Water in rivers, aquifers, and lakes naturally contains many dissolved materials, depending on atmospheric inputs, geological conditions, and climate. These materials define the water’s chemical characteristics. Its biological characteristics are defined by the flora and fauna within the water body, and temperature, sediment load, and color are important physical characteristics. Water “quality” is a function of chemical, physical, and biological characteristics but is a value-laden term because it implies quality in relation to some standard. Different uses of water have different standards. Pollution can be broadly defined as deterioration of some aspect of the chemical, physical, or biological characteristics of water (its “quality”) to such an extent that it impacts some use of that water or ecosystems within the water. Major water pollutants include organic material, which causes oxygen deficiency in water bodies; nutrients, which cause excessive growth of algae in lakes and coastal areas—known as eutrophication (leading to algal blooms, which may be toxic and consume large amounts of oxygen when decaying); and toxic heavy metals and organic compounds. The severity of water pollution is governed by the intensity of pollutants and the assimilation capacity of receiving water bodies—which depends on the physical, chemical, and biological characteristics of streamflow— but not all pollutants can be degraded, however.

Chemical river water quality is a function of the chemical load applied to the river, water temperature, and the volume of flow. The load is determined by catchment geological and land-use characteristics, as well as by human activities in the catchment: Agriculture, industry, and public water use also may result in the input of “polluting” substances. Agricultural inputs are most likely to be affected by climate change because a changing climate might alter agricultural practices. A changing climate also may alter chemical processes in the soil, including chemical weathering (White and Blum, 1995). Avila et al. (1996) simulated a substantial increase in base cation weathering rates in Spain when temperature and precipitation increased (although if precipitation were reduced, the effects of the higher temperature were offset). This, in turn, resulted in an increase in concentrations of base cations such as calcium, sodium, and potassium and an increase in streamwater alkalinity. Warmer, drier conditions, for example, promote mineralization of organic nitrogen (Murdoch et al., 2000) and thus increase the potential supply to the river or groundwater. Load also is influenced by the processes by which water reaches the river channel. Nitrates, for example, frequently are flushed into rivers in intense storms following prolonged dry periods.

River water temperature depends not only on atmospheric temperature but also on wind and solar radiation (Orlob et al., 1996). River water temperature will increase by a slightly lesser amount than air temperature (Pilgrim et al., 1998), with the smallest increases in catchments with large contributions from groundwater. Biological and chemical processes in river water are dependent on water temperature: Higher temperatures alone would lead to increases in concentrations of some chemical species but decreases in others. Dissolved oxygen concentrations are lower in warmer water, and higher temperatures also would encourage the growth of algal blooms, which consume oxygen on decomposition.

Streamwater quality, however, also will be affected by streamflow volumes, affecting both concentrations and total loads. Carmichael et al. (1996), for example, show how higher temperatures and lower summer flows could combine in the Nitra River, Slovakia, to produce substantial reductions in dissolved oxygen concentrations. Research in Finland (Frisk et al., 1997; Kallio et al., 1997) indicates that changes in stream water quality, in terms of eutrophication and nutrient transport, are very dependent on changes in streamflow. For a given level of inputs, a reduction in streamflow might lead to increases in peak concentrations of certain chemical compounds. Cruise et al. (1999) simulated increased concentrations of nitrate in the southeast United States, for example, but the total amount transported from a catchment might decrease. Hanratty and Stefan (1998) simulated reductions in nitrate and phosphate loads in a small Minnesota catchment, largely as a result of reductions in runoff. Alexander et al. (1996) suggest that nutrient loadings to receiving coastal zones would vary primarily with streamflow volume. Increased streamflow draining toward the Atlantic coast of the United States under many scenarios, for example, would lead to increased nutrient loadings. An increased frequency of heavy rainfall would adversely affect water quality by increasing pollutant loads flushed into rivers and possibly by causing overflows of sewers and waste storage facilities. Polluting material also may be washed into rivers and lakes following inundation of waste sites and other facilities located on floodplains.

Water temperature in lakes responds to climate change in more complicated ways because thermal stratification is formed in summer, as well as in colder regions in winter. Meyer et al. (1999) evaluated the effect of climate change on thermal stratification by simulation for hypothetical lakes. They show that lakes in subtropic zones (about latitude 30 to 45°) and in subpolar zones (latitude 65 to 80°) are subject to greater relative changes in thermal stratification patterns than mid-latitude or equatorial lakes and that deep lakes are more sensitive than shallow lakes in the subtropic zones. Hostetler and Small (1999) simulated potential impacts on hypothetical shallow and deep lakes across North America, showing widespread increases in lake water temperature slightly below the increase in air temperature in the scenarios used. The greatest increases were in lakes that were simulated to experience substantial reductions in the duration of ice cover; the boundary of ice-free conditions shifted northward by 10° of latitude or more (1,000 km). Fang and Stefan (1997) show by simulation that winter stratification in cold regions would be weakened and the anoxic zone would disappear. Observations during droughts in the boreal region of northwestern Ontario show that lower inflows and higher temperatures produce a deepening of the thermocline (Schindler et al., 1996).

The consequences of these direct changes to water quality of polluted water bodies may be profound, as summarized by Varis and Somlyody (1996) for lakes. Increases in temperature would deteriorate water quality in most polluted water bodies by increasing oxygen-consuming biological activities and decreasing the saturation concentration of dissolved oxygen. Hassan et al. (1998a,b) employed a downscaled climate model combined with GCM output to predict future stratification for Suwa Lake, Japan, on a daily basis, as well as for the prolonged summer stratification period. They predict increased growth of phytoplankton and reduced dissolved oxygen concentrations at different depths in the lake. Analysis of past observations in Lake Biwa in Japan (Fushimi, 1999) suggests that dissolved oxygen concentrations also tend to reduce when air (and lake water) temperature is higher.

Water quality in many rivers, lakes, and aquifers, however, is heavily dependent on direct and indirect human activities. Land-use and agricultural practices have a very significant effect on water quality, as do management actions to control point and nonpoint source pollution and treat wastewaters discharged into the environment. In such water bodies, future water quality will be very dependent on future human activities, including water management policies, and the direct effect of climate change may be very small in relative terms (Hanratty and Stefan, 1998). Considerable effort is being expended in developed and developing countries to improve water quality (Sections 4.5 and 4.6), and these efforts will have very significant implications for the impact of climate change on water quality.

Confidence in estimates of change in water quality is determined partly by climate change scenarios (and their effects on streamflow), but additional uncertainty is added by current lack of detailed understanding of some of the process interactions involved.

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