Aluminium, iron and manganese in near-surface waters of a tropical rainforest ecosystem

A hydrochemical investigation was undertaken in a tropical rainforest catchment to determine the aluminium, iron, and manganese concentrations in near-surface waters of a non-impacted, circum-neutral environment, with an emphasis on hydrologic controls of Al, Fe, and Mn stormflow variations. The concentrations of all three solutes increase with discharge, with antecedent moisture conditions influencing the amplitude. This increase is accompanied by a minor pH depression from about 7.3 to 6.8, and, in the case of one event, by an increase in DOC. For all three solutes, the ranges in stormflow and overland flow concentrations are nearly identical. Al, Fe, and Mn concentrations in soil and ground water are considerably lower, with the exception of Mn in shallow soil water which is similar to overland flow. We conclude that episodic increases in streamflow metal concentrations in this tropical rainforest environment are not so much the result of a pronounced pH depression, but of an overland flow-mediated input from near-surface sources such as leaf litter and topsoil.