Stream Water Monitoring Site Measurement: 
Conductivity is the ability of the water to conduct an electrical current.  Dissolved ions in the water are conductors.  The majority positively charged ions are sodium (Na⁺), calcium (Ca²⁺), potassium (K⁺), and magnesium (Mg²⁺).  The majority negatively charged ions are chloride (CI^-), sulfate (SO₄^-2), carbonate (CO₃^-2), and bicarbonate (HCO₃^-).  Nitrates (NO₃^-2) and phosphates (PO₄^-3) are minor contributors to conductivity, although very important biologically.

Salinity is a measure of the amount of salts or ions in the water.  Because dissolved ions increase salinity as well as conductivity, the two values are related.  The salts in sea water are primarily sodium chloride (NaCI).  However, other saline waters, such as Mono Lake, owe their high salinity to a combination of dissolved ions including sodium, chloride, carbonate and sulfate.

Importance of Conductivity (TDS):
Conductivity can affect the quality of water used for irrigation or drinking.  Most aquatic biota tolerate a range of conductivity.  However, the ionic composition of the water can be critical.  For example, cladocerans (water fleas) are far more sensitive to potassium chloride than sodium chloride at the same concentration. 

Conductivity will vary with water source such as ground water, water draining from agricultural fields, municipal wastewater and rainfall.  Therefore, conductivity can indicate groundwater seepage or a sewage leak. 

Acceptable Conductivity Ranges:
Below are some values of conductivity and salinity to give you an idea of possible data ranges you might encounter on this chart.  Waters that might have higher conductivity than reported here are rivers or drainage ditches dominated by subsurface agriculture return flows; ephemeral streams or pools late in the season; and naturally saline lakes or ponds.