Simply put, Electrical Conductivity (EC) is the ability of a subject, in this case water, to conduct an electrical current. Depending on the dissolved content of the water, it can either be an efficient conductor or act more as an insulator.
The standard measure for water is specific conductance (µS/cm) which is a conductivity measurement corrected to 26°C.
Salinity is a measure of the degree to which something is saline i.e. the amount of salts dissolved in the water. Salinity is a somewhat non-specific measure and difficult to measure as it includes all salts. Typically measured in Practical Salinity Units (PSUs).
Total dissolved solids (TDS) are compounds which have been dissolved into the water column and would pass through a 2-micron filter, for example minerals, salts and metals. This includes disassociated electrolytes which would contribute to salinity as well as other dissolved organic matter. Usually reported in mg/L.
Conductivity/Specific conductance is the main parameter and one of the most useful water quality parameters.
Conductivity is the easiest to measure and is found in most standard water quality surveys worldwide. EC measurements tell us how much dissolved substances are present in the water and therefore when changes in the EC baseline are detected, it can indicate a polluting event or other change in the water characteristics. Even small amounts of dissolved salts can have a significant impact on the EC level and therefore it is a useful measure of pollution detection. Generally, human impacts will tend to increase the EC level. The main concern is wastewater/sewage contamination which will have other indicators besides a change in EC level but EC remains a robust measure as EC can be high even if turbidity remains low. EC change cannot tell you the source of the pollution but will help in the identification of a pollution issue which can affect aquatic life.
Salinity is calculated from the EC measurement to understand the level of dissolved salts in the water. Different flora and fauna are adapted to exist within different salinity ranges. In freshwaters, we expect almost zero naturally occurring salinity. Where freshwater and saltwater meet, for example in estuarine environments, there will be a changing salinity gradient in what we define as Brackish water.
Salinity is important as freshwater aquatic organisms and plant life often won’t survive in even a 1 ppt salinity concentration. Salinity is a concern in industry effluent, areas where water is withdrawn for uses such as irrigation, in urban areas where de-icing is common and where there are wastewater discharges.
TDS can also be calculated from an initial conductivity reading. TDS is calculated form conductivity on the basis that the dissolved solids that are present in the sample are ionic and that the relationship to conductivity is established. A full TDS measurement is incredibly time consuming so using conductivity to calculate it is far more common.
TDS includes both organic and inorganic dissolved components. TDS can enter water bodies from just about anywhere and it is expected that all natural waters have a background level of TDS as there are natural sources for example groundwater that flows over rocks will absorb minerals from the rocks directly.
Very high levels of TDS in water can affect the taste, making the taste more salty or bitter but in general, TDS levels are more of a concern for aquatic organisms than humans. As TDS pertains to the mineral content of the water, it is important for the fish and other aquatic organisms inhabiting the water source. High TDS levels see an increase in the amount of salt which can cause fatal dehydration in some animals. TDS can also affect the local temperature pushing animals into physiological stress if it approaches or exceeds the boundaries of their tolerance. A TDS that is too low can negatively impact fish growth and even cause death.
Conductivity uses a four-electrode sensor with an easy-clean flow through design to minimise the likelihood of biofouling. Four-electrode sensors work by establishing the current required to run a fixed voltage between two pairs of electrodes that are separated by a fixed geometry of water.
If TDS or salinity is required, these also use the conductivity sensor with an adjusted calibration using the specific standard for that parameter.
