IRRIGATIVE ASSESSMENT OF SASYK WATER QUALITY

. The article is dedicated to the problem of artificial Lake Sasyk which was transformed from a salty water estuary into a freshwater reservoir at the expense of the Danube waters. In the first decades, the irrigative water conditions were not achieved due to the arrival of salts from the bottom sediments. Currently, the water quality formation process in Lake Sasyk has stabilized, but the problem of land irrigation remains urgent. The article gives an irrigation assessment of the quality of the Sasyk reservoir according to various methods considering observations in the HPS-2 region from 2007 to 2017. A detailed typification of irrigation waters based on Alekin O. A. water typing is proposed. It was found that mineralization and the content of sodium and magnesium ions in the Sasyk waters promote salinization and alkalinization of soils. For the safe use of this water for irrigation, both chemical reclamation and dilution with water are necessary.


Introduction
To irrigate the lands of the Tatarbunarsky and Saratsky regions the salt-water estuary Sasyk was transformed into a fresh-water reservoir: in 1978, it was separated from the sea by a dam and connected to the Danube by a canal. In the first decades of the existence of the Sasyk reservoir, the design water conditions were not achieved primarily because the mineralization of the Danube water in the reservoir increased due to the flow of salts accumulated in the sediments of the salt-water estuary during its existence.
The process of water quality formation in Sasyk has now stabilized (after forty years of its existence as a reservoir). But today the problem of irrigation of the Tatarbunarsky and Saratsky regions remains relevant because of the unsatisfactory quality of the irrigation water.
Many scientific and popular science works are devoted to the ecological problems of Sasyk (Lazovitskii, 2003;Kulibabin et al., 1997;Vasenko, 2004;Tymchenko, Ivanova, 2012;Ivanova, 2010). In these works, the irrigation properties of water are evaluated by the average and maximum values of indicators. But during some periods of the year, the properties of water can differ significantly from each other and not meet the averaged characteristics. During the year, water can belong to different irrigation categories (classes).
The aim of the work is to assess the irrigation properties of Sasyk waters and their variability during the warm period according to hydro-chemical observations in the area of the village of Trapivka, GNS-2 from 2007 to 2017 (according to the Odessa Regional Department of Water Resources).
The object of research is the Sasyk Reservoir. The subject of the study is the assessment of the variability of the irrigation properties of Sasyk waters.

Basic principles of irrigation water quality assessment
The irrigation water quality assessment is carried out according to four criteria: the concentration of salts; the ratio of ions (mainly sodium cations with magnesium and calcium); the concentration of toxic elements that can negatively affect agricultural plants and the environment in general; the concentration of nutrients.
Using water with high mineralization can lead to soil salinization. The salinization of soils is called an excessive accumulation of electrolyte (dissolved or absorbed) salts Na2CO3, NaHCО3, NaCl, СаCl2, Na2SО4, MgCl2, MgSО4 in the root containing layer which oppress or destroy agricultural plants, reduce the yield and its quality. Salinity can be neutral (chlorides and sulfates of sodium and magnesium) and alkaline (carbonates and bicarbonates of sodium and magnesium) (Slyusarev et al., 2014).
According to Kostyakov A., the danger of soil salinization based on the total mineralization of the irrigation water is estimated as follows: up to 1.0 g/dm 3 -is suitable for irrigation; from 1.0 to 1.5 g/dm 3cautious irrigation; from 1.5 to 3.0 g/dm 3 -it is necessary to analyse the chemical composition of salts, over 3 g/dm 3 -is not suitable for irrigation (Kostyakov, 1960).
In the USA the following irrigation water salinity classification (MG, g/dm 3 ) is used: -МG≤0.20low salinity water suitable for irrigation of most crops on the most soils; -0.20 <МG≤0.50medium salinity water used in the conditions of moderate leaching, medium salt tolerance crops can be grown without using anti-salinity agents; -0.50 <МG≤1.00high salinity water, even with good drainage, measures to control salinity may be necessary; cultures that have a high salt resistance should be selected; -1.00 <МG≤3.00very high salinity water, unsuitable for irrigation under normal conditions, irrigation is possible under the following conditions: high soil permeability, drainage, salt-resistance of crops.
The level of soil salinity is also characterized by the toxicity index. The toxicity threshold is the boundary value of the salt content in which the inhibition of agricultural crops growth and development is observed. Table 1 presents the toxicity of the main salts that are found in soil and water. Salts that are above a strip (Table 1) are harmful to plants. The most toxic ones are soda (Na2SO3, NaHCO3), chloride (NaCl) and sulfuric acid (Na2SO4) sodium, calcium chloride (CaCl2). Sulfate and magnesium chloride (MgSO4, MgCl2) have less toxicity. The mixtures of salts are always less toxic than their cleaner clusters.
Salts dissolved in water are usually represented as ions. However, sometimes the suitability of water for irrigation is determined by the ratio of the content of the certain types of soluble salts in it (Kostyakov, 1960).
The main ions can be toxic and non-toxic. Toxic ions include ions that can form toxic salts. The ions of Cland Na + are toxic, other main ions can be both toxic and non-toxic depending on their mutual equilibrium: Mg 2+ and Ca 2+ with Cl-are toxic salts, but with CО3 2and HCО3 --nontoxic; CО3 2and HCО3with Na + are the most toxic to plants, but with Mg 2+ and Ca 2+ are nontoxic (Zaydel'man et al., 2007). Such an analysis of ions toxicity is presented in the work (Zaydel'man et al., 2007). Isolation of toxic ions can be conveniently performed if the mineralization of water is presented as a sum of hypothetical salts.
Presentation of the mineral composition of water as a set of hypothetical salts is not used in practice, since in water the ions are in a bound state, and in the chemical analysis, the content of ions is determined, but these salts are rather often mentioned in the literature (Kostyakov, 1960;Zaydel'man et al., 2007;Astapov, 1958;Maksimov, 1979;Alekin, 1970).
For example, the founder of the Soviet period melioration Kostyakov A. points out the need to analyze the chemical composition of salts for the waters with the mineralization of 1.5-3.0 mg/dm 3 (Kostyakov, 1960). According to Alekin O. one can get an approximate idea of the nature of the salts that will come to the ground from this water, if we conditionally assume that when the water evaporates, salts will fall out when the ions are combined in the following sequence: cations -Ca 2+ , Mg 2+ , Na + ; anions -HCO3 -, SO4 2-, Cl - (Alekin, 1970).
The authors of the article offer the types of natural waters (I, II, III) (Alekin, 1970) for irrigation purposes, divide into subtypes (I, IIa, IIb, IIIa, IIIb, IIIc), which have different irrigation properties. Type IV is not considered since it includes waters (Alekin, 1970) which are not suitable for irrigation.

Ratio of ions
By the set of toxic salts the water of type I can be the most unfavourable for irrigation since besides sodium sulfate (Na2SO4) they can form sodium bicarbonate (NaHCO3) in the soil, and in the presence of carbonate ions (CO3 2-)common soda (Na2CO3), of all salts formed by the main ions, is the most toxic to plants. These salts cause an alkaline reaction of the soil.
-Water of type ІІ differs from water of type I because magnesium sulfate (MgSO4) which is in the last place in a number of salts toxicity (according to V. Kovda) may be added to the soil instead of carbonate and sodium hydrogen carbonate (Na2CO3 and NaHCO3).
-The difference between subtypes IIa and IIb is as follows: from waters of subtype IIa besides MgSO4, magnesium hydrocarbonate (Mg(HCO3)2) may also come into the soil. It is a non-toxic salt to plants, but it can cause alkaline soil reactions; water of subtype IIb contributes to forming another non-toxic saltgypsum (CaSO4×2H2O), instead of magnesium hydrocarbon,in the soil. Gypsum is a meliorant of alkaline soils.
Subtype IIIa in comparison with subtype IIa is more favourable, since when the water of this subtype evaporates less toxic magnesium chloride (MgCl2), instead of sodium sulfate (Na2SO4), may come into the soil. However, in the waters of this subtype, there is a large number of magnesium ions and their excessive content in water contributes to the alkalinization of the soil.
Subtypes IIIa and ІІІb differ from each other in the same way as subtypes IIa and IIb: IIIa promotes the formation of magnesium bicarbonate in the soil (Mg(HCO3)2), and IIІb promotes the formation of calcium sulphate (CaSO4).
To recalculate the ion concentration from an equivalent form (mg-eq/dm 3 ) to a weight form (mg/dm 3 ) it is necessary: to multiply the value of the equivalent concentration of an ion by its ionic weight and to divide by the valence of this ion.
The ratio of ions. The assessment of the irrigation water quality by the ion ratio is the mo st common.
In Bezdnina S. classification the percentages of sodium (Bezdnina, 2013) ions and the sum of cations are taken into account along with the mineralization of water (Fig. 2).
Antipov-Karatayev І. and Kader G. believe that the critical ratio of cations [(Ca 2+ + Mg 2+ ) / Na + ]10, at which the amount of absorbed sodium reaches 10 % of the cation exchange capacity (CEC) of the soil, is equal to 0.23 MO. Therefore, when K<0.23 MO the water is not suitable for irrigation.
At К≤0.65 water is suitable for irrigation, 0.65<К≤0.75 water is unsuitable, К>0.75 water is very unsuitable, because it causes alkalinization of soil.
Richards L.A. assesses the danger of salinization depending on the total mineralization and the value of SAR ( Table 2).
Concentration of toxic ions. For each species of plants, toxic substances may be different, therefore the list of toxic ions depends on the cultivated culture.
In general, when assessing the toxicity of individual ions, the content of boron (B 3+ ), sodium (Na + ), chloride (Cl -), heavy metals (As 3+ , Co 2+ , Cu 2+ , Pb 2+ , Ni + , Zn 2+ ), nitrates (NO3 -), total alkalinity (HCO3 -) and pH in water is taken into account. Increased content of toxic ions in irrigation water can lead to their accumulation in the leaves, cause burns of plants (this can occur when sprinkled at the daytime). Table  1.3 shows the characteristics of the irrigation water quality under the different methods of irrigation, depending on the concentration of Na + , Cl -, НСО3 -, B 3+ і NO3ions.

Assessment of water quality of the Sasyk reservoir and the analysis of the results
In order to assess water quality in the Sasyk water body, the results of the statistical processing of the initial data in the range of the village of Trapivka, HPS-2 locations for the period from 2007 to 2017 (according to the Odessa Regional Department of Water Resources, Table 4) were used.
The assessment of water quality of the Sasyk reservoir for irrigation properties is presented in Table 5.
The mineralization of Sasyk waters during the warm period (WP) is on average 1620 mg/dm 3 in the range of fluctuations from 324 to 3550 mg/dm 3 .
By the classification of Alekin O. (Maksimov, 1979) Sasyk waters belong to the chloride class, sodium group within 85-90 % of the WP.
On average, the waters of Sasyk belong to subtype IIb, but the conditions for forming their quality are such that during the WP they may be subtypes: IIa (30 %), Ib (50 %), ІІIb (10 %) and ІІIc (10 %). If the cold season is included in the review, the distribution of the water subtypes in Sasyk will be as follows: IIa (29 %), Ib (42 %), ІІIa (5 %), ІІIb (19 %) and ІІIc (5 %). That is, all subtypes of water are found except subtype I. Such a variety of water subtypes throughout the year indicates a large number of factors that influence the formation of water quality in Sasyk.
The total concentration of toxic salts (ions) is 1294 mg/dm 3 (range -96-3273 mg/dm 3 ). When the waters of Sasyk dry up, NaCl salt can be formed in an average amount of 787 mg/dm 3 (up to 1344 mg/dm 3 ). This salt, according to a degree of toxicity, corresponds to sodium bicarbonate NaHCO3. In addition, during 80 and 90 % of the WP (respectively) less toxic salts can be formed: Na2SO4 in an average of 199 mg/dm 3 (up to 1587 mg/dm 3 ) and MgSO4 -358 mg/dm 3 (up to 542 mg/dm 3 ).
Thus, with the evaporation of a 10 mm water layer on an area of 1 hectare, an average of 130 kg/ha (up to 330 kg/ha) of toxic salts can be formed. They are: 79 kg/ha (up to 130 kg/ha) will be NaCl; 20 kg/ha (up to 160 kg/ha) -Na2SO4; 36 kg/ha (up to 54 kg/ha) -MgSO4.
According to Kostyakov's A. A. classification water of Sasyk is classified as "high-risk" water, according to the US classificationwater with "very high" salinity. When using Sasyk water for irrigation there is a high risk of salinization of the soil.
According to Bezdnina's S. classification water of Sasyk belongs to category III. The waters of this category can be used for irrigation after chemical melioration and dilution with low mineral water.
Within 90 % of the WP, according to Stegler's X. classification, the water is unsatisfactory. It is limitedly suitable for the irrigation of salt-resistant crops. The danger of salinization according to Antipov-Karatayev I.N. and Kader G.M.-the water is "not suitable" during 70 % of the WP, according to Budanov A.M.-the water is "not suitable" within 90 % of the WP, according to Mozheiko A.M. and Vorotnyk T. K. the water is "very unfavourable" during 65 % of the WP and "unfavourable" during 22 % of the WP. According to the SAR indicator of the United States Department of Agriculture the danger of watering is "low" within 96 % of the WP, this does not coincide with the assessment of other methods.
According to Sobolch G. and Darab K. the amount of magnesium in the water of Sasyk has a detrimental effect on the soils (the danger of magnesium alkanization) during 80 % of the WP. According to Kelly and Libih, using Sasyk water for irrigation is impossible due to the sodium content during 87 % of the WP.

Ta ble 4
The results of monitoring the water quality of the Sasyk reservoir (warm period) (according to the Odessa Regional Department of Water Resources)

Conclusions
1. In general, the assessment of the irrigation properties of Sasyk water, according to different methods, is the same: the salinity of the waters is very high and promotes salinization of the soil; the content of sodium and magnesium creates the danger of soil alkalinity.
2. For safe using Sasyk water for irrigation, their chemical melioration and dilution with water with low mineralization are necessary.