**10.2. Iron (Fe)**

Iron is quite common in the lithosphere; its content is about 5%. It is present in mineral waters due to the process of leaching out of rocks and soil. Considerable amounts are released into surface water with wastewater from the plants processing iron and with mine water. Its typical concentration in surface water is on the level of several mg dm−3. Depending on the content of organic matter, oxygen, carbon dioxide, microorganism activity and pH, iron can be present in water in a dissolved form, as colloids and suspended mater. Some iron compounds can be assimilated by aquatic organisms.

In the water of the Turawa reservoir, the average iron concentration was 0.1338 mg dm−3 and changed in the range of 0.009–0.4145 mg dm−3. According to surface water classification in the years the studies were conducted, the reservoir water with respect to Fe content was classified as purity class I in summertime, as class II in autumn-winter period when, at the low level of water and the increased bottom exposure, iron compounds are leached out of sediments.

### **10.3. Cobalt (Co)**

Cobalt concentration in surface water is quite small, most often single to several μg dm−3. The main anthropogenic sources are industrial waste and coal burning. In non-polluted surface waters, its typical concentration is 0.05 μg dm−3 while the natural average content is about 0.02 μg dm−3. Cobalt undergoes the process of bioaccumulation in phytoplankton and accumulates in bottom sediments.

In the water of the Turawa reservoir, the average cobalt concentration was about 12.0 μg dm−3 and ranged from below detection limits to 62.6 μg dm−3.

### **10.4. Nickel (Ni)**

In water, nickel can be present in dissolved form as a two-valent cation, or in the complexes, most often as cyanide complex or in undissolved form as cyanide, sulphide, carbonate or hydroxide. In pure waters, its concentration is about 5 μg dm−3, while in strongly industrialized areas, surface waters can contain up to about 0.020 mg dm−3. Considerable amounts of nickel are emitted during combustion of diesel fuel [13].

In the water of the Turawa reservoir, the average cobalt concentration was about 6 μg dm−3 and ranged from below detection limits to 16.5 μg dm−3. According to surface water classification in force in the years studied, the reservoir water with respect to Ni content was classified as purity class I. Only occasionally, the limiting concentration for class I was slightly exceeded.
