An assessment of water quality in dam reservoirs, considering their aggressive properties

The paper presents the water classification of four small retention reservoirs on the basis of selected physical and chemical indicators, such as: conductivity, pH level, dissolved oxygen, calcium hardness, general hardness and alkalinity. Three reservoirs (Ożanna, Kamionka and Cierpisz) are within the limits in terms of average values for the second class of water quality. Water in the Nowa Wieś reservoir does not qualify as second quality class of surface water. These reservoirs have had very irregular seasonal variations of water quality. Unambiguous correlations have not been established. Aggressive properties of water have been tested on the basis of Langelier (IL) or Ryznar (IR) indexes and calcium carbonate saturation [pHs]. Waters in the Ożanna and Nowa Wieś reservoirs do not have aggressive properties. Both the Kamionka and Cierpisz reservoirs are characterized by corrosive and leaching aggressiveness.


Introduction
Large retention reservoirs are designed to protect us from extreme phenomena such as periodic floods and droughts [1].However, their retention capacity is really low.So-called small retention plays an important role in collecting water in small reservoirs by stopping or slowing down the runoff.In lowlands, small retention is mainly focused on increasing retention possibilities and preventing droughts and floods.In mountainous areas, small retention is aimed at counteracting the effects of storm water runoff.Retention reservoirs are most often multipurpose facilities.Their most important functions include water collection for municipal and agricultural purposes or flood protection.They are also used for recreational purposes, hydropower and fishing [2,3].The main problem occurring in ecosystems exposed to an uncontrolled accumulation of nutrients from the catchment area is the eutrophication process [4,5].
Dam reservoirs, which are an integral part of civilization development, have many features that distinguish them from natural lakes or rivers, hence they constitute a different category of surface water reservoirs [6].That is why the interpretation of anthropogenically accelerated processes of their water degradation is so important [7].When using dam reservoirs as water supply systems, it is important to determine water aggressiveness first.In treatment processes, especially during coagulation, the aggressiveness is increased, related to the introduction of reagents that disturb the carbonate-calcium balance or increase the content of interacting ions [8].
The most important factors that affect the corrosive aggressiveness of water are: pH (pH above 7); temperature (an increase in temperature increases the corrosivity of water); the alkalinity of water -when it is less than 1.36 mval/L, water is corrosive regardless of other corrosivity indexes; hardness (recommended value: 60 -500 mgCaCO3/L); dissolved oxygen; aggressive CO2; gases (NH3, H2S, CH4); salinity, especially Cl -and SO4 2-anions; water flow turbulence; contamination with microorganisms (microbiological corrosion) or corrosion inhibitors.Corrosive aggressiveness is determined based on the pH of calcium carbonate saturation, pHs, and Langelier (IL) and Ryznar (IR) indexes.Aggressiveness is caused by very soft water.In this case, mainly the low content of calcium ions decides.Corrosion occurs due to the dissolution and leaching of calcium carbonate.The greatest sensitivity to this type of destruction is specifically exhibited in the first period after application.Corrosion is particularly dangerous when water is in motion (water structure destruction) [9,10].
The purpose of the work was to classify the water quality of four small retention reservoirs as well as to assess their corrosive aggressiveness and leaching aggressiveness.

Materials and methods
The research was conducted on four small dam reservoirs in Southeastern Poland.Their characteristics are included in Table 1.Surface water samples were collected in 2013-2014, (from spring to autumn) from 2 or 3 sites located in the backwater, near the dam and possibly in the centre of the reservoir.The samples were taken for temperature, conductivity, pH and dissolved oxygen concentration measurement (in situ testing), using the multi-parameter Hach Lange HQ40D meter.Calcium concentration, hardness and alkalinity were determined using standard laboratory techniques in accordance with Polish Standards.The water stability criterion was determined by calculating the pH in steady state (saturation) with solid calcium carbonate according to the dependence [13]:

pHs = (9,3 + A + B) -(C + D)
( The stability of the studied waters was assessed on the basis of the Langelier (IL) and the Ryznar (IR) indexes [14]: where: pH -pH value, pHs -pH value in calcium carbonate saturation state.

Results and discussion
A detailed analysis of the obtained results of selected physico-chemical parameters in 2013-2014 showed that the quality of water taken from various points in these reservoirs (backwater, centre, dam) may turn out to be slightly different in the end.As an example, the reservoir in Kamionka was considered, in which the differences were the largest (the longest retention time of 4.8 d) in terms of conductivity, pH, dissolved oxygen, hardness and alkalinity (Figure 1).In further analyses of the water quality, sampling points were not taken into account, and the results were referred to the entire reservoir (Table 2).The water temperature of the studied reservoirs in the period of spring, summer and autumn over two years changed within the limits of 12.5 -28.5 °C.The average temperature value fluctuated around 20 °C.
The salinity of water expressed by measuring the conductivity in the Ożanna, Kamionka and Cierpisz reservoirs ranged from 247 -288 μS/cm, while the maximum values were practically twice as high as the first value (429 -502 μS/cm).Water conductivity in the Nowa Wieś reservoir was the largest and had the average value of 467 μS/cm.Seasonal conductivity changes in the studied periods were very irregular.In 2013, conductivity values in the Ożanna reservoir showed an upward trend from spring to autumn, but in 2014 higher values were reported in the spring and autumn, and the lowest in the summer.In the waters of the Kamionka and Cierpisz reservoirs and in 2013, the highest values of conductivity were marked in the summer, while in 2014 the maximum value of this parameter decreased in autumn.The conductivity in the Nowa Wieś reservoir reached its highest values in spring and autumn.The waters of the Ożanna reservoir were characterized by the highest pH values, with its maximum value reaching pH = 9.53.The pH of other analysed reservoirs ranged from 7.84 to 7.98.In the Ożanna reservoir the highest pH values occurred in the summer.
Figure 2A-D show examples of seasonal changes made to the parameters at the sampling point.

Fig. 2 .
Fig. 2. Seasonal variations of pH (A), conductivity (B), total hardness (C) and oxygen saturation of the studied reservoirs (a -beginning of June, b -end of June).

Table 2 .
Representative values of selected water indicators of analysed reservoirs.