A soil column study was conducted to evaluate the efficiency of soil to attenuate pollutants in landfill leachate. It was found that more than 60% of the initial alkalinity, COD, and total nitrogen was removed after the leachate had percolated through the soil column. Lower efficiencies were observed for the removal of dissolved cations, ammonia nitrogen, and nitrate nitrogen. When previously percolated leachate was recirculated through the soil column, significant removal (93.6%) of ammonia nitrogen was achieved. Moreover, a further reduction in COD, total nitrogen, nitrate nitrogen, and potassium was evident. Bioassay tests of seed germination and root growth of Brassica chinensis were performed to compare the phytotoxicities of untreated, percolated, and recirculated leachates. The phytotoxicity was reduced by both treatments, with the greatest detoxification observed in the leachate recirculation treatment. The values of pH and Olsen phosphorus were significantly reduced (P less than 0.01) in the treated column soil, whereas significant increases (P less than 0.05) in electrical conductivity (EC), various forms of nitrogen, exchangeable sodium percentage (ESP), potassium, magnesium, zinc, and iron were observed. EC, ESP, potassium, magnesium, sodium, iron, and various forms of nitrogen accumulated at the soil surface (10 cm). On the other hand, manganese was deposited at 20 to 50 cm below the soil surface of the treated column.

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