Water quality management
Water quality management is a system of organizational and technical measures aimed at changing the pattern of the physical and chemical properties of the water in a water body used as well as water discharged from it to a lower stretch of a river or another water body. It is carried out with the view of intensifying water self-purification processes in water reservoirs that serve as sources of utility and drinking water supply, for which they cannot manage to curtail inflow of polluting and biogenic substances through water control practice on riparian lands and catchment areas and as a consequence of growing degradation of drinking quality of water.
Exceptional variety of the natural and anthropogenic factors that shape, due to their combination, the full range of the transformation processes of the chemical composition of water in different areas around the reservoir and downstream eliminates the possibility to follow standard water quality management (WQM) procedures even for reservoirs of the same class and located in similar geographical conditions. Therefore, development of environmentally sound and cost-effective WQM measures in any reservoir irrespective of its size is a problem that would be particularly individual for each case. This problem should be solved based on:
a) thorough knowledge of the hydrologic, hydrochemical, and hydrobiological conditions of a particular reservoir and water quality requirements clearly defined by the water user in the water withdrawal area along with specifying the list of the factors that complicate the drinking water treatment technology in water supply plants;
b) multiple-choice mathematical modeling of water exchange and water body transformation processes throughout the reservoir and water withdrawal area for simulation of the effect of any measures on water quality change with different options of refinement of dispatching procedures for stream flow control taking into consideration not only its quantitative but also qualitative measures. Employment of the existing limnological models describing processes in water bodies by using simultaneous equations allows clearly defining the requirements for the reliability of source information that characterizes the current ecological health of the reservoir. This would allow identifying the most effective and economically viable measures, working out regulations for their implementation under different hydrometeorological conditions, optimizing the monitoring of the manipulated ecosystem.
The above-listed requirements for implementation of WQM measures in the reservoir are essential because impact of water ecosystem may bring about not only intensification of reservoir water self-purification, but also loss of its operation stability, degradation, which is fraught with dramatic deterioration of water quality in the water source. At the beginning of the searching for the optimum WQM method by using mathematical simulation, they carry out simulation computation of possible behavior of water composition indices during extremely high water and low water years at maximum flood water inflow to the reservoir or absence of floods. This will allow assessing: the most probable range of perennial fluctuations in the concentration of the substances that deteriorate drinking quality of water; degree of their danger for water supply in the context of current load of these substances on the water body and expected trends of economy development in reservoir catchment area and on reservoir banks in the longer term.
Any WQM method should be considered as that alternative to possible reduction of the water pollutant load on the water body through implementation of preventive water protection measures in the reservoir catchment area. Although it is always problematic to successfully implement such measures in view of the difficulties related to the organization of such preventive measures because of their magnitude and financing complications, still it would be relevant, having set the most real value of the reduction of external load on the reservoir, to assess probable change in the behavior of the considered water quality characteristics in the water body and its water withdrawal site in years and seasons with different water availability.
In the international practice of WQM, the following methods are known: change of the schedules of the main water mass release to the waterworks facility downstream and polluted water discharge; water aeration; hypolimnion water diversion; phosphorus deposition; decrease of phytoplankton biomass or macrophyte concentration; water surface shadowing; aeration of bottom sediment, or its removal or screening; destratification; construction of pre-reservoirs. Having selected the most suitable out of these methods, they carry out simulation calculations related to the assessment of their environmental and economic efficiency of the reservoir for different water availability years in comparison with the already considered options of preventive nature-conservative measures on its catchment area. Based on these calculations, they determine the environmentally optimum way of controlling water quality parameters behavior in the reservoir, the reliability of the rationale of which conforms to the today’s limnology level. The possibility of its experimental testing in small sections of the water surface, sometimes isolated from the rest part of the body of water by various materials, depends on the selected control method. If there is such a possibility, it would be reasonable to carry out a complex of appropriate observations over the ecosystem functioning within and beyond the experimental site and compare the results of these observations, and then replicate the processes on the model.
Such experimental test of the suggested WQM method will considerably increase the justification of the proposed solution for the management problem. After a decision about the implementation a developed WQM method is taken, the research will be concluded with working out of the scheme of reservoir operation mode monitoring items and scope for further improvement of its mathematic model which is becoming a reliable tool for the assessment of reservoir ecological state and its short-term forecast, needed by the management personnel for carrying out operational dispatching work. Mathematic models of selective water withdrawal are used for water quality management, when water is taken by pump stations from deep-water sites with vertically nonhomogeneous distribution of chemical and biological composition parameters, as well as for managing the quality of the water discharged downstream from such reservoirs. For operational use of such models for a concrete water withdrawal area, their algorithm can be transformed into a system of the nomograhic charts that are convenient for the water intake structure maintenance staff. At that, this structure must be constructed in the form of a water intake (decant) tower with windows placed on several horizons of water layer, while the maintenance personnel must have a thermoconductometer, oxymeter, and other remote sensors for regular monitoring over the changes in temperature, electric conductivity, oxygen concentration, and other water quality parameters (or for automatic recording of them) on a vertical before the intake tower and in the water taken (discharged downstream) from the reservoir.
Small reservoirs with a capacity of up to 0.5 km3, which carry out over-year river runoff regulation, are the most good for protection against pollution and control of the water quality formation process. The water catchment area of such reservoirs is only 50-70 times larger than the area of the water surface, which allows selecting the least developed territories and limiting development of urbanization and industrial construction there. Such reserve catchment areas with protected (reserve) land use regime are reasonable to be selected in the zones of future-oriented construction of systems of centralized drinking water supply to population from surface sources. Unlike lowland reservoirs built on large rivers the above mentioned type of reservoirs placed in upper sections of the river network is characterized by great relative deep water, poor water replacement (long flushing period) and, consequently, long average time of water body presence, i.e. the most potential self-purification ability of the water ecosystem. At the same time, they surely provide the required water yield in low-water phases of flow fluctuations over many years; also their specific land flood factor is half as much as that of similar water bodies on large rivers. Their specific shoreline is longer too, which is more favorable for the development of water flora and fauna, water self-purification, and use of those for recreation purposes. It is possible to enhance the self-purification ability of water in the reservoirs under consideration without lowering reservoir storage capacity by constructing those in the form of polysectional water body with the main section, which includes appurtenant and middle areas, and water protection sections in the upper areas of the main and lateral valleys.
Selected bibliography
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