The potential seriousness of the problem suggests that you should at least analyze the soil samples. I would also advise you to measure the electrical potentials in the area.
Salinity by itself does not cause rapid corrosion of cast iron pipes. Copper should not be affected by salinity. Soil acidity can sometimes be very high, with pH being 3 or lower. In this case, the soil will smell strongly acidic. Again, copper pipes should not have too much trouble with acidic soil, so this may not be the cause. Anerobic bacteria can cause damage to both cast iron and copper, so that may be worth checking.
Is this problem in a metropolitan area? Are there tram lines or railway lines in the proximity of this building? Are there any earth electrode stations of the tram/rail operator nearby?
At the moment you are worried about pipes. But whatever causes the cast iron to corrode can also attack reinforcement bars in concrete. If that happens, you caould be looking at a potential structural failure. So it is important to understand the failure mechanism. The solution development has to wait till you understand what causes the pipes to hole.
p.s. Do you mind signing your query?