Leaching, runoff and speciation of arsenic in a laboratory mesocosm

Leaching and runoff of arsenic (As) from the contaminated soil of an old wood impregnation plant, and fate in a recipient freshwater ecosystem, was studied in soil-water-sediment mesocosms in laboratory (0.9 m3; total water volume 200 l). During the 4-month experiment total leaching and discharge of As from regularly irrigated soil was approximately 40 mg, i.e. approximately 0.6% of total initial As content in the soil. Of the total As load, 7.5% remained in the water; 44% settled down to the shallow (water depth 5-30 cm) sediment zone; and 48.5% to the deeper (water depth 80 cm) sediment zone. The different arsenic species; arsenite [As(III)], arsenate [As(V)], monomethylarsonic acid (MMAA) and dimethylarsinic acid (DMAA), were analysed from irrigation and discharge water; mesocosm pool water; and sediment pore water using ion chromatography-inductively coupled plasma-mass spectrometry (IC-ICP-MS). The total amounts of arsenic in soil, water and sediment were determined by ICP-MS. Arsenic was leached out from the soil as As(V). In mesocosm water As(V) was the predominant dissolved species, but DMAA and particle bound species, were also detected. In shallow sediment, As(V) was the most abundant species together with some DMAA, whereas in deep sediment As(III) was the dominant species.