Sulfate ions may enter surface waters from agricultural drainage, industrial and municipal waste and drainage from mineworkings. Also sulfur dioxide released by burning coal (and from other sources) is oxidised in the atmosphere to SO3, which dissolves in raindrops as sulfuric acid (H+(aq) and SO42− (aq)). It can enter groundwater as it moves through sulfur-containing minerals, which dissolve over time.
The standard field test method for SO42−(aq) is to add an acidic solution of barium ions to the sample, to form a precipitate of barium sulfate which forms a suspension. The resulting turbidity is proportional to the sulfate concentration of the sample and can be measured using the equipment described in the next section (Section 4.1).
Alternatively, the well established thorin test is used. The reagent is a complicated looking molecule (shown in Structure 1) that forms a barium complex. It also has a rather complicated chemical name; hence we're referring to it by its common name - thorin. Sulfate ions displace barium forming barium sulfate and free thorin. This is accompanied by a colour change from red to yellow.