Zia-Abad is in Qazvin Province (NW Iran), with many farmlands that cover all parts of the area. The purpose of this research is to determine the background concentration of arsenic in stream sediments of Zia-Abad. The main geological units of the area include volcanic and sedimentary rocks composed of rhyolite, dacite, tuff, limestone, dolomite, sandstone, and shale. Several mines occur in this area, including the abandoned Shakin Pb-Ag-Si mine and many Si active mines. It is important to assess As contents in sediments because sediments with concentrations of different potentially toxic elements can have different effects on ecology. A total of 313 samples were collected from stream sediments and analyzed by ICP-MS. However, data distribution is not normal, so for converting to normal distribution, the logarithmic, and z = Ln(x-α) methods are used. The background is determined using median (Md ± 2MAD), means, and standard deviations concentrations of As. On the basis of the results of this research, it can be concluded that arsenic contents of sediments in the area are more than probable effect concentrations and potentially can have environmental impacts.
Keywords: Arsenic; Background; Md ± 2MAD method; Normalization; Stream sediments.