Geochemical factors affecting leachate composition derived from soils in reclaimed lands using laboratory fresh and saline water column experiments

Abstract

The present study compared the geochemistry of saline and freshwater percolating through columns containing soils sampled from reclaimed land adjoining Sydney Harbour, Australia. The investigation assessed the flux of contaminants from the soil and, hence, estimated the affect of reclaimed land (using construction waste or dredged sediment) on the adjoining aquatic environment. Leachate samples were collected weekly for 22 weeks and were monitored for pH, E_h (redox potential), electrical conductivity, dissolved oxygen, major elements and heavy metals. In the early stages of the experiment (until approximately Week 12), the concentrations of the major elements in water percolating from the two columns were similar. Differing pseudo-steady-state conditions were reached in the columns after approximately 14 weeks. Sodium and Cl are weakly sorbed onto surface sites of the material in the saline column. The exchange of Na with other cations results in an increase in the aqueous concentrations of Ca, K and Mg in this column. As the parallel field concentrations are considerably higher than those measured in the column experiments, and are much higher (by orders of magnitude) than those measured in seawater, results of the fresh and saline water column tests strongly indicate that elevated concentrations of trace metals probably enter the estuary by tidal action and during periods of rainfall.