3.5 Biological characteristics of natural waters In addition to the easily visible plants and animals which live in or on a river, there are many small and often microscopic species which play a vital role in maintaining the health of a river. Their relevance to water quality is discussed further in the sections that follow.
3.4.2 Radiological aspects Environmental radiation comes from a range of naturally occurring and anthropogenic sources, with the former estimated to contribute more than 98% of the radiation dose experienced by people (excluding medical exposure). Any exposure to radiation can lead to cancer, and the greater the exposure, the greater the risk. The contribution that drinking water makes to radiation intake is very small, and is due largely to naturally occurring radionuclides (isotopes of an element which are unst
3.4.1 Plant nutrients Plant nutrients are necessary in varying amounts for the growth, reproduction and well-being of growing plants. Of the major nutrients of plants, nitrogen and phosphorus are important growth-limiting factors in primary production (i.e. they are likely to run out before any other element needed by the plants). Both nitrogen and phosphorus enter watercourses from natural leaching by water of the soluble nitrates and phosphates found in soils and rocks, as well as from sewage effluent and
3.4 Chemical characteristics of natural waters Since water is such a good solvent, it is not surprising to find many different chemical substances present in it. Water, on reaching a river, will contain inorganic and organic compounds which were dissolved as rainwater percolated through the soil and rocks. In addition, some gases will dissolve in rainwater during its passage through the air. The substances present in water may be conveniently grouped into: those from dissolved gases such as o
3.3.3 Temperature All aquatic organisms have a fairly well-defined temperature tolerance range and this determines their distribution. Temperature affects the saturation concentration of dissolved oxygen (as seen in Table 2). An increase in water temperature will reduce the oxygen solubility as well as increase the metabolic activity of aquatic organisms. The combination of these two effects means that oxygen demand by organisms increases just when oxygen supply is being reduced. Coarse fish such as perc
3.3.2 Speed of flow/turbulence To get nutrients and dissolved oxygen to all parts of a body of water, good mixing is important. We have had one example in the thermal stratification in deep lakes in summer (Figure 7), where poor mixing and reduced sunlight lead to the bottom layers not being supplied with the necessary conditions for plant growth. The same principle applies to rivers and streams. Fast-moving turbulent streams mix and agitate the water, aiding the transfer of oxygen from the atmosphere to the river, and car
3.3.1 Turbidity, colour and suspended solids As water runs off the land, there are some substances which do not dissolve but are taken along as suspended solids. Then, depending on their sizes and the velocity of the river, the solid particles may settle out at a certain point or be carried on further. Quantities are affected by seasonal changes and tend to be higher in winter because of increased storm runoff due to higher rainfall and melting snow. The quantity of suspended solids (measured in g m<
3.3 Physical characteristics of natural waters A river's physical characteristics include: clarity/turbidity colour speed of flow/turbulence odour the presence of plants and macroscopic animal life. The physical characteristics are determined by location, geology and climate of the catchment area. In turn they influence the chemical and biological characteristics of the watercourse. The physical appearance may give yo
3.2 Dissolved oxygen Organic and inorganic nutrients are the basic food supply essential for maintaining the plants and animals in natural watercourses. Equally essential to aquatic life is a supply of oxygen, needed for respiration. Oxygen dissolved in the water is also needed in the biodegradation of organic matter by aerobic (oxygen-consuming) bacteria. A measure of this oxygen demand can be obtained experimentally and is defined as the biochemical oxygen demand (BOD). The BOD i
3.1 Water, the medium of life The list of necessities for the provision of life includes various nutrients and water: water is one of the basic resources needed for the process of photosynthesis. Since it is an excellent solvent, water, even in its 'natural' state, is never pure H2O but contains a variety of soluble inorganic and organic compounds. Water can also carry large amounts of insoluble material in suspension. The amounts and types of impurities vary with location and time of year, and determine some o
2.12 Summary The hydrological cycle is a complex process involving evaporation, transpiration, condensation, air circulation, precipitation, infiltration, surface run-off and percolation. Aquifers are an important source of fresh water supplying about one-third of the UK's potable water demand. Most of the world's water is present as saline water in the oceans. 2.11 Storage In a given fixed space at any phase of the hydrological cycle, there is an inflow and an outflow of water, the rates of which vary with time. The total cumulative difference between inflow and outflow is the storage. So within that space there is a body of water whose mass is not directly controlled by instantaneous values of inflow and outflow. For example, in river flow the movement of the whole body of water in the channel is generally downstream, yet a given reach contains a volume whose 2.10 Aquifers Groundwater is water that, after infiltrating and percolating through surface soils, flows into an aquifer, an underground water-bearing layer of porous rock. About one-third of the UK's drinking water is drawn from aquifers. To permit economic development, an aquifer must be able to transmit large quantities of water from one point to another and therefore it must have a high permeability. The groundwater contained in aquifers is released from springs and 2.9 Percolation Movement of infiltrated water downwards through the zone of aeration (Figure 5) is known as percolation. The infiltrated water which does not remain held by capillary forces in the surface soils moves by the action of gravity through the unsaturated layers of soil or rock until it arrives at the water table. Here the percolated water joins the body of groundwater which seeps slowly to the sea. 2.8 Surface run-off In some inland drainage areas, all water is removed by evaporation and infiltration. However, precipitation not penetrating the land surface usually runs off the surface along defined channels which have been produced by geological processes, previous storms, or possibly by people. This accelerates the process. Its eventual destination is the ocean, except, of course, where it runs to inland seas such as the Dead Sea. It is in the runoff phase of the cycle that physical intervention by humans 2.7 Infiltration Entry of precipitation through the soil surface and on downwards, by gravity, is known as infiltration. The rate at which this process can take place is governed by the permeability (a measure of the ease with which water can flow through the subsurface layer) and by the existing degree of saturation of the soil. Infiltration can be impeded by outcropping impermeable rocks or by paved areas, and also by the presence of finegrained soils with a low permeability (such as clay). At certain times 2.6 Precipitation Precipitation is defined as the depth of rainfall, or the water equivalent of snow, sleet and hail falling during a given measurement period. It may be in the form of rain, snow, sleet or hail, or in minor forms such as dew and hoar frost, but existing theories do not yet satisfactorily account for all the observed characteristics. In tropical climates, precipitation occurs as a result of the gradual coalescence of the tiny condensed droplets as they collide within the cloud l 2.5 Air circulation At this stage, air circulation enters and plays a dual role. Firstly, winds transmit moisture horizontally from one location to another. In this way, moisture derived from oceanic evaporation can be transported many miles to a land mass. Secondly, convective or vertical currents arising from unequal heating or cooling can transmit moisture upwards. When it cools, some of the water vapour condenses. It is from these currents that most precipitation develops. 2.4 Condensation As air rises it expands, owing to the decrease in pressure with height, and as it expands, in theory it cools at an average rate of 1°C for every 100 m of altitude. As the air cools, it becomes saturated with water vapour which condenses around small particles in the air. These particles may occur naturally, such as soil particles or salt particles residual to evaporation of sea spray, or they may be produced artificially during combustion. A measure of the necessary cooling to produce conde 2.3 Transpiration If there were no vegetation, the rate of evaporation from land surfaces after rain would diminish rapidly to a very low value. Plants increase this rate by transpiration. In this process, water is transferred from the soil through the roots to the leaves by osmosis and capillary action. Water evaporates from the surface of the leaves and the resulting vapour diffuses into the atmosphere. For hydrological measurements, this phenomenon is frequently lumped with evaporation because the two proce
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