There are two forms of chromium found in drinking water chromium (3) and chromium (6), these are different oxidation states of chromium and can be interconverted from one to the other. Chromium (6) is more toxic than chromium (3) and has been linked with carcinogenic (cancer causing) properties.
Unfortunately chlorination of our drinking water leads to a predominance of the oxidised chromium (6) form. The limit set for chromium is at a low level based on the total chromium assuming a high proportion of chromium (6).
Chromium is discharged into rivers from processes associated with the tanning and metal working industry.