| Themes > Science > Earth Sciences > Geology > Water and Water Cycles > Surface water > Surface water quality |
Brief Description: The quality of surface water in rivers and streams, lakes, ponds and wetlands is determined by interactions with soil, transported solids (organics, sediments), rocks, groundwater and the atmosphere. It may also be significantly affected by agricultural, industrial, mineral and energy extraction, urban and other human actions, as well as by atmospheric inputs. The bulk of the solutes in surface waters, however, are derived from soils and groundwater baseflow where the influence of water-rock interactions are important [see groundwater quality; karst activity; soil and sediment erosion; soil quality; streamflow; wetlands extent, structure and hydrology]. Selecting the variables to be measured depends on the objectives and economics of the monitoring. This is a complex matter because there are so many potential chemical, physical and biological substances that could be important in any one area. From the viewpoint of geoindicators, the following variables may be selected: 1. Basic variables
Nutrients: ammonium, nitrate, nitrite, total N, orthophosphate, total P. Major constituents and dissolved solids: Ca, Mg, Na, Cl, SO4, HCO3, TDS. Direct field measurements: acidity, alkalinity, dissolved O, pH, temperature. Selected organic compounds of environmental significance: 2,4-D; 2,4,5-T; phenol, chlorophenols, cresols, atrazine, cyperquat, paraquat, benzidine, DDT, malathion. 2. Additional parameters
Of importance to agriculture - B. Significance: Clean water is essential to human survival as well as to aquatic life. Most is used for irrigation, with lesser amounts for municipal, industrial, and recreational purposes: only 6% of all water is used for domestic consumption. An estimated 75% of the populations of developing nations lacks adequate sanitary facilities, and wastes are commonly dumped into the nearest body of flowing water. Pathogens such as bacteria, viruses and parasites make these wastes among the world's most dangerous environmental pollutants: water-borne diseases are estimated to cause about 25,000 deaths daily. Water quality data are, thus, essential for the implementation of responsible water quality regulations, for characterizing and remediating contamination, and for the protection of the health of humans and aquatic organisms. Human or Natural Cause: The water quality of a lake, reservoir or river can vary in space and time according to natural morphological, hydrological, chemical, biological and sedimentological processes (e.g. changes of erosion rates). Pollution of natural bodies of surface water is widespread because of human activities, such as disposal of sewage and industrial wastes, land clearance, deforestation, use of pesticides, mining, and hydroelectric developments. Environment where Applicable: The main environments are those where surface water is used for human consumption or other societal uses, or where important freshwater fisheries, sensitive aquatic habitats or valuable wetlands are involved. Types of Monitoring Sites: These are determined by the location of known sources of pollution, ease of access to sampling sites, presence of streamflow gauges and required facilities. For those watersheds where problems are known or suspected, the areal water quality should be determined by a network of systematically operated sampling stations. Sampling for stream water quality should be conducted at or near streamflow gauge stations to allow the computation of contaminant levels. Spatial Scale: patch / landscape to regional Method of Measurement: Sampling and analysis for water quality determination varies with site conditions and the constituents to be measured. Typically, samples are collected so as to represent any changes in depth and width of the water body or stream, and in sufficient quantity to permit replicate analyses. Frequency of Measurement: Changes in surface water quality may be quite rapid (e.g. in response to weather variations and flooding). Water samples taken from streams are usually collected at specified intervals of time. Continuous, real-time monitoring systems therefore provide the most complete information. Comprehensive analysis for water-quality monitoring is, however, expensive, and for most diagnostic purposes sample collection and analysis 4-6 times yearly may suffice, with sampling twice yearly for radionuclides and organic chemicals. Limitations of Data and Monitoring: Long-term records of key parameters such as pH, HCO3, NO3, and Cl in surface waters are of value in detecting trends in environmental quality, but they may suffer in terms of accuracy due to changes in analytical or sampling methods and personnel. Applications to Past and Future: Surface water does not preserve an archive of past changes because of rapid flow and mixing rates. However, it is important to recognize that there are close links between the chemistry of surface water and that of the bottom sediments in contact with the water. Thus, analysis of the sediment column can provide invaluable data about past trends in water quality [see sediment sequence and composition]. Increasing or decreasing trends in key parameters can warn of approaching thresholds requiring remedial action. Possible Thresholds: For each parameter measured, thresholds have been set by national and international organizations (eg. WHO), according to the purpose for which the water is being used. |
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