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Home The River Basin People and the River Governance Resource Management
The Limpopo River Basin
 Introduction
Geography
Climate and Weather
Hydrology
Water Quality
 Principles of Water Quality
 Physical Characteristics
 Water Temperature
 Dissolved Oxygen
Conventional Variables
 Chemical Parameters
 Biological Parameters
 Qualitative Characteristics
 Human Impacts to Water Quality
 Water Quality Fitness for Use
Ecology and Biodiversity
Sub-basin Summaries
 References

 



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Water Quality: Conventional Variables  

Conventional variables are water quality indicators commonly measured to establish a general picture of the aquatic environment, including the influences of the drainage basin and local environmental conditions, seasonal and annual variability, and the ability to support aquatic life.

pH

pH is a measurement of the hydrogen ion (H+) concentration in water, and characterises the acid/base balance of water. A pH of 7 represents neutral conditions, pH greater than 7 indicates basic (alkaline) conditions, and pH less than 7 indicates acidic conditions. The pH of most natural waters is between 6,0 and 8,5. One pH unit represents a tenfold difference in hydrogen ion concentration; pH 6 is ten times less acidic than pH 5. (pH is calculated as the negative logarithm of the hydrogen ion concentration: pH = –log [H+]).

pH affects both biological and chemical processes. Values of pH below 4,5 and above 9,5 are usually lethal to aquatic organisms, and even less-extreme pH values can affect reproduction and other biological processes.

pH affects the solubility of organic compounds, metals, and salts. In highly acidic waters, certain minerals can dissolve metals and other substances, releasing them into the water. pH can also determine the chemical form of some chemical components, which in turn can affect reactivity, bioavailability and toxicity. For example, many metals are more bioavailable (more readily absorbed by living things) at low pH because they become ionised. Many organic contaminants are weak organic acids, and are more likely to enter organisms at low pH because they are un-ionised.

For instance, ammonia is a common toxic chemical in water, and takes two forms: ammonia, NH3, and the ammonium ion, NH4+. The relative proportion of the two forms depends on pH and temperature. Un-ionised ammonia (NH3) is more toxic to aquatic organisms because it can cross biological membranes such as gills. As pH or temperature rises, the relative proportion of NH3, and hence the toxicity to aquatic organisms, increases.

The pH of a waterbody reflects its water inputs and the chemical characteristics of the surrounding land. The pH of runoff from the land, or of groundwater inputs to surface water, depends on the minerals and soils the water contacts as it moves through the land. Water draining from forests and swamps may be acidic, having dissolved weak organic acids from the organic matter there, whereas water moving through limestone deposits may be slightly alkanine.

A standard pH meter.
Source: Datamax 2008
( click to enlarge )

Total Dissolved Solids and Conductivity

The concentration of total dissolved solids (TDS) is a measure of the amount of dissolved material in water. TDS includes solutes such as sodium, calcium, magnesium, bicarbonate, chloride and others that remain as a solid residue after evaporation of water from the sample. Fresh water usually has TDS levels between 0 and 1 000 mg/L, depending on the geology of the region, climate and weathering, and other geographical features that affect sources of dissolved material and its transport to a water system.

Conductivity is a measure of the ability to conduct an electric current and is the opposite of resistance. The higher the concentration of ions in water, the more current it can conduct. Conductivity is therefore sensitive to the amount of dissolved solids—particularly mineral salts—in the water, and also depends on the amount of electrical charge on each ion, ion mobility, and temperature. There is a well defined relationship between TDS and conductivity for water collected from a specific region.

Expressed in units of microsiemens per centimetre (µS/cm), conductivity generally ranges between 10 and 1 000 µS/cm in most rivers or lakes that have outflows.

Suggested classification of water based on conductivity.

Conductivity

Classification

< 600 µS/cm

Freshwater

600-6 000 µS/cm

Moderately saline

> 6 000 µS/cm

Saline

Source: Talling and Talling 1965

In addition to natural weathering and input of geologic material, sources of TDS include mining, industrial and sewage effluent, and agriculture. Field measurements of conductivity can be used to delineate a pollution zone – the extent of influence of an effluent discharge or run-off waters.

High levels of total dissolved solids and conductivity render water less suitable for drinking and irrigation.

A hand-held TDS meter.
Source: HM Digital 2008
( click to enlarge )

Suspended Sediment

Suspended sediment is characterised as the mass of suspended sediment per unit volume of water, in units of mg/L, and is commonly expressed as TSS (Total Suspended Sediment). Particles suspended within the water column are usually less than 0,1 mm in diameter, and are mostly silt- and clay-sized particles. These particles are transported by flowing water, and settle out when flow is insufficient to keep them in suspension. Concentrations of suspended sediment generally increase during periods of increasing flow from rainfall, reach a maximum at or near the peak of a storm hydrograph, and decrease relatively rapidly with the hydrograph recession limb (Beschta 1996).

 



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