Sampling and area description
Urmia Lake is one of the 59 international parks of world heritage designated by the United Nations. It is located between west and east Azerbaijan in the uppermost northwest center of Iran. It has coordinates between 37 to 38 degrees and 15 minutes north latitude, and between 45 to 46 degrees east longitude. The Urmia National Park consists of three units: 1) a water unit, 2) a mountain and coastal plain unit, 3) rivers and wetlands. Urmia Lake basin has an area of 51876 km2, which comprises 38% mountainous areas, 21% hills, 11.2% flats and terraces, 9% lakes, and the remainder (20.8%) includes plains and various lands utilized for different uses. From 19 rivers flowing into the lake, 14 are permanent and 5 others are seasonal rivers. The annual water discharge is estimated to be 5317 million m3. The desiccation of Urmia Lake and lowering of the river water level entering the lake, illustrates the need to give attention to the increasing pollution of rivers and wetlands around the lake. Water diversion and management of dryland regions for agricultural practices are having a serious impact on the water quality in the Urmia Lake basin.
Twenty-five representative sediment samples from seven main western rivers of Urmia Lake basin were collected for this study (Figure 1). Each river was sampled from the upstream of mountainous region to downstream point river plain near discharge to Urmia Lake, and sometimes in-between. The vertical sediment cores (0-10 cm) was collected from each sampling site and then mixed to have a composite sample. For the Urmia Lake basin, water levels rise and overland flow occurs seasonally in the riverbed sediments, therefore most sediment samples were selected from summer during the reduction of river flow.