We can think of a water sample as consisting of pure water containing various dissolved substances (gases and solids), and particulates (substances not dissolved in the water). Solids dissolved in the water are collectively called the total dissolved solids or TDS. A straight forward method of measuring TDS is to filter out the particulates and then evaporate the water in a pre-weighed dish, getting a weight for the solids left behind from a measured amount of sample. Units for TDS would be milligrams per liter (mg/l). Not included in TDS would be dissolved gases (see dissolved oxygen). Note, that if we weighed the solids caught on the filter, we would have a value for TSS in the same sample. This approach defines dissolved versus particulate solids by the effective pore size of the filter, because any very small particulates that pass through the filter become part of the TDS weight.
Conductivity and salinity are essentially measures of the salts dissolved in a sample. Generally, this is close to the TDS. Conductivity measures how well the water sample conducts an electrical current, a property which is proportional to the concentration of ions in solution. The advantage of using conductivity over TDS is the ease with which measurements can be made. Conductivity or specific conductance (in units of microsiemens/cm or the older micromhos/cm) is the usual measure of "salts" in fresh and slightly brackish waters. However, some instruments will, by changing scales, permit conductivity measurements in saline waters.
Salinity is a special measure of dissolved solids based upon concentration of ocean salts. Technically, the measurement of salinity requires comparing a sample's TDS, conductivity, or other physico-chemical property with that of a standard sea water. Salinity is the usual measure of salts in sea water and in brackish water derived from mixing of fresh and sea water. A conversion program for conductivity to salinity is posted at the Cerrito Creek Monitoring Program website.
Water Quality Standard
Hawaii (US) Fresh water stream -- Specific conductance not more than 300 micromhos/cm. Estuary -- Salinity shall not vary more than 10% from ambient Embayment -- Salinity shall not vary more than 10% from natural or seasonal changes considering hydrologic input and oceanographic factors Open Coastal Water -- Salinity shall not vary more than 10% from natural or seasonal changes considering hydrologic input and oceanographic factors.