Measuring salinity

Fact sheet
August 2015

Salinity is the measure of the concentration of dissolved (soluble) salts in water from all sources.

In general:

surface water salinity is described by units of “electrical conductivity” (EC)
groundwater salinity is described by units of “parts per million” (PPM)
scientific reports use “decisiemens per metre” (dS/m) as the main unit of measure.

As temperature is important in salinity calculation (usually measured at 25ºC), most EC meters have built-in temperature compensation. This means that water samples can be measured quickly and accurately in the field.

Total dissolved solids (TDS) and total soluble salts (TSS) are measured by different processes, but can be generally thought of as the same.

Sodium Chloride (NaCl) is the dominant salt usually found in stream sampling; however other salts will also be registered with EC readings (e.g. carbonate and bi-carbonate salts, magnesium and calcium sulfates, potassium).

Commonly used units of measurement

ppm = parts per million*
mg/l = milligrams per litre*
μS/cm = microseimens per centimetre (recognised EC units)
mS/cm = milliseimens per centimetre
dS/m = decisiemens per metre

*Parts per million (ppm) are considered the same as milligrams per litre (mg/L).

Conversion for units used to measure salinity:

1 dS/m

= 1 mS/cm

= 1,000 EC μS/cm

=approximately *550 ppm

(parts per million - TDC)

(and milligrams per litre - mg/l)

*depending on the types of salts present

Simply times (x) EC (μS/cm) by .55 to get an approximate ppm equivalent.

To convert TDS - ppm back to EC units you divide the ppm value by .55.

ppm can be thought of as the same as mg/l

1 (gpg, grains per gallon)

= approximately *14 ppm

* a less common unit, but used occasionally