As(III) concentration was determined directly by HG-AFS after the samples were filtrated by a 0.22 μm membrane and diluted with 5% HNO3 solution. As(V) concentration was obtained by subtracting As(III) concentration from total As concentration. For the determination of total As concentration, thiourea and Vitamin C solutions were added after the samples were filtrated through the membrane, and then the samples diluted with 5% HNO3 solution and then stood for 24 h before analysis.ECOTOXICOLOGY AND ENVIRONMENTAL SAFETY