The mining activity in the Mining District Cartagena-La Union has generated large quantities of sterile materials which have been accumulated in silt ponds for many years; these materials present high ferric oxides, sulphates, and major concentrations of heavy metals with potential leaching due to the present acidic conditions. The oxidized materials contain significant amounts of water-soluble secondary sulphate minerals and constitute long term sources of acid drainage in these past mining environments. On site remediation activities of these abandoned landscapes require alkaline amendments, added to the oxidized tailing soil surfaces, to increase pH and immobilize the metals.

The purpose of this study was to determine the potential use of pig manure to increase pH and reduce leachable mobile metals from to the leachates. A serie of leaching experiments were performed. Three replicates of undisturbed mine soil columns with 7% of pig manure, equivalent to 3750 kg-N (ha-year)-1, were amended at the top 10 cm soil surface. The continuous leaching per 12-week period was performed with 1 litter per tilling, a rate of 7 mL min-1. Results from leaching columns show that mine alluvial soil treated with pig manure create neutral pH conditions and tend to persist at the soil surface. The leachates obtained in the soil columns, 30 cm to depth, present a maximum pH value of 2.7 in one of the replicates. However, pH slightly increases from 1.8 to 2.6, EC significantly decreases from 18.1 to 4.8 dS m-1, and Eh diminishes from 580 to 412 mV (average values). These acidic pH conditions allow a high metal dissolution in the leachates, principally for Cd, Cu, Pb and Zn.

The study shows that pig manure is an environmentally and economically attractive amendment to prevent the formation of acid mine drainage in a long-time remediation program of an abandoned mining site. The experiments allow us will determine the required doses of pig manure to obtain a long pH neutralisation.