The enquiry was launched yesterday after a judge studied a complaint made in 2010 by the environmental group L'Escurçó.
The group cited a 2002 study showing that the semen of 53 per cent of Tarragona's men had semen which did not measure up to World Health Organization parameters.
Although the case has previously been archived on two occasions, it was re-opened in February at the behest of the public prosecutor, according to local daily El Periodico.
The judge has now asked the Civil Guard to identify chemical industry companies in Tarragona which emit substances capable of reducing male fertility.
L'Escurcó's case was based on research carried out by the Marques Institute of Gynaecology and Obstetrics which revealed that the average sperm concentration of Tarragona men was almost a third lower than in some other Spanish cities.
It also found that the average sperm motility was much lower than average: less than a quarter of that of Galician men and less than half of those in nearby Barcelona.
Emissions from Tarragona's two petrochemical industry zones were identified by L'Escurçó as a possible factor in the low figures.
Some 500 commonly used chemical compounds can have oestrogenic effects, disrupting the endocrine system of humans and interfering with fertility.
Although it is commonly thought that sperm counts are dropping worldwide, recent research using modern techniques does not support that belief.
Sperm counts and semen quality have steadily fallen in some placed but have remained stable elsewhere.