Chemical water quality and pollution
Water quality varies naturally
Natural freshwater ecosystems vary widely in water quality.
The concentrations of naturally occurring chemicals dissolved in water (such as sodium, chloride or sulphate) varies between locations. This can be due to differences in geology, distance to the sea, or the downstream progression of water from headwaters.
Likewise, the concentrations of gases such as carbon dioxide or oxygen also changes:
- where discharge from ground water carries elevated carbon dioxide;
- where higher temperatures decrease the amount of gas that can dissolve in the water;
- where the decomposition of natural organic matter (e.g. dead plants and leaves) depletes oxygen;
- where the balance between plant photosynthesis and respiration alters the balance of carbon dioxide and oxygen;
- where changes in water turbulence affect how rapidly dissolved gases exchange with the atmosphere;
- where different vertical water layers form in lakes (stratification), preventing gas exchange with the atmosphere;
- or even where altitude changes the amount of gas available (think of how climbers have to use breathing equipment because of the “thin air” on top of high mountains).
Physical factors that affect water quality, such as temperature, also vary. As another example, think about the physical changes in a river as it journeys through its landscape. From steep slopes with turbulent, fast-flowing water eroding sediment through to slow moving rivers on plains depositing sediment on river beds and river banks.
Natural variations in water quality also happen through time. Dominant effects reflect seasons, for example because of variations through the year in nutrient uptake for plant growth.
Even more substantial changes occur when hydrological events such as snowmelt, rainfall events or drought dilute or concentrate the amount of chemicals dissolved in the water.
All of these dynamic, natural variations have substantial effects on individual organisms, biological communities or ecosystem processes. Indeed, these are some of the most significant effects impacting the ecological character of freshwater ecosystems.
Pollution also has major effects
Layered over natural variations in water quality, however, are the effects of a very wide range of pollutants that arise entirely from human activities.
Intriguingly, some of the substances classed as pollutants, such as nutrient and sediments, also vary naturally in freshwater environments. It was for this reason that Sir Martin Holdgate (1979) defined environmental pollution as
“the introduction by people, into the environment, of substances or energy liable to cause interference with legitimate uses of environment”.
In other words, he considered human activities as a key source of pollution, while also linking his definition to legal standards. Others have since extended this definition to suggest that pollution occurs where contaminants disrupt normal environmental processes.
Substances classed as pollutants include:
This is a dominant source of pollution around the world. It’s still a very major issue in urban regions of the less developed world where untreated or partially treated sewage enters freshwater ecosystems. As well as the dissolved pollutants, organic wastes often contain solid particles of waste and pathogenic organisms such as bacteria.
Organic wastes can also arise from processes such as leather tanning, pulp and paper manufacture, wood processing, food processing, agricultural wastes such as cattle slurry or discharges from intensive pig or poultry units.
Major problems arise when the microbial decomposition of large organic concentrations depletes oxygen concentrations faster than they can be replenished. At worst, this can lead to a complete lack of oxygen, particularly in sediments.
Phosphates and nitrates used as agricultural fertilisers can subsequently leach into rivers and lakes downstream, dramatically increasing fluxes above natural background concentrations with consequences for algal production (‘cultural eutrophication’) and water supply or safety.
Metals such as lead, nickel, cadmium, zinc, copper, mercury or aluminium enter freshwater ecosystems from a wide range of industrial processes, water leaching from landfills, some agricultural processes, and from activities such as mining.
Gases such as chlorine, ammonia and methane, and anions, such as cyanides, fluorides and sulphites are also considered as pollutants.
Alkalis and acids
These pollutants are often the result of industrial processes but can also appear from the formation of acid rain caused by sulphur and nitrogen oxides formed by fossil fuel combustion. Acid rain can have a direct effect on water ecosystems by making the waters too acidic for life. It can also leach metals such as aluminium from soils, causing additional ecological impact in acidified waters.
Complex organic compounds
Complex organic compounds include pollutants referred to as ‘xenobiotic substances’ or Persistent Organic Pollutants (POPs). These compounds include pesticides, herbicides, PCBs, a range of hydrocarbons, solvents, surfactants, dioxins, phenolic compounds, flame retardants (eg PBDE). Arising from a wide variety of industrial, agricultural, transport and domestic sources, we’ll learn more about these pollutants in Week 3.
Oil and petroleum products
These pollutants are often the result of spillages or accidents at extraction sites or during transportation. They’re also part of a cocktail of pollutants entering water courses from road runoff.
A group of substances that are receiving increasing attention where they arise from human uses or domestic livestock treated either curatively or preventatively for a wide range of conditions. Painkillers, anti-inflammatory drugs, chemotherapeutic agents, endocrine disrupting substances, anti-depressant drugs, anti-helminthic treatments (for parasitic worms, dominantly from livestock), antibiotics or their metabolites are all now widespread in surface waters among hundreds of chemicals in this group.
Plastic pollutants vary in size (macro, micro and nano) and are currently attracting attention. They can have detrimental physical effects on organisms e.g. entanglement or obstruction, may be toxic in their own right, and can transport other pollutants (see ‘Emerging Pollutants’ in Week 3).
Solid particles of a wide range of size fractions from silts and clays to sand and gravel can enter freshwater ecosystems from agriculture, forest operations, mining, construction, urban landscapes or as part of sewage wastewater. They affect organisms by inflicting physical damage, occluding surfaces, blocking pore spaces on the beds of lakes or rivers, and reducing the transport of oxygen rich water.
Heat and light
Heat and light are also considered increasingly as pollutants, for example where warm-water effluents enter surface waters from industry or power station coolants, or where artificial light at night (ALAN) disrupts a wide range of organism behaviours.
In the next step we’ll look at how chemical water quality has changed over time and in different areas.
Over to you
- Are you aware of any legislation protecting water sources from pollution in your country?
- Do you think water pollution is regulated well-enough?
Let us know in the comments.
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