10 The main sources of suspended particulate matter (SPM) into the Baltic Sea are river supply and primary production, with both sources being of the same order of magnitude. Around 1010 kg (10 Tg) of SPM are annually introduced into the sea [19]. It has been estimated that the mean SPM concentration in freshwater entering the Baltic from rivers is 20 g/m3 [17]. This freshwater is introduced from the main rivers: Neva (2460 m3/s), Vistula (1065 m3/s), Neman (632 m3/s), Oder (573 m3/s), Kemijoki (562 m3/s) and Angermanälven (489 m3/s), where figures correspond to mean ?ows. However, a large number of small rivers more or less uniformly distributed along the coast need be considered as well and mean freshwater supplies in several basins are summarized in Table 4. In general, SPM concentrations in the Baltic are low and present low seasonal variability. Mean SPM concentrations in the surface layer are also presented in Table 4 and have been obtained from Secchi disk measurements [18]. Absolute maximum concentrations (up to 10 g/m3) are found in the east of the Gulf of Finland, due to the large discharge of Neva River. Close to the seabed, SPM concentrations are 1–2 g/m3 larger than in the surface. The fraction of fine (muddy) sediments in the seabed is generally required to calculate adsorption rates of radionuclides from the water column to the sediment (f). This information is summarized in Figure 3, where HARD soil is composed of stones, thus f = 0. The same value is given to a SAND soil. A SOFT soil consists of muds, thus f = 1. A HARD-SOFT soil is composed of stones and muds in the same proportion, thus f = 1/2. Finally, a HARD-soft soil is also composed of stones and muds but with more abundance of the first, thus, f = 1/5. FIG. 3. Types of soils in the Baltic Sea. Reproduced courtesy of the University of Seville [17].