Water Quality formation Factors
Estimation of the Influence of Possible Climate Warming on the Chemical Composition of Water (case study Western Siberia)
It is a well-known fact that a global temperature increase has been documented over the last decades [Climate Change, IPCC, Geneva, 2007]. The degree of climate changes observed in the past 50 years is different among regions. As was shown [Sherstyukov, 2009] for the major portion of European Russia (ER) between 1976 and 2006, the mean annual air temperature increased at a rate of 0.5–0.7°C per 10 yr. Positive trends in mean annual air temperature were also observed in Western Siberia (WS), but the respective rate is 0.1–0.4°C per 10 yr and is highly variable within the region. The most significant changes were detected in the extreme northwest of Siberia, where values of 0.5–0.8°C per 10 years were observed; statistically significant changes in the mean temperatures of summer months were also documented in the steppe zone of the trans-Ural region [Sherstyukov, 2009].
The scale of variations in the temperature field of the WS areas discussed here is consistent with global tendencies. For instance, according to [Pavlov, Malkova, 2005], the mean air temperature in the northern hemisphere increased in the 20th century by 0.6(±0.2)°C. The last period of warming began in the 1960s–mid 1970s. The air temperature increased in 1965–1995 in northern Russia by 1.1–1.2°C, whereas the global temperature rose by 0.5–0.6°C. Climate monitoring has shown that climate changes in Arctic and sub-Arctic regions occur more rapidly and lead to the most radical consequences influencing global atmospheric circulation. Table 1 (slide in the presentations) compares data on seasonal variations and observed tendencies of changes in air temperature for various regions of Russia. These data lead to the conclusion that the rate of increase in the mean annual and seasonal temperatures of WS is higher than that of other regions in Russia. According to [Pavlov, Malkova, 2005], the most radical changes are characteristic of the tundra.
The effect of possible climate warming on the chemical composition of waters has been estimated in a number of studies [Schlesinger, 1997; Battarbee et al., 2005; Skjelkvеle, Wright, 1998; Alcamo et al., 2002; Schindler, 2001]. Climate may significantly affect the flux of cations from catchments. Climate warming can enhance chemical weathering and the flux of basic cations from catchments to lakes [Sereda et al., 2011; Wright, Dillon, 2008] and can also intensify evaporation processes, which results in the concentration and salinization of aquatic systems. After prolonged droughts, which occur in southern regions in response to warming, the accumulation of salts in catchment becomes dangerous at the beginning of a rainy season, because salts can be rapidly transported into water reservoirs sharply increasing their salinity. When a lake is rapidly filled after a dry period, the concentrations of cations will be lower than during their more steady supply from the catchment [Dillon et al., 1997]. In boreal regions, warming and increasing precipitation result in soil enrichment in water, and adsorption–desorption processes and organic matter decomposition in soils change, which results in the liberation and removal of basic cations (and possibly metals) into water streams. An increase in mean temperature, precipitation, and storm frequency observed in England and Scandinavia is related to climate variations in the past 15–25 yr [Bjerknes, 1964; Hurrell, 1995, Rodwell et al., 1999, Evans and Monteith, 2000]. The deposition of marine aerosols will stimulate salinization processes in catchments, and the intense transport of salts into terrestrial waters during rain periods following prolonged droughts may be a hazardous phenomenon.
It is obvious that production processes are enhanced in water bodies and catchments in warm climate, which may eventually result in eutrophication [Feuchtmayr et al., 2009]. On the other hand, warming-related processes in catchments enhance plant growth, uptake of nutrient elements, including nitrogen, and oxidation–reduction processes.
Much attention has been given to the influence of climate on wetland systems, with a special emphasis on the carbon cycle. Wetlands are widespread over the whole area of Russia and especially abundant in WS. On the one hand, they are peculiar traps of pollutants and stabilizers of the global carbon cycle, and they may become sources of secondary pollution under certain conditions, for example, during climate warming [Bayley et al., 1997; Dillon et al., 1997] showed that sulfate deposition over considerable wetland areas is accompanied by their reduction under anoxic wetland conditions. This is followed by the emission of sulfur-bearing gases, which are reoxidized in the atmosphere and transported over considerable distances. These processes are intensified during warmer periods.
A very hazardous phenomenon is the impact of warming on permafrost. Furthermore, the high degree of water logging in WS coupled with the tendency of precipitation increase owing to climate warming in humid zones, which was noted in [Moiseenko, Gashkina, 2010; Pavlov, Malkova, 2005], will become more significant in the future causing the enrichment of lake water in organic matter. Other effects related to the paludification of catchments (acidification of waters and an increase in CI and mobility of some toxic metals) will also be strengthened. The warming of frozen peatlands in northern WS may increase the emission of CH4 and other greenhouse gases into the atmosphere. The intensification of thermokarst processes will result in an increase in the number and area of lakes.
Thus, the results of investigations published in recent years highlighted the problem of the influence of climate variations on the development of water chemistry. However, controversial estimates were reported in the literature on the consequences of warming for terrestrial waters. The character of temperature influence is determined primarily from changes in the hydrologic conditions of water formation and biogeochemical cycles, i.e., the amount of precipitation, existence of a snow cover, state of bedrocks in the catchment, degree of their depletion in exchange bases, accumulation of P (or acid forming agents) in the catchment on the historical timescale, microbiological activity, and acceleration of plant growth.
The results of our studies of lake water chemistry in a latitudinal gradient (from tundra to arid zones) revealed general trends of changes in the chemical composition of water depending on temperature gradient; this provided a scientific basis for the theoretical prediction of possible changes in the hydrochemistry of lakes in response to a temperature increase. The chemical composition of water is controlled by a number of factors, which may enhance or suppress the geochemical processes of water formation. Let us consider the influence of climate variations via changes in the main mechanisms of the development of water chemistry on the basis of the results of our investigations.
Influence of temperature on the hydrochemistry of WS lakes. No special long term investigations of the influence of climate warming on the chemical composition of lake water were carried out in WS. Hydrochemical changes can be predicted only on the basis of correlations between the general characteristics of the chemical composition of lake water and temperature characteristics of the area. Such relations can be estimated on the basis of the extensive investigations of 2010–2012 on the hydrochemical parameters of WS lakes from tundra to steppe (Fig.1, 2 in the presentatons).
Factor and regression analysis demonstrated that the climate parameter, the sum of active temperatures (Σt > 10°C), is closely related to the water salinity indicator (Σions), which increases in response to climate warming. A correction was introduced to account for the degree of paludification in the catchment (Km), which strongly influences the chemistry of natural waters in WS. The following regression equation was derived:
Σions = – 6.98 + 13.55exp(0.002Σt > 10°C) – 2.69Km (1)
It was found that this empirical equation is most useful for the middle taiga, southern taiga, and forest steppe. The influence of permafrost is significant in northern regions, and relations between landscape components are different; therefore, another relation was derived:
Σions = 202.39 – 0.37Σt > 10°C+ (Σt > 10°C)2 1.442Krunof – 0.31Km (2)
which includes corrections for both the degree of paludification, Km, and annual runoff, Krunoff. The increasing role of surface runoff in the formation of the total salinity of lake water can be explained by the fact that a higher fraction of water is transported from the catchment to the lake basin in the permafrost zone.
The prediction of changes in lake water salinity for the permafrost using Eq. (2) suggested that a temperature increase of 1.0°C may have different effects on salinity; in particular, a slight increase in salinity is observed in the tundra, and a slight decrease is characteristic of the northern taiga. This is related to the effects of two opposing phenomena. On the one hand, the thawing of salt-bearing rocks of marine origin, which are widespread in the Yamal and Gydan peninsulas, will enhance the chemical weathering and leaching of ions; an increase in precipitation will exert diluting effects in permafrost zones.
Calculations using Eqs. (1) and (2) showed that temperature changes of 1.0–10.0°C will not significantly increase the accumulation of ions in lake water in all of the natural zones. The salinity will even decrease in waterlogged taiga areas, because freshwater mono-mineralic (90% quartz) sand and sandy loam fluvial and alluvial deposits, which are common in the taiga zone, cannot supply significant amounts of ions even at intense chemical weathering. Therefore, further freshening of taiga lakes can be expected in response to climate warming and a corresponding increase in soil moisture content.
A temperature increase activates eutrophication owing to the enhancement of the removal of P, N, and dissolved organic matter in all zones except for tundra; however, no significant correlation was observed between the P content and the climate parameter.
An increase in moisture content in landscapes will result in the progressive development of wetland systems. The ubiquitous water logging of the taiga zone of WS is very important for the evolution of the chemical composition of surface and ground waters. Before penetrating into the saturation zone of an aquifer system, precipitation percolates through a peat layer and acquires unfavorable physicochemical properties (low content of dissolved oxygen, low salinity, high acidity, high organic matter content, and high oxygen demand). Correspondingly, all the aforementioned hydro-chemical characteristics will rise.
Water soluble salts and carbonates are especially abundant in the rocks of the northern Yamal Peninsula, the fraction of freshwater glacial and fluvial sediments increases southward and inland at the expense of salt-bearing marine deposits [Yamal Peninsula, 1975]. There is no significant increase in salinity with increasing temperature even in the forest steppe and steppe zones of WS, in contrast to ER, where the predicted increase is as high as 25% [Moiseenko and Gashkina, 2010].
Summarizing the analysis of landscape–hydrological and hydro-chemical transformations of the water of small lakes in response to climate changes, it should be noted that a temperature increase is the primary cause and the first member in the chain of rather significant changes in natural processes. An increase in air temperature will be followed, especially in spring and summer months, by a temperature increase in soils and rocks, an increase in the depth of seasonal thawing of soils in the catchments of lakes in the permafrost zone, and acceleration of erosion processes in humid areas. The tundra, forest tundra, and taiga zones will show an increase in the amount of precipitation, runoff to lakes, and the rate of biological cycling in plant communities and soils of lake catchments. In contrast, the forest steppe and semiarid zones will show an increase in the salinization of soils and lake waters, whereas a decrease in the amount of precipitation will reduce water runoff from catchments, but this process will be less evident compared with that predicted for the lake water of ER.
An important consequence of transformations in natural complexes owing to climate warming in the WS taiga will be a change in the hydrologic and biochemical balance of wetland ecosystems. An increase in the amount of precipitation will enhance paludification, which will result in enrichment in metal humates, a pH decrease owing to the release of free H+ from initially frozen peatlands, and general intensification of reducing processes. The reduction of Fe and Mn compounds in gleyed soils will further enrich lake waters in Fe-bearing organometallic compounds, which will result in an increase in already high CI.
The currently observed global and regional climate warming will affect in the future the biogeochemical cycles of the catchments of lakes and the total salinity and ionic compositions of waters will change accordingly. Dramatic changes in water salinity are not expected in humid zones owing to the mutually compensating effects of chemical weathering, thawing of frozen rocks, and dissolution of salts by the increasing amounts of precipitation and fresh soil waters and groundwaters. An increase in the seasonal variability of water salinity is predicted, especially for the semiarid and arid zones of WS. A temperature increase activates eutrophication processes owing to the more intense removal of P, N, and dissolved organic matter in all natural zones.
The influence of warm periods on the contents of nitrates, organic matter, and marine salts was noted. However, the available estimates of the consequences of a temperature increase for terrestrial waters are ambiguous. The character of temperature influence is primarily manifested through changes in the hydrologic conditions of water formation and biogeochemical cycles, i.e., the amount of precipitation, presence of a snow cover, state of bedrocks in the catchment, degree of soil depletion in exchange bases, accumulation of P (or acidforming substances) in catchment on historical timescales, microbiological activity, and enhancement of plant growth.