**6. Water hardness**

Water hardness is generally defined as the content of divalent cations, mainly calcium and magnesium in water. Hardness is divided into three categories: total hardness, temporary or bicarbonate hardness, and permanent or non-bicarbonate hardness. The total hardness or the total content of calcium and magnesium ions, generally expressed as mg CaCO3 dm−3, is most often used in the classification of surface water. Depending on geological characteristics of the catchment, it changes in a wide range, from several mg CaCO3 dm−3 to several hundred mg CaCO3 dm−3. High hardness can be generally disadvantageous if water is to be used in household and industry. On the other hand, very low (smaller than 30 mg CaCO3 dm−3) hardness can be harmful for humans.

In the studied period, the monthly average total hardness in the surface water of the Turawa reservoir ranged from 106 to 191 mg CaCO3 dm−3. The lowest measured value of the total hardness was 95 mg CaCO3 dm−3 while the highest was 210 mg CaCO3 dm−3; the monthly average hardness was 141 mg CaCO3 dm−3 (**Figure 7**).

**Figure 7.** Changes in the average monthly total water hardness of the Turawa reservoir in the hydrological years 2004– 2006.

In the studied period, the difference in the total hardness was relatively small. The water could be classified as soft or medium hard. Since the total hardness is determined by the concentration of calcium and magnesium, the changes of the monthly average concentrations of these ions were also measured. They are shown in **Figures 8** and **9**, respectively.

the Turawa reservoir was classified to purity class I with respect to that parameter in all

Water hardness is generally defined as the content of divalent cations, mainly calcium and magnesium in water. Hardness is divided into three categories: total hardness, temporary or bicarbonate hardness, and permanent or non-bicarbonate hardness. The total hardness or the total content of calcium and magnesium ions, generally expressed as mg CaCO3 dm−3, is most often used in the classification of surface water. Depending on geological characteristics of the catchment, it changes in a wide range, from several mg CaCO3 dm−3 to several hundred mg CaCO3 dm−3. High hardness can be generally disadvantageous if water is to be used in household and industry. On the other hand, very low (smaller than 30 mg CaCO3 dm−3)

In the studied period, the monthly average total hardness in the surface water of the Turawa reservoir ranged from 106 to 191 mg CaCO3 dm−3. The lowest measured value of the total hardness was 95 mg CaCO3 dm−3 while the highest was 210 mg CaCO3 dm−3; the monthly

**Figure 7.** Changes in the average monthly total water hardness of the Turawa reservoir in the hydrological years 2004–

In the studied period, the difference in the total hardness was relatively small. The water could be classified as soft or medium hard. Since the total hardness is determined by the concentration of calcium and magnesium, the changes of the monthly average concentrations of these

ions were also measured. They are shown in **Figures 8** and **9**, respectively.

measuring periods, in the hydrological years 2004–2006.

**6. Water hardness**

386 Water Quality

2006.

hardness can be harmful for humans.

average hardness was 141 mg CaCO3 dm−3 (**Figure 7**).

**Figure 8.** Changes in the average monthly calcium concentration in the waters of the Turawa reservoir in the hydrological years 2004–2006.

**Figure 9.** Changes in the average monthly magnesium concentration in the waters of the Turawa reservoir in the hydrological years 2004–2006.

Seasonal changes of calcium content were much higher than those of magnesium, which can be related to difference in solubility. The calcium content in non-polluted natural waters is generally four times higher than magnesium content. In the Turawa reservoir, it was about six times higher. The total hardness of water in the reservoir was the highest in winter, most probably due to mineralization of dead organic remains at that period.
