What do you understand by the term water quality, and what factors control its variation in time and space?
What do you understand by the term water quality, and what factors control its variation in time and space?
Water quality can be described as the physical, chemical and biological characteristics of water, and is a broad term encompassing the quality of water held within river channels and lakes, but also groundwater, water held in sol, rainwater, and water in the oceans. As far as humans are concerned water quality is of primary importance in the freshwater resources which make up only a small percentage of the Earth's water resources, and therefore I shall focus primarily on water quality in river channels and lakes for the purpose of this essay. Water quality is controlled by a multitude of factors which are clearly variable over time and space and which can be grouped into natural and man-made influences. Whether the source of water is underground (groundwater) or directly from rain washing over the land is obviously a major factor determining water quality in rivers and lakes; also of great influence will be the effects of catchment characteristics, inputs to the hydrological system including pollutant sources, and mans treatment of contaminated water.
The term water quality can be broken down further into natural water quality, defined as the physical, chemical and biological characteristics of water unaffected by human activity. Taken in a literal sense, in reality it is of course difficult to find a water body on the earth which is totally unaffected by human influences. This is because whilst evaporation does provide a source of pure (distilled) water for precipitation, this will then become increasingly concentrated with dissolved material as it moves through the atmosphere and in the subsequent stages of the hydrological cycle as it comes into contact with organic matter, soil and rock material. Even Arctic precipitation contains a certain degree of constituents discharged into the atmosphere from human influences. For this reason it is often the case that natural water quality has become separate from water quality, which is looked at regardless of whether affected by human activity, simply as a result of a perceived absence of water pollution. This is the introduction by man of substances or energy that can harm living resources; pose a hazard to human health; hinder aquatic activities; or restrict the use of water for economic or amenity purposes.
In terms of natural factors that control water quality, water flowing in a river channel will have evolved through a complex series of interactions with the soil, rock and biota of the catchment system, and therefore such interactions must be considered as spatial and temporal variations in water quality. It must be noted here that in terms of spatial variation, differences will always occur between and along river catchments, as well as vertically in water bodies (particularly in groundwater supplies). Equally, temporal change can occur diurnally, seasonally and as longer term trends. Thus when looking at variations in water quality over time and space one must also always consider the different scales at which the observations of change are occurring.
The quality of precipitation falling on a catchment area is an important factor which partly controls water quality. Water falling as precipitation will not be chemically pure, as a host of particles will be found in solution within precipitation. These include naturally occurring materials such as dust, pollen, volcanic dust, bacteria and fungal spores and chemicals such as chlorides, sodium, sulphate, magnesium, calcium and potassium from evaporated sea water. Spatial variations in water quality can therefore exist as concentrations of material with terrestrial origin will dominate non-humid continental interiors, whereas coastal regions will show higher concentrations of sea ...
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The quality of precipitation falling on a catchment area is an important factor which partly controls water quality. Water falling as precipitation will not be chemically pure, as a host of particles will be found in solution within precipitation. These include naturally occurring materials such as dust, pollen, volcanic dust, bacteria and fungal spores and chemicals such as chlorides, sodium, sulphate, magnesium, calcium and potassium from evaporated sea water. Spatial variations in water quality can therefore exist as concentrations of material with terrestrial origin will dominate non-humid continental interiors, whereas coastal regions will show higher concentrations of sea salts and marine-based particles.
Temporal variations in water quality can also occur as a result of climatic changes to precipitation. Seasonal and annual patterns result in changing wind direction and air mass movement which lead to variations in chemical compositions of water, for example in North Carolina calcium concentrations (resultant from soil dust) are far higher in summer months due to a dominance of continental air flow at this time. Short term variations can occur during rain storms when the progressive exhaustion of aerosol decline results in a marked decrease in ion concentrations from the onset of the storm event.
The quality of water in a catchment is also controlled greatly by changes that occur throughout the interception process. Enrichment of rainwater occurs due to the leaching of metabolites and exudates from plant tissue and the wash of atmospheric aerosols, and this will tend to acidify the water. Differing vegetation over space can cause marked variations in chemical compositions resulting from this enrichment process, for example Ninglard (1970) revealed that enrichment was up to twice as great under mature strands of conifers than from hardwood regions. Temporal variations can also occur seasonally as a reflectance of canopy cover and rates of nutrient cycling which also vary annually. Also, storm events will reduce the susceptibility of vegetation to leaching over time by decreasing the availability of aerosols on leaf surfaces.
The lithology of the catchment area is another factor which will controls water quality over space and time, as about 60% of the total natural dissolved load of rivers is derived from rock weathering (Walling and Webb, 1996). The composition of stream solutes is closely linked to the rock type underlying a catchment. For example Miller (1961) noted that solute concentrations in streams draining easily weathered sandstone catchments were 10 times greater than in those draining quartzite. In general, chalk and limestone catchment areas tend to lead to clear, hard waters rich in calcium and magnesium salts, and impervious rocks, such as granite, lead to turbid, soft waters due to their resistance to weathering which produces lower solute concentrations. Spatial variations will therefore obviously occur between geologically different catchment areas and along a river's course if the rock type changes.
Spatial and temporal variations in water quality are also determined by groundwater flows into a rivers catchment system. This is largely dependent on the nature of precipitation into the catchment, the nature of the soil through which the water will percolate, the geology of the aquifer and the quality of the existing groundwater, which may be thousands of years old. This demonstrates the importance of temporal variations in groundwater quality which may be very long term. Spatial variations occur as solute concentrations in groundwater tend to increase with depth due to a lack of flushing soluble compounds by new water in deep groundwater stores.
Large surface water storage (e.g. lakes and reservoirs) can also determine variations in water quality through time and space. As the water remains stationary fro long periods at a time bacterial activity helps to remove organic matter and flocculation and settlement of small particles are more likely to occur. This can therefore improve water quality. This is further helped by the reduction in turbidity which has been known to promote toxicity in water. Fine sediment in suspension appears to exacerbate the toxic effect of certain chemicals and the sediment may also tend to absorb leached nutrients. For example Walling in 1990 noted that phosphates in particular are preferentially absorbed by fine sediments. In some cases however water quality in large water bodies can be altered in a negative way due to spatial and temporal variations. Often during summer months the water in a still surface body will undergo thermal stratification due to the heating of the top layers and corresponding decrease in their density. This leads to two non-mixing layers in the water body - the upper (epilimnion) and the lower (hypolimnion), separated by a small thermocline boundary. This is shown in the diagram below. The epilimnion layer, being exposed to warmth and sunlight, promotes algal bloom which leads to the deoxygenation of the hypolimnion layers, which can become stagnant. The result is a spatial variation in water quality with depth as lower layers become rich in iron, manganese, ammonia and phosphates released under aerobic conditions by sediments and microbes on the lake floor breaking down decaying organic matter.
Turning now to human factors which control water quality over time and space, we can generally distinguish between two principle categories: pollution, and treatment. Man-made pollution can originate from municipal, industrial or agricultural sources. Municipal waste is primarily composed of human excreta containing numerous pathogenic organisms. Industrial wastes vary massively dependent upon the type and intensity of activity, and may contain a wide variety of organic and inorganic chemicals. Spatial variations will obviously result from differing locations of outfall points of both municipal and industrial waste, where concentrations of compounds will increase with proximity to the point source and reduce overall water quality.
Agricultural pollution, on the other hand, is harder to pin-point in such a manner as it will tend to enter waterways in a diffuse way as chemicals percolate into groundwater or are washed into surface flow. Agriculture has long been a source of water pollution, starting earliest with the clearing of land for farming which increased runoff velocities and sediment yields. But both the range and degree of agricultural pollution has increased dramatically over the last half-century - agriculture is now the dominant source of non-point pollution (i.e. pollution entering the drainage system over a wide area) in over 50% of American rivers (Jones, 1997). Perhaps the most important source of agricultural pollution now results from the addition of fertilizers to the land, used to correct nutrient deficiencies which reduce crop growth. These contain phosphorous, potassium, sulphur, and notably nitrogen, which is not freely absorbed by the soil but moves in water flux. Temporal variations in nitrogen concentration in streamflow for an arable catchment will occur as nitrogen fertiliser is applied in spring or early summer, and losses are greatest during winter due to minimal plant uptake. Spatial variations in compound concentrations will obviously reflect the land use of a catchment area and the farming methods applied.
Agriculture can also affect water quality through alterations in vegetation and crop type. Removal of a certain crop type will leave certain nutrients that were previously absorbed to be leached out of the soil into neighbouring water stores. The removal of vegetation altogether is a human activity which has great repercussions for water quality. Deforestation or the removal of crops will weaken the bonding of the soil, resulting in increased soil erosion and sediment load to the river channel and so a decrease in water quality. Conversely the planting of forests in upland areas for commercial forestry can also lead to spatial variations in water quality. These are caused by the increased capture of atmospheric pollutants, alterations to throughfall chemistry and the acidic nature of the litter layer under conifers, as well as increased evaporation losses, which increase solute concentrations in the remaining water.
Human activity, especially in urban catchment areas, will also necessarily result in an increased collection of numerous chemical substances in the river basin system. These range from lead deposits, automobile exhaust condensate and other organic deposits, the effects of which are magnified in settlements lacking adequate garbage and sewage systems. Spatial variations will therefore occur between settlement sites and also between urban and non-urban areas. Temporal variations can also be identified over the long-term, for example in Britain between 1940-1980 nitrogen and lead concentrations in water quality increased eight-fold as part of an overall intensification of human activities.
It is important to note, however, that man's influence on water quality will not always be detrimental. By the use of various chemicals and treatment, humans can purify and improve water quality, as is shown by the numerous treatment plants which purify water before it is pumped to our houses. These use processes such as chlorination, flocculation, decantation, absorption by activated carbon and disinfection to remove large amounts of soluble chemical compounds, organics materials and particulates found in water. Careful land management and the modified/controlled use of agricultural chemicals is also human action which can help to improve the quality of our water resources, and naturally these will vary both spatially and temporally, being more likely to occur in more technologically advanced regions of the developed world.
It is clear, therefore, that many factors control the variation of water quality through time and space. Many of these are natural factors resulting from changes in the geological location of the water resources or climatic alterations in precipitation. However, man induced effects on water quality through the use of agriculture and industry, most usually in a negative sense with the associated increased chemical concentrations into water sources, also play a major role. Large scale water treatment programmes to improve the quality of our drinking water must also not be forgotten., in conclusion therefore it seems fair to say that water quality will vary greatly both spatially and temporally in both short and long-term scales due to a variety of natural and human controlling factors.
Bibliography
Ward, R and Robinson, M Principles of Hydrology (London, McGraw-Hill, 2000)
Jones, J Global Hydrology (Harlow: Longman, 1997)
Walling, D and Webb, B Water quality: physical characteristics. (1996)
