alox), and Si (Siox) contents were significantly lower in the PF domains than in the rest of identified flow domains. On the other hand, both Cu and Pb pseudo-total contents increase while As as well as S pseudo-total contents decrease, in all cases, as water flow rate increases in the soils. Multivariant discriminant analyses showed that the characteristics of the PF domain set it apart from the other indentified flow domains being the former dominated by high Cu and Pb concentrations as well as low Feox contents as compared to the other flow domains characterized by accounting with lower water flow rate. However, while PF phenomena induce a significant accumulation of Cu in domains with higher water flow rate, they do not change its metal distribution in the nominal solid fractions. Instead, PF phenomena not only increase Pb total concentration in the volume of sediment affected but also induce a metal mass transfer from insoluble solid phases to more readily soluble ones such as amorphous Fe/Al oxyhydroxide phases. Our results confirm that PF domains may act as a metal storage domain or enhance their leaching depending on the metal species and, therefore, on their tendency to be adsorbed, precipitated or dissolved under the specific geochemical conditions reached in these domains.