## 3 Geological Setting  
Due to its location between two important mountain ranges--Turkestan and Zarafshon--the Zarafshon catchment's basin covers a variety of lithologies of which clastic material represents the main component of sediments and bordering soils. According to [22], the actual Turkestan and Zarafshon ranges evolved during Oligocene from a Jurassic basin filled with continental, shallow-water marine and lagoonal sediments of which crustal origin seems to influence its present geochemistry. This was the main reason for which we have chosen UCC [17] and NASC [18] as the most appropriate descriptor of investigated material. At present, Djidjikrut, Fondarya, both tributaries of the Yagnob, as well as Mogiyon and its tributaries have the most important contribution to the mineral composition of Zarafshon sedimentary material [23,24]. The Djidjikrut River flowing in the Ayni district passes through clays, marks, limestones, dolomites and salts, significantly influencing the mineral composition of water and neighboring sediments and soils. At the confluence with the Zarafshon (Yagnob river), is located the Djidjikrut ore field related to Zarafshon-Gissar mercury-antimony belt, and dating to Middle and Upper Paleozoic. Antimonite, cinnabar and metacinnabarite are the main minerals of the Djidjikrut ore field, while pyrite and marcasite with an admixture of Co, Ni, Cu, As, Sb, Au, Te and Bi, realgar and orniment, as well as hematite and sphalerite are common minerals too. For this reason, at the confluence of the Djidjikrut and Yagnob rivers, on the northern slope of the Gissar range, the Anzob mining and processing plant is operating [5] with the final release of Hg-Sb concentrate, an important source of downstream contamination (Figure A1). The Yagnob River flows westward through the narrow mountain gorge along the southern edge of the Zarafshon mountains, being the first tributary of the Fondarya River. The total length of the Yagnob river is 114 km with a basin area of 1660 km\({}^{2}\), while the mineral composition of sediments and soils of its upper reaches is still poorly investigated. Downstream, near the confluence with Fondarya, the Fon-Yagnob coal deposit occupies almost the entire left-bank site between the Djidjikrut and Pasrud tributaries [25]. Here, due to exothermic oxidation of the sub-surface coal layers, rich in pyrite and marcasite, a self-ignition process contaminated neighboring locations. Here, the paralava, a product of complete melting and (partial) recrystallization of local rocks generated by the high temperature attained during underground coal combustion, consists of almost melted sediments enclosing coal seams [26, 27, 28]. It is supposed that the underground coal fire may contribute to diagenesis of some REE minerals such as godovikovite, etremovite, maskanite, and so on [27, 28, 29, 30]. The Fondarya River with a significant catchment basin of 3230 km\({}^{2}\) is formed by the confluence of the Yagnob and Iskandardarya, which then crosses the Zarafshon ridge and flows into the Zarafshon river. In addition, the Fondarya is fed by the waters of the Sarvoda river which originates from the Alovdin lakes, as well as the lower and upper Kumag tributaries, Chore and Pete (Figure 1a). It should be noted that in the upper flows of the Iskandardarya river operates the mining and processing plant \"Tolko-Gold\", which extracts and processes Au, Ag, Hg, and Sb ore as well as fluorite. The mining operation is also carried out at the gold deposit in the Kumargi Bolo gorge. The Chore River gorge is classified as gold-quartz deposits at the Turkestan-Chore zones of Central Tajikistan [31]. This deposit was discovered in 1960 and intensively exploited during 1974-1984 mainly for pyrite and arsenopyrite which represented more than 95% of all ore minerals together with small amounts of antimonites, chalcopyrite, sphalerite, pyrrhotite, and others [32]. The Pete tributary flows between the Pete-Takfon ore field hosting Takfon, Pete, Simich, and other deposits.