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Surface water quality

NAME: Surface water quality

(With contributions from W.M. Edmunds, C. Neal, W. Osterkamp, P. D. Jones, and J. Ridgway. Revised 2004)

BRIEF DESCRIPTION: The quality of surface water in rivers and streams, lakes, ponds and wetlands is determined by the chemistry of precipitation and by interactions of runoff (surface water) with soil and rock, transported solids (organics, sediments), groundwater and the atmosphere. Surface-water quality may also be significantly affected by agriculture, industry, mineral and energy extraction, urbanization and other human actions, as well as by atmospheric inputs. In rivers, the bulk of the solutes in the headwaters are derived from soils and groundwater baseflow, where the influence of water-rock interactions is important [see [see groundwater quality; karst activity; soil and sediment erosion; soil quality; streamflow; wetlands extent, structure and hydrology]. Lower down the system or catchment anthropogenic inputs from diffuse or point sources may be significant. Selecting what to measure is a complex matter because there are many potential chemical, physical and biological substances that enter freshwaters from natural and human influenced sources, and these may vary from area to area. In the case of the transition elements and many micro-organic contaminants, they reside in both dissolved and particulate forms, and both forms may have a considerable bearing on environmental impacts. Thus the choice of water quality indices depends on the objectives and economics of the monitoring program: different uses require different standards of water quality. From the viewpoint of geoindicators, the following determinands may be selected:

1. Basic variables

2. Additional parameters

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 lack adequate drinking water and sanitary facilities as human, industrial and animal 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 25,000 deaths daily. Thus water quality data are 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). Major storms may cause significant longer-term (c. 10 years) changes in water quality. Pollution of natural bodies of surface water is widespread because of human activities, such as disposal of sewage and industrial wastes, acidification of rainfall by gas emissions to the atmosphere, land clearance, deforestation, use of pesticides, mining, and hydroelectric developments.

ENVIRONMENT WHERE APPLICABLE: All environments 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 measured using 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 loads.

SPATIAL SCALE: patch/landscape to regional (individual stream to whole catchment(s))

METHOD OF MEASUREMENT: Sampling and analysis for water quality determination varies with site conditions and the constituents to be measured. For large water bodies sampling patterns should be designed to take account of any likely lateral or vertical variations in quality. Samples are typically, collected 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 provide the most complete information, but are restricted to those parameters for which reliable sensors exist. 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 and extent of records due to changes in analytical or sampling methods, personnel, financial constraints and organisational restructuring. Long-term records are often missing for many determinands of environmental concern (e.g. transition metals, micro-organic pollutants and pathogens). Many heavy metals, trace elements, radionuclides and organic compounds (such as pesticides) adsorb readily onto fine-grained suspended sediment. Thus water analyses yielding transport rates of these contaminants are dependent on accurate discharge data for fluvial sediment.

APPLICATIONS TO PAST AND FUTURE: Surface water usually does not preserve an archive of past changes because of rapid flow and mixing rates. Nonetheless, it is important to recognize that there are close links between the chemistry of surface water and that of the bottom and flood plain sediments in contact with the water. For example, many transition metals, and perhaps some of the micro-organic pollutants, are present in urban and industrial catchments due to a historical legacy of mining and urban activity that stretches back through the industrial revolutions and earlier. Thus, analysis of the flood plain and sediment column can provide invaluable data about past trends in water quality [see sediment sequence and composition]. In addition, there may also be an indirect legacy of changing patterns in industry, such as the closure of mines with consequential flooding and increased mobilization of metals via groundwater seepages to the stream and river channels. Increasing or decreasing trends in key parameters can warn of approaching thresholds requiring remedial action.

POSSIBLE THRESHOLDS: For the majority of the parameters measured, 0thresholds have been set by national and international organizations (eg. WHO), depending on the purpose for which the water is being used. There is a possibility of thresholds being breached when bottom and flood plain sediments are disturbed and substances buried in the past (chemical time bombs) become remobilized to degrade water quality.


Anon 1996 Environmental Indicators of Water Quality in the United States. US EPA.

Meybeck, M., D.Chapman & R.Helmer (eds) 1989. Global freshwater quality - a first assessment. Oxford: Basil Blackwell.

Nixon, S. et al. 2003. Europe's water: An indicator-based assessment. European Environment Agency.

Vorosmarty, C.J., C. Leveque, C. Revenga et al 2005. Fresh Water. Chapter 7 of Ecosystems and Human Well-being: Current State and Trends, Volume 1. Washington: Island Press

OTHER SOURCES OF INFORMATION: Environment, water/hydrology, public health and river agencies, FAO, GEMS / UN Water Program, IAHS, WHO, FAO, UK ECN.

RELATED ENVIRONMENTAL AND GEOLOGICAL ISSUES: There are many causes of changes in the quality of surface water, including acid precipitation, urbanization, mining, agricultural development, land clearance, industrial change and deforestation, as detailed above.

OVERALL ASSESSMENT: Surface water quality is one of the most fundamentally important environmental variables to be monitored, and it is of particular value as an indicator of short-term improvement or deterioration in the environment.

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