Conventional Variables

Conventional variables are water quality indicators commonly measured to establish a picture of the general status of the aquatic environment, including the impacts of the drainage basin and local environmental conditions, seasonal and annual variability, and the ability to maintain aquatic flora and fauna.

pH

The pH value is a measurement of the hydrogen ion (H⁺) concentration in water, and characterises the acidity and basicity of a solution. A pH of 7 represents neutral conditions, pH values >7 indicate basic (alkaline) conditions, and pH values <7 indicate 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⁺]).

Laboratory setup for electrical conducivity measurement. Source: Heinze 2008 ( click to enlarge )

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

pH affects both biological and chemical processes. Values of pH <4.5 and >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. Highly acidic waters can dissolve certain minerals and metals. 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 as non-ionised contaminants.

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.

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 resistivity. 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.

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.

Suspended Sediment

Suspended sediment is defined as the mass of suspended sediment per unit volume of water, expressed in mg/L, and is commonly referred to 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).